# Lesson Explainer: Effects of Temperature and Concentration on Rates of Reactions Science

In this explainer, we will learn how to describe and explain the effect temperature and concentration have on the rate of chemical reactions.

The speed at which a chemical reaction takes place is known as the rate of reaction. Usually, the rate of reaction describes how some variable changes over a certain period of time. A common way to measure the rate of a chemical reaction is to measure how the concentrations of the reactants and products change over a certain period of time.

### Definition: Rate of Reaction

The rate of reaction measures how reactant or product concentrations change per unit time.

The rate of a chemical reaction can be affected by many factors. By changing some of these factors, the rate of reaction can be increased or decreased.

The factors that affect the rate of reaction include surface area, temperature, concentration, and the addition of catalysts. We will focus on temperature and concentration.

In order for two particles to react, they must first collide. In addition, the particles must have a certain amount of energy when they collide.

Any factor that can increase the frequency of collisions, or the energy of the particles, will likely increase the rate of reaction.

### Example 1: Identifying in Which Box of Particles the Number of Collisions Will Be Greatest

The boxes below represent a chemical reaction between the red and the blue particles. In which box will the number of collisions be greatest?

A chemical reaction occurs when reactants collide with each other. The greater the number of collisions that occur, the more likely the reaction to happen and the faster the rate of reaction.

There are several factors that can affect the rate of reaction. However, from the question and diagram, we can see that we are given four boxes each containing different numbers of particles. The size of the box is also the same in each case.

If the particles are moving randomly, then the more particles there are, the more collisions there are likely to be.

We can see from the diagram that box A contains the greatest number of particles. Therefore, the number of collisions is likely to be greatest in box A.

One way to increase the number of collisions is by increasing the temperature. As the temperature increases, the particles gain energy and move faster. The faster the particles move, the more likely they are to collide with each other.

In the diagram below, the larger the arrow, the faster the particle is moving. At higher temperatures, the particles have more energy and so a larger arrow.

The effect of temperature on the rate of reaction can easily be demonstrated in a laboratory experiment. In this experiment, one effervescent tablet is put into a flask that contains hot water and a second tablet is put into a different flask that contains cold water.

The tablet reacts with the water to produce carbon dioxide gas. The experimental setup is shown below.

By measuring the volume of gas produced in each experiment, the rates of reaction can be determined and compared.

The results of this experiment are shown in the graph below:

At the higher temperature, the particles have more energy and move around faster. This increases the number of collisions between particles and increases the rate of reaction.

A faster rate of reaction increases the volume of gas produced at the start of the reaction, resulting in a steeper line on the graph. However, as the mass of the tablet and volume of water remain constant, the final amount of gas produced is the same.

### Example 2: Relating Temperature to the Frequency of Collisions between Molecules

The boxes below each contain an equal number of reactant molecules. The boxes are heated to different temperatures. Which box will have the greatest frequency of collisions between molecules?

In order for two reactant molecules to react, they have to collide. There are several factors that can increase the number of collisions between reactant molecules. One of these is temperature.

We are told that each box contains the same number of reactant molecules, so the frequency of collisions is not going to be affected by a different number of molecules. However, the temperature of each box is different, and so, the main effect on the frequency of collisions will be the change in temperature.

As the temperature increases, the reactant molecules gain energy and move faster. The faster the molecules are moving, the more likely they are to collide and the greater the frequency of collisions will be.

The higher the temperature, the greater the frequency of collisions between molecules. Looking at the diagram, we can see that the box with the highest temperature is box D. Therefore, the answer is box D.

Temperature is a very important factor for controlling the rate of reactions in food. Placing food in a cool place, such as a refrigerator or freezer, slows down the chemical reactions that spoil food. As a result, food can be preserved and last longer.

High temperatures are often used when cooking food. The higher temperature increases the rate of reaction and helps cook food quicker and more thoroughly.

The effect of concentration on the rate of reaction can be explained by looking at the frequency of collisions.

Consider the reaction between the purple particles A and the green particles B shown in the diagram below.

If the concentration of B is increased, then the number of particles of B present increases. This is shown in the diagram below.

An increase in the number of particles will result in an increase in the number of collisions. A greater number of collisions causes an increase in the rate of reaction.

The effect of concentration on the rate of reaction can be demonstrated using the reaction of iron wool and oxygen.

Iron wool, also known as steel wool, can be burned in the presence of oxygen. However, the speed and intensity of this reaction changes when the concentration of oxygen changes.

When burned over a Bunsen burner, the iron wool is being burned in air. Air contains of oxygen, a medium to low concentration. The rate of reaction is quite low, and the iron wool burns relatively slowly.

However, when burned in pure oxygen the reaction is much more rapid and intense. The concentration of pure oxygen is , much greater than air. The increase in oxygen concentration increases the rate of reaction and results in a more vigorous and fast reaction.

These two experiments are shown in the image below.

### Example 3: Explaining the Different Rates of Combustion in Air Compared with Pure Oxygen

Why is the combustion of aluminum in air slower than in pure oxygen?

1. The temperature of oxygen in air is greater than in pure oxygen.
2. The temperature of pure oxygen is greater than air.
3. The concentration of oxygen in air is less than in pure oxygen.
4. The concentration of oxygen in air is greater than in pure oxygen.

The process of combustion usually refers to the reaction of a substance with oxygen. Here, aluminum is reacted with oxygen under two different conditions.

The combustion of aluminum in air is most likely performed using a Bunsen burner. Air usually contains around oxygen, a relatively low amount of oxygen.

The combustion of aluminum with pure oxygen most likely involves conditions where there is oxygen. We can see that the difference between burning in air and in pure oxygen is the amount, or concentration, of oxygen present.

From this, we can conclude that the difference in the rate of combustion is because of the different concentrations of oxygen. Our answer is therefore likely to be either C or D.

Concentration can affect the rate of reaction by changing the number of reactant molecules present. The more reactant molecules there are, the greater the number of collisions that will occur between them and the faster the rate of reaction is.

As concentration increases, the rate of reaction increases.

The combustion of aluminum in air is slower because the concentration of oxygen is lower than in pure oxygen. This statement matches with choice C, and so our answer is C.

Another experiment that shows the effect of concentration on the rate of reaction is the reaction of magnesium with hydrochloric acid.

In this experiment, one conical flask contains dilute hydrochloric acid and a different flask contains concentrated hydrochloric acid. Into each conical flask is placed an identical piece of magnesium of the same size and mass.

The chemical equation for the reaction between magnesium and hydrochloric acid is

Therefore, by measuring the volume of hydrogen gas produced over time, any change in the rate of reaction can be determined.

The setup of this experiment is shown in the image below:

By plotting a graph of the volume of hydrogen gas produced against time, the rates of reaction for each experiment can be determined. A graph showing the rate of reaction for dilute and concentrated hydrochloric acid is shown below:

The graph shows that a greater volume of hydrogen gas is produced over a short period of time when concentrated hydrochloric acid is used. This shows that the rate of reaction increases as the concentration increases.

As the concentration of hydrochloric acid increases, the number of acid particles present increases. As a result, there is a greater number of collisions between the acid and the magnesium particles, and so, there is an increase in the rate of reaction.

### Example 4: Ordering Experiments with Differing Concentration by Their Rate of Reaction

A chemist performs a series of experiments to determine the effect of concentration on the rate of a reaction. They pour an equal amount of hydrochloric acid of different concentrations into four test tubes, then they place an identical piece of magnesium ribbon into each of the test tubes. The experiment setup is shown below.

From slowest to quickest, what is the likely ordering of the rate of reaction for the four experiments?

There are several factors that can affect the rate of reaction. These include concentration and surface area. In the experiment, the volume of hydrochloric acid used is kept the same. An identical piece of magnesium is also used, and so, the surface area and mass are kept the same.

The only factor that is changing is the concentration of hydrochloric acid. The concentration is greatest for experiment D and lowest in experiment B.

For a reaction to occur, the reactant molecules must collide with each other. Increasing the number of collisions increases the rate of reaction.

When the concentration is increased, the number of acid particles present in the solution increases. The increased number of acid particles will result in a greater number of collisions and therefore a faster rate of reaction.

If the rate of reaction increases as the concentration increases, then the order of the rate reaction from slowest to quickest will correspond to the order from the lowest to the greatest concentration.

From slowest to quickest, the likely ordering is B, C, A, D.

### Example 5: Identifying Which Set of Conditions Gives the Greatest Rate of Reaction

In a series of experiments, a student changes both the concentration and the temperature. The conditions for each experiment are shown below. In which conical flask is the rate of reaction likely to be highest?

The rate of a reaction is affected by both temperature and concentration. For a reaction to occur, reactant particles must collide with each other. Any factor that increases the number of collisions is likely to increase the rate of reaction.

As the temperature increases, the particles are given more energy and can move faster. As a result, there is likely to be a greater number of collisions and a faster rate of reaction. Therefore, the rate of reaction increases as the temperature increases.

As the concentration increases, the number of reactant particles increases. With a greater number of particles present, there is likely to be a greater number of collisions and a faster rate of reaction. Therefore, the rate of reaction increases as the concentration increases.

From the two statements above, we can conclude that the rate of reaction is likely to be highest when both the temperature and the concentration are greatest.

In the diagram above, we can see that the highest temperature is and the highest concentration is 2 mol/L, which occurs in experiment C.

The rate of reaction is therefore likely to be highest for experiment C.

### Key Points

• The rate of reaction measures how reactant or product concentrations change per unit time.
• For a chemical reaction to occur, reactant particles must collide with each other.
• Generally, as the number of collisions between reactant particles increases, the rate of reaction increases.
• When the temperature increases, the particles gain more energy and the number of collisions increases, causing the rate of reaction to increase.
• The effect of temperature on the rate of reaction can be seen experimentally by reacting effervescent tablets with water and measuring the volume of gas produced.
• Increasing the concentration increases the number of particles present. There is a greater number of collisions, and so, the rate of reaction increases.
• The combustion of substances such as iron wool in pure oxygen is faster than in air because the concentration of oxygen is lower in air.
• The effect of concentration on the rate of reaction can be seen experimentally by reacting magnesium with different concentrations of hydrochloric acid and measuring the volume of gas produced.