Lesson Video: Effects of Temperature and Concentration on Rates of Reactions Science • Third Year of Preparatory School

Video Transcript

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

In a chemical reaction, reactants are converted into products. As the reaction occurs, the concentration of the reactants will decrease and the concentration of the products will increase. The rate of reaction measures how reactant or product concentration, mass, or volume changes per unit of time. This represents the speed at which a chemical reaction takes place.

On a graph of concentration versus time, the lines on the graph represent the rate of reaction. The steeper the line, the faster the rate of reaction. The rate of reaction depends in part on the number of collisions between particles. In order for two particles to react, they need to collide with one another with a certain amount of energy. The greater the number of collisions, the more likely a reaction will occur, and the rate of reaction will increase. In this video, we will focus on two factors that can increase the number of collisions between particles and therefore increase the rate of reaction.

Let’s think about temperature. When we increase the temperature of a substance, the particles gain energy and move faster. The faster the particles move, the more collisions they are likely to have. So increasing the temperature will increase the number of collisions between particles. And as the number of collisions increase, the rate of reaction will increase. So increasing the temperature will increase the rate of reaction. Let’s see how we can prove this to be true with an experiment.

In this experiment, we will place an effervescent tablet in water. When the tablet reacts with the water, carbon dioxide gas will be produced. We can measure the volume of carbon dioxide produced with a gas syringe. We’ll perform the experiment with room-temperature water and boiling water. We will be able to see that the experiment using boiling water will bubble more rapidly, the gas syringe will fill more quickly, and the tablet will dissolve more quickly than the experiment using room-temperature water. All of these signs indicate that the rate of reaction at a higher temperature was faster. Eventually, both tablets will have completely dissolved, the reactions will end, and the same volume of gas will be produced.

Let’s analyze the results of these experiments. We can see from the plots that initially the line on the graph for the boiling water experiment is steeper than the line for the room-temperature water experiment. So the rate of reaction using boiling water is faster. We can also see that although the reaction with boiling water is faster than the reaction with room-temperature water, both reactions produced the same volume of gas.

In our everyday lives, we take advantage of the effect that temperature has on the rate of reaction. We store our food in the refrigerator. The lower temperature slows down the chemical reactions that spoil food, making our food last longer. When we cook an egg, we can increase the temperature to increase the rate of the reactions that cause the egg to cook. This will allow us to cook the egg faster.

Now that we’ve looked at how temperature affects rate of reaction, let’s think about concentration. Concentration is a measure of the amount of a substance in a particular volume. Increasing the concentration means that there is more substance in the same volume. We might refer to this as being concentrated. Decreasing the concentration means that there is less substance in the same volume. We might refer to this as being dilute.

So how does concentration affect rate of reaction? Let’s consider a reaction between orange and pink particles. We know that the particles must collide with the right amount of energy in order for a reaction to occur. So there will be times when the particles collide and don’t react. If we increase the concentration of pink particles, then there will be more pink particles available to collide with the orange particles. The number of collisions will increase, and the chance of a successful collision to form the product will also increase. This means that increasing the concentration will increase the rate of reaction. Let’s see how we could prove this to be true with an experiment.

In this experiment, we will burn iron wool, also called steel wool. When iron wool is burned, the iron reacts with oxygen. If we burn the iron wool using a Bunsen burner, the oxygen involved in the reaction is coming from the air. Air is about 21 percent oxygen. The iron wool will burn but relatively slowly. But what if we increased the concentration of oxygen by placing some lit iron wool in a flask containing 100 percent oxygen? The reaction in 100 percent oxygen would be more intense and rapid. We would see that the light given off is brighter, and the iron wool would stop burning more quickly than the iron wool burned over the Bunsen burner. These signs indicate that increasing the concentration increased the rate of reaction.

Let’s analyze the results of a different experiment. In this experiment, magnesium metal is placed into a flask containing hydrochloric acid. When these two substances react, one of the products is hydrogen gas. We can measure the volume of hydrogen gas produced by attaching a gas syringe to the flask. We can perform this experiment with dilute and concentrated hydrochloric acid. We will see that the experiment using concentrated hydrochloric acid will bubble more rapidly, the gas syringe will fill more quickly, and the magnesium will be used up more quickly than in the experiment using dilute hydrochloric acid. All of these signs indicate that the rate of reaction was faster when a greater concentration of hydrochloric acid was used. Eventually, the magnesium in both experiments will be used up, the reactions will end, and the same volume of hydrogen gas will be produced.

Let’s analyze the results of these experiments. We can see from the plots that initially the line on the graph for the concentrated hydrochloric acid experiment is steeper than the line for the dilute hydrochloric acid experiment. So the rate of reaction using concentrated hydrochloric acid is faster. We can also see that although the reaction with concentrated hydrochloric acid was faster, both reactions produced the same volume of gas.

We’ve examined how temperature and concentration affect rate of reaction. Let’s apply our understanding to some questions.

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 with one another, they have to collide. There are several factors that can affect the number of collisions between reactant molecules. Two of these factors are concentration and temperature. Concentration is a measure of the amount of substance in a given volume. We are told, and can see, that each box contains the same number of red and blue reactant molecules. So the concentration of reactant molecules is the same in each box. But the temperature of each box is not the same.

Let’s consider this picture of orange and pink particles. At this temperature, each particle moves at a particular speed and collisions between particles are occurring. As we increase the temperature, the particles gain energy and move faster. The faster they move, the more likely they are to collide. So the frequency of collisions will increase. This means that the box which will have the greatest frequency of collisions between molecules is the box which has the highest temperature. Box (D), at 80 degrees, has the highest temperature and will therefore have the greatest frequency of collisions. So the correct answer is (D).

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? (A) A, C, D, B. (B) B, C, A, D. (C) C, D, B, A. (D) D, C, A, B. Or (E) C, A, B, D.

In each of the test tubes, an identical piece of magnesium ribbon is submerged in the same volume of hydrochloric acid. The test tubes differ in the concentration of hydrochloric acid used, given here in moles per liter. Concentration is a measure of the amount of substance in a given volume. We need to determine how the different concentrations of hydrochloric acid in each test tube will affect the rate of reaction. Let’s clear some space at the top of the screen.

The rate of a reaction measures how reactant or product concentration, mass, or volume changes per unit of time. We can think of this as the speed of a chemical reaction. In order for a chemical reaction between two particles to occur, the particles must collide with one another with a certain amount of energy. This means that some collisions won’t result in a reaction. But if we can increase the total number of collisions, we can increase the chance that a reaction will occur, thus increasing the rate of reaction.

So how does changing the concentration affect rate of reaction? Let’s consider the sample of pink and orange particles. These particles will have a certain number of collisions with one another. Increasing the concentration of pink particles means that there will be more pink particles available to collide with the orange particles. The number of collisions will increase, and the rate of reaction will increase. So increasing the concentration increases the rate of reaction.

We want to order the experiments from slowest to quickest. If increasing the concentration increases the rate of reaction, then we should put the experiments in order from lowest concentration to highest concentration. At 0.1 moles per liter, (B) has the lowest concentration, followed by (C), then (A), then (D), with the highest concentration at 5.0 moles per liter. So the correct answer for the likely ordering of the rate of reaction for the four experiments from slowest to quickest is answer choice (B) B, C, A, D.

Let’s wrap up this video by reviewing what we’ve learned. The rate of a reaction measures how reactant or product concentration, mass, or volume changes per unit of time. We can think of this as the speed of a chemical reaction. Reactant particles must collide with one another in order for a chemical reaction to occur. In general, as the number of collisions between the reactant particles increases, the rate of reaction increases.

As the temperature increases, the particles gain more energy and the number of collisions increases. This causes the rate of reaction to increase. We discussed how effervescent tablets react faster with warmer water than colder water.

Increasing the concentration increases the number of particles present, so the number of collisions increases. This increases the rate of reaction. We saw how iron wool burns faster in pure oxygen than in air because the concentration of oxygen is lower in air. We also saw that magnesium reacts faster with concentrated hydrochloric acid than with dilute hydrochloric acid.