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Determination of reaction order.

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Order reaction , s.j.shah
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Determination of reaction order.

  1. 1. Order of reaction •Order of a reaction refers to the power dependence of the rate of concentration of each reactant. •Order is an experimentally determined parameter. •For first order reaction , rate is dependant on the concentration of a single species.
  2. 2. Determination of reaction order ❖Exact order of a reaction can be determined experimentally. ❖The methods to determine reaction order are: •Graphical Method •Substitution Method •Half life Method •Van’t Hoff differential Method •Ostwald’s isolation Method
  3. 3. Graphical Method ▪In presence of single reactant. ▪Rate versus concentration graph: •‘Log(a-x)’ vs ‘t’ : straight line = first order reaction. •‘1/(a-x)’ vs ‘t’ : straight line = second order reaction. •‘1/(a-x)^2’ vs ‘t’ : straight line = third order reaction.
  4. 4. Substitution Method •Hit and trial method. •Integrated rate law method. •Concentration and time will be given. •The value of k is determined . •If the value of k is constant ,the used equation gives the order of reaction.
  5. 5. Half-life method • Only used when the rate law involve one concentration term. • t(1/2) ∞ a1-n t(1/2) = k’a1-n log t(1/2) = log k’ + (1-n)loga • Graph can be drawn where the slope is (1-n) , n = order of reaction. • If different concentrations are given the ratio of half life is taken. , • By taking logs and rearranging,
  6. 6. Van’t Hoff differential method •The rate of a reaction varies as the ‘n’th power of the concentration of the reactant , where, n= reaction order. • Thus, for two different initial concentrations C1 and C2, equations can be written in the form: , • Taking logarithm and subtracting, •Or , n = [log(-(dC1)/dt)-log((dC2)/dt)] ÷ [logC1 - log C2] ❑ nA Product rate = k[A] ⁿ dxldt = k[A] ⁿ log(dxldt) = logk + nlogA y = c + mx (Slope = n)
  7. 7. Ostwald’s isolation method • Initial rate method. • Initial rate of reaction is determined by varying the concentration of the reactants while others are kept constant. • For a reaction given by: aA + bB → cC + dD • Rate ∝ [A]x[B]y ⇒ Rate = k[A]x[B]y , where , • [A] and [B] are concentration of reactants . • k is the constant. • x and y are the partial reaction order. • If [B] is kept constant, • R(1) = k[A1]x • R(2) = k[A2]x • R(1) / R(2) = ( [A1]/[A2])x
  8. 8. Thank You.. Soniya . M . Sunil B. Pharm 3rd year Mar Dioscorus College of Pharmacy

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