Enthalpy of Formation
Introduced| English |
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| enthalpy of formation/enˈθælpi ɒv fɔːˈmeɪʃn/ |
Building compounds from scratch
- Imagine making a compound straight from its raw elements.
- The heat of that assembly is tabulated for thousands of substances.
- With those values, you can find any reaction's heat.
- No lab needed -- just a table and some subtraction.
The heat of formation
- The enthalpy of formation 生成焓 is the heat to make 1 mol from its elements.
- It is measured with those elements in their standard states.
- A pure element in its standard state has a formation enthalpy of zero.
The enthalpy of formation is for making exactly 1 mole of a substance from its elements.
It is defined per mole of the compound formed.
Products minus reactants
- $\Delta H_{rxn} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$.
- Add up the products' formation enthalpies, then subtract the reactants'.
- Multiply each one by its coefficient first.
The reaction enthalpy from formation data is...
$\Delta H_{rxn} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$.
You must multiply each formation enthalpy by its coefficient before summing.
Coefficients scale the amount of each substance.
Why elements count as zero
- An element in its standard form is the starting point, so no heat is needed to make it.
- $\text{O}_2$, $\text{N}_2$, and solid carbon all have $\Delta H_f = 0$.
- This simplifies the arithmetic.
Enthalpy of formation
ΔHf is the energy change when one mole of a compound forms from its elements.
What is the standard enthalpy of formation of $\text{O}_2$ gas?
An element in its standard state has $\Delta H_f = 0$.
An element in its standard state has a formation enthalpy of ____.
No formation step is needed, so it is zero.
Products' total $\Delta H_f = -400$; reactants' total $= -250$.
- $\Delta H_{rxn} = -400 - (-250) = -150\ \text{kJ}$.
- The reaction is exothermic.
Products' $\sum \Delta H_f = -300$; reactants' $= -100$. Find $\Delta H_{rxn}$ (in kJ).
$\Delta H_{rxn} = -300 - (-100) = -200\ \text{kJ}$.
The formula is products minus reactants -- watch the double negatives when $\Delta H_f$ values are negative. Elements in their standard state contribute zero, so do not look them up. And multiply each formation enthalpy by the equation's coefficient before summing.
The enthalpy of formation is the heat to make 1 mol of a substance from its elements, and it is zero for an element in its standard state. Any reaction's heat is $\Delta H_{rxn} = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$, each scaled by its coefficient.