Thermochemistry
Explore the energy changes in chemical reactions -- how to measure, calculate, and predict enthalpy changes using Hess's law, bond energies, and calorimetry.
Enthalpy Changes (ΔH)
Enthalpy (H) is the total heat content of a system at constant pressure. The enthalpy change (ΔH) is the heat energy absorbed or released during a chemical reaction. Reactions can be classified as exothermic (ΔH < 0) or endothermic (ΔH > 0).
Exothermic (ΔH < 0)
- • Heat is released to surroundings
- • Temperature of surroundings increases
- • Examples: combustion, neutralisation
Endothermic (ΔH > 0)
- • Heat is absorbed from surroundings
- • Temperature of surroundings decreases
- • Examples: photosynthesis, thermal decomposition
Standard enthalpy notation: ΔH° is measured under standard conditions (25°C, 100 kPa, 1 mol/L). Common types include ΔH°c (combustion), ΔH°f (formation), and ΔH°neut (neutralisation).
Hess's Law
Hess's law states that the total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same. This allows us to calculate enthalpy changes that are difficult to measure directly.
Hess's Law Energy Cycle
Reactants
ΔH (direct route)
→
Products
Intermediates
(e.g. elements)
ΔH = ΔH1 + ΔH2
Using formation enthalpies: ΔH°rxn = ΣΔH°f(products) - ΣΔH°f(reactants). The standard enthalpy of formation of elements in their standard states is zero.
Using combustion enthalpies: ΔH°rxn = ΣΔH°c(reactants) - ΣΔH°c(products). Note the subtraction is reversed compared to formation enthalpies.
Bond Energies and Calorimetry
Bond energy is the energy required to break one mole of a particular bond in the gaseous state. We can estimate ΔH using bond energies, and calorimetry allows us to measure enthalpy changes experimentally.
Key Equations
ΔH = Σ(bonds broken) - Σ(bonds formed)
Bond energy method
q = mcΔT
Calorimetry equation
q
Heat energy (J)
m
Mass of solution (g)
c
Specific heat capacity (4.18 J/g/°C for water)
Important: Breaking bonds is endothermic (requires energy), while forming bonds is exothermic (releases energy). If more energy is released forming bonds than is consumed breaking them, the reaction is exothermic overall.
Key Vocabulary
Enthalpy (H)
The total heat content of a chemical system at constant pressure. We measure changes in enthalpy (ΔH), not absolute values.
Calorimetry
The experimental technique of measuring heat changes in chemical reactions using an insulated container (calorimeter) and a thermometer.
Bond Energy
The energy required to break one mole of a particular covalent bond in the gaseous state, measured in kJ/mol. These are average values.
Activation Energy (Ea)
The minimum energy reactants must have to form products. Catalysts lower Ea but do not change ΔH.
Worked Examples
50 mL of 1.0 mol/L HCl is mixed with 50 mL of 1.0 mol/L NaOH. The temperature rises by 6.8°C. Calculate the enthalpy of neutralisation.
Step 1: Total volume = 100 mL, so mass = 100 g (assuming density of water). c = 4.18 J/g/°C.
Step 2: q = mcΔT = 100 × 4.18 × 6.8 = 2842 J = 2.842 kJ.
Step 3: Moles of HCl = 0.050 × 1.0 = 0.050 mol. ΔH = -q/n = -2.842/0.050 = -56.8 kJ/mol.
Answer: ΔHneut = -56.8 kJ/mol (negative because the reaction is exothermic).
Use Hess's law to find ΔH for: C(s) + ½O2(g) → CO(g), given: C(s) + O2(g) → CO2(g), ΔH = -393 kJ/mol; CO(g) + ½O2(g) → CO2(g), ΔH = -283 kJ/mol.
Step 1: Label: Equation 1: C + O2 → CO2, ΔH1 = -393 kJ/mol.
Step 2: Reverse Equation 2: CO2 → CO + ½O2, ΔH2 = +283 kJ/mol.
Step 3: Add: C + O2 + CO2 → CO2 + CO + ½O2. Cancel CO2 and simplify: C + ½O2 → CO.
Answer: ΔH = -393 + 283 = -110 kJ/mol.
Estimate ΔH for H2 + Cl2 → 2HCl using bond energies: H-H = 436 kJ/mol, Cl-Cl = 242 kJ/mol, H-Cl = 431 kJ/mol.
Step 1: Bonds broken: 1 × H-H + 1 × Cl-Cl = 436 + 242 = 678 kJ.
Step 2: Bonds formed: 2 × H-Cl = 2 × 431 = 862 kJ.
Step 3: ΔH = bonds broken - bonds formed = 678 - 862 = -184 kJ/mol.
Answer: ΔH ≈ -184 kJ/mol. The reaction is exothermic (more energy released forming bonds than absorbed breaking them).
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
An exothermic reaction has:
Question 2
According to Hess's law, the enthalpy change of a reaction:
Question 3
When 200 g of water is heated by 5°C in a calorimeter, the heat energy absorbed is (c = 4.18 J/g/°C):
Question 4
Breaking chemical bonds is:
Question 5
A catalyst affects a reaction by:
Key Concepts Summary
- ●Exothermic reactions release heat (ΔH < 0); endothermic reactions absorb heat (ΔH > 0).
- ●Hess's law: ΔH is independent of the reaction pathway, enabling indirect calculation of enthalpy changes.
- ●Bond energies: ΔH = energy to break bonds - energy released forming bonds.
- ●Calorimetry: q = mcΔT measures heat changes experimentally.
- ●Catalysts lower activation energy but do not change the overall ΔH of a reaction.