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Chemistry A2 OCR > 5.1, rates > Flashcards

5.1, rates Flashcards

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1
Q

What is the definition for the rate of reaction?

A

The change in concentration of a reactant or product per unit time.

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2
Q

What is the order of the reatction?

A

The order resepct to a reactant is the power to which the concentration of the rate is raised in the rate equation.

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3
Q

What is the rate constant, K?

A

is the constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equations.

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4
Q

Give the definition for the rate equation.

A

for a reaction A + B -> C is given by: rate = k[A]m[B]n where m is the order of a reaction with respect to A and n is the order of reaction with respect to B.

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5
Q

Give the definition for the overall order.

A

The overall order of a reaction is the sum of the individual orders, n + m

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6
Q

What is the equation for measuring rates of reaction?

A

rate of reaction = change in concentration of reaction or products / time

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7
Q

What are the units for the ROR?

A

measured in moldm-3s-1, but if more appropriate moldm-3min-1.

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8
Q

How can you measure change in concentration to calculate the ROR?

A

Concentration is difficult to measure, measuring mass change or gas volume prodcued is usually easier.

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9
Q

How is the effect of individual reactants on the ROR described?

A

If more than one reactant is involved in a reaction, each reactant can affect the rate of the reaction differently. The effect of the individual reactants is decribed by stateing an order with respect to each reactant.

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10
Q

How do you express the effect of reactant A (its concentration affects the ROR) on the ROR?

A

rate(insert symbol for proportional)[A]x

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11
Q

What is a zero order reaction.

A

rate = [A]0

  • The rate is unaffected by changing the concentratio of A.
  • Note that any number to the power of O is equal to one.
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12
Q

What is a first order reaction?

A

rate = [B]1

  • If [B] increases by two time, the rate increases by two times.
  • If [B] invcreases by three times the rate increases by three times.
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13
Q

What is a second order reaction?

A

rate = [C]2

  • If [C] increases by two times the rate increases by 22
  • If [C] increases by three times the rate increases by 32 = 9 times.
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14
Q

How does expression for the effect of concentration of a reaction become a rate equation?

A

The sign for proportional can be removed if a constant is added into the equation. Chemists use the rate constant K. The rate constant links the concentrations and orders of reactants to the rate. The expression then becomes the rate equation.

rate = k[A]x[B]y[C]z

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15
Q

What reactants are not included in the rate equation?

A

If a reactant is 0 order it is not included in the rate equation.

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16
Q

How would you present a 0, 1st, and 2nd order reactant in a rate equation, if the reactants were A, B and C?

A

rate = k[B]1[C]2

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17
Q

What is the overall order for the reaction rate = k[B]1[C]2 ?

A

3

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18
Q

How do you calculate the value for a rate constant?

A

The rate constant is calculated by submitting values for concentration and rates into the rate equation and rearranging to find k.

Zero order, rate = k[A]0

k = rate/1

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19
Q

How do you calculate rate constants for a first order reaction?

A

rate = k[A]

k = rate/[A]

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20
Q

How do you calculate rate contants for a second order reaction?

A

rate = k[A]2

k = rate/[A]2

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21
Q

How do you work out the units for the rate constant, k?

A

The units of K depend on the verall order of the rate reactiion. The units of k are determined by substituting units for rate and concentration into the rate equation.

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22
Q

What will be the units of k for a zero order reaction, k = rate/1?

A

mol dm-3s-1

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23
Q

What will be the units of k for a first order reaction, rate = k[A]?

A

units of k = (moldm-ss-1) / (mol dm-3)2 = s-1

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24
Q

How would you calculate the rate constant for a third order reaction?

A

rate = k[A]2[B]

k = rate / [A]2[B]

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25
Q

What are the units for a rate constant of a third order reaction?

A

units of k = moldm-3s-1 / (mol dm-3)3 = dm6mol-2s-1

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26
Q

What is the half-life of a reactant?

A

The half-life of a reactant is the time taken for the concentration of the reactant to reduce by half.

27
Q

What experimental rechniques may chemists use to measure the concentration of reactants or prodcts in a reaction.

A

Acids and bases

  1. PH changes carrying out titrations
  2. PH changes by using a PH meter

Reactions that produce gasses

  1. The change in volume or pressure
  2. the loss of mass of reactants

Reactions that form visual changes

  1. The formation of a precipitate
  2. A colour change. (colorimeter)
    3.
28
Q

How can the rate of a reaction be found by using a concentration/time graph?

A

The rates of reaction can be founf by plotting the concentration on a coc tome graph and taking the gradient of a tangent/ (y/x).

29
Q

What symbol is given for the half life of a reaction?

A

Half life is given the symbol t 1/2

30
Q

Draw a zero order conentration time graph.

A
31
Q

Drawn a first order concentration time graph.

A
32
Q

What does a first order reaction produce on a concentration time graph?

A

A curved graph with constant half life’s.

33
Q

How can you determine the half life from a first order recation on a concentration time graph.

A
  • You can use the value of a half life from a first order reaction to determine the rate constant.
  • They are linked by the equaion K = ln (natural log) 2 / t1/2
34
Q

How do you calcuate initial rate on a concentration/time graph?

A
  • Initial rate = t=O.
  • If a tagent line is drawn at T = O on a conc/time graph, the gradient of the line is equal to the initial rate.
35
Q

What is the link between ROR and a reaction with a visible change?

A

For example a clock reaction:

  • When there is a measured time taken for a visible change to occur the shorter the time taken for the change to occur the fatser th ereaction must be.
  • (sign for proportional) 1/t
  • So when a grpah is plotted, 1/t is taken as a good approximation of the intial rate.
36
Q

Describe the iodine clock reaction.

A

The iodine clock reaction.

If a solution of hydogen peroxide, H2O2, is added to a solution of potassium iodide, KI, in the presence of starch, acid and sodium thiosulfate, Na2S2O3, the colourless solution will sudenly turn blue. The reaction occurs in two steps:

  1. H2O2 (aq) + 2I-(aq) + 2H+ → I2(aq) + 2H2O(l)
  2. 2S2O32-(aq) +I2 (aq) → S4O62-(aq)

The amount of time that passes before this colour change occurs is a measure of the initial rate.

37
Q

Draw a zero order rate - concnetration graph.

A
38
Q

What does the graph of a zero order rate/conc graph show?

A

Changes in concentrations of this reaction will have no effect on the rate.

39
Q

Draw a first order rate/conc graph.

A
40
Q

What does a first order rate/conc graph tell us?

A
  • The rate is proportional to [B]1.
  • If the conc of reactant B is doubled th rate will be doubled.
41
Q

How do you determine the rate constant form a first order rate/conc graph?

A

Rate is proportional [raectant] this means that rate = K [reactant].

If we rearrange this to find K; k = rate/conc

42
Q

Draw a second order rate/conc graph.

A
43
Q

What does a second order rate/conc graph tell us?

A

Rate is proportional to [C]2.

If the conc of a reactant C is doubled, the rate will increase by 22.

44
Q

What is the definition for a raection mechanism?

A

A reaction mechanism is a series of steps that, togther, amke up the overall reaction.

45
Q

What is the definition for a rate determinig step?

A

The rate determining step is the slowest step in the reaction mechanism of a multistep reaction.

46
Q

What is the definition for an intermediate?

A

An intermediate is a species formed in one step of a multi-step reaction that is used up in a subsequent step, and is not seen as either a reactant or a product of the overall equation.

47
Q

What reactants won’t be included in the rate detrermining step?

A

Reactants with an order of zero do not appear in the rate equation. If they have no influence on the rate, they cannot be involved in the rate-determining step.

48
Q

Suggest the rate determining step for the example of bromoethene;

  • CHsBr + OH- → CH3OH + Br-
  • the rate equation is rate = K[CH3Br]
A

This shows that the concentration of CH3Br affects the ROR and only this is involved in the rate determining step.

It also suggests that the rate determining step must involve the C-Br bond breaking before the OH- can take it’s place.

Step 1. CH3Br → CH3 + Br-

Step 2. CH3+ + OH- → CH3OH (CH3+ must be involved in the overall equation)

The species involved in the two steps cancell each other out to leave the overall equation.

49
Q

What can we infer from the reactant being first or second order that you can use to solve a rate equation?

A

If the order is one with respect to a reactant the rate determing step involves just one molecule

If the order is second with respect to a reactant the rate determining step involves just two molcules. Doubling the conc of each of the two molecule swill double the rate so overall the rate is quadroupled.

50
Q

What steps must be followed to determine a reaction mechanism?

A

The reaction mechanism must be wored out by looking at the rate equation and ensurig that;

  1. A rate-determining step is given taht involves the number of molecules shown by the order for that reactant.
  2. Subsequent steps are shown that eventually generate the products shown in the balanced equation.
  3. Any intermediate generated is not present in the overall equation.
51
Q

What is the rate equation?

A

Reaction rate depends on both the rate constant and the concentrations of the reactants present in the rate equation:

rate = k[A]x[B]y

The larger the value of k, the faster the reaction.

52
Q

What is the effect of raising the temperature on K?

A

Raising the temperature speeds up the rate of most reactions by increasig the rate constant, K.

For many reactions, the rate apporximately doubles for each 10oC increase in temperature, → greater number of particles that wil exceed the activation energy.

Typically, doubling the rate will double the value of K.

53
Q

Why does incvreasing the temperature increase the value of k?

A
  • The key factor affecting the recation rate is the number of collisions that exceed the activation energy as is explained in the Bolzman distribution.
  • Collisions are also more frequent.
54
Q

What is the definition of an Arrhenius plot?

A

An Arrhenius plot is a graph of ln k = ln A - Ea/R x 1/T, (on data sheet) where ln k is plotted against 1/T.

55
Q

What is the Arrhenius equation?

A

The Arrhenius equation is used to describe, mathmatically, the exponential relationship between the rate constant and temperature.

56
Q

What do al the symbols in the Arrhenius eqation stand for?

A

k = Ae - Ea/RT

  • k = rate constant
  • Ea = activation energy
  • T = temerature
  • e = mathmatical consatnt with a value of 2.71828 - known as the inverse natural log.
  • A = pre - exponanetial factor (you will need to explain this)
  • R = gas constant.
57
Q

What does the Arrhenius equation tell us?

A

Temperature, T, ad the rate constant, K, are related exponentially.

As temperature increass, the rate constant increases.

58
Q

How would adding a catayst affect the Arrhenius equation?

A

Adding a catalyst provides an alternative reaction path with a lower activation energy, Ea. A lower value of Ea = a higher rate constant k.

59
Q

What graphs are refered to as Arrhenius plots?

A

If a given reaction is carried out at varying temperatures and the value of the rate constant is calculated for each temperature, a graph of ln k can be plotted against 1/T. Such graphs are reffered to as Arrhenius plots.

60
Q

What patern does the logarithmic of the Arrhenus equation follow?

A

y = mx +c

ln k = ln A - Ea/R X 1/T

y = c m x

61
Q

What can an Arrhenius plot be used to identify?

A
  • A: the intercept on the graph is equal to ln A
  • the activation energy, Ea : thee gradient is equal to -Ea/R
62
Q

How is the natural log related to e in the Arrhenius equation?

A

Natural log (ln) and e are inversely related.

x = ey (where y is the power e is raised by) and ln x=y

63
Q
A

Chemistry A2 OCR (5 decks)

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