Reading an Energy Profile
Introduced| English |
|---|
| energy profile/ˈenədʒi ˈprəʊfaɪl/ |
| activation energy/ˌæktɪˈveɪʃn ˈenədʒi/ |
| transition state/trænˈsɪʃn steɪt/ |
A hill the reaction must climb
- Every reaction has a hill to get over before it can finish.
- A graph draws that hill as the atoms rearrange.
- Its height tells you how hard the reaction is to start.
- Its two ends tell you whether heat is released or absorbed.
The reaction-coordinate diagram
- An energy profile 能量图 plots energy along the reaction's progress.
- Reactants sit on the left and products on the right.
- A hill rises between them.
The peak is the barrier
- The peak height above the reactants is the activation energy 活化能.
- At the very top sits the transition state 过渡态.
- A higher peak means a slower reaction.
The transition state is located at the...
The transition state sits at the very top of the barrier.
A higher activation energy makes a reaction slower.
Fewer collisions can clear a taller barrier.
Uphill or downhill overall
- If products sit lower than reactants, energy is released (exothermic).
- If products sit higher, energy is absorbed (endothermic).
- The difference between the two ends is $\Delta H$.
Read the energy profile
Find the activation energy, the enthalpy change, and how the reactants climb to the transition state.
Reactants at $40\ \text{kJ}$, products at $25\ \text{kJ}$. The value of $\Delta H$ (in kJ)?
$\Delta H = 25 - 40 = -15\ \text{kJ}$ (products minus reactants).
If the products are lower in energy than the reactants, the reaction is...
Lower products mean net energy is released -- exothermic.
Reactants at $50\ \text{kJ}$, peak at $120\ \text{kJ}$, products at $30\ \text{kJ}$.
- Activation energy $= 120 - 50 = 70\ \text{kJ}$.
- $\Delta H = 30 - 50 = -20\ \text{kJ}$, so it is exothermic.
Reactants at $40\ \text{kJ}$, peak at $100\ \text{kJ}$. The activation energy (in kJ)?
$E_a = 100 - 40 = 60\ \text{kJ}$ (peak minus reactants).
Activation energy is measured from the reactants up to the ____.
It is the height of the peak above the reactants.
Activation energy is measured from the reactants up to the peak, not from zero. $\Delta H$ is the difference between products and reactants (the two ends), independent of the peak height. And a large activation energy makes a reaction slow even if it is strongly exothermic.
An energy profile plots energy along a reaction. The peak above the reactants is the activation energy, with the transition state at the top. Whether products end lower (exothermic) or higher (endothermic) than reactants gives $\Delta H$. Barrier height sets speed; the end difference sets $\Delta H$.