5.2 Speed Of Reaction

Speed Of Reaction

By the end of the topic, students should be able to: □ Describe a suitable method for investigating the effect

of a given variable (e.g. concentration) on the speed of reaction. □ Interpret the results of a reaction rate experiment (e.g. as graphs or as a table of readings). □ Describe how concentration, pressure, particle size and temperature affect the rate of a reaction. □ Explain the above four effects in terms of collisions between the reacting particles. □ Describe the features of catalysts (including enzymes). □ Explain the meaning of activation energy and how catalysed reactions have lower activation energies. Please tick in the box if you can do any of the above.

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Speed of Reaction • Different chemicals reactions take place at different speeds. • Table 1 below gives some other examples of reactions that proceed at different speeds. Very fast Moderately fast Slow • Explosion of • Reactions of • Rusting of iron petrol-air metals or in air. mixture carbonates • Reaction of with dilute • Precipitation magnesium acids. reactions with cold water. • Fermentation (conversion of fruit juice into alcohol). Table 1 Different reactions proceed at different speeds.

Measuring the speed of reaction The speed of a reaction can be found by measuring these quantities at regular time intervals: a) the mass of the reactant that remains b) the volume of the products that was produced The method chosen depends upon the reaction being studied. It is usual to record the mass or total volume at regular intervals and plot a graph. The readings go on the vertical axis and the time goes on the horizontal axis. Measuring the speed of reaction from changes in mass This method works best for reactions which produce gases such as carbon dioxide. For example, the speed of reaction between calcium carbonate and hydrochloric acid can be studied this way. EXPERIMENT 1 To study the speed of reaction between calcium carbonate and dilute hydrochloric acid.

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Figure 1 An experiment to study the speed of reaction by measuring the mass at different time intervals.

Procedure 1. The apparatus is set up as shown in Figure 1. The cotton wool in the mouth of the conical flask is used to prevent acid spray i.e. to stop the acid from splashing out as the reaction takes place. 2. The mass of the system is recorded. This includes the mass of the marble chips (calcium carbonate), dilute hydrochloric acid, conical flask, small test tube, string and cotton wool. 3. 3. The conical flask is shaken to mix the marble chips and acid. The stopwatch is immediately started. 4. The mass of the system is recorded at one-minute intervals. Result Plot a graph of mass of system against time from a sample set of results from Experiment 1.

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Measuring speed of the reaction from changes in volume The reaction between a reactive metal and a dilute acid is considered fast. For example, magnesium reacts with dilute hydrochloric acid according to the equation: Magnesium + dilute hydrochloric acid  magnesium chloride+ hydrogen ___________________________________________________________ _____ As the reaction proceeds, the total volume of hydrogen gas produced increases. The speed of the reaction can therefore be determined by collecting and measuring the volume of hydrogen produced at regular time intervals. EXPERIMENT 2 To study the speed of reaction between dilute hydrochloric acid and magnesium.

Figure 2 An experiment to study the speed of reaction by measuring the volume of gas evolved.

Procedure 1. The apparatus is set up as shown in Figure 2. The layer of oxide on the magnesium ribbon is removed using a piece of sandpaper. This ensures that magnesium reacts with the dilute hydrochloric acid. The magnesium ribbon is then put in a small test tube. 2. The conical flask is shaken to mix the magnesium ribbon and acid. The stopwatch is started at the same time.

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3. The volume of hydrogen collected in the gas syringe is recorded every half-minute. Result Plot a graph of the volume of hydrogen produced against time from a sample set of results from Experiment 2.

• How can we estimate the change of speed of reaction from a graph? - The shape of the graph tells us whether the speed of reaction changes or remains the same as time passes. - The _________ the gradient, the __________ the speed of the reaction. On the two graphs below, label which reaction is faster and slower.

 Please proceed to do exercise from: WORKSHEET 1

Collision theory of Reaction

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The theory that we use to explain how different variables change the rate of reaction is called the collision theory. For a reaction to take place, the particles of the substances that are reacting have to collide. If they collide, with enough energy then they will react. The minimum amount of kinetic (movement) energy that two particles need if they are going to react when they collide is called the activation energy.

There are therefore two main ways of increasing the rate of a reaction: 1) increase the number of collisions 2) increase the amount of movement (kinetic) energy so that more collisions lead to a reaction

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A thought experiment on collision theory A group of students are taken into the school hall and blindfolded. They are asked to move around the hall. If two students crash into each other and both students fall over, then they stay lying on the ground - they have "reacted". How can we increase the rate at which students fall down? There are two obvious choices: 1) Put more students into the hall. This will lead to more collisions between students. 2) Ask the students in the hall to run more quickly. This will also lead to more collisions and to a greater chance of a collision leading to the students falling over. We will use this "thought experiment" to try and explain rates of reaction. Factors affecting the speed of reaction You will be expected to remember the factors that affect the speed of reactions, and to plot or interpret graphs from rate experiments. Factors affecting the speed of reactions are: • Concentration of solution • Pressure • Particle size of solids • Temperature • Catalyst 1) Concentration of solution

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Looking back to the “students in the Hall” thought experiment, we are putting more students into the Hall. 2) Pressure

Looking back to the “students in the Hall” thought experiment, we are putting the students into a smaller Hall. 3) Particle size of solids

4) Temperature If the temperature is increased, the rate increases because not all particles react when they collide. They must have a certain minimum energy to react when they collide. At a higher temperature, the particles have more energy so they react more often when they collide.

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5) Catalyst If a catalyst is added, the rate increases because catalysts allow the reaction to go by a different route. The minimum energy needed for a reaction when particles collide is less, so more reactions take place.

The graph above summarises the differences in the rate of reaction at different temperatures, concentrations and size of pieces. The steeper the line, the greater the rate of reaction. Reactions are usually fastest at the beginning when the concentration of reactants is greatest. When the line becomes horizontal the reaction has stopped.  Please proceed to do exercise from: WORKSHEET 2

Catalysts • What is a catalyst? ___________________________________________________________ ___________________________________________________________ __________ 9

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• What are the characteristics of a catalyst? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________ • What are the factors affecting the speed of catalysed reactions? The speed of catalysed reactions is affected by changes in conditions, just like reactions without catalysts. The speed of a catalysed reaction is increased by: ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ _________________________ • How does a catalyst work? Catalysts provide an alternative route for the reaction. In the catalysed route, ______ energy is needed to break bonds and so the activation energy is ________. As a result, bond breaking takes place more easily and more often when particles collide. This is why catalysed reactions are faster. The effect of catalysts on activation energies can be shown by energy profile diagrams such as shown in Figure 3.

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Figure 3 the energy profile diagrams showing the effect of a catalyst on activation energy.

A simple analogy of how catalysts work is a group of cyclists riding up and over a hill. The hill route is the normal one, requiring a lot of energy. The short cut through the tunnel is like the catalysed route. It requires less energy and the cyclists, like the reaction, go faster. (Figure 4)

Figure 4 showing how catalyst work by taking an alternate pathway.

• What are the common catalysts for some industrial processes? In many industrial processes, catalysts are used to speed up various reactions. Below are some examples: a) Ironb) Vanadiumc) Platinum – d) Aluminium oxide or silicon (IV) oxide – Enzymes • Enzymes are substances that catalyse the chemical reactions in plants and animals. They are often called biological catalysts.

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Figure 5 showing how an enzyme works.

• What are the properties of enzymes? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ____________________ • What are the uses of enzymes in industry? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ _______________  Please proceed to do exercise from: WORKSHEET 3 WORKSHEET 4

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