Energy Cycles In Reactions

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Bond Enthalpy Basics

Bond breaking is endothermic: energy must be supplied to overcome the attraction between atoms.
Bond formation is exothermic: energy is released when new bonds form.
The energy required to break a particular bond is called the bond dissociation enthalpy (or bond enthalpy).
The energy released when a bond forms is equal in magnitude but opposite in sign to the energy needed to break it.
If more energy is released than absorbed, the reaction is exothermic; products are more stable than reactants.
If more energy is absorbed than released, the reaction is endothermic; products are less stable than reactants.

Average Bond Enthalpy

Bond energies are affected by the molecular environment, so an average bond enthalpy is used.
Average bond enthalpy is defined as the energy needed to break one mole of a specified type of bond in gaseous molecules, averaged over similar compounds.
For example, the average C–H bond enthalpy in methane is found by dividing the total bond dissociation enthalpy by the number of C–H bonds.
The first C–H bond is easier to break than the second because remaining hydrogens are pulled closer to the carbon.
Average bond enthalpies are compared across similar compounds to obtain an accepted value.

Bond Enthalpy Calculations

Use bond enthalpies to calculate ΔH when experimental measurement is not possible.
Formula: ΔH = Σ(bonds broken) + Σ(bonds formed).
Bonds broken: values are positive (endothermic); bonds formed: values are negative (exothermic).
Always draw full displayed structures to account for every bond.
Multiply bond enthalpies by coefficients from the balanced equation.
Show all steps: bond enthalpy calculations often carry marks for working even if the final answer is wrong.

Hess's Law

Formulated by Germain Hess in 1840; based on the law of conservation of energy.
Hess's Law: The total enthalpy change in a chemical reaction is independent of the route taken, as long as initial and final conditions are the same.
Energy cannot be created or destroyed, only changed in form; total energy in a closed system is constant.
Used to calculate enthalpy changes that cannot be found experimentally, e.g., formation of propane from carbon and hydrogen.
For a two-step route: ΔH₂ = ΔH₁ + ΔHᵣ, so ΔHᵣ = ΔH₂ − ΔH₁.
You do not need to learn the law word for word, but you must understand the principle.

Hess's Law Calculations

Two common methods: using cycles or using equations.
In cycles: if you follow the direction of an arrow, add the quantity; if you go against the arrow, subtract the quantity.
When using equations: reverse an equation → reverse the sign of ΔH; multiply an equation → multiply ΔH by the same factor.
Always put brackets around values and add the mathematical operator in front to avoid sign errors.
Cancel common terms on both sides after combining equations.
Both methods are valid; choose the one that seems easier for the problem.

Calculating Enthalpy Changes Using ΔHf⦵

Standard enthalpy of formation (ΔHf⦵) is the enthalpy change when one mole of a compound is formed from its elements under standard conditions.
In a Hess cycle, elements are placed at the bottom; arrows point upwards from elements to compounds.
General formula: ΔHᵣ = ΣΔHf⦵(products) − ΣΔHf⦵(reactants).
ΔHf⦵ of an element in its standard state is zero.
Multiply each ΔHf⦵ by the number of moles from the balanced equation.
Enthalpy of formation data are provided in Section 13 of the Data Booklet.

Calculating Enthalpy Changes Using ΔHc⦵

Standard enthalpy of combustion (ΔHc⦵) is the enthalpy change when one mole of a substance burns completely in oxygen under standard conditions.
In a Hess cycle, combustion products are placed at the bottom; arrows point downwards.
General formula: ΔHᵣ = ΣΔHc⦵(reactants) − ΣΔHc⦵(products).
Combustion products of both reactants and products are placed at the bottom of the cycle.
When going against the arrow direction, reverse the sign of the enthalpy value.
Ensure the number of atoms of each element is balanced when drawing the cycle.

Born-Haber Cycles (HL)

A Born–Haber cycle is a specialised Hess's Law cycle used to determine lattice enthalpy of ionic compounds.
Lattice enthalpy (ΔHlatt) is the energy change when one mole of an ionic compound is separated into its component gaseous ions; it is endothermic (positive).
First ionisation energy (ΔHie1) is the energy to remove one mole of electrons from one mole of gaseous atoms; endothermic.
Second ionisation energy (ΔHie2) is the energy to remove one mole of electrons from one mole of gaseous 1+ ions; endothermic.
Enthalpy of atomisation (ΔHat) is the energy to form one mole of gaseous atoms from the element in its standard state; endothermic.
First electron affinity (ΔHea1) is the energy change when one mole of gaseous atoms gains one mole of electrons; can be exothermic, endothermic, or neutral.
Second electron affinity (ΔHea2) is the energy change when one mole of gaseous 1− ions gains one mole of electrons; endothermic due to repulsion.
In Born-Haber diagrams: endothermic processes go upward, exothermic processes go downward.

Born-Haber Cycle Calculations (HL)

Apply Hess's Law: ΔHlatt = −ΔHf + ΔH1, where ΔH1 is the sum of all steps from elements to gaseous ions.
For a compound like MgCl₂, double the values for atomisation and electron affinity of chlorine because two Cl⁻ ions are formed.
Include both first and second ionisation energies when forming a 2+ ion (e.g., Mg²⁺).
The cycle can be used to calculate any unknown step if the others are known.
Always check stoichiometry: multiply enthalpy values by the number of moles involved.
You may need to refer to the Data Booklet for specific values; ensure you select the correct data for each step.
You will not be asked to draw an entire Born-Haber cycle from scratch, but you may need to complete a partially drawn one.
Show electrons in ionisation steps and ensure arrows point in the correct direction.

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Practice questions

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1.What is the correct definition of average bond enthalpy?
Easy
- AThe energy required to break one covalent bond in a gaseous molecule
- BThe energy released when one mole of covalent bonds is formed in gaseous molecules
- CThe average energy required to break one mole of a specified type of bond in gaseous molecules
- DThe energy required to break one mole of all bonds in a substance in its standard state
2.Which statement best describes what is meant by the average HI bond enthalpy?
Easy
- AThe energy stored in a covalent bond.
- BThe energy required to break one covalent bond in the gas phase.
- CThe energy required to break one mole of the HI bonds in the gas phase.
- DThe energy released when two atoms form a covalent bond.
3.Breaking a chemical bond is an endothermic process.
Easy
True or false?
4.Which quantity gives the best indication of the relative strength of the hydrogen bonds between water molecules in the liquid state?
Medium
- AEnthalpy changes of vaporisation
- BBond dissociation energies
- CEnthalpy of formation
- DActivation energy
5.What is the correct definition of lattice enthalpy?
Easy
- AEnthalpy change when one mole of solid ionic compound is separated into its component gaseous ions
- BEnthalpy change when one mole of electrons is removed from one mole of gaseous atoms
- CEnthalpy change when one mole of solid ionic compound is formed from its gaseous ions under standard conditions
- DEnthalpy change when one mole of a compound is formed from its elements
6.Which of the following processes are endothermic in a Born-Haber cycle? (select all that apply)
Medium
- AAtomisation of a solid metal
- BFirst ionisation energy
- CFirst electron affinity of a halogen
- DSecond electron affinity of oxygen
- ELattice enthalpy (separation into gaseous ions)
7.Which equation represents the second electron affinity of nitrogen?
Medium
- A½N₂(g) + 2e⁻ → N²⁻(g)
- BN⁻(g) + e⁻ → N²⁻(g)
- CN(g) + 4e⁻ → 2N²⁻(g)
- DN(g) + e⁻ → N⁻(g)
8.Which steps are endothermic in the Born-Haber cycle for the formation of LiCl? I. ½Cl₂(g) → Cl(g); II. Cl(g) + e⁻ → Cl⁻(g); III. Li(s) → Li(g)
Medium
- AI and II only
- BI and III only
- CII and III only
- DI, II and III

More questions

9.Which is the correct equation to calculate the enthalpy of reaction using enthalpy of formation data?
Medium
- AΔHr = ΣΔHf products + ΣΔHf reactants
- BΔHr = ΣΔHf products - ΣΔHf reactants
- CΔHr = ΣΔHf reactants - ΣΔHf products
- DΔHr = ΣΔHf reactants + ΣΔHf products
10.Iron(II,III) oxide, Fe₃O₄, is reduced by graphite: Fe₃O₄(s) + 2C(s) → 3Fe(s) + 2CO₂(g). The standard enthalpy of formation values for Fe₃O₄ and CO₂ are -1118 kJ mol⁻¹ and -394 kJ mol⁻¹ respectively. Which calculation gives the standard enthalpy of reaction?
Medium
- A-330
- B1512
- C-1906
- D330
11.Hydrogen peroxide slowly decomposes: 2H₂O₂(l) → 2H₂O(l) + O₂(g). Given ΔHf⦵: H₂O₂(l) = -188 kJ mol⁻¹, H₂O(l) = -286 kJ mol⁻¹. Which calculation gives the correct enthalpy of reaction?
Medium
- A-98
- B-196
- C-662
- D196
12.The standard enthalpy of formation of an element in its standard state is zero.
Easy
True or false?
13.The first stage in the industrial production of nitric acid from ammonia is: 4NH₃(g) + 5O₂(g) ⇌ 4NO(g) + 6H₂O(g). Using ΔHf⦵ values (kJ mol⁻¹): NH₃(g) = -46.1, NO(g) = +90.3, H₂O(g) = -241.8, what is ΔH⦵ for this reaction?
Medium
- A-1274 kJ mol⁻¹
- B-905.2 kJ mol⁻¹
- C905.2 kJ mol⁻¹
- D-378.2 kJ mol⁻¹
14.Titanium can be extracted by reducing rutile with carbon: TiO₂(s) + 2C(s) → Ti(s) + 2CO(g). Given ΔHf⦵(TiO₂) = -890 kJ mol⁻¹ and ΔHf⦵(CO) = -110.5 kJ mol⁻¹, what is the standard enthalpy change of this extraction reaction?
Medium
- A+669 kJ mol⁻¹
- B+779.5 kJ mol⁻¹
- C-779.5 kJ mol⁻¹
- D-669 kJ mol⁻¹
15.The combustion of ethanol is: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l). Given ΔHf⦵(CO₂) = a, ΔHf⦵(H₂O) = b, ΔHf⦵(C₂H₅OH) = c (all in kJ mol⁻¹), what is the standard enthalpy change of combustion of ethanol?
Medium
- A2a + 3b + c
- B2a + 3b − c
- C−2a − 3b + c
- D−2a − 3b − c
16.Propanone has the molecular formula C₃H₆O. Given the enthalpy of combustion of carbon is x kJ mol⁻¹, of hydrogen is y kJ mol⁻¹, and of propanone is z kJ mol⁻¹, what is the enthalpy change of formation of propanone?
Medium
- Ax + y - z
- B3x + 3y - z
- Cz - 3x + 3y
- D3x + 3y + z
17.Which of the following statements are needed to make the definition of Hess's Law complete? (select all that apply)
Medium
- AProviding that the reactants are the same
- BProviding that the products are the same
- CProviding that the conditions at the start and the end of the reaction are the same
- DProviding that the reaction is exothermic
- EProviding that the reaction is carried out at constant pressure
18.The hydration enthalpy of anhydrous copper(II) sulfate cannot be measured directly. It can be found indirectly by determining the solution enthalpies of anhydrous and hydrated copper(II) sulfate. Which are the correct labels for the enthalpies shown in the Hess cycle?
Medium
- AΔH₁ = solution enthalpy of anhydrous CuSO₄, ΔH₂ = solution enthalpy of hydrated CuSO₄, ΔH₃ = hydration enthalpy of anhydrous CuSO₄
- BΔH₁ = hydration enthalpy of anhydrous CuSO₄, ΔH₂ = solution enthalpy of anhydrous CuSO₄, ΔH₃ = solution enthalpy of hydrated CuSO₄
- CΔH₁ = solution enthalpy of hydrated CuSO₄, ΔH₂ = hydration enthalpy of anhydrous CuSO₄, ΔH₃ = solution enthalpy of anhydrous CuSO₄
- DΔH₁ = solution enthalpy of anhydrous CuSO₄, ΔH₂ = hydration enthalpy of anhydrous CuSO₄, ΔH₃ = solution enthalpy of hydrated CuSO₄
19.The reaction 2N₂(g) + 6H₂(g) → 4NH₃(g) can be calculated from: 4NH₃(g) + 3O₂(g) → 2N₂(g) + 6H₂O(l) ΔH₁ = -1530 kJ mol⁻¹ and H₂(g) + ½O₂(g) → H₂O(l) ΔH₂ = -288 kJ mol⁻¹. What is ΔH for the first reaction?
Medium
- A-198 kJ mol⁻¹
- B+198 kJ mol⁻¹
- C-1266 kJ mol⁻¹
- D-1818 kJ mol⁻¹
20.What is the enthalpy change for 4FeO(s) + O₂(g) → 2Fe₂O₃(s) given: 2Fe(s) + O₂(g) → 2FeO(s) ΔH = -544 kJ and 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s) ΔH = -1648 kJ?
Medium
- A-560 kJ mol⁻¹
- B+560 kJ mol⁻¹
- C-2192 kJ mol⁻¹
- D-1088 kJ mol⁻¹
21.Given: N₂(g) + O₂(g) → 2NO(g) ΔH = +180 kJ and 2NO(g) → 2NO₂(g) + O₂(g) ΔH = +112 kJ, what is ΔH for N₂(g) + 2O₂(g) → 2NO₂(g)?
Medium
- A+68 kJ mol⁻¹
- B-68 kJ mol⁻¹
- C+292 kJ mol⁻¹
- D-292 kJ mol⁻¹
22.Given: Br₂(l) + F₂(g) → 2BrF(g) ΔH = x kJ and Br₂(l) + 3F₂(g) → 2BrF₃(g) ΔH = y kJ, what is the enthalpy change for BrF(g) + F₂(g) → BrF₃(g)?
Medium
- Ax – y
- By - x
- C½(–x + y)
- D½(x – y)
23.In a Born-Haber cycle for the formation of LiCl, which of the following steps are endothermic? I. ½Cl₂(g) → Cl(g); II. Cl(g) + e⁻ → Cl⁻(g); III. Li(s) → Li(g)
Medium
- AI and II only
- BI and III only
- CII and III only
- DI, II and III
24.Which of the following processes are exothermic? (select all that apply)
Medium
- ANa(g) → Na⁺(g) + e⁻
- BCl(g) + e⁻ → Cl⁻(g)
- CNaCl(s) → Na⁺(g) + Cl⁻(g)
- DNa(s) → Na(g)
- ENa⁺(g) + Cl⁻(g) → NaCl(s)
25.The lattice enthalpy of an ionic compound is always positive because separating ions is an endothermic process.
Easy
True or false?
26.The second electron affinity of oxygen is exothermic because the negative ion attracts the electron.
Easy
True or false?
27.Match each enthalpy change with its correct definition.
Medium
- Lattice enthalpy
- First ionisation energy
- Enthalpy of atomisation
- First electron affinity
- Energy change when one mole of gaseous atoms is converted into one mole of gaseous ions each with a single positive charge
- Energy change when one mole of an ionic compound is separated into its component gaseous ions
- Energy change when one mole of gaseous atoms is converted to one mole of gaseous ions, each with a single negative charge
- Energy change when one mole of gaseous atoms is formed from the element in its standard state
28.Place the steps of a Born-Haber cycle for NaCl in the correct order from start to finish.
Medium
- Na(s) + ½Cl₂(g) → NaCl(s)
- Na(s) → Na(g)
- ½Cl₂(g) → Cl(g)
- Na(g) → Na⁺(g) + e⁻
- Cl(g) + e⁻ → Cl⁻(g)
- Na⁺(g) + Cl⁻(g) → NaCl(s)
29.A Hess cycle is used to find the enthalpy of combustion of methane using enthalpy of formation data. The cycle has elements placed at the bottom. Which expression gives the correct enthalpy of combustion?
Medium
- AΔHc = ΣΔHf(reactants) – ΣΔHf(products)
- BΔHc = ΣΔHf(products) – ΣΔHf(reactants)
- CΔHc = ΣΔHf(products) + ΣΔHf(reactants)
- DΔHc = ΣΔHf(reactants) + ΣΔHf(products)
30.Which of the following statements about bond enthalpy calculations are correct? (select all that apply)
Medium
- ABond breaking is endothermic and bond making is exothermic.
- BThe enthalpy change of a reaction can be estimated from average bond enthalpies.
- CAverage bond enthalpies are always exact for a specific bond in a specific molecule.
- DThe enthalpy change for bonds broken is given a negative sign in the calculation.
- EIf more energy is released than absorbed, the reaction is exothermic.
31.The standard enthalpy change of formation of an element in its standard state is always zero.
Easy
True or false?
32.Place the following steps of a Born–Haber cycle for the formation of NaCl from its elements in the correct order, starting from the elements in their standard states and ending with the formation of the solid ionic compound.
Medium
- Na(s) + ½Cl₂(g) → Na(g) + Cl(g)
- Na(g) + Cl(g) → Na⁺(g) + Cl⁻(g)
- Na⁺(g) + Cl⁻(g) → NaCl(s)
- Na(s) + ½Cl₂(g) → NaCl(s)
33.The standard enthalpy of combustion of methanol is –726 kJ mol⁻¹. Using ΔHf⦵(CO₂) = –394 kJ mol⁻¹ and ΔHf⦵(H₂O) = –286 kJ mol⁻¹, what is the standard enthalpy of formation of methanol, CH₃OH(l)?
Hard
- A–239 kJ mol⁻¹
- B+239 kJ mol⁻¹
- C–954 kJ mol⁻¹
- D–726 kJ mol⁻¹
34.A student determines the enthalpy change for the reaction between zinc and copper(II) sulfate solution using a polystyrene cup and a thermometer. Which improvement would most reduce heat loss to the surroundings and give a more accurate result?
Medium
- AUse a larger volume of copper(II) sulfate solution
- BPlace a lid on the polystyrene cup and use a second cup as a sleeve
- CStir the mixture more vigorously with the thermometer
- DUse powdered zinc instead of granulated zinc
35.A student is planning an experiment to determine the enthalpy change of combustion of ethanol using a spirit burner and a calorimeter. Which of the following precautions would improve the accuracy of the result? (select all that apply)
Hard
- AUse a draught shield around the burner
- BMeasure the mass of the burner before and after burning
- CUse a large excess of ethanol in the burner
- DStir the water continuously during heating
- EPlace the thermometer in the flame to measure the flame temperature
36.In a calorimetry experiment, the heat released by the reaction is calculated using the mass of the solution, its specific heat capacity, and the temperature change.
Easy
True or false?
37.Match each enthalpy change in a Born–Haber cycle with the correct description.
Medium
- Enthalpy of atomisation
- First ionisation energy
- First electron affinity
- Lattice enthalpy
- Energy change when one mole of gaseous atoms is converted into one mole of gaseous 1+ ions
- Energy change when one mole of an element in its standard state is converted into one mole of gaseous atoms
- Energy change when one mole of an ionic compound is separated into its component gaseous ions
38.Place the following steps of a Born–Haber cycle for the formation of calcium oxide, CaO, in the correct order from start to finish, starting from the elements in their standard states.
Medium
- Ca(s) + ½O₂(g) → Ca(g) + O(g)
- Ca(g) → Ca⁺(g) + e⁻
- Ca⁺(g) → Ca²⁺(g) + e⁻
- O(g) + e⁻ → O⁻(g)
- O⁻(g) + e⁻ → O²⁻(g)
- Ca²⁺(g) + O²⁻(g) → CaO(s)
39.A student calculates the lattice enthalpy of magnesium oxide using a Born–Haber cycle. The data include the first and second ionisation energies of magnesium and the first and second electron affinities of oxygen. Which of the following is the correct expression for the lattice enthalpy, ΔH_latt, in terms of the enthalpy of formation, ΔH_f, and the sum of the other steps, ΔH₁?
Hard
- AΔH_latt = -ΔH_f + ΔH₁
- BΔH_latt = ΔH_f + ΔH₁
- CΔH_latt = -ΔH_f - ΔH₁
- DΔH_latt = ΔH_f - ΔH₁
40.Which statement describes what happens to energy when a chemical bond is broken?
Easy
- AEnergy is released to the surroundings.
- BEnergy is required from the surroundings.
- CNo energy change occurs.
- DEnergy is both released and absorbed equally.
41.Calculate the lattice enthalpy of KCl (in kJ mol⁻¹) given: ΔHf(KCl) = -437, ΔHat(K) = +90, ΔHat(Cl) = +122, ΔHie(K) = +418, ΔHea(Cl) = -349. Use ΔHlatt = -ΔHf + [(ΔHat K) + (ΔHat Cl) + (ΔHie K) + (ΔHea Cl)].
Medium
42.Calculate the lattice enthalpy of MgO (in kJ mol⁻¹) given: ΔHf(MgO) = -602, ΔHat(Mg) = +148, ΔHat(O) = +248, ΔHie1(Mg) = +736, ΔHie2(Mg) = +1450, ΔHea1(O) = -142, ΔHea2(O) = +770. Use ΔHlatt = -ΔHf + [sum of all steps from elements to gaseous ions].
Hard
43.Complete the equation for the enthalpy of reaction using standard enthalpies of formation.
Medium
ΔHr⦵ = ΣΔHf⦵(products) ____ ΣΔHf⦵(reactants)
44.Complete the equation for the enthalpy of reaction using standard enthalpies of combustion.
Medium
ΔHr⦵ = ΣΔHc⦵(reactants) ____ ΣΔHc⦵(products)
45.Given the bond enthalpies (kJ mol⁻¹): H–H = 436, Cl–Cl = 243, H–Cl = 431, calculate the enthalpy change for H₂(g) + Cl₂(g) → 2HCl(g). Include the sign in your answer.
Medium
46.Complete the equation for calculating the enthalpy of reaction from standard enthalpies of formation. Use the symbols ΔHr, ΔHf, products, and reactants.
Medium
ΔH____ = ΣΔH____(____) – ΣΔH____(____)
47.What is the name of the enthalpy change when one mole of an ionic compound is separated into its component gaseous ions?
Medium
48.Use the bond enthalpies N≡N = 945 kJ mol⁻¹, H−H = 436 kJ mol⁻¹ and N−H = 391 kJ mol⁻¹ to calculate the enthalpy change, in kJ mol⁻¹, for the Haber process reaction N₂(g) + 3H₂(g) → 2NH₃(g). Include the sign.
Medium
49.Using average bond enthalpies C−H = 414, C≡C = 839, O=O = 498, C=O = 804 and O−H = 463 kJ mol⁻¹, calculate the enthalpy of combustion of ethyne, in kJ mol⁻¹, for C₂H₂(g) + 2½O₂(g) → H₂O(g) + 2CO₂(g). Include the sign.
Medium
50.Calculate the lattice enthalpy of KCl, in kJ mol⁻¹, given ΔHf(KCl) = −437, ΔHat(K) = +90, ΔHat(Cl) = +122, ΔHie(K) = +418 and ΔHea(Cl) = −349 kJ mol⁻¹.
Medium
51.Calculate the lattice enthalpy of MgO, in kJ mol⁻¹, given ΔHf(MgO) = −602, ΔHat(Mg) = +148, ΔHat(O) = +248, ΔHie1(Mg) = +736, ΔHie2(Mg) = +1450, ΔHea1(O) = −142 and ΔHea2(O) = +770 kJ mol⁻¹.
Hard
52.The standard enthalpy of combustion of methanol is −726 kJ mol⁻¹. Using ΔHf(CO₂) = −394 kJ mol⁻¹ and ΔHf(H₂O) = −286 kJ mol⁻¹, calculate the standard enthalpy of formation of methanol, in kJ mol⁻¹. Include the sign.
Medium
53.Explain why the enthalpy change for the reaction 3C(s) + 4H₂(g) → C₃H₈(g) cannot be measured directly, and state how it can be determined instead.
Hard
54.State Hess's Law and explain how it can be used to calculate an enthalpy change that cannot be measured directly.
Medium
55.Explain why the first electron affinity of oxygen is exothermic but the second electron affinity is endothermic.
Hard
56.Explain why the lattice enthalpy of an ionic compound cannot be measured directly, and describe how a Born–Haber cycle allows it to be determined.
Hard
57.Explain why the lattice enthalpy of an ionic compound cannot be measured directly by experiment, and describe how a Born–Haber cycle allows it to be determined indirectly. Include the relevant enthalpy changes in your answer.
Challenge
58.Calculate the lattice enthalpy of KCl (in kJ mol⁻¹) given: ΔHf(KCl) = -437, ΔHat(K) = +90, ΔHat(Cl) = +122, ΔHie(K) = +418, ΔHea(Cl) = -349. Use ΔHlatt = -ΔHf + [(ΔHat K) + (ΔHat Cl) + (ΔHie K) + (ΔHea Cl)].
Medium
- A718 kJ mol⁻¹
- B862 kJ mol⁻¹
- C790 kJ mol⁻¹
- D646 kJ mol⁻¹
59.Calculate the lattice enthalpy of MgO (in kJ mol⁻¹) given: ΔHf(MgO) = -602, ΔHat(Mg) = +148, ΔHat(O) = +248, ΔHie1(Mg) = +736, ΔHie2(Mg) = +1450, ΔHea1(O) = -142, ΔHea2(O) = +770. Use ΔHlatt = -ΔHf + [sum of all steps from elements to gaseous ions].
Hard
- A4193 kJ mol⁻¹
- B3431 kJ mol⁻¹
- C4574 kJ mol⁻¹
- D3812 kJ mol⁻¹
60.Given the bond enthalpies (kJ mol⁻¹): H–H = 436, Cl–Cl = 243, H–Cl = 431, calculate the enthalpy change for H₂(g) + Cl₂(g) → 2HCl(g). Include the sign in your answer.
Medium
- A-147 kJ mol⁻¹
- B-165 kJ mol⁻¹
- C-183 kJ mol⁻¹
- D-201 kJ mol⁻¹
61.Use the bond enthalpies N≡N = 945 kJ mol⁻¹, H−H = 436 kJ mol⁻¹ and N−H = 391 kJ mol⁻¹ to calculate the enthalpy change, in kJ mol⁻¹, for the Haber process reaction N₂(g) + 3H₂(g) → 2NH₃(g). Include the sign.
Medium
- A-93 kJ mol⁻¹
- B-75 kJ mol⁻¹
- C-84 kJ mol⁻¹
- D-102 kJ mol⁻¹
62.Using average bond enthalpies C−H = 414, C≡C = 839, O=O = 498, C=O = 804 and O−H = 463 kJ mol⁻¹, calculate the enthalpy of combustion of ethyne, in kJ mol⁻¹, for C₂H₂(g) + 2½O₂(g) → H₂O(g) + 2CO₂(g). Include the sign.
Medium
- A-1230 kJ mol⁻¹
- B-984 kJ mol⁻¹
- C-1107 kJ mol⁻¹
- D-1353 kJ mol⁻¹
63.Calculate the lattice enthalpy of KCl, in kJ mol⁻¹, given ΔHf(KCl) = −437, ΔHat(K) = +90, ΔHat(Cl) = +122, ΔHie(K) = +418 and ΔHea(Cl) = −349 kJ mol⁻¹.
Medium
- A862 kJ mol⁻¹
- B718 kJ mol⁻¹
- C646 kJ mol⁻¹
- D790 kJ mol⁻¹
64.Calculate the lattice enthalpy of MgO, in kJ mol⁻¹, given ΔHf(MgO) = −602, ΔHat(Mg) = +148, ΔHat(O) = +248, ΔHie1(Mg) = +736, ΔHie2(Mg) = +1450, ΔHea1(O) = −142 and ΔHea2(O) = +770 kJ mol⁻¹.
Hard
- A4574 kJ mol⁻¹
- B3812 kJ mol⁻¹
- C4193 kJ mol⁻¹
- D3431 kJ mol⁻¹
65.The standard enthalpy of combustion of methanol is −726 kJ mol⁻¹. Using ΔHf(CO₂) = −394 kJ mol⁻¹ and ΔHf(H₂O) = −286 kJ mol⁻¹, calculate the standard enthalpy of formation of methanol, in kJ mol⁻¹. Include the sign.
Medium
- A-238 kJ mol⁻¹
- B-190 kJ mol⁻¹
- C-214 kJ mol⁻¹
- D-262 kJ mol⁻¹

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