#### Key Terms for this topic (Tier 3 vocabulary)

Collision theory – activation energy – catalyst – surface area – concentration – pressure – rate – precipitation – gas syringe – tangent – proportional – reversible – equilibrium.

Rates of Reactions

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#### What everyone needs to know:

You can measure the rate of a chemical reaction at any point using either of these formulae:

Amount of reactant used/time taken or

Amount of product formed/time taken.

The units of rate can be in g/s or cm3/s, it depends on what you are measuring above. You need to be able to calculate these rates and interpret them from a graph. The steeper the curve, the faster the reaction.

Rate can be changed by changing: Temperature, Concentration, Surface area of solids, Pressure of gases and by adding a catalyst.

Required practical 11 – Investigate how changes in concentration affect rate. Must be measured by collecting a gas and by seeing a colour change or turbidity.

Rates can be explained by collision theory. For particles to react, they must collide with enough energy for them to react. Increasing the temperature means that they have more energy and collide more often. By increasing the concentration or pressure, particles collide more often because there are more particles per unit volume. Increasing the surface area allows more reactants to be in contact with each other and adding a catalyst lowers the activation energy which means that the reactions can happen at a lower temperature. You need to link this last one to the energy profiles seen in the last topic.

Some reactions are reversible and have this double headed arrow symbol symbol ⇌ in their equations. This simply means that the reaction can go either way depending on certain conditions. One direction is exothermic so the other way is endothermic.

Equilibrium is a balance point. It does not mean that the reactions have stopped, it means that the rate of the forward reaction is the same as the rate of the reverse reaction, this is why it is called a “dynamic equilibrium”.

#### Extra topics needed for the Higher papers:

When calculating rates, you need to be able to measure rate involving moles. e.g. mol/s

As well as looking at graphs and stating that the steeper the curve, the faster the reaction, you need to be able to draw a tangent and calculate the gradient to give the rate at that exact time.

Dynamic equilibrium is affected by certain conditions as described by Le Chatelier’s Principle.

• Heating will cause the reaction to move in the endothermic direction.
• Increasing the concentration of reactants will cause more products to be made and vice versa.
• Increasing the pressure of a gas will cause equilibrium to move to the side with the fewest number of moles.
• The catalyst has no impact on the point of equilibrium as it will in crease the rate of both the forward and the backward reaction equally. It simply allows it to reach equilibrium faster.