ࡱ> JLIq` bjbjqPqP 86::hhhhhhh2222$VD!2^" $Sh hhh"hhhh |27:0!=!qF=!=!h4!$$|4"hhhhhh Summary of Topic 2.8 Kinetics 2.8a Factors influencing rate Temperature: An increase in temperature increases the rate of reaction. Concentration: An increase in the concentration of a solution of reactants increases the rate of reaction with respect to that reactant. Pressure: Increasing the pressure of a gas involved in a reaction increases the rate of reaction. Increasing a gas pressure is like increasing the concentration of the gas. Surface area of reactants: An increase in the surface area of a solid reactant increases rate of reaction. The surface area of a solid is increased if it is broken into smaller pieces. Catalyst: The use of a catalyst increases the rate of reaction. Task 2.8a Complete and extend the table below: variable (factor)Value (low/high)high or low ratecatalystpresentcatalystNot present 2.8b Explaining rates using collision theory Most reactions involve collisions between reacting particles. E.g see below.  HYPERLINK "http://monte.chem.ttu.edu/group/animations/image/h2br_hot.gif" http://monte.chem.ttu.edu/group/animations/image/h2br_hot.gif The collision theory is used to explain changes in reaction rate. In a reaction between 2 gaseous substances A and B, a molecule of A must collide with a molecule of B before reaction can occur. The number of collisions in a given time, the collision frequency, controls the rate of reaction. The greater the collision frequency the greater the rate of reaction. Not every collision leads to a reaction. A reaction takes place during a collision if the molecules hit at the correct angle or orientation and if they have enough energy. This amount of energy per mol of molecules is called the activation energy. The theory is extended to cover liquids and solids. Temperature Increasing the temperature increases the speed of the reacting particles and faster particles collide more often than slow ones. The increase in the number of collisions leads to an increase in the collision frequency and rate of reaction. Increasing the temperature also gives the particles more energy so that they collide with more violence. Energetic particles have a better chance of their collisions leading to a reaction. Concentration The concentration of a substance, normally a solution, is the amount in a given volume. concentration = amount {units = mol/dm3 or M} volume In a higher concentration solution there are more particles to react therefore there are more collisions and a higher collision frequency. As a reaction depends on collisions happening, a higher collision frequency leads to a faster reaction rate. If we were doing a reaction with acid and we double the number of acid particles, we double the number of collisions and therefore are likely to double the reaction rate. Pressure Increasing the pressure of a gas puts more gas molecules into a given volume. There will be more collisions and a higher collision frequency leading to a higher rate of reaction. Surface area Surface area is controlled by the particle size of a solid. A powder has a higher surface area than lumps and therefore a powder has more atoms or ions exposed on its surface in a position to react. More collisions take place between the ions or molecules in the surrounding liquid. The collision frequency in increased and so is the rate of reaction.See animations of particles when factors above change reaction rate.  HYPERLINK "http://www.southaxholme.doncaster.sch.uk/subjects/animations/showcase%20page%206/collision%20theory.swf" http://www.southaxholme.doncaster.sch.uk/subjects/animations/showcase%20page%206/collision%20theory.swf Task 2.3b Draw diagrams of particles to show slow and fast reactions caused by changing the factors above. 2.8c The Maxwell-Boltzmann model  INCLUDEPICTURE "http://www.drbateman.net/asa2sums/sum2.3/maxbol1.gif" \* MERGEFORMATINET  As the temperature is raised the average energy of the molecules increases. The proportion of molecules with the activation energy (see section under graph) is greater at higher temperatures. A small increase in temperature gives a large increase in reaction rate.  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