Chemistry • Year 11 • Module 4 • Lesson 7
Enthalpy of Formation
Nail the core vocabulary, the definition of standard enthalpy of formation, the zero-convention for elements, and the products-minus-reactants formula.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: standard enthalpy of formation (ΔH°f), standard state, formation equation, stoichiometric coefficient, thermodynamic stability, standard conditions, reference baseline, exothermic, endothermic, enthalpy change of reaction (ΔH°rxn). 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The enthalpy change when exactly one mole of a compound is formed from its elements in their standard states at 25°C and 100 kPa. | |
| 1.2 | The most stable physical form of an element at 25°C and 100 kPa (e.g. O2(g), C(graphite), H2(g)). | |
| 1.3 | A balanced chemical equation that shows the formation of one mole of a compound from its constituent elements in standard states. | |
| 1.4 | The number in front of a species in a balanced equation; must be applied to ΔH°f values before summing. | |
| 1.5 | A compound with a large negative ΔH°f is said to have this property — it is energetically stable relative to its elements. | |
| 1.6 | Temperature of 25°C (298 K) and pressure of 100 kPa; all substances in standard states; solutions at 1 mol L−1. | |
| 1.7 | The level set at zero in the ΔH°f method: all elements in their standard states. | |
| 1.8 | A reaction that releases heat to the surroundings; ΔH is negative. | |
| 1.9 | A reaction that absorbs heat from the surroundings; ΔH is positive. | |
| 1.10 | Calculated as ΣΔH°f(products) − ΣΔH°f(reactants). |
2. True or false — with correction
For each statement circle T or F. If false, rewrite the statement correctly on the line below. 12 marks (1 for T/F, 1 for correction where needed)
2.1 The standard enthalpy of formation of O2(g) is defined as zero because oxygen is an element in its standard state. T / F
2.2 When calculating ΔH°rxn using the ΔH°f method, you subtract the sum of product formation enthalpies from the sum of reactant formation enthalpies (reactants minus products). T / F
2.3 A compound with a positive ΔH°f stores more energy than its elements and is therefore energetically less stable relative to those elements. T / F
2.4 The bond energy method and the ΔH°f method always give exactly the same ΔH for the same reaction. T / F
2.5 Stoichiometric coefficients must be multiplied by the corresponding ΔH°f value before the sum is taken. T / F
2.6 In the ΔH°f method, fractional coefficients (e.g. ½ O2) are not permitted in the formation equation. T / F
3. Fill-the-blank paragraph
Complete the paragraph using words from the word bank below. Each word or phrase is used once only. 9 marks
Word bank: products, elements, zero, experimentally, standard state, stoichiometric coefficients, products minus reactants, liquid, approximate, one mole
The standard enthalpy of formation, ΔH°f, is defined as the enthalpy change when _______________ of a compound is formed from its _______________ in their _______________. By convention, the ΔH°f of any element in its standard state is _______________. The formula for a reaction is: ΔH°rxn = ΣΔH°f(_______________) − ΣΔH°f(reactants), which is often remembered as “_______________”. Each ΔH°f value must be multiplied by the corresponding _______________ before summing. Because ΔH°f values are measured _______________ for real substances in their actual states (e.g. water in the _______________ state at 25°C), this method is more accurate than bond energies, which are _______________ averages.
4. Function recall
Answer each question in 1–2 sentences using precise chemical terms. 10 marks (2 each)
4.1 Why is the ΔH°f of all elements in their standard states defined as zero?
4.2 What does a negative value of ΔH°f tell you about the energy of a compound relative to its constituent elements?
4.3 Why must the state symbol (s), (l), (g) or (aq) always be included when writing a formation equation?
4.4 In the calculation for CH4(g) combustion, the formula gives ΔH°rxn = −890.3 kJ mol−1 while the bond energy method gives −674 kJ mol−1. State two reasons why the ΔH°f method is more accurate.
4.5 Write the formation equation for CO2(g) from its elements. Then state the value of ΔH°f[CO2(g)].
5. Concept map
Draw labelled arrows between the six terms below inside the box. Each arrow must carry a linking phrase (e.g. “is measured from”, “equals zero for”, “is scaled by”, “gives”). Aim for at least 5 labelled arrows. 5 marks
Supplied terms: ΔH°f (enthalpy of formation) · elements in standard states · one mole of compound · stoichiometric coefficient · ΔH°rxn · products minus reactants.
Q1 — Term–definition matches
1.1 standard enthalpy of formation (ΔH°f) • 1.2 standard state • 1.3 formation equation • 1.4 stoichiometric coefficient • 1.5 thermodynamic stability • 1.6 standard conditions • 1.7 reference baseline • 1.8 exothermic • 1.9 endothermic • 1.10 enthalpy change of reaction (ΔH°rxn).
Q2 — True / false with correction
2.1 True. O2(g) is the standard state of oxygen; no formation reaction is needed.
2.2 False. Correction: the formula is products minus reactants: ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants).
2.3 True. Positive ΔH°f means the compound is above the element baseline — it stores extra energy and is less thermodynamically stable relative to its elements.
2.4 False. Correction: the two methods give different values for the same reaction. The ΔH°f method uses experimentally measured data for actual states, giving a more accurate result; the bond energy method uses averages and assumes all species are gaseous.
2.5 True.
2.6 False. Correction: fractional coefficients (e.g. ½O2) are entirely acceptable — and sometimes necessary — in formation equations, because the requirement is exactly 1 mol of product.
Q3 — Fill-the-blank paragraph (in order of blanks)
one mole → elements → standard state → zero → products → products minus reactants → stoichiometric coefficients → experimentally → liquid → approximate.
Q4 — Function recall
4.1 No chemical reaction occurs when an element forms from itself; there is no change in chemical bonds and therefore no enthalpy change. The zero value is a defined reference point, not a measured quantity.
4.2 A negative ΔH°f means the compound is at a lower energy level than its constituent elements. Energy was released when the compound formed, so the compound is more thermodynamically stable relative to its elements.
4.3 The enthalpy of formation depends on the physical state of each substance. For example, ΔH°f[H2O(l)] = −285.8 kJ mol−1 but ΔH°f[H2O(g)] = −241.8 kJ mol−1 — a 44 kJ mol−1 difference. Omitting the state symbol would make the ΔH°f value ambiguous.
4.4 (i) The ΔH°f method uses experimentally measured values specific to each compound, not averages across bond types. (ii) It correctly accounts for the physical state of each substance (e.g. H2O(l) at 25°C), whereas the bond energy method assumes all species are gaseous and misses the condensation energy of water.
4.5 Formation equation: C(graphite) + O2(g) → CO2(g). ΔH°f[CO2(g)] = −393.5 kJ mol−1.
Q5 — Sample concept map
A correct map should include arrows such as:
- ΔH°f — is the enthalpy change when forming → one mole of compound
- one mole of compound — is formed from → elements in standard states
- elements in standard states — have ΔH°f = → zero (award credit if labelled correctly)
- ΔH°f — is multiplied by → stoichiometric coefficient
- stoichiometric coefficient (× ΔH°f) — summed as → products minus reactants
- products minus reactants — gives → ΔH°rxn
Any chemically valid linking phrases accepted. Award 1 mark per correctly labelled arrow (min. 5).