Define activation enthalpy.

Activation enthalpy is the minimum energy that a colliding pair of molecules must possess between them for a reaction to take place.

Give the symbol and the usual units for activation enthalpy.

The symbol is Eₐ and the usual units are kJ mol⁻¹.

What is plotted on the two axes of an enthalpy profile?

Enthalpy is plotted on the vertical axis against the progress of the reaction on the horizontal axis.

What does the peak of an enthalpy profile represent?

It represents the transition state, the highest-enthalpy arrangement on the way from reactants to products, in which bonds are partly broken and partly formed.

How is the activation enthalpy read off an enthalpy profile?

It is the vertical rise from the reactant level up to the top of the peak.

OCR A-Level Chemistry B H433

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Activation enthalpy and enthalpy profiles

3 things to know
Components 01, 02 and 03

Overview

Recall questions on the definition, symbol and units of activation enthalpy, why most collisions fail, and reading a profile.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

What is activation enthalpy, what are its symbol and units, and why do most collisions fail?

Numbers

What is plotted on an enthalpy profile, what are Eₐ and ΔH when reactants lie at 0, the peak at +150 and products at −80 kJ mol⁻¹, and what are Eₐ forward, ΔH and Eₐ reverse when the peak is 60 and the products 25 kJ mol⁻¹ above the reactants?

Key term

Why do most collisions fail to produce a reaction?

Questions from this subtopic

Each one with the model answer underneath.

1
Define activation enthalpy.
Model answer
Activation enthalpy is the minimum energy that a colliding pair of molecules must possess between them for a reaction to take place.
2
Why do most collisions fail to produce a reaction?
Model answer
Most colliding pairs have less energy than the activation enthalpy, so bonds cannot begin to break and the molecules simply bounce apart unchanged.
3
Give the symbol and the usual units for activation enthalpy.
Model answer
The symbol is Eₐ and the usual units are kJ mol⁻¹.
4
What is plotted on the two axes of an enthalpy profile?
Model answer
Enthalpy is plotted on the vertical axis against the progress of the reaction on the horizontal axis.
5
What does the peak of an enthalpy profile represent?
Model answer
It represents the transition state, the highest-enthalpy arrangement on the way from reactants to products, in which bonds are partly broken and partly formed.
6
How is the activation enthalpy read off an enthalpy profile?
Model answer
It is the vertical rise from the reactant level up to the top of the peak.

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