Factors Affecting Reaction Rates

What is rate of reaction?

As we defined at the start, rate of reaction is a measure of how quickly either reactants are used up, or products are formed, in a chemical reaction. In other words, it is a change of concentration of reactants or products compared to time. The units of rate of reaction vary, but they are usually $\mathrm{mol}{\mathrm{dm}}^{-3}{\mathrm{s}}^{-1}$, $\mathrm{g}{\mathrm{s}}^{-1}$ or ${\mathrm{cm}}^3{\mathrm{s}}^{-1}$.

Measuring rate of reaction

There are a few different ways of measuring the rate of a reaction. They depend on the products and reactants involved in the reaction. For example:

• If your reactants are solid, liquid, or aqueous and one of your products is a gas, you can allow the gas to leave the reaction container and measure the container's change in mass.
• Alternatively, you could turn the reaction container into a closed system by attaching a gas syringe, then catching and measuring the volume of gas given off.
• If one of your products is a precipitate that forms a cloudy suspension, you could measure the light that passes through the solution.
• You could also measure a change in pH.

For each of these methods, take measurements at regular time intervals until the reaction is complete. You can then move on to graphing the rate of your reaction.

Graphing rate of reaction

Once you've made your measurements, you can draw a graph and use it to find out the rate of the reaction at any specific time period. The graph will typically take the form of a curve. Here's an example that measures the volume of gas given off in a reaction:

Fig. 1 - A graph showing how the volume of gas given off changes with time in a reaction

You'll notice:

• The curve is initially steep. This means the volume of gas given off changes rapidly. The initial rate of reaction is therefore very fast.
• The curve then levels off. This means that the rate of reaction slows down. When all of the reactants are used up, the reaction eventually stops.

If we measure the change in mass, the graph looks slightly different. The curve starts off high and then gets lower. This is because mass is decreasing as some of the reactants turn into gaseous products and leave the system.

Fig. 2 - A graph showing how mass changes with time in a reaction

To measure the overall rate of reaction, you divide the change in whatever you were measuring, be it mass or volume, by the time taken for the reaction. To find the rate of reaction at a specific point, you need to draw a tangent to the curve and calculate its gradient. You can find out more about this in Chemical Kinetics.

What causes a reaction?

If you've read Collision Theory, you'll know that in order to react, particles need to collide with the correct orientation and sufficient energy. This energy is known as the activation energy.

The reaction between two particles is like a three-step process. Firstly, do they collide? Secondly, are they orientated in the correct way? Thirdly, do they have enough energy? If the answer is "no" at any stage of the process, then a reaction won't happen - it is as simple as that.

What factors affect the rate of reaction?

So, in order to react, particles need to collide with the correct orientation and sufficient energy. Because the particles are constantly moving, we can't really control their orientation, but we can influence two other things: the rate of collisions and the energy of the particles. Any factors that affect these two variables will affect the rate of reaction. These include:

• Temperature
• Concentration
• Pressure
• Surface area
• The presence of a catalyst

Temperature

When we heat a system, we supply it with energy. This energy is transferred to the particles inside of the system. Some of it is transferred as kinetic energy. This means that the particles move faster. Because they are moving faster, they collide more frequently, and so the rate of collisions increases. This increases the rate of reaction.

However, heating a mixture has another effect that is even more significant than increasing the rate of collisions. Because the particles have more energy, on average, more of them meet or exceed the activation energy needed for a particular reaction. This means that there is an increased chance of the particles reacting when they collide - the chance of a successful collision increases.

Maxwell-Boltzmann distribution

Here's a Maxwell-Boltzmann distribution graph. It shows the energy distribution for particles at two temperatures. So, it is a useful way of showing the effect caused by heating a reaction. Here, temperature Y is higher than temperature X.

Fig. 3 - A Maxwell-Boltzmann distribution for the same reaction at two different temperatures

The area under the graph to the right of the activation energy line tells us the number of particles that meet or exceed the activation energy. You can clearly see that the area under Y is greater than the area under X. This means that a greater number of particles meet or exceed the activation energy, and thus there is a greater chance of a successful reaction when they collide.

In summary, increasing the temperature of a system not only increases the number of collisions per second but increases the proportion of successful collisions.

Concentration

When looking at the effect of concentration on the rate of reaction, it can help to define what concentration actually is.

If we increase the concentration of a solution, we increase the number of solute particles in a particular volume. This means that there is an increased chance of collision between a solute molecule and another reactant - the frequency of collisions increases. Typically, we do this by adding more of the solute and taking away some of the solvent, keeping the overall volume the same.

Fig. 4 - Increasing the concentration of a solution increases the rate of reaction

Increasing the concentration of a solution also increases the rate of reaction if one of the reactants is a solid. There is still an increased chance of a solute particle colliding and reacting with the solid, as shown below:

Fig. 5 - Increasing concentration also increases the rate of reaction if one of the reactants is solid

Pressure

Increasing the pressure of a gas has much the same effect as increasing the concentration of a solution. In gases, pressure, volume, and number of particles are directly related. So, if you want to increase the pressure of a gas but keep its number of particles the same, you must decrease the volume. This results in a higher concentration of gaseous particles and increases the rate of reaction.

Fig. 6 - Increasing the pressure of a gas increases the rate of reaction

Surface area

Dissolving a solid tablet in a beaker of water can take a long time. But if you crush it up into a fine powder, it dissolves much more quickly. This is because it has a larger surface area and there are more molecules exposed on its surface. Only molecules on the surface of a solid can collide and react with other particles, so increasing its surface area increases the rate of reaction.

Fig. 7 - Increasing the surface area of a solid increases the rate of reaction

Catalysts

The final factor we'll look at today is the presence of a catalyst.

Catalysts don't affect the individual energies of particles themselves, nor how often they collide. Instead, they work by decreasing the activation energy requirements of a reaction. Thus, on average, more of the particles meet or exceed the activation energy, therefore there is a greater chance of a successful collision. The rate of reaction increases.

Let's now take a look at the action of catalysts on energy profiles and Maxwell-Boltzmann distributions.

Energy profile

Here's an energy profile for an exothermic reaction, shown both with and without a catalyst. The overall energy change for both reactions is the same. However, the activation energy is lower for the reaction involving a catalyst:

Fig. 8 - The energy profile for an exothermic reaction shown both with and without a catalyst

Maxwell-Boltzmann distribution

Now let's look at a Maxwell-Boltzmann distribution for a reaction with and without a catalyst. The activation energy for the reaction without a catalyst is marked ${\mathrm{E}}_{\mathrm{a}}1$. The activation energy for the reaction with a catalyst is marked ${\mathrm{E}}_{\mathrm{a}}2$. Note that the overall energies of the particles don't change. Instead, ${\mathrm{E}}_{\mathrm{a}}2$ is simply lower than ${\mathrm{E}}_{\mathrm{a}}1$ and so a greater number of particles meet or exceed this energy.

Fig. 9 - The effect of a catalyst on the activation energy of a reaction, shown using a Maxwell-Boltzmann distribution

Applications of factors affecting rate of reaction

Finally, let's discuss some applications of factors that increase the rate of a reaction.

Temperature

Keeping food in a refrigerator helps stop it going bad. This is because the low temperature slows down the activity of microorganisms by lowering the rate of all of their reactions.

Surface area

An example of using surface area to increase the rate of reaction is the Haber process, used to make ammonia. In this reaction, iron is used as a catalyst. However, the iron is powdered to increase its surface area and increase the rate of reaction.

Catalysts

Catalysts are frequently used by those working in industry. Because they aren't used up in the reaction, they offer a cheap way to increase the rate of reaction and hence increase the reaction's yield. Even if the catalyst is expensive to purchase, you only need to buy it once - you can then reuse it many times!