Energetics (Edexcel IAL Chemistry Unit 2)

Edexcel International A-Level Chemistry· 6.1-6.11· 25 min read

1. Enthalpy Fundamentals & Standard Conditions★★☆☆☆⏱ 5 min

📘 Definition

Enthalpy Change ($\Delta H$)

Heat change measured at constant pressure, reported in kJ mol⁻¹. Exothermic reactions release heat (negative $Δ H$ ), endothermic reactions absorb heat (positive $Δ H$ ).

Example:

Combustion of methane has $Δ H = - 890$ kJ mol⁻¹, so it is highly exothermic.

Enthalpy level diagrams plot enthalpy on the y-axis against reaction progress on the x-axis. For exothermic reactions, products are at a lower enthalpy level than reactants, with a downward arrow for $Δ H$ . For endothermic reactions, products are higher than reactants, with an upward arrow for $Δ H$ .

📐 Worked Example

Classify the thermal decomposition of calcium carbonate ( $Δ H = + 178$ kJ mol⁻¹) as exothermic or endothermic, and describe its enthalpy level diagram.

1
1. The $Δ H$ value is positive, so the reaction is endothermic, absorbing heat from the surroundings.
2
1. The enthalpy level diagram will have reactants (CaCO₃(s)) at a lower level than products (CaO(s) + CO₂(g)), with an upward arrow between them labelled $Δ H = + 178$ kJ mol⁻¹, and an activation energy hump above the reactant level.

2. Calorimetry & Enthalpy Calculations★★★☆☆⏱ 6 min

📘 Definition

Calorimetry Formula

$q = m c Δ T$

Calculates heat transferred to a surrounding medium, where $q$ = heat (J), $m$ = mass of medium (g), $c$ = specific heat capacity (4.18 J g⁻¹ K⁻¹ for water), $Δ T$ = temperature change (K or °C).

Example:

A 100 g water sample heated by 10 K absorbs $q = 100 * 4.18 * 10 = 4180$ J of heat.

To convert $q$ to molar enthalpy change ( $Δ H$ in kJ mol⁻¹): divide $q$ by 1000 to convert to kJ, then divide by the moles of the limiting reactant. For aqueous reactions, assume solution density = 1 g cm⁻³, so volume in cm³ equals mass in g.

📐 Worked Example

25 cm³ of 2 mol dm⁻³ HCl is mixed with 25 cm³ of 2 mol dm⁻³ NaOH, and the temperature rises by 13.2 °C. Calculate the standard enthalpy of neutralisation.

1
1. Total mass of solution = 25 + 25 = 50 g. $Δ T = 13.2$ K (equal to °C change).
2
$q = 50 * 4.18 * 13.2 = 2758.8 \text{ J} = 2.7588 \text{ kJ}$
3
1. Moles of limiting reactant (H⁺ or OH⁻) = 0.025 dm³ * 2 mol dm⁻³ = 0.05 mol, so moles of water formed = 0.05 mol.
4
1. Reaction is exothermic, so $Δ H$ is negative:
5
$\Delta_{neut} H = - \frac{2.7588}{0.05} = -55.2 \text{ kJ mol}^{-1} \text{ (3 sig figs)}$

Exam tip:

Always include the correct sign for $Δ H$ ; 1 mark is usually reserved for the sign in calculation questions.

3. Hess's Law & Enthalpy Cycles★★★★☆⏱ 7 min

📘 Definition

Hess's Law

The total enthalpy change for a reaction is independent of the route taken, as long as initial and final conditions are identical. This allows calculation of enthalpy changes for reactions that cannot be measured directly.

Example:

Enthalpy of formation of methane can be calculated from combustion enthalpies of C, H₂ and CH₄.

For formation data: $Δ_{r} H = \sum Δ_{f} H (p r o d u c t s) - \sum Δ_{f} H (r e a c t an t s)$
For combustion data: $Δ_{r} H = \sum Δ_{c} H (r e a c t an t s) - \sum Δ_{c} H (p r o d u c t s)$

📐 Worked Example

Calculate $Δ_{r} H$ for the reaction $C_{2} H_{4} (g) + H_{2} (g) \to C_{2} H_{6} (g)$ using $Δ_{f} H$ data: $Δ_{f} H (C_{2} H_{4}) = + 52$ kJ mol⁻¹, $Δ_{f} H (C_{2} H_{6}) = - 85$ kJ mol⁻¹.

1
1. $Δ_{f} H$ of H₂(g) (element in standard state) is 0 kJ mol⁻¹.
2
$\sum\Delta_f H(products) = -85 \text{ kJ mol}^{-1}$
3
$\sum\Delta_f H(reactants) = 52 + 0 = 52 \text{ kJ mol}^{-1}$
4
$\Delta_r H = (-85) - (52) = -137 \text{ kJ mol}^{-1}$

4. Core Practical 2: Enthalpy Measurement by Hess's Law★★★☆☆⏱ 4 min

Core Practical 2 measures the enthalpy change of a reaction that cannot be measured directly (e.g. decomposition of KHCO₃) by measuring enthalpy changes for two related reactions (reaction of K₂CO₃ and KHCO₃ with HCl) and constructing a Hess cycle.

Cooling curve correction accounts for heat loss to surroundings: plot temperature against time, then extrapolate the cooling trend back to the time of mixing to get the true maximum temperature change. Common sources of uncertainty include heat loss, incomplete reaction, and approximation of solution specific heat capacity.

📐 Worked Example

A student records an uncorrected temperature rise of 3.7 °C for a reaction. After cooling curve extrapolation, the true $Δ T$ is 4.3 °C. Calculate the percentage error from uncorrected temperature data.

1
1. Calculate the difference between true and measured $Δ T$ : $4.3 - 3.7 = 0.6$ °C.
2
$\text{Percentage error} = \frac{0.6}{4.3} * 100 = 14.0\% \text{ (3 sig figs)}$

Exam tip:

You will almost always be asked to evaluate at least one source of experimental error for this practical, with a suggested improvement.

5. Bond Enthalpies & Reactivity★★★★☆⏱ 5 min

📘 Definition

Mean Bond Enthalpy

Average energy required to break 1 mole of a given covalent bond in gaseous molecules, across a range of compounds. Bond breaking is endothermic (positive value), bond making is exothermic (negative value).

Example:

The mean C-Cl bond enthalpy is +346 kJ mol⁻¹, so breaking 1 mole of C-Cl bonds in gaseous molecules absorbs 346 kJ of energy.

Reaction enthalpy from bond enthalpies is calculated as: $Δ_{r} H = \sum$ (bond enthalpies of bonds broken) - $\sum$ (bond enthalpies of bonds formed). Limitations: values are averages, so only approximate for specific molecules, and only apply to gaseous species. Lower bond enthalpy means weaker bonds that break first, determining reaction reactivity.

📐 Worked Example

Calculate $Δ_{r} H$ for the reaction $C H_{4} (g) + B r_{2} (g) \to C H_{3} B r (g) + H B r (g)$ using bond enthalpies: E(C-H)=413, E(Br-Br)=193, E(C-Br)=276, E(H-Br)=366 kJ mol⁻¹.

1
1. Bonds broken: 1 C-H, 1 Br-Br: total = 413 + 193 = 606 kJ mol⁻¹.
2
1. Bonds formed: 1 C-Br, 1 H-Br: total = 276 + 366 = 642 kJ mol⁻¹.
3
$\Delta_r H = 606 - 642 = -36 \text{ kJ mol}^{-1}$

6. Common Pitfalls

Wrong move:

Forgetting to convert $q$ from J to kJ when calculating $Δ H$ in kJ mol⁻¹.

Why:

$q$ is measured in J from the calorimetry formula, but $Δ H$ units require kJ, so conversion is mandatory.

Correct move:

Divide $q$ by 1000 before dividing by the moles of limiting reactant.

Wrong move:

Using the formation enthalpy formula for combustion data (sum products minus sum reactants).

Why:

Combustion cycles have arrows pointing from reactants/products to combustion products, so the sign convention is reversed.

Correct move:

For combustion data, use $Δ_{r} H = \sum Δ_{c} H (r e a c t an t s) - \sum Δ_{c} H (p r o d u c t s)$ .

Wrong move:

Omitting the negative sign for exothermic enthalpy changes.

Why:

The sign is a required part of the enthalpy value, indicating direction of heat flow.

Correct move:

Always include the sign for $Δ H$ , even if the question asks for the 'enthalpy change' without explicitly mentioning sign.

Wrong move:

Using bond enthalpy calculations for reactions involving liquid or solid species without adjusting for phase changes.

Why:

Mean bond enthalpies are only defined for gaseous molecules, so phase changes add extra enthalpy changes not accounted for.

Correct move:

Convert all species to gaseous form first by adding enthalpy of vaporisation/fusion values if required.

Wrong move:

Using total solution volume instead of moles of limiting reactant to calculate molar enthalpy change.

Why:

$Δ H$ is reported per mole of reaction, not per volume of solution.

Correct move:

Identify the limiting reactant first, calculate its moles, then divide total enthalpy change by this value.

7. Quick Reference Cheatsheet

Concept	Formula/Rule	Key Exam Note
Enthalpy sign convention	Exothermic = -ve, Endothermic = +ve	Heat released = negative $Δ H$ , always include the sign
Calorimetry	$q = m c Δ T$ , $Δ H = - q / (1000 * n)$	$c = 4.18$ J g⁻¹ K⁻¹ for water, m = volume (cm³) for aqueous solutions
Hess's Law ( $Δ_{f} H$ )	$Δ_{r} H = \sum Δ_{f} H (p r o d u c t s) - \sum Δ_{f} H (r e a c t an t s)$	$Δ_{f} H$ of elements in standard state = 0
Hess's Law ( $Δ_{c} H$ )	$Δ_{r} H = \sum Δ_{c} H (r e a c t an t s) - \sum Δ_{c} H (p r o d u c t s)$	Combustion products are CO₂(g) and H₂O(l)
Bond enthalpy	$Δ_{r} H = \sum$ (bonds broken) - $\sum$ (bonds formed)	Only valid for gaseous species, values are averages

8. Frequently Asked

What are standard conditions for enthalpy measurements?

Standard conditions are defined as 100 kPa pressure, 298 K (25°C) temperature, 1 mol dm⁻³ concentration for aqueous solutions, and all elements present in their most stable standard state.

Why is the standard enthalpy of formation of elements zero?

By definition, there is no enthalpy change when an element is formed from itself in its standard state, so $Δ_{f} H^{⊖}$ (element, standard state) = 0 kJ mol⁻¹.

Why are enthalpy values calculated from bond enthalpies only approximate?

Mean bond enthalpies are average values across a range of different gaseous compounds, so they do not exactly match the bond energy in a specific molecule, and only apply to gaseous species.

Going deeper

official_specEdexcel IAL Chemistry Unit 2 SpecificationRefer to section 6.1-6.11 for full syllabus details

What's Next

Now that you have mastered Unit 2 energetics, move on to Group Chemistry, which uses enthalpy concepts to explain reactivity trends for Group 1 and Group 7 elements. Practice past paper questions on Hess's Law cycles and calorimetry calculations, as these are high-frequency questions worth 4-6 marks each. Make sure you can evaluate practical errors and apply cooling curve corrections for Core Practical 2, as these are common extended response questions. This content also forms the foundation for thermodynamics in Unit 4, so solidifying your understanding now will save you time when you cover more advanced enthalpy topics later.