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IB Chemistry. Rates of Reaction. Contents. Introduction. Effect of temperature. Effect of concentration. Effect of surface area. Effect of catalysts. Summary activities. Rates of Reaction. Reaction. Rate. Rates of reactions.
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IB Chemistry Rates of Reaction
Contents Introduction Effect of temperature Effect of concentration Effect of surface area Effect of catalysts Summary activities Rates of Reaction
Reaction Rate Rates of reactions The speed of different chemical reactions varies hugely. Some reactions are very fast and others are very slow. The speed of a reaction is called the rate of the reaction. What is the rate of these reactions? rusting slow explosion very fast chemical weathering of rocks very slow sodium and water fast rotting fruit slow
Reactions, particles and collisions Reactions take place when particles of reactants collide with a certain amount of energy. This energy is called activation energy, and is different for each reaction. The rate of a reaction depends on three things: • the frequency of collisions between particles; • the energy with which particles collide. • the geometry with which particles collide. If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other.
Changing the rate of reactions Anything that increases the number of successful collisions between reactant particles will speed up a reaction. What factors speed up reactions? • Increased temperature; • increased concentration of dissolved reactants, and increased pressure of gaseous reactants; • increased surface area of solid reactants; • use of a catalyst.
hydrochloricacid magnesiumchloride + + magnesium hydrogen Measuring rates of reaction Measuring the rate of a reaction means measuring the rate of change over a period of time. This means measuring the change in the amount of a reactant or the amount of a product. What can you measure to calculate the rate of reaction between magnesium and hydrochloric acid? • The amount of magnesium used up (g/min). • The amount of hydrochloric acid used up (cm3/min). • The amount of magnesium chloride produced (g/min). • The amount of hydrogen produced (cm3/min).
fast slower very slow stopped reactant A reactant B product 0% 25% 75% 100% percentage completion of reaction Slower and slower! Reactions do not proceed at a steady state. They start off at a certain speed, then get slower and slower until they stop. As the reaction progresses, the concentration of reactants decreases. This reduces the frequency of collisions between particles and so the reaction slows down.
Contents Effect of catalysts Rates of Reaction Introduction Effect of temperature Effect of concentration Effect of surface area Summary activities
The effect of temperature on collisions How does temperature affect the rate of particle collision?
Temperature The higher the temperature, the faster the rate of a reaction. In many reactions, a rise in temperature of 10°C causes the rate of reaction to approximately double. Why does increased temperature increase the rate of reaction? • At a higher temperature, particles have more energy. This means they move faster and are more likely to collide with other particles. • When the particles collide, they do so with more energy, and so the number of successful collisions increases.
Temperature and food Food goes off because chemical reactions take place. Why does food remain usable for much longer if it is kept in a freezer? The low temperature in the freezer means that particles will move much slower and with less energy than if they were at room temperature. This means that there are fewer successful collisions and so a slower rate of reaction.
Temperature and cooking Before microwave ovens were common, many people used pressure cookers to cook food more quickly. In a pressure cooker, water doesn’t boil until it reaches about 115°C. How does this help cooking? The higher temperature means that particles move more quickly and with more energy. This means that there are more successful collisions between particles, and the food cooks more quickly.
sodiumchloride sulfurdioxide sodiumthiosulfate hydrochloricacid + + + + sulfur water SO2(g) H2O(l) Na2S2O3(aq) 2HCl(aq) 2NaCl(aq) S(s) + + + + Temperature and rate of reaction The reaction between sodium thiosulfate and hydrochloricacid produces sulfur. Sulfur is solid and so it turns the solution cloudy. The effect of increasing temperature on the rate of reaction can be measured by comparing how long it takes the solution to turn cloudy at different temperatures.
5. Repeat the experiment at different temperatures using the same volume of reactants. Compare how long it takes the cross to disappear. Sodium thiosulfate and hydrochloric acid To run the experiment investigating the effect of temperature on the rate of reaction: 1. Mark a cross on a piece of paper. 2. Add a known amount of sodium thiosulfate to a beaker, and place it on the piece of paper. 3. Add a known amount of hydrochloric acid to the beaker and immediately start a stop-clock. The solution will begin to turn cloudy. 4. As soon as the cross can no longer be seen, stop the clock and note the time.
increasing time Sodium thiosulfate and hydrochloric acid When looking down into the beaker, the cross will become fainter over time: The time taken for the cross to disappear can be used as the time of the reaction.
Contents Effect of catalysts Rates of Reaction Introduction Effect of temperature Effect of concentration Effect of surface area Summary activities
low concentration high concentration Concentration The higher the concentration of a dissolved reactant, the faster the rate of a reaction. Why does increased concentration increase the rate of reaction? At a higher concentration, there are more particles in the same amount of space. This means that particles are more likely to collide with other particles.
magnesiumchloride hydrochloricacid + + magnesium hydrogen + + MgCl2 (aq) Mg(s) 2HCl (aq) H2 (g) Reaction between acid and metal Reactive metals such as magnesium react with acid to produce hydrogen gas. The effect of increasing concentration on the rate of reaction can be measured by comparing how quickly hydrogen is produced using different concentrations of hydrochloric acid.
glasstube rubber connector gas syringe conicalflask rubber bung hydrochloric acid magnesium ribbon Mg + HCl: experiment set-up What equipment do you need for the experiment investigating the effect of concentration on the rate of reaction?
Magnesium and hydrochloric acid To run the experiment investigating the effect of concentration on the rate of reaction: 1. Measure out a fixed volume of hydrochloric acid into the conical flask. 2. Add a known mass of magnesium to the flask, immediately attach the gas syringe and start a stop-clock. 3. Measure the volume of hydrogen collected in the syringe at regular intervals until no more gas is produced. 4. Repeat the experiment using a different concentration of hydrochloric acid but using the same volume of acid and the same mass of magnesium. Compare the rate at which hydrogen is produced.
low pressure high pressure Pressure Why does increasing the pressure of gaseous reactants increase the rate of reaction? As the pressure increases, the space in which the gas particles are moving becomes smaller. The same number of particles but in a smaller space. The gas particles become closer together, increasing the frequency of collisions, and so increasing the rate of reaction.
Contents Effect of catalysts Contents Rates of Reaction Introduction Effect of temperature Effect of concentration Effect of surface area Summary activities
Surface area Any reaction involving a solid can only take place at the surface of the solid. If the solid is split into several pieces, the surface area increases. slow rate This means that there is an increased area for the non-solid reactant particles to collide with. The smaller the pieces, the larger the surface area. This means more collisions and a faster rate of reaction. fast rate
calciumchloride carbondioxide calciumcarbonate hydrochloricacid + + + water CaCO3(aq) 2HCl (aq) CaCl2(aq) H2O(aq) CO2(g) + + + Reaction between a carbonate and acid Marble chips are made of calcium carbonate. They react with hydrochloric acid to produce carbon dioxide. The effect of increasing surface area on the rate of reaction can be measured by comparing how quickly the mass of the reactants decreases using marble chips of different sizes.
cotton wool ‘plug’ conicalflask hydrochloricacid calciumcarbonatechips weighing scales CaCO3 + HCl: experiment set-up What equipment do you need for the experiment investigating the effect of surface area on the rate of reaction?
Calcium carbonate and hydrochloric acid To run the experiment investigating the effect of surface area on the rate of reaction: 1. Measure out a fixed volume of hydrochloric acid into a conical flask and place the flask on weighing scales. 2. Add a fixed mass of calcium carbonate chips to the flask, and place a cotton wool plug in the neck. This stops the liquid from spitting while allowing the CO2 to escape. 3. Begin taking mass readings straight away, and continue until there is no further change in mass. 4. Repeat the experiment using the same mass of calcium carbonate but of a smaller chip size, and the same volume of hydrochloric acid. Compare the rate at which the mass of reactants decreases.
Contents Rates of Reaction Introduction Effect of temperature Effect of concentration Effect of surface area Effect of catalysts Summary activities
What are catalysts? What are catalysts? Catalysts are substances that change the rate of a reaction without being used up in the reaction. Catalysts are very important in industry because products can be made more quickly, saving time and money.They can also avoid having to use high temperatures, so they can save fuel and reduce pollution. Catalysts are also very important in living cells. Biological catalysts are special types of protein called enzymes.
Examples of catalysts Many catalysts are transition metals or their compounds. Different reactions use different catalysts. For example: • Nickel is a catalyst in the production of margarine (hydrogenation of vegetable oils). • Iron is a catalyst in the production of ammonia from nitrogen and hydrogen (the Haber process). • Platinum is a catalyst in the catalytic converters of car exhausts. It catalyses the conversion of carbon monoxide and nitrogen oxide into the less polluting carbon dioxide and nitrogen.
How do catalysts work? How do catalysts work? For a chemical reaction to take place: • energy is needed to break existing bonds, so new bonds can be formed; • the reacting parts of particles need to be brought together. Different catalysts work in different ways, but most solid catalysts work by lowering the amount of energy needed for the reaction to take place. Catalysts work by lowering the activation energy of a reaction.
platinum wire oxygen molecule hydrogen molecule How a platinum catalyst works When hydrogen and oxygen are mixed in a jar, there is no reaction. If a platinum wire is added, the gases react instantly with a loud pop, producing water. How does platinum catalyse this reaction? The gas molecules are brought together onto the surface of the platinum. They are adsorbed. The molecules are much closer together and their bonds are weakened, lowering the activation energy of the reaction. The larger the surface area of the platinum, the quicker the reaction.
hydrogenperoxide + water oxygen + 2H2O2(aq) 2H2O (l) O2 (g) Catalysts never produce more product – they just produce the same amount but quicker. Decomposition of hydrogen peroxide Hydrogen peroxide decomposes into water and oxygen. Without a catalyst, this reaction is very slow, and can take months. With a catalyst such as manganese (IV) oxide, the reaction takes minutes.
Contents Rates of Reaction Introduction Effect of temperature Effect of concentration Effect of surface area Effect of catalysts Summary activities
Glossary • activation energy – The amount of energy needed for a reaction to begin. • adsorption – The formation of a layer of molecules on the surface of a solid. • catalyst – A substance that changes the rate of a reaction without being used up. • concentration – The amount of particles in a given volume. • enzyme – A biological catalyst. • rate of reaction –The speed with which a particular chemical reaction progresses.
