Transcription
Let's see how quickly we can cover everything you need to know for Ed EXL GCSE chemistry paper 1. This is good for higher and Foundation Tier, double combined and triple separate. That's topics 1 to 5: key concepts, states of matter, chemical changes, extracting metals, and equilibria. And the special triple separate chemistry one topic we're going to be Bez in it.
So, pause the video if you need a bit more time to get your head around something you see. Let's go.
Substances. Stuff are made of atoms. The different types or elements of atoms there are are represented in the periodic table by a symbol. A compound is a substance that contains two or more different types of atoms chemically bonded together. For example, the chemical formula for water is H2O. It's made up of hydrogen and oxygen atoms. For every one oxygen atom, there are two hydrogen atoms. If there's no number after a symbol, there's an invisible one there.
These atoms change what they're bonded to and how they're bonded through chemical reactions. We can represent a reaction with a word equation and a chemical equation using symbols. As atoms are not created or destroyed in any chemical reaction, there must be the same number of each type of atom on both sides. So, sometimes we must balance equations.
Pro tip: start balancing atoms that are only in compounds. So, with this one, let's go with the carbons first. There's one on the left, one on the right, so that's all good. Hydrogens: there are four on the left, only two on the right. Now, we can't change the small numbers because that would change what the compound is. So, what we can do is put numbers in front of elements or compounds to multiply them up. Stick a two in front of the H2O. We now have 2 * 2 hydrogens, so that's four. That's also doubled the oxygen in it, however. So, now we have four oxygens on the right, still only two on the left. So, doubling this O2 on the left takes care of that. If there's an element in a reaction, like oxygen here, we always finish balancing that as there's no knock-on effect.
The idea of what atoms are like came about gradually. JJ Thompson discovered that atoms are made up of positive and negative charges. He came up with the plum pudding model of the atom: a positive charge with lots of little electrons dotted around it. It was Ernest Rutherford who found that the positive charge must actually be incredibly small. We now call this the nucleus, and the electrons must orbit relatively far away from it. He discovered this by finding that most alpha particles fired at a thin leaf of gold atoms went straight through, proving that atoms must be mostly empty space.
Neils Bohr later discovered that electrons exist in shells or orbitals. Then James Chadwick discovered that the nucleus must also contain some neutral charges. He called them neutrons, while the positive charges are called protons. Protons and electrons have equal and opposite charges, so we just say they're plus one and minus one, relatively speaking. Neutrons have a charge of zero. Protons and neutrons have essentially the same mass, so we say they have a relative mass of one. Electrons are very light in comparison, so we say they have a mass of zero, or just very small, depending on the situation.
The periodic table tells us everything we need to know about an atom. The bottom number is the atomic number. That's the number of protons in the nucleus. This is what determines what element you have. Every atom has an overall neutral charge, so that means they must have the same number of electrons as protons. If an atom gains or loses electrons, it's now called an ion, not an atom. The top number is the mass number, or relative atomic mass, or RAM for short. It tells you how many protons and neutrons are in the nucleus. So, that must mean that this carbon atom, carbon 12, has six neutrons on top of its six protons to make that 12. However, you can get a carbon atom with seven neutrons instead, so its relative mass is 13. These are what we call isotopes: atoms of the same element but different numbers of neutrons.
You might see a number that isn't a whole number for the mass. This is because periodic tables sometimes show the average mass for all of the isotopes of that element found in the world. For example, if you have some chlorine gas, it turns out that 75% of the atoms will have a mass of 35, while 25% of the atoms will be 37. These are what we call their relative abundance. To find the average, we just pretend that we have 100 atoms. We add up the total masses of all the isotopes, then just divide by 100. That's why chlorine's average relative atom mass is 35.5.
The periodic table is incredibly useful, but how was it made? Before it, scientists just put elements in order of their atomic weights. Some were then grouped together if they were seen to have similar properties, but still using the atomic weight order. Dimitri Mendeleev then came along and grouped elements together based on their properties, even if the order didn't follow atomic weight. Using this method, he found there were gaps in his table. He asserted that these elements were yet to be discovered. In time, he was proven correct, showing that his table was indeed correct.
Like we said, electrons exist in shells around the nucleus. The shells fill up from the inside with a maximum of two on the first shell, eight on the second and third shells, then we only go to two on the fourth shell. That's 20 electrons altogether, which brings us to a calcium atom. After this, we get into the transition metals where things get a little bit crazy, so we leave that until A-level chemistry. So, we only care about the electron configuration going up to 2, 8, 8, 2. Magnesium has 12 electrons, so its electron configuration, for example, would be 2, 8, 2.
The modern periodic table can be split up into different sections. For example, everything to the left of this staircase is called a metal. Metal atoms always donate electrons to gain an empty outer shell of electrons. Again, slightly weird with transition metals, but we don't think about their shells. To the right of the staircase, non-metals. They always accept electrons to gain a full outer shell. The column an atom is in is called the group. It tells you how many electrons an atom has in its outer shell. Again, the transition metals work in a really weird way, so they don't get their own group. In fact, it turns out this is because they can donate a different number of electrons when they bond to different things.
The atoms in group one are called the alkali metals. They all have one electron in their outer shell, which they give away (donate) when they bond to something, so they have similar properties, like when they react with water. The further down the group you go, though, the further that outer electron is from the nucleus, so the electrostatic attraction is weaker between the negative electron and the positive nucleus. This means that the electron is more readily donated. This means the metals get more reactive as you go down to the group.
Group seven are what we call the halogens. They're essentially the opposite. They have seven electrons in their outer shell, so they need one more to gain a full outer shell. The further down the group you go, the less readily an electron is accepted onto that shell that's further away from the nucleus, so they get less reactive down the group. Their boiling points also increase down the group.
Two, group zero, sometimes referred to as group eight, are called the noble gases. They already have an empty or full outer shell, just depends on your perspective, so they don't react. In reality, they can react under special conditions, so we just say they're very unreactive. We don't really say group eight anymore, though, because some people thought that helium might feel a little left out as it only has two electrons in its outer shell.
As electrons are negative themselves, metals become positively charged when they lose them. They always form positive ions. All of group one lose one electron when they turn into an ion, so all of their ions are one plus. But again, we don't write the one, we just put plus. Group two lose two electrons to get an empty outer shell, so their ions are all two plus. Group seven gain one electron each, so all their ions are minus. Group six's ions are all two minus. The atoms in group three, four, and five don't really form ions, except for aluminium, which is 3+.
Like we said, transition metals can donate different numbers of electrons. For example, an iron ion can be Fe2+ or Fe3+. It can donate two or three electrons, so we give them the names iron two and iron three to distinguish between them. Transition metals are generally harder and less reactive than the alkali metals. They also form colored compounds.
Metal atoms bond to each other through metallic bonding. Essentially, a lattice or grid of ions is formed with a sea of delocalized electrons around them. Delocalized just means they're not exactly on the atom. As these electrons are free to move, metals make good conductors of electricity and heat.
Metals bond to non-metals through ionic bonding. Like we said, a group one metal needs to lose an electron, while a group seven atom needs to gain one. It's a match made in heaven. For example, a lithium atom donates or loans its outer electron to the chlorine. We can draw a dot and cross diagram to show where the electrons end up. You can choose which one belongs to which. We only need to draw the outer shell for each. Don't forget to put brackets and the charge of the ions.
When it comes to ionic bonding, the charges of all ions in an ionic compound must add up to zero. So, Li+ and Cl- is all good. So, this is the chemical formula for it. Same with beryllium oxide: Be2+ and O2-. Beryllium chloride, on the other hand, well, the beryllium needs to lose two electrons, while a chlorine only needs one. So, that means there must be two chlorines, or chloride ions, for every beryllium. So, Be2+ and two lots of Cl- adds up to zero. So, that means the chemical formula is BeCl2. Sorted.
Ionic compounds consist of lots of repeating units of these ions in a lattice to form a crystal. They have high melting points and boiling points due to the strong electrostatic forces that need to be overcome, and they can conduct electricity, but only in liquid form, that is molten, or when dissolved in solution. That's because the ions are free to move in both cases and they carry charge.
You can also get molecular ions. For example, OH- is a hydroxide ion and consists of a hydrogen atom and an oxygen atom. So, magnesium would need two of these to make magnesium hydroxide. Here are a few other examples, by the way. I spell sulfate with a 'ph' instead of an 'f' because I'm stubborn and refuse to adopt the American spelling. You'll get the mark either way. Any ionic compound can be called a salt, not only sodium chloride, your table salt. The name is always the metal ion (positive ion or cation, we can call it) followed by the non-metal ion or anion. Anion names are different from their normal names, like we've just seen. It's not sodium chlorine, but sodium chloride. Some people remember which way around cations and anions are by liking cats, and they say cations are positive.
Non-metals bond to each other with covalent bonding to form molecules. They do this by sharing electrons to gain full outer shells. For example, chlorine gas is Cl2. Each chlorine atom shares an electron with the other, so they're both happy. Never write down "happy" in the exam, though. Here's the dot and cross diagram. We can also draw the structural formula for molecules with just symbols and lines. We could also say that every one of these represents a dot-cross electron pair. Each oxygen needs two extra electrons, so O2 is a result of each oxygen atom sharing two electrons each. As such, this is a double covalent bond. Nitrogen, N2, is one of the few molecules with a triple bond. In covalent bonding, the number of electrons an atom needs is the same as the number of bonds it must make. Hydrogen can only ever make one bond, carbon makes four bonds, etc. Here's a few more. If you're not in a rush, pause the video and have a go with them, and here are the answers.
These above are what we call simple molecular or simple covalent structures: individual molecules that can mix together. These have relatively low boiling points as there are only weak intermolecular forces between them that need to be overcome with heating. Be careful, though, there's not covalent bonds being broken. Like we said, and unlike ionic compounds, these can't conduct electricity even as liquids.
Giant covalent bonding is similar to the lattice nature of ionic compounds. Atoms form covalent bonds to other atoms, which form bonds to other atoms, and so on, until what we have in effect is one giant molecule. Diamond is an example of this. It's a crystal of carbon atoms bonded to each other. That's why it's so hard and has such a high melting point. You would have to break the covalent bonds in order to do that, and they're incredibly strong.
Graphite is only made of carbon as well, but it's not diamond, so it's an allotrope of carbon, made out of the same atoms bonded together in a different way. Graphite consists of layers of carbons with three bonds each in a hexagonal structure. Where's the fourth bond, though? Well, the spare delocalized electrons form special weak bonds between the layers, which means that it can conduct electricity because the electrons can move between the layers, as well, and it also means the layers can slide over each other easily, which is why it's used in pencils.
As a side note, metal alloys are stronger than pure metals. Having mixtures of metals means that we have different size atoms, and that disrupts the regular lattice, so layers can't slide over each other as easily. Back to carbon allotropes: graphene is just a single layer of graphite. Fullerenes are 3D structures of carbon atoms. For example, Buckminsterfullerene is a spherical, football-like structure consisting of 60 carbon atoms. Each fullerene that has a tube shape are called nanotubes.
Just for triple, real quick: nanoparticles is the term given to structures that are between 100 and 2,500 nanometers in size, whereas particles bigger than this are called coarse particles, like dust. Surface to volume ratio is just one divided by the other. If the length of a side of a cube doubles, that means this ratio halves. As nanoparticles are tiny, this ratio is huge for them, which means that fewer could be needed to fulfill a purpose compared to larger ones.
Total mass of all substances is conserved in a chemical reaction. Like we said earlier, that must mean the atoms that go in must come out, so we must balance equations. To the end, we already know about relative atomic mass, but if it's a compound, we can add these up to give the relative formula mass. We just add up the individual RAMs. So, CO2 is 12 plus 2 lots of 16, so that's 44. Some reactions produce a gas product, which, if it leaves the reaction vessel, will result in a seeming decrease in mass of the reactants.
A mole is just a specific number of atoms or molecules, but we don't really need to know the number. It's just a way of comparing amounts of substances, as we can't deal in individual numbers of atoms or molecules. If you're foundation, you don't need to deal in moles, by the way. If you have as many grams of a substance as its relative atomic or formula mass, you have one mole. So, one mole of carbon has a mass of 12 g. That means we calculate the number of moles of something we have like this: moles = g / RAM, where RAM is short for relative atomic mass, but it also could be relative formula mass. This is an equation worth remembering.
Let's take our methane combustion reaction from earlier. Like we said, in order to balance this, we'd need two oxygen molecules per one molecule of methane. We'd need double the moles of oxygen to methane. So, here's how a question could go: How many grams of water would be made if 64 g of methane reacted completely with oxygen? We need to get from the mass of one thing to the mass of another, so we use moles as the middleman. The process is this: Mass -> Moles -> Moles -> Mass. We switch from one to the other at the halfway mark. So, a mass of 64 g of methane, how many moles is that? Moles = g / RAM, so that's 64 / 16, that's 4 moles of methane. But look, there's no number in front of the methane, but there is a two in front of the water, which means we must have double the moles of water, so that's 8 moles. By the way, we can say that the stoichiometry is 1 to 2, that just means the ratio of moles of one substance to another in a reaction. So, what we have to do then is turn that back into mass using our equation by rearranging it. Put it into a triangle if you have to, and cover up mass: g = moles * RAM. So, that's 8 moles * water's RAM of 18, that's 144 g of water made.
You could also be given the mass in kilograms or even tons. The great thing is that because this is all relative, we can just put those masses into our equation instead of grams, and so long as you stick with that unit for the whole question, you'll still end up with the right answer. Of course, we can also use moles to predict how much of a reactant we would need in a reaction. As you can see, we need two moles of oxygen to every one mole of methane. If we had that one mole of methane but only one mole of oxygen, that means that not all of the methane would react. Some would be left behind. We say that the oxygen is the limiting reactant in this case; it ran out first.
The concentration of solutions can be given in g/dm³, where a dm³ is 1,000 cm³, but it's often useful to convert this into moles/dm³ instead. If 1 mole of HCl is dissolved in 1 dm³ of water, we've made hydrochloric acid at a concentration of 1 mole/dm³. Sometimes we shorten this to just one molar.
Polymers are super long-chain alkanes made up of repeating sections made from monomers. Poly just means lots, mono just means one. For example, lots of ethenes, the monomer, can be joined together through addition polymerization to make polyethene, or polythene, that's the polymer. These monomers must have a double bond in. Note that even though it makes a long alkane, we still use the name of the alkene it's made from. It's polyethene, not polyethane. As you can see, this happens because the double bond splits, so a carbon can bond to the next monomer and so on. Thankfully, we only have to draw the repeating unit with brackets around it and the bonds coming out with an 'n' on the outside, showing that there are lots of these joined together.
Solid, liquid, and gas are the three main states of matter. For example, water can be ice, a solid, where the particles or molecules in this case vibrate around fixed positions. It can also be liquid water, where the molecules are still touching but are free to move past each other. And it can also be a gas, water vapor, we call it, when it's water, where the particles are far apart and move randomly, and they also have the most energy and so move quickly. As molecules in a gas are far apart, gases can be compressed, while solids and liquids cannot.
To melt or evaporate a substance, you must supply energy, usually in the form of heat, to overcome the electrostatic forces of attraction between the particles. We don't say we're breaking bonds in this case. Note that none of these make a new substance, so these have to be physical changes, again, not chemical reactions. We're not breaking any chemical bonds. In chemical reaction equations, we indicate what state of substance is in with state symbols: (s) for solid, (l) for liquid, (g) for gas, and also (aq) for aqueous, that means dissolved or in solution, again, like salt in water.
Obtaining pure substances is very important when it comes to chemistry. One way to tell if a substance is pure or not is by testing to see what its melting point or boiling point is. If it's pure, it should be a very specific temperature. A formulation is a mixture that has been specially designed to be useful in a very specific way with very specific quantities of different substances used to make things like paints, fuels, alloys, fertilizers. Think of George's Marvellous Medicine as being the ultimate formulation.
A mixture is any combination of any different types of elements and compounds that aren't chemically bonded together. For example, air is a mixture of oxygen, nitrogen, and more. Solutions are mixtures too, like salt water, a mixture of water and sodium chloride. You can separate large insoluble particles from a liquid using filtration, like sand from water, as sand can't dissolve. Crystallization can leave a solute (that's the solid) dissolved in a liquid behind after you evaporate the solvent from a solution, like salt from water. Similar distillation involves heating the solution as well, but this time the gas is cooled so it condenses back into a liquid. You can also do this at different temperatures to separate the different liquids of a mixture, as they will have different boiling points. This is called fractional distillation. These are all physical processes, though, and not chemical reactions, because no new substances are being made.
Chromatography is a way of separating substances in a mixture, for example, pigments in inks or drugs in a urine sample. The stationary phase, often special chromatography paper or just filter paper, is what the substances move up with the help of the mobile phase, often just water, which rises up the paper due to capillary action, dragging lighter particles further up the stationary phase. We draw the line at the bottom in pencil so it doesn't move with the solvent. The water then, at the end of the process, we measure how far the solvent has moved and also how far the substance or substances have moved too, and these are both measured from that starting line. We can then calculate an Rf value. That stands for a retention factor, which is just a ratio of how far a spot has moved compared to the solvent, so that ends up being a number between 0 and 1. We can compare Rf values of our spots with known Rf values to identify what's in our mixture.
The pH scale is a logarithmic scale, base 10. It's not linear. What does that mean? Well, an acid contains H+ ions, and an acid that has a pH of three will have 10 times the concentration of these compared to an acid of pH four. pH 3 would have a 100 times the concentration of H+ ions compared to an acid of pH 5, and so on. Alkali work in a similar way, but with OH- ions instead. The higher you go, the greater the concentration.
A strong acid is one that dissociates or ionizes completely when in solution, like hydrochloric, nitric, and sulfuric acids. Weak acids, on the other hand, only partially dissociate, like ethanoic, citric, and carbonic acids. The pH of an acid depends on both its strength and concentration. If hydrochloric acid and ethanoic acid have the same concentration, the hydrochloric acid will have the lower pH as it's stronger.
We can test for hydrogen by holding a burning splint over the test tube, which will produce a squeaky pop. Oxygen will relight a glowing splint. Carbon dioxide will turn limewater cloudy when bubbled through it. Chlorine gas will bleach damp blue litmus paper, that means turn it white. Metals more reactive than hydrogen can displace it from an acid, so most metals react with hydrochloric acid and sulfuric acid, for example. This produces a salt. Alkalis, they have a pH greater than seven, react with acids less than seven to produce a salt and water. If the quantities used are correct according to their stoichiometry, they will neutralize each other completely to leave no unused reactants. Here's an example: sodium hydroxide and hydrochloric acid makes sodium chloride and water, neutral pH of 7. If sulfuric acid is used, a metal sulfate is made. Nitric acid, metal nitrate. These salts are left in solution, that is dissolved in water.
When any substance dissolves, its ions partially dissociate, as does the water actually, into H+ and OH- ions. We can obtain solid crystals of a dissolved salt by warming gently so the water evaporates. We saw briefly earlier that metals vary in their reactivity, as some donate their electrons more readily than others. Here's the reactivity series for the most common metals we consider. You can see that hydrogen and carbon have also snuck in there, that's because it's often necessary to compare the reactivity of metals to those in order to predict what will happen in a reaction.
A more reactive metal will displace a less reactive metal from a compound, that is, kick it out. For example, if you place zinc in copper sulfate solution, you'll see copper forming on the lump of zinc. The zinc displaces the copper to form zinc sulfate, kicking the copper out of the compound. We know that alkali metals react with water. The reaction happens because, for example, potassium is more reactive than hydrogen, so in essence, it displaces it from the water, leaving potassium hydroxide and hydrogen gas is produced. We can use this when it comes to extracting metals from their ores found in the ground. Any metal less reactive than carbon can be displaced by it, for example, iron can be displaced from iron oxide with carbon. This is called smelting.
We can also say that the iron oxide has been reduced; it's the opposite of oxidation because oxygen is lost. Even if oxygen is not involved in a reaction, we can still say that reduction and oxidation happen depending on whether a reactant loses or gains electrons. The mnemonic is OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons. That is, the iron ions in the iron oxide are positive, of course, 'cause they're metals, and they gain electrons to turn back into atoms. They become neutral; they've been reduced. Here's the half or ionic equation for this. We should never really have a minus in any half equations, so think carefully about which side the electron should go on depending on whether it's oxidation or reduction.
Electrolysis is for everyone. If you melt an ionic compound, let's say aluminium oxide, it can conduct electricity as the ions can move. We know that from earlier. By passing a current through it using inert electrodes, that means they won't react, like carbon. The positive metal ions, or cations (Al³⁺ in this case), they move to the negatively charged electrode, we call that the cathode, where they receive electrons and turn into atoms. Cations are always reduced at the cathode, so in this case, solid aluminium is formed on the cathode. The negative ions, or anions (O²⁻ in this case), move to the positive electrode, the anode, where they lose electrons. In this case, oxygen gas (O₂) is formed. Anions are always oxidized at the anode. This is one way of purifying metals or extracting them from compounds, say if displacing with carbon isn't an option due to their reactivity. In this case of aluminium oxide, the oxygen produced at the graphite carbon anode reacts with the anode itself, so these need to be replaced every so often. Again, specifically for this case, aluminium oxide is mixed with cryolite to reduce its melting point, making it cheaper to extract the aluminium.
We can also do electrolysis with ionic substances in solution. Say, sodium chloride solution. We know that the solution is a mixture of Na⁺, Cl⁻, H⁺, and OH⁻ ions, as they're all partially dissociated. But what will be attracted to and reduced at the cathode? The Na⁺ or the H⁺? Well, it comes back to reactivity. The more reactive ion stays in solution, while the less reactive one moves to the electrodes. That's the H⁺ in this case. That's why hydrogen gas is made at the cathode here. If the metal is less reactive than hydrogen, say copper in copper sulfate solution, it forms on the cathode instead, and the H⁺ ions stay in solution. That actually makes an acid. If there is a halide ion present, like the Cl⁻ here, it is oxidized at the anode. If there's no halide ion in solution, the oxygen from the OH⁻ is oxidized instead, and oxygen gas is produced.
Extracting metals from the earth is a huge industry. We use them for electrical appliances, batteries, for building, and more. Most metals can be obtained from their ore after mining by electrolysis or displacement reactions. A couple of new ways of extracting metals are being developed, especially for copper, as we need a lot of it for, say, electrical wiring. Phytomining uses the fact that plants absorb minerals from the soil into their roots. Grow a crop in an area with copper-rich soil, then burn the plants to be left with copper in the ash. Biorefining uses bacteria that make leachate solutions that contain metal compounds, and we can get the metal from those. Both of these ways are pretty terrible, though, as they yield incredibly small amounts of the metal.
An LCA, or life cycle assessment, is the thought process carried out in order to predict a new product's impact on the environment. You need to consider a few things: extraction and processing of raw materials, manufacturing and packaging, its use over its lifetime, disposal at the end of its life, then transportation at each of these stages too. We can reduce our impact by reducing the use of products in general. We can reduce the materials needed to make them, the energy required, and also the waste produced. We can recycle materials to reduce our impact too. Glass and metal can be recycled pretty much infinitely, although energy is required to do this, but the impact on the environment is less than what it would be if we just obtained these raw materials from the earth.
Reversible reactions are pretty self-explanatory. Once the products are made, they're able to return to their original reactants. The prime example here is the Haber process. Hydrogen and nitrogen react to make ammonia, which can also break down back into the separate gases again. More on what ammonia is used for later. In a closed system, that is, no particles or energy going in or out, both reactions will continually take place. Eventually, the quantity of particles on both sides will reach a point at which the rates of both the forward and reverse reaction will be the same. So, that means there will be no more overall change in the quantities on both sides. Remember, that's not saying that the reaction is stopped, per se; it's just that there's no more overall change. That is, until a condition is changed which will affect these rates.
Le Chatelier's principle states: if a system at equilibrium is subjected to a change, the system will adjust to counteract that change. Sounds awfully vague, so let's see what that means in practice. There are a greater number of moles on the left than the right of this reaction, which means that the reactants take up more space. Therefore, if you increase the pressure of all of these gases, we say this favors the forward reaction. That is, the rate of the forward reaction will increase until equilibrium is once again reached, but that will happen when there's a greater proportion of ammonia than there was before. We could also say that the position of equilibrium is shifted to the right. Reducing the pressure would of course do the opposite by shifting it to the left instead.
Concentration follows the same principle when it comes to solutions, by the way. Naturally, if you remove molecules from one side of the reaction, the position of equilibrium shifts in that direction, so more is produced. Increasing the temperature, in essence, means it's harder for a reaction to produce heat. That means that a hotter temperature favors the endothermic reaction. In this case, that's the reverse reaction. You could also think of it like this: an endothermic reaction requires energy being put in, so a higher temperature supplies that. A colder temperature will favor the exothermic reaction. In this case, that's the forward reaction. As a rule of thumb, any reaction that involves the breaking down of one reactant (ammonia in this case) that's going to be endothermic. In any reversible reaction, if the forward reaction is exothermic, the reverse reaction must be endothermic, and vice versa.
Like we saw at the start, the Haber process is used to make ammonia, which can be used for fertilizers. Nitrogen can be easily taken from the air, whereas hydrogen can be obtained from electrolyzing water. The gases are passed over a catalyst at around 450°C and a pressure of 200 atmospheres. Like we saw with Le Chatelier, a high pressure favors the forward reaction. However, we can't have too low a temperature, otherwise the rate of reaction will be too slow, so that 450° is a compromise to balance yield and rate of reaction. The ammonia produced is removed, and the unreacted nitrogen and hydrogen are recycled, ready to make more ammonia.
Titrations are only for triple. This is how we deduce the concentration of an acid or an alkali. We use a glass pipette to measure out an unknown volume of alkali and put it in a conical flask with a few drops of an indicator like methyl orange. We put the acid of unknown concentration in a burette above the flask. We open the tap and let it drip into the flask slowly while we swirl it. When it turns pink, we close the tap, and if it stays pink after we swirl it, that shows that neutralization has occurred. You can also do a rough titration to get a rough value for the volume needed. To do this, then do another, and then add a drop at a time near the end point to get a more accurate value.
Let's say that it's sodium hydroxide and sulfuric acid. Here's the balanced equation. So, let's say that we have 50 cm³ of 0.2 moles/dm³ sodium hydroxide. First, we need to turn that volume into dm³, so we divide by 1,000, so that's 0.05 dm³ of the alkali. Multiply that by the concentration, and we get 0.01 moles. From the stoichiometry of 1 to 2 for the acid and alkali, we can see that we need half the number of moles of acid to neutralize it, so that's 0.005 moles of acid needed. Now we can use our actual volume of acid measured. Finally, we just calculate the concentration by doing moles divided by volume. That's 0.005 / 0.125 dm³ (which is how we converted it), which gives us a concentration of 0.4 mol/dm³. Don't forget that units are your friends.
If you forget what calculation you're supposed to do, triple only. Now, until the next topic: chemical changes.
In many reactions, we want to make as much product as possible. More often than not, though, there will be some reactants left behind over at the end, like we know. For example, if a reaction is reversible, like the Haber process to make ammonia (more about that in paper 2), you'll always end up with hydrogen and nitrogen at the end. In this case, when it's reached equilibrium.
Percentage yield merely tells you how much product is actually made compared to how much you could have made in theory had all the reactants reacted. For example, if you start with 20 g of reactants here but only end up with 10 g of ammonia, the percentage yield is 50%. You must be given the actual masses involved in questions on this, so you can't predict what the yield would be just from the equation.
Atom economy, on the other hand, tells you how much of a desired product you get out of a reaction compared to the mass of the reactants that went in. You use relative atomic or formula masses to do this. Like to think of atom economy as efficiency of mass. We calculate it like this: (RAM of desired product / total RAM of reactants) * 100.
Back to the methane reaction. Sometimes this is done in greenhouses to make CO₂ for the plants. It's an incredibly important gas, necessary for life to thrive, you see. The RAM of CO₂ is 44, so that goes on top of our equation. Now, we could calculate the RAM of the reactants, but there's a nifty shortcut we can take here because this is also the same as the RAM of all of the products due to conservation of mass, as we know. So, we might as well use that, seeing that we've already got the RAM for one product. Add on two lots of 18, so that's 44 divided by the total of 80 * 100, that's 55%.
One mole of any gas takes up a volume of 24 dm³ regardless of its relative mass. This is true for RTP (room temperature and pressure), that's 20°C and a pressure of one atmosphere. You must be able to convert moles to volume and back by multiplying or dividing by 24.
Corrosion is when materials are destroyed slowly over time by chemical reactions. For example, iron and steel rust when the iron reacts with oxygen and water. Other metals corrode in a similar fashion, like the Statue of Liberty now green copper oxide on the outside. We just reserve the term rust exclusively for iron. We can coat a metal with a more reactive metal that corrodes before the other. We then call that a sacrificial metal. Zinc is an example. Coating a sheet of another metal with this is called galvanizing.
Alloys are mixtures of different metals. Bronze is an alloy of copper and tin. Brass, copper and zinc. Even gold jewelry isn't usually pure gold; it would be too soft. It's combined with silver, copper, and zinc. 24 karat is 100% gold, 18 karat being 75%, etc. Steel is an alloy of iron and carbon, which makes it stronger than pure iron. If it contains chromium or nickel, it's a stainless steel, which is more resistant to corrosion. Alloys are usually stronger than pure metals because the different size atoms disrupt the regular lattice, which means the layers cannot slide over each other as easily. Aluminium is used in an alloy when we need a low density.
Finally, just for triple: cells or batteries. They contain chemicals that can produce a potential difference of voltage to power electrical appliances. The basic composition is two different metals in contact with an electrolyte. Non-renewable batteries stop working when the reactants are used up. Rechargeable batteries can be recharged when a supplied current causes the reverse reaction to occur. Hydrogen fuel cells work in a similar way. Water is split up into hydrogen and oxygen by electrolysis. When they recombine, a voltage is produced.
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