Presentation on theme: "The Effect of Concentration on Rate"— Presentation transcript:
1 The Effect of Concentration on Rate Chemical Ideas 10.3The Effect of Concentration on Rate
2 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 distanceinflation = rate of change of pricesm/s%/yearwhen taking about rate you MUST be clear about units being used
3 rate at which products are converted to reactants rate of reaction
4 EACH SECOND!!! 0.0001 mol O2 formed 0.0002 mol H2O formed mol H2O2 used upEACH SECOND!!!
5 Measuring the rate of a reaction. measure the change in amount of a reactant or product in a certain timeDecide on a property of reactant or product that you can measure.Measure the change in property over a certain timeFind the ratechange of propertytime
7 concentration of H2O2 at start / mol dm3 Initial rate / (cm3 of O2(g))s-10.400.510.320.410.240.160.210.080.10
8 rate = constant x [H2O2(aq)] This graph shows us that rate is directly proportional to the concentration of hydrogen peroxiderate = constant x [H2O2(aq)]rate [H2O2(aq)]
9 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]
10 rate = k [H2O2(aq)] [catalase] This is the rate equation for the reactionthe constant k is called the rate constantk varies with temperature, therefore you must always state the temperature at which measurements are made.
11 Order of a Reaction For a reaction in which A & B are reactants … A + B productsThe general rate equation is…rate = k [A]m [B]nm 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
12 decomposition of hydrogen peroxide rate = k [H2O2(aq)] [catalase]The reaction is first order with respect to H2O2The 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
13 For the reaction 2Br (g) Br2 (g) Rate equation israte = k [Br]2S2O82-(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
14 rate = k [BrO3-] [Br-] [H+]2 BrO3-(aq) + 5Br-(aq) + 6H+ (aq) 5H2O (l) + 3Br2 (aq)rate = k [BrO3-] [Br-] [H+]2you cannot predict the rate equation for a reaction from it’s balanced equation
17 Chemical Ideas 10.3 (again) Knowing how concentration affects rate can tell us something about the way reactions occur.
18 Remember … rate = k [A]x [B]y [C]z The rate of any reaction can be expressed in terms of the concentrations of its reactantsrate = k [A]x [B]y [C]zx,y & z are the order of the reaction with respect to that reaction.If they =1 the number is not shown
19 decomposition of hydrogen peroxide rate = k [H2O2(aq)] [catalase]The reaction is first order with respect to H2O2The 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
20 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
21 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!
22 Finding the Order of a Reaction you cannot predict the rate equation for a reaction from it’s balanced equationTo 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.
23 Progress Curve MethodRate is calculated by drawing tangents to the curve at various pointsCan then find the order with respect to a reactant/producttedious & inaccurate (unless using a PC?)
24 Initial Rates Method – drawing tangents most usedSeveral 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
25 producing graphs first order zero order second order second order [A] raterate[A][A]second ordersecond orderraterate[A][A]2
26 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 smallRate low – reaction time large.Average rate 1/t .Graph of 1/t against concentration.
27 half-lives methodYou can use the progress curve to determine half-lives for the reaction.If they are constant then the reaction is first order.
28 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.
29 first order w.r.t. (CH3)3CBr +OH-first order w.r.t. (CH3)3CBrCH3zero order w.r.t. OH-CH3COH+Br-CH3rate = k[(CH3)3CBr]
30 slow Ξfast + + CH3 Br C CH3 C+ Br- CH3 C+ CH3 C OH OH- step one step twoOH-+Ξfast
31 mechanism of enzyme catalysed reactions … when the substrate concentration is low for the reactionrate = 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 thenrate = k [E]
32 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.