Chapter 18 - Rates of Reactions Flashcards Preview

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Flashcards in Chapter 18 - Rates of Reactions Deck (57)
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1
Q

What is the shorthand for the concentration of A

A

[A]

2
Q

What is the order of reaction?

A

The power of the concentration. It can change with different reactants and affects the rate of the reaction in different ways.

3
Q

What is the equation of rate to concentration

A

Rate ∝ [A] ^n

4
Q

What does zero order mean

A

This means that the concentration of a reactant has no effect on the rate.
The reaction is zero order with respect to the reactant.

5
Q

What does first order mean

A

A reaction is first order with respect to a reactant when the rate depends on its concentration raised to the power of 1.
If the concentration is multiplied by 2, then the rate if also multiplied by 2.

6
Q

What does second order mean

A

If the concentration is multiplied by 2, the rate of the reaction is multiplied by 4.

7
Q

The rate equation

A

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

It gives a mathematical relationship between the concentration of the reactants and the reaction rate. K is the reaction rate.

8
Q

Overall order

A

M+N

9
Q

How to find out the units of k

A
  1. Rearrange to make k the subject
  2. Substitute units into that expression
  3. Cancel the common units.
10
Q

How can you calculate the overall order from experimental orders

A

You should get the tangent and the rate of the reaction. Then compare each repeat of the experiment. How does changing the concentration of a reactant change the initial rate? If you *2 the conc, and the rate is *2, then that is the first order of that reactant.

11
Q

What is continuous monitoring

A

They can be plotted from continuous measurements taken during the course of a reaction.

12
Q

Examples of continuous monitoring

A

Monitoring by gas collection
Monitoring by mass loss
If no gases are produced, then a colour change?

13
Q

Explain a colorimeter

A

The wavelength of the light passing through a coloured solution is controlled using a filter. The amount of light absorbed by a solution is measured.

14
Q

Zero order in a conc-time graph

A

A straight line with a negative gradient.

The gradient = k.

15
Q

First order in a conc-time graph

A

Downward curve with a decreasing gradient over time.
Half life: the time for the concentration to halve is constant and called the half life.
The rate constant can be figured out using this value.

16
Q

Second order in a conc-time graph

A

Also a downward curve, steeper at the start and tailing off more slowly.

17
Q

Half life.

A

T1/2.
Time taken for half the reactant to be used up.
1st order relationship can be confirmed from a conc-time graph by measuring the number of successive half-lives. If they are the same, then the reaction is 1st order with respect to that reactant.

18
Q

Determining k for a 1st order reaction by calculating rate constant from the rate.

A

Draw tangent and calculate rate. Then put this in the rate equation to calculate k.

19
Q

Determining k for a 1st order reaction by using half-life

A

K= ln2/ t1/2.

20
Q

PAG for using a colorimeter

A
  1. Prepare solution of known conc.
  2. Select filter
  3. Zero the colorimeter with water
  4. Measure the absorbance
  5. Plot a calibration curve
  6. Carry out reaction
  7. Measure absorbance of unknown curve
  8. Using calibration curve, get the concentration.
21
Q

Why are rate- concentration graphs good?

A

They offer a direction route in the link between rate and the concentration in the rate equation.

22
Q

On a rate- concentration graph, what does zero order look like?

A

A horizontal straight line with 0 gradient.

This means that the y-intercept is equal to k.

23
Q

On a rate- concentration graph, what does first order look like?

A

A straight line through origin.
Where the rate is proportional to the concentration.
The gradient is equal to the gradient.

24
Q

On a rate- concentration graph, what does second order look like?

A

An upward curve with an increasing gradient.
Because the rate is equal to the K * concentration squared.
So plot another graph, rate against the concentration^2.
This gives you a straight line graph, going through the origin, where the gradient = k.

25
Q

Log-log graphs

A
A log(rate) against log(concentration) is a good way to find the order and the rate constant. 
The gradient = Order n
The y-intercept = Log k.
26
Q

Clock reactions

A

Measure the time taken for a certain amount of product to form as you vary the concentration of one of the reactants.
There is usually an observable endpoint, such as a colour change or precipitate.
You assume that the average rate = initial rate.
It needs to be repeated many times.

27
Q

How accurate are clock reactions?

A

The shorter the period of time over which an average rate is measured, the less the rate changes over that time period, because it stays the same as the initial rate.

28
Q

PAG : Iodine Clock Procedure

A

Change the conc of one reactants, leaving the others the same.
The solution begins colourless and then changes to blue-black when the iodine-starch is made.
The rate is proportional to 1/t.
So from the results, you would plot a graph of 1/t against concentration and then calculate an order with respect to a reactant.

29
Q

What is a reaction mechanism

A

The series of steps that make up a reaction.

30
Q

What is a rate-determining step

A

The slowest step in the sequence of steps in a reaction.

The rate equation only includes reacting species and orders involved in the rate-determining step.

31
Q

How does increasing the temperature affect the rate constant?

A
  1. It shifts the Boltzmann Distribution to the right, so there’s a higher proportion of molecules that reach the Activation Energy
  2. The particles move and collide more frequently.
32
Q

What is affected more by the increasing temperature (for the rate constant)?

A

The increased frequency of collisions comparatively smaller in comparison to the increase in proportion of molecules that exceed Ea from the shift in Boltzmann Distribution.

33
Q

What is the Arrhenius Equation

A

K = A e-Ea/RT

34
Q

What is R?

A

The gas constant- 8.134.

35
Q

What does the exponential Factor in the Arrhenius Equation mean?

A

The proportion of molecules that exceeds Ea and have sufficienty energy for a reaction to take place.

36
Q

What does the pre-exponential Factor in the Arrhenius Equation mean?

A

The frequency of collisions with the correct orientation. It is constant over small temperature changes, but may change slightly over large temperature changes.

37
Q

How can the Arrhenius Equation be rearranged?

A

ln k = -Ea/ R * 1/T + ln A.

38
Q

On a graph of ln k against 1/T /K-1, what does the equation show?

A

Gradient = -Ea /R

Y- intercept = Ln A.

39
Q

How can you tell that a graph is first order?

A

Use the half life!!

40
Q

How can you answer a half life questions

A

Say that it takes x sec for the concentration to halve, and then x sec again for the concentration to halve again.
Since the half life is constant, it means that the reaction is first order with respect to Y.

41
Q

IF THEY ASK YOU THE RATE

A

JUST FIND THE GRADIENT OF THE TANGENT.

RATE = TANGENT!!!

42
Q

What is ln2/ half life

A

The rate constant.

43
Q

What is the effect of doubling the initial concentration on the half life

A

NO CHANGE

44
Q

What happens to the rate constant if you have a higher pressure (or increase in concentration)?

A

NO CHANGE.

45
Q

Why may something be at zero order

A

That substance may be in excess.

46
Q

State 2 reasons why a reaction may be unlikely to happen

A

The stoichiometry in the rate equation doesn’t match that in the overall equation.
Also, there are x molecules trying to react together at the same time, which seems unlikely.

47
Q

Define the rate of a reaction

A

The speed at which the reactants are used up and converted into products.

48
Q

Define the order of a reactant

A

How much the concentration effects the rate of the reaction, and is determined by the rate equation.

49
Q

Define the order of a reaction

A

The power dependence of the rate on the concentration of each reactant.

50
Q

Define the rate constant

A

A constant of proportionality which relates the concentration of each reactant to the rate.

51
Q

What changes the rate constant

A

Temperature, catalyst.

52
Q

Define the RDS

A

The slowest step in a reaction mechanism.

53
Q

What is the graph plotted for the initial reaction

A

1/t - Concentration graph.

But you can then go on to create a log-log graph, where the gradient is equal to the order of the reaction.

54
Q

the oxidation of iodide ions by hydrogen peroxide under acidic conditions.

A

H2O2 + 2I- + 2H+ –> 2H20 + I2.

All aq, but water is liquid.

55
Q

Testing for iodine - in the iodine clock reaction

A

Add a small amount of sodium thiosulfate (Na2S2O3)

It should turn dark blue- black.

56
Q

Iodine and thiosulfate reaction equation

A

2S2O3 + I2 –> S4O6 2- + 2I-.

All aq.

57
Q

Describe and explain what is happening to the rate of reaction as the reaction proceeds -
a graph of volume of gas produced against time. Increases and then plateaus off.

A

The rate of reaction is high at the start of the reaction, decreases as the reaction proceeds, and reaches zero when the reaction stops . The reaction slows as the concentration of the acid decreases (OR amount of magnesium remaining decreases) hence there are fewer collisions per unit time