Chemistry • Year 11 • Module 4 • Lesson 1
Enthalpy & Energy Profile Diagrams
Lock in the key vocabulary, the exothermic/endothermic comparison, and the five features of a correctly labelled energy profile diagram.
1. Term–definition match
The eleven definitions below are shuffled. In the right-hand column, write the matching term from this list: enthalpy change (ΔH), exothermic reaction, endothermic reaction, activation energy (Ea), transition state (activated complex), energy profile diagram, reaction coordinate, thermochemical equation, state function, standard conditions, enthalpy. 11 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | A measure of the total energy stored in the bonds of a chemical system at constant pressure. | |
| 1.2 | The heat energy exchanged between a system and its surroundings at constant pressure during a reaction; equals H(products) − H(reactants). | |
| 1.3 | A reaction in which energy is released to the surroundings; ΔH is negative. | |
| 1.4 | A reaction in which energy is absorbed from the surroundings; ΔH is positive. | |
| 1.5 | The minimum energy that colliding particles must possess for the reaction to occur. | |
| 1.6 | The highest-energy, unstable arrangement of atoms that exists at the peak of an energy profile diagram. | |
| 1.7 | A graph of enthalpy versus the progress of reaction, showing reactants, products, transition state, Ea and ΔH. | |
| 1.8 | The x-axis variable on an energy profile diagram; represents the progress of the reaction, not time. | |
| 1.9 | A balanced chemical equation that includes the ΔH value and state symbols for all species. | |
| 1.10 | A property that depends only on the initial and final states, not on the pathway taken; ΔH is an example. | |
| 1.11 | 25°C and 100 kPa — the reference conditions at which standard enthalpy changes (ΔH°) are measured. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the correct version on the line provided. 10 marks (1 T/F + 1 correction where needed)
2.1 In an exothermic reaction, the products have higher enthalpy than the reactants. T / F
2.2 The activation energy arrow on an energy profile diagram starts at the reactant enthalpy level and ends at the transition state peak. T / F
2.3 When a cold pack feels cold in your hand, the chemical reaction is releasing heat to the surroundings. T / F
2.4 Reversing a thermochemical equation flips the sign of ΔH. T / F
2.5 Doubling the coefficients in a thermochemical equation doubles the magnitude of ΔH. T / F
3. Cloze — fill the blanks
Complete the paragraph using the word bank below. Each word is used once. 8 marks
Word bank: activation energy • endothermic • exothermic • negative • positive • surroundings • released • transition state
A reaction that transfers heat to the ______________ is called ______________, and its ΔH value is ______________. By contrast, a reaction that absorbs heat is called ______________, and its ΔH value is ______________. On an energy profile diagram, the peak of the curve represents the ______________. The minimum energy needed to reach this peak from the reactant level is called the ______________. In an exothermic reaction, energy is ______________ when the products form.
4. Function recall
Answer each in 1–2 precise sentences. Use lesson terminology. 10 marks (2 each)
4.1 What does the sign of ΔH tell you about the direction of energy flow between system and surroundings?
4.2 Why is activation energy (Ea) always measured from the reactant enthalpy level to the peak — never from the x-axis?
4.3 Why must state symbols (s), (l), (g), (aq) be included in every thermochemical equation?
4.4 What does it mean for ΔH to be a state function? Give one practical implication for chemists.
4.5 When an instant cold pack (ammonium nitrate dissolving in water) is cracked open, the pack feels cold. What does this tell you about the direction of energy flow and the sign of ΔH for this reaction?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “is measured by”, “determines”, “requires”). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: ΔH · exothermic reaction · endothermic reaction · activation energy (Ea) · transition state · energy profile diagram
Q1 — Term–definition matches
1.1 enthalpy • 1.2 enthalpy change (ΔH) • 1.3 exothermic reaction • 1.4 endothermic reaction • 1.5 activation energy (Ea) • 1.6 transition state (activated complex) • 1.7 energy profile diagram • 1.8 reaction coordinate • 1.9 thermochemical equation • 1.10 state function • 1.11 standard conditions.
Q2 — True / False with correction
2.1 False. In an exothermic reaction, products have lower enthalpy than reactants — the difference is released as heat (ΔH < 0).
2.2 True. Ea is always measured from the reactant enthalpy level to the peak of the curve.
2.3 False. A cold pack feels cold because the reaction is absorbing heat from the surroundings (including your hand) — the reaction is endothermic (ΔH > 0).
2.4 True. Reversing a thermochemical equation flips the sign of ΔH (e.g. −890 becomes +890 kJ mol−1).
2.5 True. ΔH is a property of the equation as written; doubling coefficients doubles ΔH.
Q3 — Cloze answers (in order)
surroundings • exothermic • negative • endothermic • positive • transition state • activation energy • released.
Q4.1 — Sign of ΔH and energy flow
A negative ΔH means the system loses energy — heat flows from the system to the surroundings (exothermic). A positive ΔH means the system gains energy — heat flows from the surroundings into the system (endothermic).
Q4.2 — Why Ea is from reactant level, not x-axis
The y-axis represents enthalpy, and the absolute position of the reactant level on the y-axis is arbitrary (enthalpy has no absolute zero). Only differences between levels are meaningful. Ea is the energy gap between the reactant level and the peak — not the total height of the peak from zero.
Q4.3 — Why state symbols are compulsory
The physical state of a substance affects its enthalpy content. For example, water as liquid (l) releases more energy than water as gas (g) because condensation releases latent heat. Omitting state symbols makes the ΔH value ambiguous and loses marks in HSC.
Q4.4 — ΔH as a state function
A state function depends only on initial and final states, not the pathway. For chemists this means ΔH for a multi-step reaction can be calculated by adding the ΔH values of the individual steps (Hess’s Law), without needing to measure every possible route.
Q4.5 — Cold pack and energy flow
The pack feels cold because heat energy flows from the surroundings (your hand) into the reaction mixture — the dissolution of NH4NO3 is endothermic. ΔH > 0 (positive).
Q5 — Sample concept map
Correct arrows include (any direction with valid linking phrase):
- energy profile diagram — displays → ΔH
- energy profile diagram — shows → activation energy (Ea)
- energy profile diagram — has a peak called the → transition state
- activation energy (Ea) — is energy to reach → transition state
- ΔH — is negative in → exothermic reaction
- ΔH — is positive in → endothermic reaction
Award 1 mark per chemically valid labelled arrow (minimum 6). Causal direction must be correct.