Chemical Reactions

• Rate answers “how fast”. Equilibrium answers “how far”. Redox answers “who gave, who took”. Before you write, check which of the three the question actually asked — answering a different one is the easiest way to write something true that earns nothing.
• Has at least one new chemical substance been formed? Yes → chemical change. No → physical change.
• Every evaluation answer has the same skeleton: this measurement is imperfect because …, which makes the readings too high / too low / too coarse, and the fix is … because it removes that specific cause.
• In a closed system: (1) the rate of the forward reaction equals the rate of the reverse reaction, and (2) the concentrations of reactants and products remain constant.
• Typical conditions are the ones that make the most ammonia per day at an acceptable cost — not the ones that would make the highest percentage of ammonia if you waited forever.
• Four columns, every time: rate · position and yield · safety · cost. Then a verdict. An answer missing the verdict has not evaluated anything.
• OIL RIG — Oxidation Is Loss of electrons, Reduction Is Gain of electrons.
• An oxidising agent oxidises something else, and is itself reduced. A reducing agent reduces something else, and is itself oxidised.

• State that a catalyst increases the rate of a reaction, lowers the activation energy and is chemically unchanged at the end.
• State that some reactions are reversible, shown by the symbol \(\rightleftharpoons\).
• State that a closed system reaches equilibrium when the forward and reverse rates are equal and concentrations no longer change.
• State the equation, raw materials and typical operating conditions of the Haber process.
• State the equation, raw materials and typical operating conditions of the Contact process.
• State the four rules used to assign oxidation numbers.
• State that a Roman numeral in a name gives the oxidation number of that element.
• Describe the difference between a physical change and a chemical change.
• Describe collision theory in terms of particles per unit volume, collision frequency, kinetic energy and activation energy.
• Describe practical methods for following a reaction by change in mass and by volume of gas formed.
• Describe how heating hydrated copper(II) sulfate and hydrated cobalt(II) chloride, and adding water to the anhydrous solids, reverses the direction of change — with the correct colours.
• Describe oxidation and reduction in terms of oxygen transfer, electron transfer and oxidation number change.
• Explain the effect of concentration, gas pressure, solid surface area, temperature and a catalyst on rate, each time using collision theory.
• Explain why a catalyst reaches equilibrium sooner without changing the position of equilibrium.
• Explain each Haber and each Contact operating condition as a compromise between rate, equilibrium yield, safety and cost.
• Explain why an oxidising agent is itself reduced, and a reducing agent is itself oxidised.
• Predict the effect of a change in temperature, pressure, concentration or catalyst on the position of a given equilibrium, using information supplied in the question.
• Predict which curve on a rate graph belongs to which set of conditions, and predict the final amount of product.
• Predict the oxidation number of an element in an unfamiliar compound or ion.
• Evaluate whether the loss-of-mass method or the gas-collection method is the better choice for a stated reaction, and name a limitation of each.
• Evaluate an industrial proposal to change a temperature, pressure or catalyst, separating the effect on rate from the effect on yield and including safety and cost.
• Interpret and evaluate rate data presented as a table or a graph.

Why it matters. Five separate syllabus outcomes ask you to explain the effect of a different factor on rate. They are not five explanations to memorise — they are one explanation, applied five times. Build the chain properly here and the next four sections become almost free.

Dynamic Equilibrium

The state reached by a reversible reaction in a closed system when the forward and reverse reactions are proceeding at exactly equal rates, so that the concentrations of all reactants and products remain constant. Both reactions continue: nothing has stopped, and the concentrations need not be equal to one another.

Collision Theory

The model that explains reaction rates by requiring reacting particles to collide, and to collide with combined energy at least equal to the activation energy. Rate is therefore governed by the frequency of successful collisions rather than by the total number of collisions.

Chemical Change

A change in which one or more new chemical substances are formed. Bonds within the reactants are broken and new bonds are made, so the atoms are regrouped into products with different formulae and different properties from the starting materials.

Surface Area

The total area of a solid that is exposed to the surrounding reactant. Breaking a fixed mass of solid into smaller pieces exposes surfaces that were previously buried inside, so the surface area rises while the mass stays the same, and the reaction rate rises with it.

Gas Collection Method

A practical technique for following a reaction rate by measuring the volume of gas produced at regular time intervals, using either a gas syringe attached to the reaction flask or an inverted measuring cylinder filled with water standing in a trough. The gas must be insoluble in water for the second version to work.

Catalyst

A substance that increases the rate of a chemical reaction by providing an alternative reaction pathway of lower activation energy, and that is chemically unchanged at the end of the reaction. A catalyst does not alter the enthalpy change of the reaction and does not increase the yield of product at equilibrium.

Rate of Reaction

A measure of how quickly reactants are used up or products are formed, expressed as the change in a measurable quantity such as mass, gas volume or concentration divided by the time taken. It is found from the gradient of a graph of that quantity against time.

Physical Change

A change in which no new chemical substance is formed. The particles are rearranged in space or given more energy, but each particle keeps its own chemical identity and formula, so melting, boiling, dissolving and changing shape are all physical changes.

Oxidation Number

A number assigned to an element in a compound or ion that records how many electrons it has effectively gained or lost compared with the uncombined element. It is found using four rules, and a change in it during a reaction is what identifies oxidation and reduction.

Catalyst and Equilibrium

The principle that a catalyst increases the rates of the forward and reverse reactions by the same factor, because it lowers the activation energy of both by the same amount. Equilibrium is therefore reached in a shorter time, while the position of equilibrium, the equilibrium yield and the enthalpy change are all unaltered.

Haber Process

The industrial manufacture of ammonia from nitrogen and hydrogen over an iron catalyst. Nitrogen comes from the air and hydrogen from methane, and the reversible reaction is run at about 450 degrees Celsius and 20000 kilopascals, which is 200 atmospheres, as a compromise between equilibrium yield, reaction rate and plant cost.

Initial Rate

The rate of a reaction at the instant it begins, found from the gradient of the tangent drawn to the curve at time zero. It is the fastest rate of the whole reaction, because the reactants are at their highest concentration and the greatest area of solid is exposed at that moment.

Reducing Agent

A substance that reduces another substance and is itself oxidised in the process. It gives electrons to the other species, or takes oxygen from it, and its own oxidation number therefore increases.

Collision Frequency

The number of collisions between reacting particles per unit time. It rises when there are more particles in a given volume, as when a solution is made more concentrated or a gas is compressed, and it is one of the two routes by which any factor can change a reaction rate.

Pressure Effect on Equilibrium

The response of a gaseous equilibrium to a change in pressure. Raising the pressure moves the position of equilibrium towards the side of the equation with fewer gas molecules, and lowering it moves the position towards the side with more; if both sides have equal numbers of gas molecules, changing the pressure does not move the position at all.

Loss of Mass Method

A practical technique for following a reaction rate in which the reaction vessel stands on a balance and its mass is recorded at regular time intervals. It works only when a product leaves the apparatus, almost always as a gas, so that the falling mass measures the amount of product formed.

Position of Equilibrium

A description of how far a reversible reaction has proceeded when equilibrium is reached, in other words the relative amounts of reactants and products in the equilibrium mixture. It moves to the right when more product is present at equilibrium and to the left when more reactant is, and it is changed by temperature, pressure and concentration but never by a catalyst.

Reversible Reaction

A reaction in which the products can react together to re-form the original reactants, so that the change can proceed in both directions. It is written with the equilibrium symbol, a pair of opposed half-arrows, instead of a single arrow.

Contact Process

The industrial manufacture of sulfur trioxide, used to make sulfuric acid, by the reversible oxidation of sulfur dioxide over a vanadium(V) oxide catalyst. Sulfur dioxide is obtained by burning sulfur or by roasting sulfide ores, oxygen comes from the air, and the reaction is run at about 450 degrees Celsius and only 200 kilopascals, which is 2 atmospheres.

Oxidising Agent

A substance that oxidises another substance and is itself reduced in the process. It takes electrons from the other species, or supplies oxygen to it, and its own oxidation number therefore decreases.

Successful Collision

A collision between reacting particles in which the particles have a combined energy at least equal to the activation energy, so that bonds break and products form. The rate of a reaction is set by how many successful collisions occur per unit time, not by the total number of collisions.

Redox Colour Test

The use of a colour change as evidence that a redox reaction has occurred. Acidified aqueous potassium manganate(VII) is decolourised from purple when a reducing agent reduces manganese from plus seven to plus two, and colourless aqueous potassium iodide turns brown when an oxidising agent oxidises iodide ions to iodine.

• 1. “It is a physical change because you can reverse it.” Why temptingMost physical changes are easy to reverse, so the correlation is real. RepairReversibility is a correlation, not a definition. Heating hydrated copper(II) sulfate is chemical and reverses in seconds; cutting paper is physical and cannot be undone. Ask instead whether a new substance has formed. TransferClassify “anhydrous cobalt(II) chloride turning pink in damp air” and justify it. Chemical: a different compound, the hydrate, has formed.
• 2. “There was a colour change, so a chemical reaction happened.” Why temptingColour change is on every list of signs of a reaction. RepairThose signs are supporting evidence, not proof. Iodine sublimes from grey solid to purple vapour with no new substance at all. TransferGive one colour change that is not a chemical reaction and one that is. Iodine subliming; copper(II) sulfate crystals turning white on heating.
• 3. “More collisions, so the reaction is faster.” Why temptingIt is half of the right answer, and it sounds like a full one. RepairInsert the word successful: collisions in which the particles have combined energy at least equal to the activation energy. Almost all collisions achieve nothing. TransferRewrite “higher concentration means more collisions so it is faster” as a full-mark sentence.
• 4. “Increasing the concentration makes the particles move faster.” Why tempting“Faster reaction” slides into “faster particles”. RepairAverage particle speed depends on temperature alone. Concentration changes how crowded the particles are, so they meet more often at the same speed. TransferWhich of the five rate factors do change particle speed? Only temperature.
• 5. “Grinding the solid increases its concentration.” Why temptingBoth changes make a reaction faster, so the words get swapped. RepairSolids do not have a concentration; concentration is defined for a solute in a solution. Grinding increases the surface area, exposing more particles. TransferState what is unchanged when a lump is ground to powder. The mass, the number of particles present and the final amount of product.
• 6. “Powdered reactant gives more gas in total.” Why temptingThe powder curve is above the lump curve for most of the graph. RepairIt is above only until the lump catches up. Both plateau at the same height, because the plateau is fixed by the limiting reactant, and grinding changes no amounts. TransferTwo curves plateau at different heights. What must have been changed? The amount of limiting reactant.
• 7. “Heating lowers the activation energy.” Why temptingHeating and catalysis both make more collisions succeed. Repair\(E_\mathrm{a}\) is a fixed property of the reaction pathway. Heating raises the particles towards the barrier; only a catalyst lowers the barrier, by offering a different pathway. TransferOn an energy profile, which of the two changes would you be able to see? The catalyst; heating changes nothing on the diagram.
• 8. “Heating gives all the particles enough energy to react.” Why temptingIt sounds like a stronger version of the right answer. RepairIt raises the fraction of particles with energy at or above \(E_\mathrm{a}\). Many still fall short — which is why reactions have a rate rather than finishing instantly. TransferIf every particle had enough energy, what would the graph look like? Vertical, then flat: the reaction would be instantaneous.
• 9. “The catalyst is used up, so you have to keep adding it.” Why temptingIt clearly takes part, so it feels like a reactant. RepairIt is regenerated. Filter it off at the end, dry it and weigh it: the mass is unchanged. That is the experimental evidence, and it is worth quoting. TransferWhy does a catalyst not appear in the equation? It is neither consumed nor produced, so it cancels from both sides.
• 10. “A catalyst increases the yield.” Why temptingMore product appears sooner, which looks like more product. RepairIt lowers the activation energy of forward and reverse reactions equally, so both rates rise by the same factor. Equilibrium arrives sooner at exactly the same composition. TransferExplain why iron is used in the Haber process. To reach equilibrium fast enough at \(450\,{}^\circ\mathrm{C}\) — not to raise the yield.
• 11. “The flat part of the graph shows equilibrium.” Why temptingNothing is changing, which is what equilibrium looks like. RepairIn an open flask the curve flattens because a reactant has run out and the reaction has stopped. Equilibrium needs a closed system and two reactions still running at equal rates. TransferWhat two things does a vessel need before it can reach equilibrium? A reversible reaction, and a seal so that no product escapes. Marble and acid fails the first test — carbon dioxide, calcium chloride and water do not react back — so sealing that flask alone would achieve nothing.
• 12. “At equilibrium the amounts of reactants and products are equal.” Why tempting“Equilibrium” and “balance” suggest equality. RepairThe rates are equal; the concentrations are constant. The Contact process sits at equilibrium with about \(98\%\) product. TransferFill the gap: “the concentrations remain ______.” Constant.
• 13. “At equilibrium the reaction has stopped.” Why temptingNothing observable changes. RepairBoth reactions continue at equal, non-zero rates. That is exactly what the word dynamic is doing in “dynamic equilibrium”. TransferWhat would happen to the colour of a sealed \(\mathrm{NO_2}\)/\(\mathrm{N_2O_4}\) tube if the forward reaction really stopped? It would keep changing until one substance ran out.
• 14. “Raising the pressure always moves an equilibrium to the right.” Why temptingIt does in both industrial examples in this chapter. RepairIt moves towards the side with fewer gas moles, whichever side that is — and does nothing at all if the counts are equal. TransferPredict the effect of higher pressure on \(\mathrm{H_2(g) + I_2(g) \rightleftharpoons 2HI(g)}\). No shift: two moles of gas on each side.
• 15. “Adding an inert gas at constant volume raises the pressure, so the equilibrium shifts.” Why temptingThe pressure gauge really does read higher. RepairWhat matters is the number of reacting molecules per unit volume, and that is unchanged. No shift. TransferWhat would shift it? Reducing the volume, which genuinely crowds the reacting molecules.
• 16. “Heat acts as a catalyst.” Why temptingBoth speed reactions up. RepairA catalyst lowers \(E_\mathrm{a}\) and leaves the position of equilibrium alone. Heat leaves \(E_\mathrm{a}\) alone and does move the position. They are opposites in both respects. TransferWhich of the two changes the yield? Heat.
• 17. “The Contact process runs at 200 atmospheres.” Why temptingThe two processes share a temperature, so the pressures get shared too. RepairContact runs at \(200\ \mathrm{kPa}\), which is \(2\) atmospheres. Haber runs at \(20\,000\ \mathrm{kPa}\), which is \(200\) atmospheres. Both numbers contain “200”, and the unit is the whole difference. TransferGive both pressures with units, then say why they differ.
• 18. “Industrial conditions are chosen to give the highest possible yield.” Why temptingYield is what the chemistry is about. RepairThey are chosen to give the most product per day at an acceptable cost and risk. \(450\,{}^\circ\mathrm{C}\) is deliberately not the yield-maximising temperature. TransferWhat temperature would maximise the Haber yield, and why is it not used? As low as possible; the rate would be far too slow.
• 19. “The species oxidised is the oxidising agent.” Why temptingThe two phrases share a word and differ by three letters. RepairThe species oxidised gave electrons away, causing reduction elsewhere, so it is the reducing agent. An oxidising agent is itself reduced. TransferIn \(\mathrm{2Mg + O_2 \rightarrow 2MgO}\), name both agents. Magnesium reducing; oxygen oxidising.
• 20. “The Roman numeral is the charge on the ion.” Why temptingFor simple ions such as \(\mathrm{Fe^{3+}}\) in iron(III) chloride, the two do coincide. RepairThe Roman numeral is the oxidation number of one element. In manganate(VII) manganese is \(+7\) while the whole \(\mathrm{MnO_4^{\,-}}\) ion carries only \(1-\). TransferState the oxidation number of chromium in dichromate(VI) and the charge on the ion. \(+6\) per chromium atom; the ion is \(2-\).

• The two-sentence rule for “explain” in this topic. Sentence one names the particle-level change. Sentence two names the consequence for successful collisions or for the position of equilibrium. An answer with only the first sentence has described rather than explained, and an “explain” question is asking for the second one.
• The word that carries the explanation. A rate explanation is only complete when it contains the word successful, or an equivalent such as “collisions with energy greater than or equal to the activation energy”. “More collisions, so faster” leaves out the entire energy condition, which is the half of collision theory that does the explaining.
• Where pressure applies. A pressure change only affects the rate if at least one reactant is a gas. Increasing the pressure above a solution of hydrochloric acid does essentially nothing to the rate, because liquids are almost incompressible — the particles were already touching. If a question mentions pressure, check for a \((\mathrm{g})\) state symbol before you write anything.
• Say “surface area”, never “concentration”. A solid does not have a concentration; concentration is defined for a solute in a solution. “Grinding the solid increases its concentration” names the wrong quantity, so the sentence is wrong however sound the reasoning around it is.
• Why the plug is cotton wool and not a bung. The reaction is vigorous and throws fine droplets of acid upwards. If those droplets escaped, the mass loss would be too large and would not be due to gas alone. Loose cotton wool lets the carbon dioxide through while trapping the spray — it makes the measurement valid without sealing the system.
• Comparing two curves in one sentence each. Say something about the steepness and something about the plateau, and say what each one tells you. “Curve A is steeper at the start, so its initial rate is higher; both curves level off at the same volume, so both experiments contained the same amount of limiting reactant.” That single sentence pattern answers most graph-comparison questions in this topic.
• The rule that links the last two lines. An improvement is only worth writing if it fixes the limitation you named. “Repeat the experiment” does not fix a systematic error such as a leak or a soluble gas — repeating it just gives the same wrong answer three times. Match the repair to the fault.
• The blue trap. “Blue” appears twice in that table, on opposite sides. For copper(II) sulfate, blue is the hydrated form. For cobalt(II) chloride, blue is the anhydrous form. A useful anchor: cobalt chloride paper is used to test for water, and a test must start in a state that visibly changes, so the dry paper is blue and turns pink when water arrives.
• Read the last two columns together. Rows one and two are the reason industrial chemists cannot simply pick the temperature that gives the best yield: for an exothermic forward reaction, the temperature that maximises yield is also the temperature at which you would wait forever. That tension is the whole of the compromise argument in Sections 6.3F and 6.3G.
• The row worth memorising is the third. Whenever the gas counts are equal, the correct answer to “what happens to the position of equilibrium?” is nothing. Students who have learned “pressure pushes it right” as a slogan get this wrong every time. Pressure still speeds the reaction up — but it does not move the position.
• Read the last column. All four changes shorten the time to equilibrium, so “it gets there faster” never distinguishes them and never answers a question about yield. The column that discriminates is the second one — and only one row in it says “no change”.
• Give the pressure in the unit the question uses. \(20\,000\ \mathrm{kPa}\) and \(200\) atmospheres are the same pressure, and the syllabus states both. Quoting both costs nothing and covers you whichever unit a question is set in.
• The pressure is the discriminating fact. Both processes run at \(450\,{}^\circ\mathrm{C}\), and both use a catalyst, so the temperature and the idea of a catalyst do not tell the two apart. The pressures differ by a factor of a hundred: \(200\) atmospheres for Haber, \(2\) atmospheres for Contact. Both numbers contain the digits “200”, so check the unit every time before you write one of them down.
• The sentence that finishes an evaluation. “The chemistry favours this change, but the cost and safety consequences outweigh the gain, so the proposal should be rejected.” Or the reverse. An evaluation is the judgement plus the reasons that support it; a list of effects with no verdict has stopped one step short.
• Two values you will use constantly. In its compounds hydrogen is almost always \(+1\) and oxygen almost always \(-2\). These are not extra rules to memorise — they follow from rules 2 and 3 applied to the simple compounds you already know — but having them ready makes every ledger a one-line calculation.
• Evidence first, name second. “Identify the oxidising agent” is asking for two things: which species it is, and how you know. Write the oxidation number change, or the electron transfer, and then name the agent. A bare name is an assertion; the evidence is what makes it an answer — and, since the inversion is so easy to get backwards, writing the evidence first is also the best way to catch your own error.
• Using this list under exam conditions. Nine of the twenty errors above (numbers 3, 4, 7, 8, 10, 12, 14, 16, 19) are triggered by a single word slipping out of place. If you have thirty seconds at the end of a paper, re-read your rate and redox sentences hunting for those words: successful, fraction, constant, fewer, and the direction of the agent inversion.
• How to use these across a revision week. Do not work through all eighty in one sitting. Take one heading per day, answer every prompt cold, and mark only whether you produced the answer without help. Return the following day to the ones you missed. Spaced, effortful retrieval is what moves this topic into long-term memory; re-reading the chapter does not.
• Marking yourself honestly. Award a mark only where your answer contains the idea in the marking point, not merely a related word. If you wrote “more collisions” where the point says “more successful collisions”, that is not a mark. These marking points are original to this chapter and are a guide to completeness, not a Cambridge mark scheme, so treat your score as a map of what to revise rather than as a prediction. Whatever the total, the useful step is the same: list the sections that produced the losses and return to those, rather than repeating the paper.
• If you have only one hour before the exam. Read the seven danger zones, the nine comparison matrices, and the five-line specifications of the Haber and Contact processes. Those three pages are the densest summary of the topic in the chapter: the recall facts, the distinctions that are easiest to merge, and the errors that turn correct chemistry into a wrong answer.

• Topic 6 can be assessed on any of the papers, and the same content is asked for in different ways on each. A multiple-choice paper tests recognition: which curve, which condition, which species is oxidised. A structured paper tests extended reasoning: explain, predict, evaluate. A practical paper tests method design and evaluation, using the two techniques in Section 6.2.4. What changes between them is not the chemistry — it is the command word.
• The two-sentence rule for “explain” in this topic. Sentence one names the particle-level change. Sentence two names the consequence for successful collisions or for the position of equilibrium. An answer with only the first sentence has described rather than explained, and an “explain” question is asking for the second one.
• Rate explanations need the whole chain, not the conclusion. “Faster” on its own explains nothing, because it repeats the question.
• Equilibrium predictions have two halves: the direction of the shift, and the reason drawn from information given in the question (the sign of \(\Delta H\), or the gas-mole counts).
• Industrial conditions need the compromise. Naming a temperature states a fact; the explanation is why a lower one is not used.
• Redox identification needs the evidence before the label. Show the oxidation numbers or the electrons, then name the agent.