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Chemical Ideas 10.3 The Effect of Concentration on Rate

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rate Listen: [ rāt ] n.Listen: A quantity measured with respect to another measured quantity. speed = rate of change of distance inflation = rate of change of prices when taking about rate you MUST be clear about units being used m/s %/year

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rate of reaction rate at which products are converted to reactants

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mol O 2 formed mol H 2 O formed mol H 2 O 2 used up EACH SECOND!!!

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Measuring the rate of a reaction. 1.Decide on a property of reactant or product that you can measure. 2.Measure the change in property over a certain time 3.Find the rate measure the change in amount of a reactant or product in a certain time change of property time

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concentration of H 2 O 2 at start / mol dm 3 Initial rate / (cm 3 of O 2 (g))s

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This graph shows us that rate is directly proportional to the concentration of hydrogen peroxide rate [H 2 O 2(aq) ] rate = constant x [H 2 O 2(aq) ]

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The concentration of the enzyme catalase also affects the rate of the reaction … rate = constant x [catalase] We can combine the two equations to get … rate = constant x [H 2 O 2(aq) ] x [catalase] rate = k [H 2 O 2(aq) ] [catalase]

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This is the rate equation for the reaction the constant k is called the rate constant k varies with temperature, therefore you must always state the temperature at which measurements are made. rate = k [H 2 O 2(aq) ] [catalase]

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Order of a Reaction For a reaction in which A & B are reactants … –A + B products The general rate equation is… – rate = k [A] m [B] n m and n are powers to which the concentration must be raised. usually have values of 0, 1 or 2. m & n are called the order of the reaction

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decomposition of hydrogen peroxide rate = k [H 2 O 2(aq) ] [catalase] The reaction is first order with respect to H 2 O 2 The reaction is also first order with respect to catalase. The overall order of a reaction is given by (m + n). the reaction is overall second order

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For the reaction 2Br (g) Br 2 (g) Rate equation is –rate = k [Br] 2 S 2 O 8 2- (aq) + 2I - (aq) SO 4 2- (aq) + I 2 (aq) –rate = k [S 2 O 8 2- (aq) ] [I - (aq) ] cannot you cannot predict the rate equation for a reaction from it’s balanced equation

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BrO 3 - (aq) + 5Br - (aq) + 6H + (aq) 5H 2 O (l) + 3Br 2 (aq) rate = k [BrO 3 - ] [Br - ] [H + ] 2 cannot you cannot predict the rate equation for a reaction from it’s balanced equation

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Half Lifes

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Chemical Ideas 10.3 (again) Knowing how concentration affects rate can tell us something about the way reactions occur.

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Remember … The rate of any reaction can be expressed in terms of the concentrations of its reactants rate = k [A] x [B] y [C] z x,y & z are the order of the reaction with respect to that reaction. If they =1 the number is not shown

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decomposition of hydrogen peroxide rate = k [H 2 O 2(aq) ] [catalase] The reaction is first order with respect to H 2 O 2 The reaction is also first order with respect to catalase. The overall order of a reaction is given by (m + n). the reaction is overall second order

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half-lives (t ½ ) Reactions which are first order will show a curve that is identical to radioactive decay! Time taken for half of a reactant to get used up in the reaction

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For a first order reaction the half- life is always constant no matter what the starting amount! zero order & second order reactions do not have this feature

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Finding the Order of a Reaction To find out the order of a reaction it is necessary to carry out practical experiments. The data can then be used to determine the order of the reaction. cannot you cannot predict the rate equation for a reaction from it’s balanced equation

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Progress Curve Method Rate is calculated by drawing tangents to the curve at various points Can then find the order with respect to a reactant/product tedious & inaccurate (unless using a PC?)

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Initial Rates Method – drawing tangents most used Several experimental runs are completed (as in activity EP6.3). Initial rate is calculated by drawing tangents at the origin. We then plot initial rate against concentration

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producing graphs [A] [A] 2 [A] rate zero order first order second order

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Initial Rates Method – reciprocal of time Measuring how long to produce a small fixed amount of one of the products. Time taken is called the reaction time. Rate is high – reaction time small Rate low – reaction time large. Average rate 1 / t. Graph of 1 / t against concentration.

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half-lives method You can use the progress curve to determine half-lives for the reaction. If they are constant then the reaction is first order.

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rate equations & rate mechanisms when we know the rate equation we can link it to the reaction mechanism. We can then work out the rate determining step.

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CH 3 BrC + OH - CH 3 Br - COH + first order w.r.t. (CH 3 ) 3 CBr zero order w.r.t. OH - rate = k[(CH 3 ) 3 CBr]

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CH 3 BrC + OH - CH 3 COH + step one CH 3 C+C+ Br - CH 3 C+C+ step two Ξfast slow

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mechanism of enzyme catalysed reactions … when the substrate concentration is low for the reaction –rate = k[E][S] ([E] is concentration of enzyme) we can deduce from this that the rate determining step involves one enzyme molecule & one substrate molecule. Following steps are faster. Substrate concentration high then –rate = k [E]

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WHY? – why are some steps slow & others fast? One reason = different steps have different activation enthalpies. –Large activation enthalpy, only a small number of molecules pass over it each second so rate of reaction is slow. –Small activation enthalpy, greater proportion of molecules can pass each second, hence a faster rate.

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