# Factors Affecting Reaction Rates

Maltase, amylase, protease, lipase - these are all examples of digestive enzymes. They are responsible for turning that juicy burger you ate for lunch, topped with cheese and lettuce and sandwiched between a bun, into small molecules that can be used by the body. Theoretically, we could digest our food without enzymes. However, this would take a very long time. You'd die of starvation before your body could get the nutrients it needed. Enzymes are therefore great examples of how we can increase the rate of a reaction.

#### Create learning materials about Factors Affecting Reaction Rates with our free learning app!

• Flashcards, notes, mock-exams and more
• Everything you need to ace your exams

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 the change in concentration of reactants or products over time. Factors affecting rate of reaction are variables we can manipulate in order to speed up or slow down reactions.

• We will learn about increasing the rate of reaction in chemistry.
• To start with, we'll look at what rate of reaction actually is, and briefly explore how you measure it.
• We'll then look at different factors that affect rate of reaction, including temperature, concentration, surface area, pressure, and the presence of a catalyst.
• Finally, we'll investigate some processes that manipulate conditions in order to increase the rate of reaction.

## 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.

Activation energy is the minimum amount of energy needed to start a chemical reaction. It takes the symbol ${\mathrm{E}}_{\mathrm{a}}$.

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.

Concentration is the amount of a substance in a particular volume.

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

Actually, increasing the concentration of some reactants doesn't always increase the rate of reaction. It all depends on the order of reactants for each particular species. For some species, when you double their concentration, you double the rate of reaction. For some species, doubling their concentration quadruples the rate of reaction. But for some species, doubling their concentration has no effect on the rate whatsoever. You can find out more about this in Rate Equations.

### 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

The pressure, volume, number of moles and temperature of a gas are all related by something called the gas constant, R. You can read more about it in Ideal Gas Law.

### 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

Increasing the surface area of a solid only impacts the rate of reaction if the solid reacts with a liquid, gas, or aqueous solution.

However, this doesn't just work for reactants - increasing the surface area of a solid catalyst can increase the rate of reaction too. We'll look at catalysts next.

### Catalysts

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

Catalysts are substances that increase the rate of a reaction without being chemically changed themselves in the process.

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.

There are several theories about how catalysts work. The first looks at transition states and the second focuses on adsorption.

All reactions have a transition state. This is the point in the middle of the reaction with the highest energy level, where some of the bonds have been broken but not all of the new bonds have been formed. The transition state often contains intermediates, which are molecules that are created from the reactants that themselves react further to give the products. Intermediates only exist for a split second; this is what the activation energy is used for - to make these intermediate molecules.

The most common catalytic theory is that catalysts react with some of the reactants to form more stable intermediates than those formed in the original reaction. This requires less energy. The intermediates then react to form the products of the reaction, regenerating the catalyst in the process. This creation of more stable intermediates most often occurs when you use homogenous catalysts.

For example, reactant AB may react with catalyst X to form the intermediates AX and B. AX then reacts with reactant C to form AC and X. X cancels out on each side of the equation. Overall, you've produced AC and B and regenerated the catalyst in the process. The equations are shown below:

$\mathrm{AB}+\mathrm{X}\to \mathrm{AX}+ \mathrm{B}\phantom{\rule{0ex}{0ex}}\mathrm{AX}+\mathrm{C}\to \mathrm{AC}+ \mathrm{X}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\mathrm{Overall}:\mathrm{AB}+\mathrm{C}\stackrel{\mathrm{X}}{\to }\mathrm{AC}+\mathrm{B}$

Another idea is that reactant particles form weak bonds with the surface of the catalyst, which hold them in place with just the right orientation. This means that there is an increased chance of particles reacting when they collide with each other. The process of binding to the catalyst is called adsorption.

Adsorption may also weaken the bonds found within the reactants, making them easier to break. The new products then detach from the catalyst, which is known as desorption. Adsorption and desorption most often occur when you use heterogenous catalysts.

Enzymes are biological catalysts. They work inside of living organisms, speeding up chemical reactions without being used up in the process. Again, they do this by lowering the activation energy of a reaction. Common examples of enzymes include:

• The digestive enzymes we looked at at the start of the article.
• Lysozyme, an enzyme found in saliva and tears that helps kill pathogens.
• Alcohol dehydrogenase, an enzyme found in the liver that breaks down ethanol into ethanal.
• PSI and PSII are important enzyme complexes involved in photosynthesis.

For more on these biological catalysts, check out Enzymes.

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!

One example of using catalysts in industry is the production of margarine. To make margarine, unsaturated oils with C=C double bonds are hydrogenated, turning them into saturated molecules. This requires bubbling hydrogen gas through the oil in the presence of a nickel catalyst.

You might have heard of trans fats. These are also made in margarine production. They are produced when the oils used aren't fully hydrogenated. Instead, the high temperatures used cause some of the C=C double bonds to flip to the trans-isomeric state, forming a trans fat. Trans fats are getting an increasingly bad reputation due to their link to cardiovascular disease.

Nickel is a transition metal. In fact, many transition metals make good catalysts. This is because they easily adopt many different oxidation states. If you want to learn more about their properties, head over to Transition Metals.

## Factors Affecting Reaction Rates - Key takeaways

• Rate of reaction is a measure of either how quickly reactants are used up, or how quickly products are formed, in a chemical reaction. In other words, it is the change in concentration of reactants or products over time.
• We can measure rate of reaction by measuring change in mass, change in pH, or by measuring the production of a gas.
• Increasing the temperature of a system supplies the particles with more energy. This increases the rate of reaction by increasing both the frequency of collisions and the proportion of successful collisions.
• Increasing the concentration of a solution or the pressure of a gas increases the rate of reaction by increasing the frequency of collisions. This is because there are more particles in a given volume.
• Increasing the surface area of a solid increases the rate of reaction. This is because there are more particles exposed to the surrounding liquid or gas.
• Adding a catalyst increases the rate of reaction. This is because catalysts lower the activation energy needed for a reaction.

What are the ways to speed up a chemical reaction?

You can increase the rate of a reaction in the following ways:

• Increasing the pressure of reactions involving gases.
• Increasing the concentration of reactants.
• Increasing the surface area of solid reactants.
• Increasing the temperature.

How do you control factors that affect rate of reaction?

To control the rate of a reaction, you need to control several factors such as temperature, pressure, and concentration. You can do this, for example, by heating the reaction to a constant temperature or continuously distilling off the products of the reaction, in order to keep the concentrations of the reactants the same.

How do environmental factors affect the rate of enzyme-catalysed reactions?

Enzymes are biological catalysts. This means that they increase the rate of reactions. They are affected by variables like temperature, pH, and the concentration of the molecules they act on.

What factors affect reaction rate?

Factors that affect reactant rate include the concentration of the reactants, the surface area of solids, the pressure of gases, temperature, and the presence of a catalyst.

Why is it important to increase rate of reaction?

Increasing the rate of reactions is useful because it can speed up chemical processes. In industry, this saves time and money. For example, many industrial reactions use catalysts to increase the rate of reaction in order to increase their yield.

StudySmarter is a globally recognized educational technology company, offering a holistic learning platform designed for students of all ages and educational levels. Our platform provides learning support for a wide range of subjects, including STEM, Social Sciences, and Languages and also helps students to successfully master various tests and exams worldwide, such as GCSE, A Level, SAT, ACT, Abitur, and more. We offer an extensive library of learning materials, including interactive flashcards, comprehensive textbook solutions, and detailed explanations. The cutting-edge technology and tools we provide help students create their own learning materials. StudySmarter’s content is not only expert-verified but also regularly updated to ensure accuracy and relevance.

##### StudySmarter Editorial Team

Team Factors Affecting Reaction Rates Teachers

• Checked by StudySmarter Editorial Team