Chemical Ideas 10.3 - University of York

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  • 1.Chemical Ideas 10.3 The Effect of Concentration on Rate
  • 2.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 when taking about rate you MUST be clear about units being used m/s %/year
  • 3.rate of reaction rate at which products are converted to reactants
  • 4.0.0001 mol O2 formed 0.0002 mol H2O formed 0.0002 mol H2O2 used up EACH SECOND!!!
  • 5.Measuring the rate of a reaction. Decide on a property of reactant or product that you can measure. Measure the change in property over a certain time 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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  • 8.This graph shows us that rate is directly proportional to the concentration of hydrogen peroxide rate  [H2O2(aq)] rate = constant x [H2O2(aq)]
  • 9.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.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 [H2O2(aq)] [catalase]
  • 11.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
  • 12.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
  • 13.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
  • 14. 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
  • 15.Half Lifes
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  • 17.Chemical Ideas 10.3 (again) Knowing how concentration affects rate can tell us something about the way reactions occur.
  • 18.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
  • 19.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
  • 20.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.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
  • 22.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. you cannot predict the rate equation for a reaction from it’s balanced equation
  • 23.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?)
  • 24.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
  • 25.producing graphs [A] [A] [A]2 [A] rate rate rate rate zero order first order second order second order
  • 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 small Rate low – reaction time large. Average rate  1/t . Graph of 1/t against concentration.
  • 27.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.
  • 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.CH3 CH3 CH3 Br C + OH- CH3 CH3 CH3 Br- C OH + first order w.r.t. (CH3)3CBr zero order w.r.t. OH- rate = k[(CH3)3CBr]
  • 30.CH3 CH3 CH3 Br C + OH- CH3 CH3 CH3 C OH + step one CH3 CH3 CH3 C+ Br- CH3 CH3 CH3 C+ step two Ξfast slow
  • 31.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]
  • 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.