Enthalpy change and reaction pathways
| English | Chinese | Pinyin |
|---|---|---|
| enthalpy change | 焓变 | hán biàn |
| exothermic | 放热 | fàng rè |
| endothermic | 吸热 | xī rè |
| activation energy | 活化能 | huó huà néng |
| standard conditions | 标准条件 | biāo zhǔn tiáo jiàn |
| combustion | 燃烧 | rán shāo |
| neutralisation | 中和 | zhōng hé |
Enthalpy change 焓变, ΔH
- Every reaction takes in or gives out energy.
- That energy change (at constant pressure) is the enthalpy change, $\Delta H$.
- Its sign tells you the direction of energy flow.
A reaction that releases heat to the surroundings has a negative value of ______.
Exothermic reactions have ΔH < 0.
Exothermic 放热 vs endothermic 吸热
- Exothermic: gives out heat → products lower than reactants → $\Delta H < 0$.
- Endothermic: takes in heat → products higher → $\Delta H > 0$.
- The "hill" between them is the activation energy 活化能 — the least energy needed to react.
Reaction energy profiles
Ea = barrier · ΔH = products − reactants
The hump is the activation energy; whether products sit below or above reactants sets ΔH.
In an exothermic reaction:
Exothermic reactions release heat; products sit below reactants, so ΔH is negative.
The activation energy is:
Activation energy is the height of the "hill" — the minimum energy needed to start the reaction.
Standard conditions 标准条件 and types
- Values are compared under standard conditions ($298\ \text{K}$, $101\ \text{kPa}$), shown by $^{\ominus}$.
| Symbol | Enthalpy change of |
|---|---|
| $\Delta H_f^{\ominus}$ | formation — 1 mole of compound from its elements |
| $\Delta H_c^{\ominus}$ | combustion 燃烧 — 1 mole burns completely in oxygen |
| $\Delta H_{\text{neut}}^{\ominus}$ | neutralisation 中和 — 1 mole of water from acid + alkali |
An instant cold pack uses an endothermic reaction that takes in heat
The standard enthalpy change of combustion is for:
ΔHc is per mole of substance burned completely; ΔHf is formation from elements; ΔH_neut makes one mole of water.
Standard conditions for enthalpy values are:
Standard conditions are 298 K and 101 kPa, with each substance in its normal state (symbol ⊖).
Mind the sign of ΔH
- Exothermic reactions release heat and have a negative ΔH; endothermic are positive.
- ΔH is measured per mole and quoted under standard conditions.
Exothermic reactions release energy; endothermic reactions take it in
Match each enthalpy term to its meaning.
Each item links the term to its correct meaning.
You've got it
- exothermic $\Delta H < 0$ (products lower); endothermic $\Delta H > 0$ (products higher)
- activation energy = the energy hill, the least needed to react
- standard conditions: 298 K, 101 kPa ($^{\ominus}$)
- types: formation ($\Delta H_f$), combustion ($\Delta H_c$), neutralisation ($\Delta H_{\text{neut}}$)