Chemical Energetics

• 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 → sign. If the evidence does not fix the direction, the honest answer is “cannot be determined from this information”.
• 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.
• \(E_\mathrm{a}\): reactants → peak. \(\Delta H\): reactants → products. Same start, different finish.
• Every pathway mark is an endpoint mark. Say where each arrow starts and where it ends, out loud, before you move on.
• Breaking costs energy. Making pays it back. \(\Delta H\) is the balance of the account: broken minus made.

• 5.1.1 I can state that an exothermic reaction transfers thermal energy to the surroundings.
• 5.1.1b I can state that this makes the temperature of the surroundings increase, and say which reading is the evidence.
• 5.1.2 I can state that an endothermic reaction takes in thermal energy from the surroundings.
• 5.1.2b I can state that this makes the temperature of the surroundings decrease, and say which reading is the evidence.
• 5.1.3a I can state that the transfer of thermal energy during a reaction is called the enthalpy change, \(\Delta H\), of the reaction.
• 5.1.3b I can state and justify that \(\Delta H\) is negative for an exothermic reaction, using the direction of transfer rather than a memorised rule.
• 5.1.3c I can state and justify that \(\Delta H\) is positive for an endothermic reaction.
• 5.1.4 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.
• 5.1.5a I can draw a reaction pathway diagram for an exothermic and for an endothermic reaction, with correct axes and correct relative level heights.
• 5.1.5b I can label the reactants level and the products level at geometrically correct points.
• 5.1.5c I can label \(\Delta H\) between the two levels and \(E_\mathrm{a}\) from the reactants level to the peak, with each arrow starting and ending where it should.
• 5.1.5d I can interpret an unfamiliar pathway diagram using information provided — and say what the diagram does not allow me to conclude.
• 5.1.6a I can state and explain that bond breaking is endothermic.
• 5.1.6b I can state and explain that bond making is exothermic.
• 5.1.6c I can explain the overall enthalpy change of a reaction in terms of the balance between the two totals, in prose, without a calculation.
• 5.1.7 I can calculate the enthalpy change of a reaction from supplied bond energies, showing the bond inventory, the two totals, the subtraction, the unit, the sign and the classification.

The one causal story. Energy is taken in to break the bonds in the reactants. Energy is given out when the bonds in the products form. Which of those two totals is bigger decides the sign of the enthalpy change, \(\Delta H\), and it decides which way the temperature of the surroundings moves. Nothing else in Topic 5 is independent of that sentence.

Enthalpy Change

The transfer of thermal energy during a chemical reaction, given the symbol delta H; it is negative when the reaction transfers thermal energy to the surroundings and positive when the reaction takes thermal energy in from the surroundings.

Activation Energy

The minimum energy that colliding particles must have in order to react; on a reaction pathway diagram it is the vertical difference between the reactants level and the peak of the curve.

Bond Energy

The energy needed to break one mole of a particular bond, quoted as a positive value in kilojoules per mole; the same value is released when that bond is formed.

Bond Breaking

The separation of bonded atoms, which requires an input of energy because it works against the attraction holding the atoms together; bond breaking is therefore an endothermic process.

Bond Making

The formation of a chemical bond between atoms, which releases energy to the surroundings; bond making is therefore an exothermic process.

Reaction Pathway Diagram

A diagram with energy on the vertical axis and progress of reaction on the horizontal axis, showing the reactants level, a peak, and the products level, on which the activation energy and the enthalpy change of a reaction can be labelled.

Endothermic Reaction

A reaction that takes in thermal energy from its surroundings, so the temperature of the surroundings decreases and the enthalpy change of the reaction is positive.

Exothermic Reaction

A reaction that transfers thermal energy from the reacting chemicals to their surroundings, so the temperature of the surroundings increases and the enthalpy change of the reaction is negative.

• 1. “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.
• 2. “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.
• 3. “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.
• 4. “\(\Delta H\) 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.
• 5. “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 it is the activation energy of the reverse reaction.
• 6. “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 \(\Delta H\).
• 7. “I drew an exothermic profile with the products 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. Check the level order before you add a single label.
• 8. “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.
• 9. “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.
• 10. “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.
• 11. “\(\mathrm{O{=}O}\) 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.
• 12. “\(\mathrm{C_2H_6}\) 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.
• 13. “\(\Delta H = \) 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.
• 14. “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”.
• 15. “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.
• 16. “I added a second, lower curve to show the catalysed route.” Repair Catalysts are Topic 6, not Topic 5. 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. Multiple-choice items ask you to classify, read a profile or pick a correct \(\Delta H\); structured items ask you to define, draw, label, interpret, explain or calculate. The command word fixes the shape of the answer before you write a word of chemistry.
• 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 all seven command words below 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 bond energies, it wants a calculation and the answer ends with a sign, a unit and a classification. If it supplies a diagram with no numbers, it wants an interpretation and any numerical claim you make is unsupported.
• 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.