Rates of Chemical Reactions

What we are learning: (component knowledge)

Ways of measuring the rate of a reaction:
To measure how fast a reaction is happening, you either need to be able to measure how much is being used up per unit of time or how much of a product is being made per unit of time. The best involves the production of a gas as this can be collected in a gas syringe and the volume recorded every 5s or 10s then plotted.
Another method will give the time for a reaction to run to completion, this can't be plotted during the course of a reaction, but it can be used to compare several where we have changed one variable. The best example of this reaction is the "disappearing cross" reaction with sodium thiosulfate. These are both required practicals.

Factors affecting Rate & Collision Theory:
Collision theory is all about how often particles (of reactants) collide and whether or not they have sufficient energy (activation energy)to make the collision a successful one. There are 4 ways of speeding up a chemical reaction.
The first way to increase the rate of a reaction is to increase the temperature. When the temperature increases, the particles move faster and so collide more often. As these particles have more energy, they are more likely to have the activation energy which will result in more successful collisions per second
The second way to increase the rate is to increase the concentration of a solution, by doing this, there will be more particles in the same unit of volume, this means that there will be more successful collisions per second. This is the same as increasing the pressure in a gas, the particles are pushed closer together which is the same as increasing the concentration.
The third way is to increase the surface area of the solid reactants, by doing this, there will be more reactants in contact with other reactants, this will result in more successful collisions per second
Finally, you can add a catalyst which will lower the activation energy. This will mean that more of the particles will have activation energy so without increasing the temperature, there will be more successful collisions per second

Practical - measuring how concentration or temperature affects rate:
Here, you need to complete a required practical. For this one, you will either add different concentrations of hydrochloric acid to a sodium thiosulfate solution in a conical flask that is sitting on a card that has a cross drawn on it or you may use thiosulfate solution of differing temperatures. In this method, you will start your timer when you add the HCl and stop it when you can no longer see the cross through the top of the conical flask. You will then be able to plot time taken for the cross to disappear (dependent variable on the Y-axis) against either the temperature or concentration (independent variable on the X-axis).

Practical - measuring how surface area affects rate:
For this second one, you need to complete a different required practical. Here you will add a specific mass of limestone chips (small surface area) to a conical flask containing hydrochloric acid that has a delivery arm attached by a pipe to a gas syringe. Once you add the limestone, add the bung to the top and start the timer. Record the volume of CO2 released every 5 seconds. You repeat this for smaller pieces of limestone (larger surface area) and finally for limestone powder (largest surface area). You can then plot time vs volume and you will see the rate as the steepness of the three graphs. Steeper = faster. You can draw tangents and calculate the rate in cm3/s.

Catalysis and energy profiles:
A catalyst will speed up a reaction but the catalyst never gets used up in the process. A catalyst is specific to a reaction so it will only speed up the specific reaction and will have no effect at all in others. Enzymes are biological catalysts, without them, reactions in nature would not be able to happen in their natural conditions.
In the presence of a catalyst, the reaction is provided with an alternative pathway, this new pathway will still produce the safe final product, however, it will not need as much energy to get it started (activation energy). In energy profiles that we saw last module, we have seen the "hump" that shows the activation energy. When you add a catalyst to the reaction, the "hump" reduces in height.

Reversible reactions and Le Chatelier's principle:
In a reversible reaction, reactants can become products and products can become reactants. When the reaction is a equilibrium, it has not stopped. We call this dynamic equilibrium and the rate of the forward reaction is equal to the rate of the reverse reaction. Because energy cannot be created or destroyed, one direction will be endothermic and the other will be exothermic and their values will be equal and opposite.
In a closed system (nothing enters or leaves), the equilibrium will shift in order to try and compensate for any external changes. This is called Le Chatelier's principle. These are the ways in which it will move to try and compensate. If you heat the mixture, the equilibrium will move in the endothermic direction (to absorb some of the heat). If you remove a product, it will move to the right to replace some of the product that you took. In a gas, if you increase the pressure, the equilibrium will move to the side with the fewest number of moles of gas.

The Haber Process:
Fritz Haber, along with engineer Carl Bosch, came up with the Haber-Bosch process (commonly known and the Haber process). This process uses the reversible reaction:
N2 + 3H2 ⇋ 2NH3
This reaction uses both a reversible reaction and a catalyst (iron). This is a fascinating reaction and it is the best way of making synthetic fertilisers. Without this reaction, there is no way that we could grow enough food to feed billions of the people alive on our planet.

Key words/terms for this topic

Activation Energy      Catalyst      Closed system      Collision      Collision Theory      Collisions per Second      Concentration      Conditions      Dynamic equilibrium      Frequency of collisions      Gas Syringe      Gradient      Heat Energy      Le Chatelier's principle      Precipitation      Pressure      Proportional      Rate of reaction      Reversible reaction      Surface Area      Tangent     

Curriculum Health Check:

Q: A rate will be increased by cramming more particles into the same unit volume. This could also be described as...

A: Increasing the temperature

B: Increasing the surface area

C: Increasing the concentration

D: Increasing the volume

What you need 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 ⇋ 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 Tier 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.

This page was updated on: 2nd March 2024