Energy Diagrams
Introduced| English | Meaning |
|---|---|
| enthalpy/enˈθælpi/ | A state function equal to internal energy plus the product of pressure and volume. |
Drawing heat as height
- Picture reactants and products as blocks at different heights.
- The higher one holds more stored chemical energy.
- Drop from high to low and heat spills out.
- Climb from low to high and heat must be poured in.
Reactants and products as levels
- An energy diagram draws reactants and products as horizontal levels.
- Their heights show how much enthalpy 焓 (stored energy) each holds.
- The gap between them is $\Delta H$.
Energy diagrams: exothermic products sit below the reactants, endothermic above
The vertical gap between the reactant and product levels is $\Delta H$.
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That height difference is the enthalpy change.
The exothermic shape
- Exothermic: the products sit lower than the reactants.
- Energy is released as the reaction "falls."
- So $\Delta H$ is negative.
On an energy diagram, products below reactants means the reaction is...
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Lower products mean energy was released -- exothermic.
An exothermic reaction has a positive $\Delta H$.
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False. Products lower in energy than reactants give a negative $\Delta H$, which is exothermic. A positive $\Delta H$ is endothermic.
The endothermic shape
- Endothermic: the products sit higher than the reactants.
- Energy is absorbed as the reaction "climbs."
- So $\Delta H$ is positive.
An endothermic energy diagram
When a reaction absorbs energy, the products sit higher than the reactants.
An endothermic reaction has products that are...
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Absorbing energy raises the products above the reactants.
Products at $30\ \text{kJ}$, reactants at $80\ \text{kJ}$. Exothermic or endothermic?
- The products are lower, so energy was released.
- $\Delta H = 30 - 80 = -50\ \text{kJ}$, which is exothermic.
Reactants at $60\ \text{kJ}$, products at $90\ \text{kJ}$. The value of $\Delta H$ (in kJ)?
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$\Delta H = 90 - 60 = +30\ \text{kJ}$ (endothermic).
$\Delta H$ equals products minus ____.
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$\Delta H$ = final (products) minus initial (reactants).
Read $\Delta H$ as products minus reactants (final minus initial). Products lower means exothermic and a negative $\Delta H$; products higher means endothermic and positive. Do not confuse this enthalpy diagram (heights are stored energy) with a kinetics profile's activation-energy hump.
An energy diagram draws reactants and products as levels whose heights are their enthalpy. Products lower than reactants means exothermic (negative $\Delta H$); products higher means endothermic (positive $\Delta H$). The vertical gap, products minus reactants, is $\Delta H$.