Rate of reaction

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Created by
Erin Jeavons

Cards (33)

  • How can rate be found?
    From the gradient.
    Gradient= change in y / change in x
  • How can rate be found on a curved line?
    Rate can be found from the gradient on a tangent.
  • Whats the rate equation?
    Rate= K[A]a[B]b
    Rate= moldm-3s-1
    K= rate constant.
    [A][B]= concentration of substances.
    a,b= orders of reaction.
  • Orders of reaction?
    An order is the power in which a concentration is raised to and how it affects the rate of reaction.
  • Zero order?
    Changes in concentration has no effect on rate
  • First order?
    Changes in concentration has a proportional change on rate.
  • Second order?
    Changes in concentration has a squared proportional change on rate.
  • The rate constant- K?
    The larger the value of K, the faster the reaction.
    As we increase temp, particles have more ke and collide more often, which increases the rate.
    The concentration of the substances remains constant.
  • Calculating initial rate from a graph?
    Take gradient of tangent from 0 mins.
  • Workout rate equation for A + B + C > D + E
    Repeat experiment but change conc of A,B,C one at a time in each experiment.
    Workout the initial rate for each experiment.
    Record the conc of reactants and their initial rates in a table.
    From this we can workout their orders and write a rate equation.
  • Calculate initial rate for experiment 4?
    2NO + Cl2 > 2NOCl
    Experiments initial [NO] initial [Cl2] Initial rate
    1 0.2 0.1 0.63
    2 0.2 0.3 1.92
    3 0.8 0.1 2.58
    4 0.5 0.5 ?
    Workout order of NO. Experiment 1+3, NO and initial rate has 4x. NO is 1st order.
    Workout order of Cl2. Experiment 1+2, Cl2 and initial rate has 3x. Cl2 is 1st order.
    Rate equation= K[NO][Cl2]
    Calculate K. K=rate/[NO][Cl2]. K=0.63/(0.2x0.1)= 31.5 mol-1dm3s-1
    Rate= 31.5 x 0.5 x 0.5= 7.88 moldm-3s-1
  • Calculate rate for table with none constant conc?
    A + B
    Experiments initial [A] initial [B] initial rate Change [A] only rate
    1 0.15 0.24 0.42x10-3
    2 0.45 0.24 3.78x10-3
    3 0.90 0.12 7.56x10-3 15.1x10-3
    Workout order of A, experiment 1+2 has 3x. Rate has 9x so A is 2nd order.
    Workout order of B by calculating rate if we only changed A. Experiment 2+3 A 2x, so rate must be 4x as A is 2nd order to give 15.1x10-3.
    Bs rate is about half of 15.1x10-3. So B is 1st order.
    Rate= K[A]2[B]
  • The iodine clock experiment?
    H2O2(aq) + 2H+(aq) + 2I-(aq) > 2H2O(l) + I2(aq)
    Add sodium thiosulfate and starch to reaction mixture.
    Sodium thiosulfate reacts immediately with I2 produced.
    When there is no more sodium thiosulfate, I2 reacts with starch to give a black colour.
    Varying the I2 and H2O2 conc and keeping the rest constant will result in varying time taken for colour change. From, this we can calculate order of reaction.
  • The change in pH of a reaction?
    pH of a reaction may change if H+ ions are used up or produced.
    A pH meter can measure the pH of a reaction at regular intervals.
  • Amount of mass lost?
    For reactions that produce a gas, place reaction on a balance and measure mass lost.
  • Volume of gas produced?
    Measure amount of gas produced using a gas syringe.
  • Colorimeter?
    Used for reactions with colour change.
    Measures the absorbance of light by a coloured sample.
    The more conc a sample, the darker the colour and hence more light absorbed.
    eg I2 + CH3COCH > CH3COCH2I + I- + H+
    Brown + colourless > All products colourless.
  • Straight line graph?
    Rate is constant, Changing conc doesn't change rate. Zero order.
  • Shallow curve graph?
    Rate changes in equal amounts to conc. 1st order.
  • Steep curve graph?
    Rate changes squared compared to changing conc. 2nd order.
  • What is the rate determining step?
    The slowest step in a multi stage reaction.
    The whole reaction rate depends on how fast the rate determining step is.
    Reactants that appear in the rate equation, affect the rate of reaction and must be in the rate determining step.
  • Rate determining step example?
    Step 1: A + B > 2C (fast)
    Step 2 : 2C > D (slow)
    Step 3: D + E > F + G (fast)
    Step 2 is the rate determining step. Reactant C must appear in the rate equation. C is an intermediate of A + B, so A + B must be in the equation too.
    Rate= K[A][B][C]2
    C is a catalyst as it doesnt appear in the overall equation.
  • Workout the rate determining step?
    eg Step 1: NO + NO > N2O2
    Step 2: N2O2 + O2 > 2NO2
    Rate= K[NO]2[O2]
    Start from step 1 and score off atoms needed to match the ratio in the rate equation.
    We have 2NO but still need O2, so continue to step 2.
    We have O2 and have now accounted for all molecules in the equation.
    Rate determining step is step 2.
  • Reaction mechanism found from rate determining step?
    eg (CH3)3CBr + OH- > (CH3)3COH + Br-
    Mechanism 1: (CH3)3CBr + OH- > (CH3)3COH + Br- (slow)
    Mechanism 2: (CH3)3CBr > (CH3)3C+ + Br- (slow due to breaking strong C-Br bond.
    (CH3)3C+ + OH- > (CH3)3COH (fast due to good chance of collision.
    Rate= K[(CH3)3CBr]
    Rate equation tells us only (CH3)3CBr can appear in the rate determining step. This means mechanism 2 will be the rate determining step as it matches the ratio in the rate equation.
  • Arrhenius equation?
    K= Ae -Ea/RT
    Rate constant= Arrhenius constant x ex -activation energy/8.31 x temp
  • Arrhenius important?
    As Ea gets smaller, K gets bigger. Therefore as Ae drops, rate of reaction increases as more particles have sufficient energy to react when colliding.
    As we increase temp, K increases as particles have more Ke, therefore more likely to collide, so rate of reaction increases.
  • Calculate using Arrhenius equation?
    eg Calculate Ae of reaction at 330K and K=1.3x10-4. Arrhenius constant= 4.55x1013 and gas constant is 8.31.
    Write out Arrhenius equation. K=Ae -Ea/RT
    Take the natural loh (In) of both sides.
    Ink= InA - Ea/RT
    Rearrange to move Ea to 1 side. Ea/RT= InA - Ink
    Multiply both sides by RT to get Ea on its own.
    Ea= (InA - InK) x RT
  • Using Arrhenius to calculate activation energy?
    Plot a graph of Ink against 1/T.
    The gradient represents -Ea/R
    Ink= -Ea/RT + InA
  • How can this data be used to workout the rate expression?
    Experiment initial [A] initial [B] initial rate
    1 0.12 0.26 2.1x10-4
    2 0.36 0.26 1.89x10-3
    3 0.72 0.13 3.78x10-3
    In experiment 1+2, B is constant. A increased by 3x, so is 2nd order.
    In experiment 2+3, A increases by 2x, so rate should increase 2x^2, but only increases 2x.
    Therefore halving B, halves rate, so B is 1st order.
    Rate= K[A]2[B]
  • rate = k[E]
    Explain why doubling the temperature has a much greater effect on the rate of the reaction than doubling the concentration of E?
    Reaction occurs when molecules have E>Ea .
    Doubling T by 10 °C causes many more molecules to have sufficient energy.
    Whereas doubling [E] only doubles the number with this energy.
  • State the effect, if any, on the value of the rate constant k when the temperature is lowered but all other conditions are kept constant?
    Lowered as fewer particles/collisions have energy > E
  • A student carried out the experiment using a flask on the laboratory bench. The student recorded the time taken for the reaction mixture to turn blue. State one way this method could be improved. Explain why the accuracy of the experiment would be improved.
    Constant temp as rate would be affected from temp change.
  • How can you tell that H+ acts as a catalyst in this reaction?
    Appears in rate equation but does not appear in overall equation