Enthalpy Changes | OCR A-Level Chemistry A (H432)
Enthalpy Changes
- 89 questions
- 8 subtopics
- Module 3: Periodic table and energy
- Component 01 and Component 03
Energy changes in reactions: exothermic and endothermic profiles, the standard enthalpy changes and how each is defined, calculating an enthalpy change from bond enthalpies, Hess' law and enthalpy cycles, and measuring an enthalpy change by calorimetry..
It covers exothermic and endothermic reactions and enthalpy profiles, standard conditions and the standard enthalpy changes, bond enthalpies, q = mcΔT and calorimetry, calculating enthalpy change from bond enthalpies, why average bond enthalpies give only an approximation, hess' law and enthalpy cycles from combustion and formation data, working through an enthalpy cycle and the sign conventions and calorimetry and correcting for heat loss.
Sample questions from Enthalpy Changes
Answer each one closed book first, then open the answer.
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Exothermic and endothermic reactions and enthalpy profiles
A reaction absorbs energy from its surroundings. What sign does its ΔH have?
Show the answer
Positive (ΔH > 0), indicating an endothermic reaction. -
Standard conditions and the standard enthalpy changes
Define the standard enthalpy change of formation (ΔfH⦵).
Show the answer
The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions. -
Bond enthalpies, q = mcΔT and calorimetry
Explain why a reaction is exothermic overall in terms of bond energies.
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A reaction is exothermic overall when the energy released in forming new bonds is greater than the energy required to break the bonds in the reactants. -
Calculating enthalpy change from bond enthalpies
State the formula used to calculate enthalpy change of reaction from bond enthalpies.
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ΔH = Σ E(bonds broken) − Σ E(bonds formed). -
Why average bond enthalpies give only an approximation
Why are enthalpy changes calculated using average bond enthalpies only approximate?
Show the answer
Because average bond enthalpies are mean values that do not account for variations in bond strength in specific molecular environments. -
Hess' law and enthalpy cycles from combustion and formation data
In a combustion enthalpy cycle, what do the arrows from reactants point towards?
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The combustion products, using ΔcH values. -
Working through an enthalpy cycle and the sign conventions
How must stoichiometric coefficients be treated when using enthalpy values in an enthalpy cycle?
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They must be used as multipliers for the enthalpy values. -
Calorimetry and correcting for heat loss
In the equation q = mcΔT, what does each symbol represent?
Show the answer
q = heat energy transferred, m = mass of substance being heated, c = specific heat capacity, ΔT = temperature change.
The 8 subtopics
One subtopic is one session. Work down the list.
| Subtopic | What it covers | Questions |
|---|---|---|
| Exothermic and endothermic reactions and enthalpy profiles | Recall questions on the sign of ΔH for each type, what the axes of an enthalpy profile show, where the products sit, and how activation energy appears. | 11 |
| Standard conditions and the standard enthalpy changes | Recall questions on standard conditions and standard states, and on the definitions of the standard enthalpy changes of reaction, formation, combustion and neutralisation. | 9 |
| Bond enthalpies, q = mcΔT and calorimetry | Recall questions on average bond enthalpy, why bond breaking is endothermic and bond making exothermic, q = mcΔT, and what calorimetry measures. | 13 |
| Calculating enthalpy change from bond enthalpies | Recall questions on ΔH = Σ(bonds broken) − Σ(bonds formed), identifying which bonds break and form, and working the sum through. | 9 |
| Why average bond enthalpies give only an approximation | Recall questions on why a bond's strength varies between molecules, why the values are averages, and the gaseous-state assumption behind them. | 9 |
| Hess' law and enthalpy cycles from combustion and formation data | Recall questions on the statement of Hess' law and why it holds, the two cycle constructions, and the formulae that follow from each. | 13 |
| Working through an enthalpy cycle and the sign conventions | Recall questions on reversing a reaction's sign, treating stoichiometric coefficients, and the order of steps in solving an unfamiliar cycle. | 13 |
| Calorimetry and correcting for heat loss | Recall questions on q = mcΔT in practice, why a polystyrene cup and a lid are used, why stirring matters, and correcting for heat loss by extrapolation. | 12 |
How the guide is worked
Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.
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Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
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Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
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Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.
Read the full method, the return schedule and the research behind it.
Nearby sections
OCR A-Level Chemistry A Active Recall Guide
Every section, not just this one. 1,913 questions with their answers.