Enthalpy change and reaction pathways
Introduced| English | Meaning |
|---|---|
| enthalpy change/enˈθælpi tʃeɪndʒ/ | The heat transferred in a reaction or process at constant pressure. |
| exothermic/eɡzəˈðɜːmɪk/ | Describing a process that transfers heat to its surroundings. |
| endothermic/ˌendəʊˈθɜːmɪk/ | Describing a process that takes in heat from its surroundings. |
| activation energy/ˌæktɪˈveɪʃn ˈenədʒi/ | The minimum energy that reacting particles need for a successful reaction. |
| standard conditions/ˈstændəd kənˈdɪʃnz/ | 298 K and 100 kPa (and 1 mol dm^-3 for solutions), used for standard enthalpy values. |
| combustion/kəmˈbʌstʃn/ | An exothermic reaction of a fuel with an oxidising agent, usually oxygen. |
| neutralisation/ˌnjuːtrəlaɪˈzeɪʃn/ | A reaction between an acid and a base that forms a salt, usually with water. |
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 ______.
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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:
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Exothermic reactions release heat; products sit below reactants, so ΔH is negative.
The activation energy is:
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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:
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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:
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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.
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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 ($\Delta H_{\text{neut}}$)