Introduction to Enthalpy of Reaction
Introduced| English |
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| enthalpy of reaction/enˈθælpi ɒv rɪˈækʃn/ |
Putting a number on the heat
- Every reaction gives off or takes in a definite amount of heat.
- We can label it with an exact value, in kilojoules.
- The sign tells you the direction; the size tells you how much.
- That single number is one of chemistry's most useful.
The enthalpy of reaction
- The enthalpy of reaction 反应焓 ($\Delta H_{rxn}$) is the heat at constant pressure.
- Negative means exothermic; positive means endothermic.
- It is usually quoted per mole of reaction.
A reaction with $\Delta H = -200\ \text{kJ}$ is...
A negative $\Delta H$ releases heat -- exothermic.
The enthalpy of reaction is the heat measured at constant ____.
$\Delta H$ is the heat at constant pressure.
A positive enthalpy of reaction means the reaction is endothermic.
Positive $\Delta H$ absorbs heat -- endothermic.
Thermochemical equations
- A thermochemical equation writes $\Delta H$ next to the reaction.
- Double the amounts and $\Delta H$ doubles too.
- Reverse the reaction and $\Delta H$ flips its sign.
Reversing a reaction changes its $\Delta H$ by...
The reverse reaction has the opposite $\Delta H$.
Scaling with amount
- $\Delta H$ scales with how much reacts.
- Half the moles gives half the heat.
- Always match the $\Delta H$ to the equation's coefficients.
Enthalpy scales with amount
Enthalpy of reaction is energy per mole, so the total heat is proportional to how much reacts.
If burning 1 mol releases $500\ \text{kJ}$, how much does burning 3 mol release (in kJ)?
$\Delta H$ scales with amount, so $3 \times 500 = 1500\ \text{kJ}$.
If a reaction of 2 mol releases $600\ \text{kJ}$, how much does 1 mol release (in kJ)?
Half the moles gives half the heat, so $600/2 = 300\ \text{kJ}$.
$\text{CH}_4 + 2\text{O}_2 \to \text{CO}_2 + 2\text{H}_2\text{O}$, with $\Delta H = -890\ \text{kJ}$.
- Burning 1 mol of methane releases $890\ \text{kJ}$.
- Burning 2 mol releases $2 \times 890 = 1780\ \text{kJ}$.
$\Delta H$ scales with amount -- double the moles, double the heat. Reversing a reaction flips the sign of $\Delta H$. And a negative $\Delta H$ means heat is released (exothermic); a common slip is to read the sign backwards.
The enthalpy of reaction $\Delta H_{rxn}$ is the heat at constant pressure -- negative for exothermic, positive for endothermic, quoted per mole. In a thermochemical equation it scales with the amounts and flips sign when the reaction is reversed.