Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- The enthalpy change of reaction, ΔH, is the heat transferred by a reaction at constant pressure. Negative: released (exothermic). Positive: absorbed (endothermic).
- ΔH is quoted per mole of reaction: for the amounts in the balanced equation, with the state symbols shown.
- Particle view: breaking and forming bonds changes the chemical potential energy. In an exothermic reaction the products have less potential energy, so their particles move faster and are briefly hotter than the surroundings. Energy then flows to the surroundings until thermal equilibrium. An endothermic reaction leaves the products colder, and energy flows in from the surroundings.
- In a calorimeter, q_reaction = −q_surroundings.
Key relationships
| Relationship | Symbols and units | Use it to |
|---|---|---|
| moles of reaction = n ÷ coefficient | n in mol (limiting reactant, or any product formed) | turn an amount of one substance into moles of reaction |
| q = moles of reaction × ΔH | q in kJ, ΔH in kJ mol⁻¹ | find the heat released or absorbed |
| equation × k → ΔH × k; reverse → −ΔH | adjust ΔH to match the equation you need | |
| q_soln = mcΔT = −q_rxn | m in g, c in J g⁻¹ °C⁻¹ | link the reaction to a temperature change |
Assumptions behind the numbers
- The reaction happens at constant pressure, so ΔH equals the heat of reaction.
- The reaction goes to completion with the limiting reactant used up.
- Products are in the states shown in the equation.
- In calorimetry, no energy is lost to the cup or the air, and the solution behaves like water.
Mistakes to avoid
- Forgetting the coefficient. For 2NH₃ in the equation, divide moles of NH₃ by 2.
- Using the excess reactant to count the reaction. Find the limiting reactant first.
- Wrong sign. Released energy means q is negative for the system.
- Reversing without changing the sign of ΔH.
- Multiplying grams by ΔH. Convert to moles first.
- Saying bond breaking releases energy. It always needs energy.
Quick self-check
- CH₄ + 2O₂ → CO₂ + 2H₂O(l), ΔH = −890 kJ mol⁻¹. q for 2.00 mol CH₄? (−1780 kJ)
- A reaction has ΔH = +50 kJ mol⁻¹. q for 0.25 mol of reaction? (+12.5 kJ, absorbed)
- N₂ + 3H₂ → 2NH₃, ΔH = −92 kJ mol⁻¹. ΔH for 6NH₃ → 3N₂ + 9H₂? (+276 kJ mol⁻¹)
- A reaction in water makes the temperature fall. Is ΔH positive or negative? (Positive: the system absorbed energy from the water.)
Next: practice questions.