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Boardworks AS Chemistry Kinetics

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Presentation on theme: "Boardworks AS Chemistry Kinetics"— Presentation transcript:

1 Boardworks AS Chemistry Kinetics

2 Boardworks AS Chemistry Kinetics
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

3 What is collision theory?
Boardworks AS Chemistry Kinetics Collision theory states that for a reaction to occur: particles must collide particles must have sufficient energy particles must collide with the correct orientation. Teacher notes It could be pointed out that the term ‘particles’ can refer to atoms, molecules or ions. The third point (collision with the correct orientation) applies only to molecules and molecular ions, since atoms are symmetrical. Most collisions do not result in reaction because they do not meet the second and third criteria.

4 Boardworks AS Chemistry Kinetics
Activation energy Boardworks AS Chemistry Kinetics Teacher notes See the ‘Energetics’ presentation for more information about energy and bond breaking.

5 Effect of surface area on collisions
Boardworks AS Chemistry Kinetics Only the particles on the surface of a solid will collide with particles of the other reactant. If the surface area is increased, more particles will be on the surface and able to collide with particles of the other reactant. This means that there will be more collisions in total and therefore more reactive collisions. Surface area can be increased by decreasing the size of the reactant particles. Powders have a very large surface area. increasing surface area

6 Reaction of marble chips with acid
Boardworks AS Chemistry Kinetics The effect of changing surface area on the rate of reaction can be explored by reacting marble (calcium carbonate) chips and an acid such as 2 mol dm-3 hydrochloric acid. CaCO3(s) + 2HCl(aq) ® CaCl2(s) + CO2(g) + H2O(l) The carbon dioxide gas evolved can be collected and its volume measured over time. The rate at which it is produced is a measure of the rate of reaction. Teacher notes Students could be asked to produce a plan of how to run this experiment. It could be highlighted that measuring the rate at which a product is evolved is a common way of measuring rate of reaction. The rate at which gas is evolved is the gradient of a graph of volume evolved against time. An alternative way to measure rate of reaction is to measure the rate at which a reactant is consumed. This is often more difficult and so is a less common method. By repeating the experiment with marble chips of different sizes, the effect of surface area on the rate of reaction can be examined.

7 Effect of surface area on rate: graph
Boardworks AS Chemistry Kinetics

8 Effect of concentration on rate
Boardworks AS Chemistry Kinetics Teacher notes The animation will run continuously for 90 seconds unless paused. Students should be encouraged to note that for a higher concentration of reactants (the box on the right), there are more collisions in total and therefore more reactive collisions. A higher number of particles per unit volume increases the probability of collisions, and therefore their frequency. However, the ratio of reactive/total collisions (or the proportion of collisions that are successful) is not affected by concentration, being approximately 1:3 in both cases.

9 Effect of concentration on rate: graph
Boardworks AS Chemistry Kinetics

10 Effect of gas pressure on rate
Boardworks AS Chemistry Kinetics Teacher notes The animation will run continuously for 30 seconds unless paused. Students should be encouraged to note that for higher pressure (the box on the right), there are initially more collisions between the two types of reactant and therefore a faster initial rate of reactive collisions. In particular, students should be encouraged to note the rate at which the reactions occur in each case, how this rate changes as time progresses, and relate their observations to the changing proportions of reactants and product. A higher number of particles per unit volume increases the probability of collisions, and therefore their frequency. However, the ratio of reactive/total collisions (or the proportion of collisions that are successful) is not affected by pressure, being approximately 1:3 in both cases. This simulation involves reducing the volume in order to increase the pressure. Another way to increase the pressure while keeping the volume constant would be to introduce more particles into the vessel. This would also increase collision frequency and is another way of looking at the same explanation. It could be pointed out that increasing the pressure in a gas phase reaction is like increasing the concentration of reacting solutions. Both increase the number of particles in a given volume and thereby increase the collision frequency.

11 Effect of pressure on rate: graph
Boardworks AS Chemistry Kinetics

12 Effect of temperature on particles
Boardworks AS Chemistry Kinetics Teacher notes Students should be encouraged to notice that increasing the temperature increases both the average particle speed and the number of collisions per second. They should be able to relate the increase in speed to the increase in collisions. It could also be pointed out that at constant temperature, the collisions per second and particle speed do not remain exactly constant.

13 Effect of temperature on rate
Boardworks AS Chemistry Kinetics Increasing the temperature of the reaction mixture increases the rate of reaction in the following two ways: 1. At higher temperatures, the particles are moving faster, so collide more frequently. A higher number of collisions in total means a higher number of successful collisions. 2. At higher temperatures, a higher proportion of the molecules have the activation energy or more. This means that a higher proportion of collisions is successful.

14 Effect of temperature on rate: graph
Boardworks AS Chemistry Kinetics

15 Collision theory summary
Boardworks AS Chemistry Kinetics

16 Rate of reaction summary
Boardworks AS Chemistry Kinetics

17 Boardworks AS Chemistry Kinetics
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

18 Distribution of particle speeds
Boardworks AS Chemistry Kinetics Teacher notes The Maxwell–Boltzmann distribution is a theoretical model developed from kinetic theory. Ludwig Eduard Boltzmann (1844–1906) was an Austrian physicist. James Clerk Maxwell (1831–1879) was a Scottish mathematician and theoretical physicist. Maxwell used statistical methods to derive the Maxwell distribution, which gives the distribution of velocities of particles in a gas at a given temperature. This was later generalized by Boltzmann to give a distribution function for the distribution of states of a system.

19 Energy distribution curves
Boardworks AS Chemistry Kinetics

20 The effect of changing temperature
Boardworks AS Chemistry Kinetics

21 Small temperature changes
Boardworks AS Chemistry Kinetics The Maxwell–Boltzmann distribution shows that for a small increase in temperature, there is a relatively large increase in the number of particles with at least the activation energy. no. of particles energy Ea no. particles with Ea almost doubled A small increase in temperature therefore leads to a large increase in rate. The increase in collision frequency is also a factor, but its effect is small compared to the increase in energy.

22 Effect of temperature summary
Boardworks AS Chemistry Kinetics

23 Boardworks AS Chemistry Kinetics
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

24 Effect of catalysts on rate: graph
Boardworks AS Chemistry Kinetics

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What do catalysts do? Boardworks AS Chemistry Kinetics

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How do catalysts work? Boardworks AS Chemistry Kinetics Catalysts increase the rate of reactions without being used up during the reaction. One way in which this occurs is for the catalyst to be changed during the reaction, then changed back in a second reaction with one of the reactants or products. This is an alternative reaction pathway. An example is the oxidation of sulfur dioxide: SO2(g) + ½O2(g) ® SO3(g) Teacher notes It could be pointed out that an advantage of the catalyst not being consumed during the reaction is that only a small amount of catalyst is needed. This is an economic advantage, particularly since catalysts are often expensive materials. For example the catalyst in catalytic converters in cars is platinum, palladium or rhodium, all of which are expensive metals. See the ‘Alkanes’ presentation for more information about catalytic converters. This is catalyzed by vanadium(V) oxide: SO2(g) + V2O5(s) ® SO3(g) + V2O4(s) The catalyst is re-formed by reacting with oxygen: V2O4(s) + ½O2(g) ® V2O5(s)

27 Catalysts and energy distribution curves
Boardworks AS Chemistry Kinetics

28 Heterogeneous catalysts
Boardworks AS Chemistry Kinetics There are two types of catalysts: heterogeneous and homogeneous. Heterogeneous catalysts are in a different phase to the reactants. The catalyst is usually a solid and the reactants are liquids or gases (e.g. solid catalysts for gas reactions in catalytic converters). Photo credit: Astrid & Hanns-Frieder Michler / Science Photo Library A catalytic converter, a device which reduces the toxic emissions from an internal combustion engine by converting the harmful exhaust by-products into relatively harmless ones. Oxidation catalysts convert hydrocarbons to carbon dioxide and water. Three-way catalysts convert oxides of nitrogen back into nitrogen. A 90 percent reduction of pollution emission has been achieved without loss of engine performance or fuel economy. The addition of a catalytic converter to a car necessitates the use of unleaded fuel since the lead compounds added to regular petrol as anti- knocking agents can 'poison' the catalyst. Teacher notes See the ‘Alkanes’ presentation for more information about catalytic converters. See the ‘Alkenes’ presentation for more information about the production of poly(e)thene. Industrial examples of heterogeneous catalysis include the iron catalyst used in ammonia production and the Ziegler–Natta catalyst used in poly(e)thene production.

29 Homogeneous catalysts
Boardworks AS Chemistry Kinetics Homogeneous catalysts are in the same phase as the reactants. The catalyst and the reactants are usually liquids, such as the hardener added to fibreglass resin. Photo credit: Martyn F. Chillmaid / Science Photo Library Catalyst being added to a beaker of fibreglass resin. The resin (poured from the tin at left) is liquid, and needs to have this catalyst added to it to make it harden. The hardener contains free radicals that initiate the setting process, triggering a cascade of bond formation that turns the liquid resin into a hard, glassy solid. Fibreglass is a fibrous form of glass that is tougher than ordinary glass. The resin is Fastglas from the David's company. The hardener is benzoyl peroxide (BPO). See A500/684 for another example of a catalytic hardening process. Another example of homogeneous catalysis is the destruction of atmospheric ozone catalyzed by chlorine free radicals. In this reaction the catalyst and reactants are in the gas phase.

30 Advantages of catalysts
Boardworks AS Chemistry Kinetics Using a catalyst means that a reaction can take place at the same rate as the uncatalyzed reaction, but at a lower temperature and/or pressure. This has the following advantages, which are particularly important in industry: lower energy demands… …therefore less CO2 produced… …therefore less environmental impact… …and lower production costs. Teacher notes Biological washing powders will only allow efficient washing at lower temperatures than non-bio if the enzymes present are active enough at the temperature used. Most biological washing powders will not work well below about 30°C. One potential problem with catalysts is safe disposal, because some catalysts are toxic. A non-industrial example is enzyme catalysis in biological washing powders, allowing efficient washing at a lower temperature.

31 Catalysts: true or false?
Boardworks AS Chemistry Kinetics

32 Boardworks AS Chemistry Kinetics
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

33 Boardworks AS Chemistry Kinetics
Glossary Boardworks AS Chemistry Kinetics Teacher notes activation energy – The minimum amount of energy particles need to have if a collision between them is to result in a reaction. catalyst – A substance that speeds up a chemical reaction without itself being consumed by the reaction. It works by providing an alternative route by which the reaction can take place, which has a lower activation energy. collision theory – A theory stating that for a reaction to occur the particles must collide, have sufficient energy (at least the activation energy) and collide with the correct orientation. energy distribution curve – A graph of energy against the number of particles in a sample with those energies. Also called a Maxwell–Boltzmann curve, the energy distribution curve is a representation of the Maxwell–Boltzmann distribution. enthalpy level diagram – A diagram showing the amount of energy contained in the reaction mixture over time, beginning with the enthalpy of the reactants and ending with the enthalpy of the products. Also called an enthalpy profile diagram. heterogeneous catalyst – A catalyst in a different phase to the reactants. homogeneous catalyst – A catalyst in the same phase as the reactants. kinetic energy – The energy an object possesses due to its motion. Maxwell–Boltzmann distribution – The characteristic distribution of the energies of particles. Often plotted as a graph, called an energy distribution curve or Maxwell–Boltzmann curve, of energy against the number of particles in a sample with those energies. phase – A region of a physical system with uniform chemical composition or physical properties. For example, 'the gas phase' indicates a region composed only of gas. rate of reaction – The rate at which reactants are consumed or products are formed in a reaction. transition state – A state through which a reaction passes between the reactants and the products. It is the state that has the highest energy, represented by the highest point on an enthalpy profile diagram or enthalpy level diagram.

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What’s the keyword? Boardworks AS Chemistry Kinetics

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Multiple-choice quiz Boardworks AS Chemistry Kinetics


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