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A-Level Chemistry > Rate Equations > Flashcards

Rate Equations Flashcards

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1
Q

what is the rate equation? state the meaning and the units of each component.

A

Rate = k[A]a[B]b

  • rate = mol dm-3s-1
  • k (rate constant) = units vary
  • A/B (concentration of substance) = mol dm-3
  • a/b = order of reaction
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2
Q

what is an order of a reaction?

A
  • the power to which a concentration is raised to in the rate equation
  • tells us how the concentration affects the rate
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3
Q

what is zero order with respect to the rate equation?

A
  • changes in concentration has no effect on rate
  • e.g. if [A] doubles,
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4
Q

what is first order with respect to the rate equation?

A
  • changes in concentration has a proportional change on rate
  • e.g. if [A] doubles, then rate doubles
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5
Q

what is second order with respect to the rate equation?

A
  • changes in concentration has a squared proportional change on rate
  • e.g. if [A] doubles, then rate quadruples
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6
Q

what effect does temperature have on the rate constant?

A
  • rate constant only fixed at a particular temp
  • if temp changes so does rate constant
  • inc temp = inc k
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7
Q

what effect does rate constant have on the rate of reaction?

A

larger value of k = faster rate of reaction

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8
Q

what are the steps to calculate initial rate? use the following equation as an example.

A + B + C –> D + E

A
  1. repeat experiment several times but changing the concentrations of A, B and C in each experiment, one at a time
  2. work out the initial rate (rate at 0) for each experiment using the graph
  3. record the concentrations of reactions used for each experiments and their initial rates in a table
  4. from this table, work out the orders with respect to each reactant and write an equation (do this by working out the order with respect to A, where B and C are constant then repeat for B and C)
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9
Q

give an equation that can be used to determine the units of k in the rate equation

A

mol(1-p-q-r) dm(3p+3q+3r-3) s-1

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10
Q

outline the steps of finding the rate equation when the concentrations do not remain constant
take A and B to be the reactants

A
  1. work out order wrt A by comparing experiments where A is changing and B is constant
  2. work out order wrt B by working out what the rate would be if only A changes (use the order of A x initial rate of A and B)
  3. if there is a difference in this rate, B must have an effect on the rate so is not zero order
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11
Q

give a balanced equation for the Iodine Clock reaction

A

H2O2 (aq) + 2H+(aq) + 2I-(aq) –> 2H2O(l) + I2 (aq)

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12
Q

outline the Iodine Clock Experiment

A
  • add sodium thiosulfate and starch to the reaction mixture
  • sodium thiosulfate reacts immediately with the I2 that is produced in the reaction
  • when there is no more sodium thiosulfate left, the I2 reacts with starch giving a blue/black colour
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13
Q

what is the role of starch in the Iodine Clock Experiment?

A

acts as an indicator

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14
Q

during the iodine clock experiment, what will happen if the concentration of iodine and/or hydrogen peroxide (H2O2) is varied whilst everything else is kept constant? why is this useful?

A
  • time taken for the blue/black colour to appear will change
  • can be used to work out the order of a reaction
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15
Q

how can a change in pH be used to measure rate?

A
  • pH may change if H+ ions are produced/used up
  • a pHmeter can be used to measure the pH of a reaction at regular intervals
  • this can be used to calculate the change in H+ concentration and therefore the rate
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16
Q

how can a colour change from a reaction used to measure rate?

A
  • colorimeter used to measure colour changes by measuring absorbance
  • samples taken regularly + tested for absorbance
  • first plot calibration curve by making up a range of known concs of iodine (eg) + measuring the absorbance then plot
  • use calibration curve and absorbance to find conc of desired substance
17
Q

what does the gradient of a concentration-time graph show?

A

rate

18
Q

what would a rate-concentration graph of a zero order reaction look like? why?

A
  • horizontal line
  • changing conc doesn’t change rate
19
Q

what would a rate-concentration graph of a first order reaction look like? why?

A
  • straight diagonal line
  • changing conc changes rate equally (directly proportional)
20
Q

what would a rate-concentration graph of a second order reaction look like? why?

A
  • curved line
  • changing conc changes rate squared
21
Q

what is the rate determining step (RDS)?

A
  • slowest step in a multi-step reaction
  • rate of whole reaction depends on RDS
22
Q

can catalysts appear in the rate equation? how can these be distinguished from reactants?

A
  • can appear in rate equation
  • won’t appear in the overall equation
23
Q

how does the rate equation link to the RDS?

A

reactants that appear in the rate equation affect rate so must appear in the RDS

24
Q

outline the steps used to find the RDS using the rate equation

A
  1. find the step with the same ratio of molecules/atoms to that in the rate equation - this is the RDS
  2. if no step with this, mark off the molecules/atoms needed to match the ratio in the rate equation. stop when all accounted for
  3. the step where the last molecule is marked off is the RDS
25
Q

what factors does the Arrhenius equation link?

A

links activation energy + temp to the rate constant k

26
Q

state the Arrhenius equation and what each term means

A

k = Ae-Ea/RT

  • k = rate constant
  • A - Arrhenius constant
  • Ea = activation energy /J
  • e = euler’s number (ex button on calculator)
  • R = gas constant (8.314 J K-1 mol-1
  • T = temperature /K
27
Q

how does activation energy affect the rate constant? why?

A
  • as Ea decreases, rate constant increases
  • as Ea dec, rate increases as many more particles have enough energy to react when they collide
28
Q

how does temperature affect the rate constant? why?

A
  • inc temp = inc rate constant
  • inc temp = inc KE of particles = inc frequency of successful collisions as more particles have energy greater than or equal to the activation energy
29
Q

how can the Arrhenius equation be simplified? state the new equation

A
  • take natural logs of both sides

lnK = lnA - Ea/RT

30
Q

rearrange the Arrhenius equation so that activation energy is the subject

A

Ea = RT(lnA - lnK)

31
Q

what is an Arrhenius plot?

A
  • a graph of lnK (y axis) and 1/T (x axis)
  • can be used to work out Ea and the Arrhenius constant
32
Q

what does the gradient of an Arrhenius plot represent?

A

-Ea/R

33
Q

how can the gradient of an Arrhenius plot be used to work out the Arrhenius constant?

A

sub in value of gradient and coords of any point on the line of best fit (1/T = x coord, lnK = y coord)

lnK = (gradient x 1/T) + lnA
y = mx + c

A-Level Chemistry (23 decks)

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