Topic 7 -Rates of reaction + Energy changes

Rates practical (HCl + marble chips): what do you measure?

Measure gas produced (CO₂) over time (e.g. gas syringe).

Rates practical (HCl + sodium thiosulfate): what do you observe?

Observe time for colour change / cross to disappear (solution turns cloudy).

How do you calculate rate from time taken?

Rate ∝ 1 ÷ time (shorter time = faster rate).

Practical methods to measure rate

Measure gas volume, change in mass, colour change, concentration change, precipitate forming, or time for a reaction to finish.

How do reactions happen (collision theory)?

Particles must collide to react.

Why does increasing collision frequency increase rate?

More collisions per second → more successful reactions per second.

Why does increasing collision energy increase rate?

More particles have enough energy to react (over activation energy).

Effect of increasing temperature on rate

Rate increases because particles move faster → collisions are more frequent and more energetic.

Effect of increasing concentration on rate

Rate increases because there are more particles per volume → more frequent collisions.

Effect of increasing surface area of a solid on rate

Rate increases because more particles are exposed → collisions happen more often.

Effect of increasing pressure (gases) on rate

Rate increases because particles are closer together → more frequent collisions.

Rate on a graph (mass/volume/concentration vs time)

Rate = gradient (slope) of the graph.

How does rate change during a reaction?

Usually decreases over time because reactants get used up → fewer collisions.

What is a catalyst?

Speeds up a reaction without changing the products, and is unchanged at the end (chemically and in mass).

How does a catalyst increase rate?

Provides an alternative pathway with lower activation energy.

What is an enzyme?

A biological catalyst.

Example use of enzymes

Used in producing alcoholic drinks (fermentation).

Heat energy changes can happen in…

Salts dissolving, neutralisation, displacement, precipitation (in solution temperature changes can be measured).

Exothermic reaction definition

Gives out heat energy (temperature increases).

Endothermic reaction definition

Takes in heat energy (temperature decreases).

Breaking bonds is…

Endothermic (requires energy).

Making bonds is…

Exothermic (releases energy).

Overall energy change is exothermic when…

More energy is released making product bonds than is taken in breaking reactant bonds.

Overall energy change is endothermic when…

Less energy is released making product bonds than is taken in breaking reactant bonds.

Calculate energy change using bond energies

ΔH = (energy to break bonds) − (energy to make bonds).

Activation energy meaning

Minimum energy needed for particles to react when they collide.

Reaction profile diagram (exothermic)

Products lower energy than reactants; show activation energy as the peak from reactants.

Reaction profile diagram (endothermic)

Products higher energy than reactants; show activation energy as the peak from reactants.