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A Level Chemistry Year 2 > Reaction Rates > Flashcards

Reaction Rates Flashcards

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1
Q

What is the reaction rate?

A

The change in the amount of reactant or products per unit time

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2
Q

What is continuous monitoring?

A

A method used to monitor the progress of a reaction

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3
Q

How do you do continuous monitoring?

A

The formation or loss of one of the substances in the reaction is measured at regular intervals, the you can make a concentration-time graph for the whole reaction

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4
Q

How does the rate of reaction change as a reaction progresses?

A

It decreases

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5
Q

Why does rate of reaction decrease as time progresses?

A

The amount of reactant particles is decreasing as the reaction progresses, so the chance of successful collisions must also decrease

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6
Q

What piece of equipment do you use to collect gas produced from a reaction to measure the rate of reaction?

A

A gas syringe

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7
Q

Give an example of a reaction that you could use a gas syringe to measure the rate of reaction before (by measuring the volume of gas produced in regular time intervals)?

A

An acid + carbonate reaction because CO2 is released

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8
Q

Apart from a gas syringe, what is another method of measuring rate of reaction for a formation of a gas?

A

You could measure the change in mass of a reaction vessel at regular intervals as the gas is released to the surroundings, using a mass balance

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9
Q

What do you use to measure the colour change during a reaction?

A

Colorimeter

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10
Q

How does a colorimeter work?

A

It measures the absorbance of a particular wavelength of light by a solution

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11
Q

How can you use a colorimeter to measure the rate of reaction?

A

Set the colorimeter to measure the wavelength of light you want to measure
Calibrate the colorimeter by placing distilled water in a sample tube and then set the absorbance on the colorimeter to 0
Carry out the reaction, taking samples from the reaction mixture at regular intervals, and measure the absorbance of each one using the colorimeter

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12
Q

What is a cuvette?

A

A straight-sided clear container for holding liquid samples

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13
Q

How can you measure the rate of reaction if hydrogen or hydroxide ions are produced or used up?

A

You can monitor the progress by measuring the change in pH at regular intervals

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14
Q

Once you’ve finished the experiment to measure the rate of reaction, what do you then do?

A

Plot a concentration-time graph

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15
Q

If you measured the rate of reaction by colorimetry, how do you plot a concentration-time graph from the measured absorbances?

A

For each data point, convert the absorbance reading into concentration by using a calibration curve for the absorbance of wavelength of light by solution of different concentrations of the reactant/product you’re measuring

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16
Q

How do you convert absorbance readings from a colorimeter into concentrations?

A

Plot the data to obtain a graph of absorbance versus concentration. Then use the colorimeter to find the absorbance of the test solution, and use the graph to find the concentration of the solute in the test solution

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17
Q

What is the equation to find the pH of a solution?

A

pH = -log (H+)

H+: Concentration of hydrogen ions

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18
Q

For the experiment of measuring pH to find rate of reaction, how do you find the concentration of each data point to plot a concentration-time graph?

A

Rearrange the equation for pH to get H+ = 10^-pH

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19
Q

What is the gradient of a concentration-time graph at any point equal to?

A

Rate of reaction

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20
Q

What does a steeper gradient for a concentration-time graph mean?

A

A faster rate of reaction

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21
Q

How do you find the initial rate of reaction from a concentration-time graph?

A

Draw a tangent to the graph at time=0, the calculate the gradient

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22
Q

What is the initial rates method?

A

An experimental technique to used to work out the initial rate of reaction by timing how long it takes for a set amount of product to form or reactant to be used up

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23
Q

What can the data from the initial rates method be used for?

A

To work out the order or reaction with respect to each reactant

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24
Q

What is the overall order of a reaction?

A

The sum of the orders of each reactant in a reaction

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25
Q

What is the rate equation?

A

Rate = k[A]^m [B]^n

k: Rate constant
[A] and [B]: Concentrations respectively
m: Order of reaction in respect to A
n: Order of reaction in respect to B

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26
Q

What is the only given reaction that the rate constant ACTUALLY IS constant?

A

A reaction changing concentration and nothing else (temperature is constant)

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27
Q

What are clock reactions?

A

A method used to find the initial rate of reaction. You measure the time it takes for a given amount of product to form

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28
Q

What 3 assumptions are made for clock reactions?

A

Temperature is constant
The concentration of each reactant doesn’t change significantly
When the endpoint is seen, the reaction has not proceeded too far

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29
Q

Give an example of a clock reaction?

A

The iodine clock reaction:

H2O2 + 2I- + 2H+ —> 2H20 + I2

30
Q

Outline the Iodine clock reaction

A

Small amounts of sodium thiosulfate and starch are added to an excess amount of hydrogen peroxide and iodine ions in acid solution. The sodium thiosulfate immediately reacts with any iodine atoms that form

31
Q

What is it called if you double the reactant’s concentration, but the rate stays constant?

A

The order in respect to that reactant is 0 (zero order reaction)

32
Q

What is the order of reaction?

A

The order of reaction with respect to a particular reactant tells you how the reactant’s concentration affects the rate of reaction

33
Q

What is it called if you double the reactant’s concentration and the rate also doubles?

A

The order in respect to that reactant is 1 (first order reaction)

34
Q

What is it called if you double the reactant’s concentration and the rate quadruples?

A

The order in respect to that reactant is 2 (second order reaction)

35
Q

How do you find the order of reaction?

A

By monitoring how each reactant, one-by-one, affects the rate of reaction in an experiment

36
Q

How do you draw a rate-concentration graph from a concentration-time graph?

A

Draw tangents and find the gradient at various points on the concentration-time graph. Use the value of the gradient to plot points on the rate-concentration graph

37
Q

What order of reaction in respect to a reactant is a rate-concentration graph with gradient=0 (straight line intercepting Y-axis parallel to X-axis)?

A

Zero order reaction, as increasing concentration has no affect on rate

38
Q

What order of reaction in respect to a reactant is a rate-concentration graph that is a straight line through the origin?

A

First order reaction, as rate is proportional to concentration of reactant

39
Q

What order of reaction in respect to a reactant is a rate-concentration graph that is a curved line that goes through the origin?

A

Second order reaction, as rate is proportional to concentration of reactant^2

40
Q

What do square brackets mean in the rate equation?

A

The concentration of the reactant inside the square brackets

41
Q

How do you find the units for the rate constant?

A

By putting in the other units into the rate reaction and then simplifying

42
Q

How do you calculate the rate constant from a rate-concentration graph of a first order reaction?

A

If the overall reaction is first order, then the rate constant is the gradient of the rate-concentration graph for that reactant

43
Q

What is the half life of a reaction?

A

The time it takes for a half of the reactant to be used up

44
Q

How do you find the half life of a reaction from a concentration-time graph?

A

Read off, from the graph, the time it takes for the concentration of the reactants to halve

45
Q

Is the half life of a reaction dependant or independent of the concentration in a first order reaction?

A

Independent, as the half life is the same whatever the initial concentration is (only true for first order reactions)

46
Q

If a reaction has a constant half life, what order reaction must it be?

A

First order

47
Q

What is the equation to work out the rate constant, from a concentration-time graph where you know the half life, for a first order reaction?

A

k = (ln2)/t

ln2: Natural logarithm of 2
t: Half life

48
Q

What is the units of the rate constant?

A

S^-1

49
Q

What is the rate-determining step?

A

The slowest step in a reaction mechanism that determines the reaction rate

50
Q

How do you pick which reactants from the chemical equation are involved in the rate-determining step?

A

If a reactant appears in the rate equation, it must affect the rate, so it must be in the rate-determining step

51
Q

For predicting rate equations, what does the order of reaction in respect to a reactant show?

A

The number of molecules of that reactant that are involved in the rate-determining step

52
Q

What are the 3 rules if you need to suggest a mechanism when given the overall equation and rate equation?

A

The rate-determining step must fit with the rate equation
All of the equations in the mechanism must balance
The different steps of the reaction must add up to the overall equation

53
Q

What affect does increasing the temperature have on rate of reaction?

A

Rate of reaction increases

54
Q

Why does rate of reaction increase with an increasing temperature

A

As temperature increases, the reactant particles gain more kinetic energy so they collide more. This means a higher proportion of the reactant particles will collide with enough energy and start reacting

55
Q

How does temperature link to the rate equation?

A

Temperature doesn’t change concentration of reactants, so it must change the rate constant

56
Q

How does increasing temperature affect the rate constant?

A

Increasing temperature means a higher rate constant

57
Q

What is the definition of the Arrhenius equation?

A

An equation that links the rate constant to the activation energy and temperature

58
Q

What is the Arrhenius equation?

A

k = A e^ (-Ea / RT)

59
Q

In the Arrhenius equation, what does k mean?

A

Rate constant

60
Q

In the Arrhenius equation, what does Ea mean?

A

Activation energy / J mol^-1

61
Q

In the Arrhenius equation, what does R mean?

A

Gas constant = 8.31 J K^-1 mol^-1

62
Q

In the Arrhenius equation, what does T mean?

A

Temperature / K

63
Q

In the Arrhenius equation, what does A mean?

A

A constant number which is a pre-exponential factor

64
Q

What is the Arrhenius equation used for?

A

To predict how the rate constant will change if you change the activation energy or temperature

65
Q

What does a larger Ea mean for the rate?

A

A larger activation energy means a slower rate

66
Q

Why does a reaction with a higher activation energy have a slower rate?

A

Because a smaller proportion of particles will have enough kinetic energy to start reacting

67
Q

What does a higher temperature mean for the rate?

A

A higher temperature means a faster rate

68
Q

Why does a higher temperature mean a faster rate?

A

Means reactant particles have more kinetic energy, so they’re more likely to collide with enough energy to react

69
Q

What is the logarithmic form of the Arrhenius equation?

A

ln(k) = ln(A) + (-Ea/RT)

70
Q

What is an Arrhenius plot?

A

A graph where 1/temperature is plotted against ln(k)

71
Q

What is the gradient of an Arrhenius plot equal to?

A

(-Ea) / R

72
Q

What is the y-intercept of an Arrhenius plot equal to?

A

ln(A)

A Level Chemistry Year 2 (7 decks)

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