STUDENT SPACE · TOPIC 1

Energy transfer and reaction pathways

Interpret and label energy changes and activation energy.

The essentials

An exothermic reaction transfers thermal energy to its surroundings. The surroundings become warmer.
An endothermic reaction takes thermal energy from the surroundings. The surroundings become cooler.
Eₐ is the minimum energy that colliding particles must have to react.

Try each question before opening its hint or answer. Use paper for calculations and diagrams.

An exothermic reaction has no activation energy. Explain why this is wrong.

Need a hint for Question 1?

Mark reactants, peak and products before subtracting energy levels.

Check Question 1

It may still require an initial energy input; negative ΔH does not mean Eₐ is zero.

A mixture cools its surroundings. State the reaction type and the sign of ΔH.

Need a hint for Question 2?

If surroundings cool, where has their energy gone?

Check Question 2

Endothermic; ΔH is positive.

Reactants: 20 kJ; peak: 90 kJ; products: 55 kJ. Draw and label the pathway, ΔH and Eₐ. Calculate both changes.

Need a hint for Question 3?

Subtract reactant energy from product energy and from peak energy separately.

Check Question 3

ΔH = +35 kJ; Eₐ = 70 kJ. Products are above reactants; Eₐ runs from reactants to the peak. Label energy and reaction progress.

Use Lesson 1 in your worksheet for written practice. Your teacher will discuss those answers in class.