# 3.5 Kinetics

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• Created by: holly_b
• Created on: 04-01-17 09:40
• Kinetics
• Experimental Methods
• Iodine Clock
• When the reaction first begins the iodine that is formed reacts with sodium thiosulfate.
• Only when the thiosulfate is used up does the iodine produce a dark blue colour with starch.
• The time taken from mixing the reactants to the formation of the dark blue starch complex gives a measure of the initial rate of reaction.
• Initial rate is proportional 1/t
• Measure volume of gas formed over time.
• Measure change in mass over time.
• Measure concentration of acid by sampling at set times intervals.
• Some reactions cannot be easily monitored by methods throughout the entire chemical reaction so sampling and quenching may be used.
• Sampling
• A small amount of the sample is removed at regular intervals.
• If the catalyst is homogenous then it is sampled too so it needs to be quenched.
• Quenching
• The sample is placed into iced water to cool and dilute the mixture which stops the reaction.
• Pros
• Accurate
• Can be applied to a large number of chemical reactions
• Cons
• Time consuming
• Labour internsive.
• Homogenous mixture
• The Rate Equation
• Order of Reactions
• Order tells you how much the concentration of a reactant affects the rate.
• Individual order: the power to which a concentration is raised in the rate equation
• Overall order: the sum of all the individual orders on the rate equation, e.g. if m = 2 and n = 1, the overall order of reaction would be 2 + 1 = 3
• Graphical Determination
• For a reaction to occur reacting particles must collide with sufficient activation energy.
• Units of the Rate Constant
• Zero order: Unit =
• First order: Unit =
• Second order: Unit =
• Rate Determining Step
• Most reactions take place by a number of simple steps, the sequence being known as the mechanism. The reaction cannot proceed faster than the slowest of these steps.
• The Arrhenius Equation
• On the straight line graph of lnk (y) and 1/T (x), the gradient is -Ea/R and the y intercept is lnA.