Chemical Energetics

• Out of the reaction → surroundings warm → products lower → exothermic. Into the reaction → surroundings cool → products higher → endothermic.
• Break bonds → energy in. Make bonds → energy out. Out beats in → surroundings warm, \(\Delta H\) negative. In beats out → surroundings cool, \(\Delta H\) positive.
• The thermometer is in the surroundings. Whatever it does, the system did the opposite.
• Evidence → direction → classification. If the evidence does not fix the direction, the honest answer is “cannot be determined from this information”.
• Left is reactants, right is products, and the only question is which one is higher. Products lower → exothermic. Products higher → endothermic. No scale → no numbers.
• Three statements, one question: which way did the thermal energy go? Everything else in the Core route is that answer, said in a different form.
• Out of the system → products lower → \(\Delta H\) negative. Into the system → products higher → \(\Delta H\) positive.
• A sign is a direction. A magnitude is an amount. A unit makes it an energy. Leave any one of the three out and the answer is incomplete.

• Core 5.1.1 I can state that an exothermic reaction transfers thermal energy to the surroundings, leading to an increase in the temperature of the surroundings — and say which reading is the evidence.
• Core 5.1.2 I can state that an endothermic reaction takes in thermal energy from the surroundings, leading to a decrease in the temperature of the surroundings.
• Core 5.1.3 I can interpret reaction pathway diagrams showing exothermic and endothermic reactions: name the axes, find the reactants and products levels, compare them, classify the reaction, state what the surroundings do — and say what an unscaled diagram does not entitle me to conclude.
• Supplement 5.1.4 I can state that the transfer of thermal energy during a reaction is called the enthalpy change, \(\Delta H\), of the reaction, and that \(\Delta H\) is negative for exothermic reactions and positive for endothermic reactions — justifying each sign from the direction of transfer rather than from memory.
• Supplement 5.1.5 I can define activation energy, \(E_\mathrm{a}\), as the minimum energy that colliding particles must have to react — in those words, without adding anything the syllabus does not say.
• Supplement 5.1.6 I can draw and label reaction pathway diagrams for exothermic and endothermic reactions from information provided, including all four named features: (a) reactants, (b) products, (c) the enthalpy change of the reaction \(\Delta H\), and (d) the activation energy \(E_\mathrm{a}\), with every arrow starting and ending where it should.
• Supplement 5.1.7 I can state that bond breaking is an endothermic process and bond making is an exothermic process, and explain the enthalpy change of a reaction in terms of bond breaking and bond making — in prose, without a calculation.
• Supplement 5.1.8 I can calculate the enthalpy change of a reaction using bond energies, showing the bond inventory, the two totals, the subtraction, the unit, the sign and the classification.

Core 5.1.1 Core 5.1.2 Core 5.1.3 The Core story, in three sentences. An exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases. An endothermic reaction takes in thermal energy from the surroundings, so the temperature of the surroundings decreases. On a reaction pathway diagram those two cases look different in exactly one way: for an exothermic reaction the products level is drawn below the reactants level, and for an endothermic reaction it is drawn above.

• C1. “Exothermic means the mixture contains a lot of heat.” Repair A chemical does not contain heat, and there is no such stored substance to hold. Exothermic describes a transfer: during the reaction, thermal energy moves from the system to the surroundings. Talk about where energy went, never about what a chemical contains.
• C2. “Endothermic reactions cannot happen, because they need energy they do not have.” Repair They take the energy they need from the surroundings, which is precisely why the surroundings get colder. Photosynthesis and thermal decomposition both happen every day. Needing an input is not the same as being impossible.
• C3. “The reaction got hotter, so it is endothermic — heat went in.” Repair Two errors in one sentence. A thermometer sits in the surroundings, so a rising reading is a fact about the surroundings, and something must have supplied that energy — the system. Rising surroundings temperature means exothermic.
• C4. “This profile shows an exothermic reaction, and its products are above the reactants.” Repair Exothermic means energy left the system, so what is left behind — the products — must sit lower. Products above reactants is the endothermic picture. Establish the level order before you name the reaction type, not after.
• C5. “Reading off the diagram, the reactants are at about 250 and the products at about 100.” Repair Unless the energy axis carries a numerical scale, there is nothing to read those values from and they have been invented. An unscaled pathway diagram fixes the order of the two levels and nothing else. “A value cannot be given, because the energy axis has no scale” is a creditable answer, not an admission of defeat.
• C6. “The curve has a big hump, so the reaction must be endothermic.” Repair The peak is not what decides the type. Both exothermic and endothermic profiles have a peak, and it can be any height on either. What decides the type is the comparison of the reactants level with the products level — nothing else on the diagram is being asked about.
• S1. “\(\Delta H\) is negative, so negative energy was released.” Repair There is no negative energy. The minus sign is a direction label: it records that energy left the system. If a reaction has \(\Delta H = -184\;\mathrm{kJ\,mol^{-1}}\), then \(184\;\mathrm{kJ}\) of thermal energy is transferred to the surroundings for every mole of reaction; the minus sign says which way that energy went, not how much of it there was.
• S2. “The \(E_\mathrm{a}\) arrow goes from the bottom of the axis up to the peak.” Repair \(E_\mathrm{a}\) is measured from the reactants level to the peak, never from zero and never from the products level. Drawn from zero it is not activation energy at all; drawn from the products level it is not the forward \(E_\mathrm{a}\) either — that arrow measures the barrier for the reverse reaction, which statement 5.1.6 does not ask you for.
• S3. “The \(\Delta H\) arrow runs from the reactants up to the peak.” Repair That is \(E_\mathrm{a}\) again. \(\Delta H\) is the vertical gap between the reactants level and the products level. The peak plays no part in \(\Delta H\), which is why two reactions with very different barriers can have exactly the same \(\Delta H\).
• S4. “This reaction has a big activation energy, so \(\Delta H\) must be large and positive.” Repair The two are independent. The barrier height says how much energy colliding particles need; \(\Delta H\) says where the products ended up relative to the reactants. A strongly exothermic reaction can have a very large barrier, and a barely endothermic one can have a small barrier. You cannot infer either from the other.
• S5. “Breaking a bond releases the energy that was stored in it.” Repair The single most damaging sentence in this topic. Bonded atoms attract each other; pulling them apart works against that attraction and therefore needs an energy input. Bond breaking is endothermic. Energy is released when bonds form.
• S6. “I cancelled the unchanged bonds on the left, then counted every bond on the right.” Repair Both methods are valid; mixing them is not. Either count every bond on both sides, or count only the bonds that change on both sides. Half of each gives an answer that is wrong by exactly the energy of the bonds you cancelled on one side only.
• S7. “\(\mathrm{O{=}O}\) is a bond, so I used the \(\mathrm{O-O}\) value.” Repair When a table distinguishes single, double and triple bonds it does so because they are different quantities. \(\mathrm{O{=}O}\) is one bond with its own energy, and so are \(\mathrm{C{=}C}\), \(\mathrm{C{=}O}\) and \(\mathrm{N{\equiv}N}\). Use the row that matches the bond in the structure.
• S8. “\(\mathrm{C_2H_6}\) has eight atoms, so it has eight bonds.” Repair A molecular formula counts atoms, not connections. Draw the structure: ethane is one \(\mathrm{C-C}\) bond plus six \(\mathrm{C-H}\) bonds — seven bonds. If you cannot draw the connectivity, you cannot count the bonds, and no amount of correct arithmetic will rescue a wrong inventory.
• S9. “\(\Delta H = \) bonds made \(-\) bonds broken.” Repair Reversed. It is \(\Delta H = \sum E(\text{broken}) - \sum E(\text{made})\). Energy in comes first because it happens first and because it is the positive contribution. Reverse it and every exothermic reaction in your answer becomes endothermic.
• S10. “I got \(-822\), but energy cannot be negative, so I wrote 822.” Repair \(\Delta H\) is a signed quantity by definition, and the sign carries the physics. Removing it does not tidy the answer, it deletes the classification. Write \(-822\;\mathrm{kJ\,mol^{-1}}\) and then say “exothermic”.
• S11. “My final line was ‘\(\Delta H = -184\)’.” Repair Incomplete. The final line needs value, unit and classification: \(\Delta H = -184\;\mathrm{kJ\,mol^{-1}}\), so the reaction is exothermic. All three are part of the answer: without the unit the number is not an energy, and without the classification nothing has been said about which way the energy went.
• S12. “I added a second, lower curve to show the catalysed route.” Repair Catalysts are Topic 6, not Topic 5, at either tier. Drawing one here answers a question that was not asked, and in a “draw the reaction pathway diagram” question the extra curve can only confuse the labelling of the one that was wanted. Keep to a single pathway.

• Topic 5 is examined through the written papers. Which papers you sit depends on your route, and the command word fixes the shape of your answer before you write a word of chemistry.
• What this means for Topic 5. A Core candidate can be asked about statements 5.1.1 to 5.1.3 only. An Extended candidate can be asked about all eight, because Extended means Core plus Supplement — the Core statements are examined on Papers 2 and 4 as well. The practical papers are taken by both routes, so nothing about practical work in this chapter is Supplement-only.
• A caution about revision folklore. Nobody — not a teacher, not a revision guide, not this chapter — can tell you how often a topic appears, which diagram will be set, or how many marks it will carry. Any source that does is guessing. Prepare the command words below for your route and the question cannot surprise you.
• The three-second check before you write. Underline the command word. Underline the data you were given. If the question supplies a diagram with no numbers, it wants an interpretation, and any numerical claim you make is unsupported. If it supplies bond energies, it wants a calculation, and the answer ends with a sign, a unit and a classification.
• Supplement 5.1.8 Write the inventory as a list even when you can do the sum in your head. Laid out line by line, a slip in one subtotal stays visible as one wrong line, and the rest of the method is still there on the page to be read; buried inside a single mental calculation, the same slip surfaces only as a wrong final number with nothing behind it:
• Then the final line, every time, in this order: value, unit, classification. For example: \(\Delta H = -184\;\mathrm{kJ\,mol^{-1}}\), so the reaction is exothermic.