Factors affecting rates
Collision theory The Activation Energy is defined as the minimum energy which particles need to collide to start a reaction 3.2.2. Reactions Rates reactants products Activation Energy: EA ∆H Energy Reactions can only occur when collisions take place between particles having sufficient energy. The energy is usually needed to break the relevant bonds in one or either of the reactant molecules. This minimum energy is called the Activation Energy Effect of Increasing Concentration and Increasing Pressure At higher concentrations(and pressures) there are more particles per unit volume and so the particles collide with a greater frequency and there will be a higher frequency of effective collisions. Measuring Reaction Rates The rate of reaction is defined as the change in concentration of a substance in unit time Its usual unit is mol dm-3s -1 When a graph of concentration of reactant is plotted vs time, the gradient of the curve is the rate of reaction. The initial rate is the rate at the start of the reaction where it is fastest Reaction rates can be calculated from graphs of concentration of reactants or products In the experiment between sodium thiosulphate and hydrochloric acid we usually measure reaction rate as 1/time where the time is the time taken for a cross placed underneath the reaction mixture to disappear due to the cloudiness of the Sulphur . Na2S2O3 + 2HCl 2NaCl + SO2 + S + H2O This is an approximation for rate of reaction as it does not include concentration. We can use this because we can assume the amount of Sulphur produced is fixed and constant. Initial rate = gradient of tangent time concentration Note: If a question mentions a doubling of concentration/rate then make sure you mention double the number of particles per unit volume and double the frequency of effective collisions. Comparing rate curves Need to calculate/ compare initial moles of reactants to distinguish between different finishing volumes. e.g. the amount of product is proportional to the moles of reactant Different volumes of the same initial concentrations will have the same initial rate (if other conditions are the same) but will end at different amounts The higher the concentration/ temperature/ surface area the faster the rate (steeper the gradient) Amount of product e.g. Volume of gas Effect of Catalysts Definition: Catalysts increase reaction rates without getting used up. Explanation: They do this by providing an alternative route or mechanism with a lower activation energy so more molecules have energy above activation energy Comparison of the activation energies for an uncatalysed reaction and for the same reaction with a catalyst present. reactants Activation Energy: uncatalysed ∆H Progress of Reaction EA catalysed products Catalysts speed up the rate of reaction. This means that the use of a catalyst may mean lower temperatures and pressures can be used. This can save energy costs as there is reduced energy demand for providing high temperature and less electrical pumping costs for producing pressure. This can mean fewer CO2 emissions from burning of fossil fuels, Benefits of Catalysts Catalysts can enable different reactions to be used, with better atom economy and with reduced waste, or fewer undesired products or less use of hazardous solvents and reactants. Catalysts are often enzymes, generating very specific products, and operating effectively close to room temperatures and pressures Heterogeneous catalysts are usually solids whereas the reactants are gaseous or in solution. The reaction occurs at the surface of the catalyst. Heterogeneous catalysis When catalysts and reactants are in the same phase, the reaction proceeds through an intermediate species. Homogeneous catalysis A heterogeneous catalyst is in a different phase from the reactants A homogeneous catalyst is in the same phase as the reactants
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3.2.2 Reaction rates
Simple collision theory (a) the effect of concentration, including the pressure of gases, on the rate of a reaction, in terms of frequency of collisions (b) calculation of reaction rate from the gradients of graphs measuring how a physical quantity changes with time M3.1, M3.2, M3.5 Suitable physical quantities to monitor could include concentration, gas volume, mass, etc. Catalysts (c) explanation of the role of a catalyst: (i) in increasing reaction rate without being used up by the overall reaction (ii) in allowing a reaction to proceed via a different route with lower activation energy, as shown by enthalpy profile diagrams Details of processes are not required. (d) (i) explanation of the terms homogeneous and heterogeneous catalysts (ii) explanation that catalysts have great economic importance and benefits for increased sustainability by lowering temperatures and reducing energy demand from combustion of fossil fuels with resulting reduction in CO2 emissions HSW9,10 Benefits to the environment of improved sustainability weighed against toxicity of some catalysts.
Experimental techniques
Techniques to investigate rates of reaction measurement of the change in volume of a gas Titrating samples of reaction mixture with acid, alkali, sodium thiosulphate etc Colorimetry. Measurement of change of mass Measuring change in electrical conductivity H2O2 (aq) + 2I- (aq) + 2H+ (aq) 2H2O(l) + I2 (aq) HCOOCH3 (aq) + NaOH(aq) HCOONa(aq) + CH3OH(aq) (CH3 )2C=CH2 (g) + HI(g) (CH3 )3CI(g) BrO3 – (aq) + 5Br – (aq) + 6H+ (aq) 3Br2 (aq) + 3H2O(l) HCOOH(aq) + Br2 (aq) 2H+ (aq) + 2Br – (aq) + CO2 (g) HCOOH(aq) + Br2 (aq) 2H+ (aq) + 2Br – (aq) + CO2 (g) There are several different methods for measuring reactions rates. Some reactions can be measured in several ways This works if there is a change in the number of moles of gas in the reaction. Using a gas syringe is a common way of following this. If drawing a gas syringe make sure you draw it with some measurement markings on the barrel to show measurements can be made. This works if there is a gas produced which is allowed to escape. Works better with heavy gases such as CO2 HCOOH(aq) + Br2 (aq) 2H+ (aq) + 2Br – (aq) + CO2 (g) CH3COCH3 (aq) + I2 (aq) → CH3COCH2 I(aq) + H+ (aq) + I– (aq) Small samples are removed from the reaction mixture, quenched (which stops the reaction) and the titrated with a suitable reagent. The NaOH could be titrated with an acid The H+ could be titrated with an alkali The I2 could be titrated with sodium thiosulphate If one of the reactants or products is coloured then colorimetry can be used to measure the change in colour of the reacting mixtures The I2 produced is a brown solution Can be used if there is a change in the number of ions in the reaction mixture
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3.2.2 Reaction rates
(e) the techniques and procedures used to investigate reaction rates including the measurement of mass, gas volumes and time PAG9 HSW4 Many opportunities to carry out experimental and investigative work.
Maxwell-Boltzman
Maxwell Boltzmann Distribution The Maxwell-Boltzmann energy distribution shows the spread of energies that molecules of a gas or liquid have at a particular temperature Learn this curve carefully The energy distribution should go through the origin because there are no molecules with no energy The energy distribution should never meet the x axis, as there is no maximum energy for molecules The mean energy of the particles is not at the peak of the curve The area under the curve represents the total number of A few have low particles present energies because collisions cause some particles to slow down Only a few particles have energy greater than the EA Most molecules have energies between the two extremes but the distribution is not symmetrical (normal) Q. How can a reaction go to completion if few particles have energy greater than Ea? A. Particles can gain energy through collisions Increasing Temperature As the temperature increases the distribution shifts towards having more molecules with higher energies The total area under the curve should remain constant because the total number of particles is constant At higher temperatures the molecules have a wider range of energies than at lower temperatures. At higher temps both the Emp and mean energy shift to high energy values although the number of molecules with those energies decrease Ea Collision energy Fraction of molecules with energy higher temperature Lower temperature Ea Collision energy Fraction of molecules with energy mean Collision energy T= 25OC T= 1067OC F.Effect of Increasing Surface area Increasing surface area will cause collisions to occur more frequently between the reactant particles and this increases the rate of the reaction. If the activation energy is lower, more particles will have energy > EA, so there will be a higher frequency of effective collisions. The reaction will be faster Effect of Increasing Temperature At higher temperatures the energy of the particles increases. They collide more frequently and more often with energy greater than the activation energy. More collisions result in a reaction As the temperature increases, the graph shows that a significantly bigger proportion of particles have energy greater than the activation energy, so the frequency of successful collisions increases higher temperature Lower temperature Ea Collision energy Fraction of molecules with energy Ea catalysed Fraction of molecules with energy Collision energy Ea un catalysed With a lower activation energy more particles have energy greater than the activation energy
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3.2.2 Reaction rates
The Boltzmann distribution (f) qualitative explanation of the Boltzmann distribution and its relationship with activation energy (see also 3.2.1 c) M3.1 (g) explanation, using Boltzmann distributions, of the qualitative effect on the proportion of molecules exceeding the activation energy and hence the reaction rate, for: (i) temperature changes (ii) catalytic behaviour (see also 3.2.2 c). M3.1 HSW1,2,5 Use of Boltzmann distribution model to explain effect on reaction rates.
Credits: Neil Goalby