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RATES OF REACTION Increasing the rates of chemical reactions is important in industry because it helps to reduce costs. The rate of reaction is the speed at which a chemical change takes place. It is followed by measuring the rate at which reactants are used up or the rate at which products are formed. This allows a comparison to be made of the changing rate of a chemical reaction under different conditions. Rate = amount of reactant used time or Rate = amount of product formed time Chemical reactions can occur only when reacting particles collide with each other with sufficient energy. The minimum amount of energy which particles must have to react is called the activation energy. Reactions are fastest at the start because this is when there are the most particles. As a reaction proceeds, reactants are used up and the rate decreases. Concentration of reactant Time Since the rate of a reaction is a measure of how quickly reactants are being used up, it follows that we can determine the rate of a reaction by measuring the gradient of the above graph. TOPIC 10: SPEED OF REACTION 1 Various factors alter the rate of a reaction: the state of division of the reactants (i.e. the particle size of a solid): the smaller the particle size, the faster the reaction is. the concentration of dissolved reactants or the pressure of gases: the higher the concentration or the pressure, the faster the reaction is. the temperature of the reaction mixture: the higher the temperature, the faster the reaction is. the addition of a catalyst speeds up the reaction 1. The Particle Size of Solid Reactants The reaction used to study the effect of particle size is the reaction of calcium carbonate, in the form of marble, with dilute hydrochloric acid. CaCO3(s) + 2HCl(aq) CaCl2(aq) + H2O(l) + CO2(g) The reaction is followed by the change in mass of the reaction flask with time as carbon dioxide is given off. It would also be possible to measure the change in the volume of carbon dioxide given off with time by collecting the gas in, for example, a gas syringe. Method: A constant mass of marble chips was weighed out and placed into a 250cm 3 conical flask. 100cm3 of 1M hydrochloric acid was added from a measuring cylinder. The flask was loosely stoppered with cotton wool (to allow the gas to escape but to prevent the loss of liquid splashes) then placed onto an electronic balance. The mass of the conical flask was recorded every 15s for the first minute, then every 30s for a total of ten minutes. It is assumed that the temperature of the reaction mixture stayed constant. The experiment was repeated with three different sizes of marble chips, keeping all other variables the same. The results were tabulated and a graph of mass of carbon dioxide (y-axis) against time (x-axis) was plotted. TOPIC 10: SPEED OF REACTION 2 Results: Large Particles Time /s Mass of flask/g Mass of CO2 /g Medium Particles Mass of flask /g 0 15 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 570 600 TOPIC 10: SPEED OF REACTION 3 Mass of CO2 /g Small Particles Mass of flask /g Mass of CO2 /g Interpretation of Results: The reaction occurs only on the surface of the calcium carbonate. For a given mass of calcium carbonate, the smaller the size of the particles, the greater is the surface area and the faster the reaction. This is shown on the graph by the gradient becoming steeper as the particle size decreases. The smaller the particle size (the bigger the surface area) , the faster the reaction Since the same quantities of reactants are involved in all three reactions, the same mass of carbon dioxide is given off in all of them, if the reaction is allowed to go to completion. This is shown by all three curves levelling off at the same total mass of carbon dioxide. The calcium carbonate is in excess, so the hydrochloric acid is used up completely as the reaction takes place. Since its concentration decreases with time, the reaction becomes slower and slower. This shown on the graph by a curve of steadily decreasing gradient. The expected mass of carbon dioxide can be calculated. Since the calcium carbonate is in excess, the mass of carbon dioxide depends on the amount of hydrochloric acid used. 100cm3 of 1M HCl contains 0.1mole of HCl. CaCO3(s) + 2HCl(aq) 2 moles 0.1 mole 0.1 mole 0.1 mole TOPIC 10: SPEED OF REACTION CaCl2(aq) + H2O(l) + CO2(g) 1 mole 0.05 mole 0.05 x 44 g 2.2g 4 2. The Temperature The reaction used to study the effect of temperature is the reaction of sodium thiosulphate solution with dilute hydrochloric acid. Na2S2O3(aq) + 2HCl(aq) 2NaCl(aq) + S(s) + H2O(l) + SO2(g) The reaction is followed by the appearance of colloidal sulphur as the reaction proceeds. The formation of sulphur begins as soon as the reactants are mixed, but it takes time for observable amounts to be produced. The time taken to reach a particular point in the reaction can be determined by standing the reaction flask on a piece of paper marked with a feint cross and timing how long it takes for the cross to be obscured when looked at from above. Method: 10cm3 of sodium thiosulphate stock solution (concentration 40g per litre) were measured out into a measuring cylinder and poured into a 250cm 3 conical flask. 40cm3 of water were measured out in a similar way and added to the flask. The flask was heated gently over a Bunsen burner to a temperature slightly above the desired temperature. The flask was placed on a piece of paper marked with a feint cross. 5cm3 of 2M hydrochloric acid were measured out into a second measuring cylinder. As the acid was poured into the conical flask, a stopwatch was started and the mixture gently swirled. The initial temperature was recorded. The stopwatch was stopped when the cross, viewed from above, became obscured. The time and the final temperature were recorded. The mean of the initial and final temperatures was taken as the temperature of the reaction. The experiment was repeated for five different temperatures, keeping all other variables constant. The results were tabulated and from the results a graph of 1000/ time (y-axis) against temperature (x-axis) was plotted. (1000/ time is a measure of the rate) TOPIC 10: SPEED OF REACTION 5 Results: Initial temp. /oC Final temp. /oC Mean temp. /oC 1000/time /s- Time /s 1 Interpretation of Results: The graph is an exponential curve: the rate of reaction increases rapidly with temperature, a rise in temperature of 10oC approximately doubling the rate. The higher the temperature, the faster the reaction In the reaction between thiosulphate ions and hydrogen ions, as the ions collide covalent bonds are broken and new bonds are formed. Since energy is needed to break bonds, the colliding particles must have a minimum energy on collision sufficient to break the bonds. This energy is known as the activation energy (Ea). bonds breaking bonds forming Ea Chemical Energy reactants H products Only those collisions with energy greater than or equal to the activation energy will result in a reaction. Reaction path An increase in temperature increases the rate of reaction in two ways: TOPIC 10: SPEED OF REACTION 6 The particles collide more energetically The particles of a substance have a range of different energies, the average energy being proportional to the temperature in Kelvins. As the temperature increases, the particles move faster (i.e. they have more kinetic energy), and the proportion of particles with higher energies increases. Therefore, the number of collisions with energy greater than or equal to the activation energy rises rapidly as the temperature increases, and so the rate rises rapidly. This is the major effect. The particles collide more frequently The particles move around faster and therefore there is a greater chance that they will be involved in a collision. This is a minor effect. 3. The Concentration of Reactants The reaction used to study the effect of concentration is the reaction of sodium thiosulphate solution with dilute hydrochloric acid. Na2S2O3(aq) + 2HCl(aq) 2NaCl(aq) + S(s) + H2O(l) + SO2(g) The reaction is followed by the appearance of colloidal sulphur as the reaction proceeds. The formation of sulphur begins as soon as the reactants are mixed, but it takes time for observable amounts to be produced. The time taken to reach a particular point in the reaction can be determined by standing the reaction flask on a piece of paper marked with a feint cross and timing how long it takes for the cross to be obscured when looked at from above. Method: 50cm3 of sodium thiosulphate stock solution (concentration 40g per litre) were measured out into a measuring cylinder and poured into a 250cm 3 conical flask. The flask was placed on a piece of paper marked with a feint cross. 5cm 3 of 2M hydrochloric acid were measured out into a second measuring cylinder. As the acid was poured into the conical flask, a stopwatch was started and the mixture gently swirled. The stopwatch was stopped when the cross, viewed from above, became obscured. The time was recorded. TOPIC 10: SPEED OF REACTION 7 Volume of Na2S2O3 (40g/l) /cm3 50 40 30 20 10 Volume of H2O /cm3 0 Time /s 1000/ time /s-1 10 20 30 40 The experiment was repeated for five different concentrations, keeping all other variables constant. A graph of 1000 /time (y-axis) against volume of Na2S2O3 (x-axis) was plotted. Since the total volume of the reaction mixture is constant (at 55cm 3) the concentration of Na2S2O3 is proportional to its volume. Results: Constants: total volume of Na2S2O3 solution 2M hydrochloric acid temperature conical flask & cross TOPIC 10: SPEED OF REACTION 8 50cm3 5cm3 20oC x 30 x 20 x 1000/ time /s-1 x 10 x 0 10 20 30 Volume of Na2S2O3 /cm3 40 50 Interpretation of Results: Your graph should be a straight line through the origin, therefore the rate of the reaction is directly proportional to the concentration of sodium thiosulphate. The more concentrated the solution, the faster the reaction To react, the reacting particles must collide; therefore the rate will be faster the greater the number of collisions there are in a given volume in a given time. The more concentrated the solution is, the greater the number of particles there are in a given volume and therefore the greater the frequency of collisions. It is important to note that only a very small proportion of the total number of collisions is successful and leads to a reaction. TOPIC 10: SPEED OF REACTION 9 Effect of Pressure Pressure is important only in reactions involving gases. Pressure affects gaseous reactions in the same way that the concentration affects reactions in solution. As the pressure is increased, the greater the number of particles there are in a given volume and therefore the greater the number off collisions in a given time. Therefore, as the pressure increases, the rate increases. 4. Addition of a Catalyst The reaction used to study the effect of a catalyst is the decomposition of hydrogen peroxide: 2H2O2 2H2O + O2 The reaction is catalysed by several metal oxides; the compound used here is manganese(IV) oxide. The reaction is followed by collecting the oxygen given off and measuring its volume at regular intervals of time. The gas may be collected either in a gas syringe or over water in a burette. A graph of volume of oxygen (y-axis) against time (x-axis) is plotted. A catalyst is a substance which increases the rate of a chemical reaction but is not used up in the reaction. A catalyst works by providing an alternative reaction route which has a lower activation energy. TOPIC 10: SPEED OF REACTION 10 Interpretation of Results: The reaction is faster the more catalyst there is present; this is shown by the increasing steepness of the curves. The reaction takes place on the surface of the catalyst. Increasing the mass of catalyst increases the surface area and therefore speeds up the rate. A catalyst does not affect the outcome of a reaction; the same product is formed but in a shorter time. A catalyst works by weakening bonds, which lowers the activation energy for the reaction. If the activation energy is lowered, more particles have enough energy to react and therefore the reaction goes faster. Ea Chemical Energy Ea cat reactants H products A catalyst is not used up in the reaction but is recovered unchanged at the end. Reaction path A catalyst can be used over and over again to speed up the conversion of reactants to products. Different reactions need different catalysts. Biological catalysts Enzymes are large molecules that speed up the chemical reactions inside cells. Enzymes are a type of protein and, like all proteins, they are made from long chains of different amino acids. They catalyse reactions such as fermentation, respiration and photosynthesis and also speed up the breakdown of fats and other food stains in biological washing powders. Each enzyme will only speed up one reaction as the shape of the enzyme molecule needs to match the shape of the molecule it reacts with (the substrate molecule). The part of the enzyme molecule that matches the substrate is called the active site. If the shape of the enzyme changes, it’s active site may no longer work. We say the enzyme has been denatured. They can be denatured by high temperatures or extremes of pH. Note that it is wrong to say the enzyme has been killed. Although enzymes are made by living things, they are proteins, and not alive. TOPIC 10: SPEED OF REACTION 11 Photochemical reactions Photochemical reactions get the energy the need from light. The more intense the light, the faster the reaction goes. Two examples of photochemical reactions are photosynthesis and the reaction in film photography. Photosynthesis turns carbon dioxide and water into sugar and oxygen. It is catalysed by chlorophyll. You have probably done an experiment in Biology lessons to show the effect of changing light intensity on the rate of this reaction. In photography, light falls on a film coated with silver bromide. When light is exposed to the film, the silver ions break down (reduced) to silver metal. The more light that hits the film, the more silver ions are reduced to silver. When the film is developed any unreacted silver bromide is removed leaving silver behind on the film. So the areas exposed to most light have the most silver and appear darkest. This is what we call the ‘negative’. TOPIC 10: SPEED OF REACTION 12 Topic 10: The speed of a reaction Summary questions 1 2 TOPIC 10: SPEED OF REACTION 13 3 4 TOPIC 10: SPEED OF REACTION 14 5 6 TOPIC 10: SPEED OF REACTION 15 TOPIC 10: SPEED OF REACTION 16 7 TOPIC 10: SPEED OF REACTION 17