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**The Effect of Concentration on Rate**

Chemical Ideas 10.3 The Effect of Concentration on Rate

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**A quantity measured with respect to another measured quantity.**

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

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**rate at which products are converted to reactants**

rate of reaction

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**EACH SECOND!!! 0.0001 mol O2 formed 0.0002 mol H2O formed**

mol H2O2 used up EACH SECOND!!!

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**Measuring the rate of a reaction.**

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

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**concentration of H2O2 at start / mol dm3 **

Initial rate / (cm3 of O2(g))s-1 0.40 0.51 0.32 0.41 0.24 0.16 0.21 0.08 0.10

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**rate = constant x [H2O2(aq)]**

This graph shows us that rate is directly proportional to the concentration of hydrogen peroxide rate = constant x [H2O2(aq)] rate [H2O2(aq)]

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**rate = k [H2O2(aq)] [catalase]**

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 [H2O2(aq)] x [catalase] rate = k [H2O2(aq)] [catalase]

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**rate = k [H2O2(aq)] [catalase]**

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.

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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 [H2O2(aq)] [catalase] The reaction is first order with respect to H2O2 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) Br2 (g)**

Rate equation is rate = k [Br]2 S2O82-(aq) + 2I- (aq) SO42- (aq) + I2 (aq) rate = k [S2O82-(aq) ] [I- (aq) ] you cannot predict the rate equation for a reaction from it’s balanced equation

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**rate = k [BrO3-] [Br-] [H+]2**

BrO3-(aq) + 5Br-(aq) + 6H+ (aq) 5H2O (l) + 3Br2 (aq) rate = k [BrO3-] [Br-] [H+]2 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 … rate = k [A]x [B]y [C]z**

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 [H2O2(aq)] [catalase] The reaction is first order with respect to H2O2 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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**Time taken for half of a reactant to get used up in the reaction**

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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**zero order & second order reactions do not have this feature**

For a first order reaction the half-life is always constant no matter what the starting amount!

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**Finding the Order of a Reaction**

you cannot predict the rate equation for a reaction from it’s balanced equation 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.

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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 first order zero order second order second order [A]**

rate rate [A] [A] second order second order rate rate [A] [A]2

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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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**first order w.r.t. (CH3)3CBr**

+ OH- first order w.r.t. (CH3)3CBr CH3 zero order w.r.t. OH- CH3 C OH + Br- CH3 rate = k[(CH3)3CBr]

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**slow Ξfast + + CH3 Br C CH3 C+ Br- CH3 C+ CH3 C OH OH- step one**

step two OH- + Ξfast

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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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