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Chemistry Unit 3 > ✅3.5: Chemical Kinetics > Flashcards

✅3.5: Chemical Kinetics Flashcards

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1
Q

Define the term quenching

A

The sudden stopping or significant slowing of a chemical reaction to allow for analysis to occur without the reaction proceeding further. It is usually undertaken by cooling and diluting, such as by adding the sample to cold water

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2
Q

Define activation energy

A

The minimum amount of energy requires for a collision to be successful

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3
Q

Finish the sentence:

Chemical kinetics is…

A

The study of rates of chemical reactions

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4
Q

What do you need to work out the rate or a chemical reaction?

A

Need to work out how much of a reactant has been used up, or how much of a product has been produced in a set period of time

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5
Q

Name some examples of changes in reaction over time that you can measure.

A

Mass of reactants
Volume/pressure of gas
Colour of solution
Other electromagnetic absorption

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6
Q

Name a distinct example of a colour change reaction

A

The iodine click reaction which shows a distinct colour change after a set amount of compound has reacted

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7
Q

Describe when quenching is useful in measuring the rate of a chemical reaction

A

When it is not possible to collect data on the progress of a reaction through the entire chemical reaction

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8
Q

Describe the process of quenching

A

1) Remove a small amount of the reaction mixture at regular time intervals (sampling)
2) immediately place sample into iced water cooling and diluting the mixture
3) Analyse the sample using an appropriate method e.g titration, each sample must be analysed individually to attain information on the progress of the reaction

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9
Q

Why must you place the sample into iced water immediately during the quenching process?

A

It slows down the reaction (less KE) effectively stopping it.

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10
Q

Name some advantages of quenching

A

-can be used for a large range of reactions

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11
Q

Name some disadvantages of sampling and quenching

A
  • labour intensive method
  • time intensive (each sample must be analysed) Therefore time intervals used between measurements tend to be longer than in colorimetric method which can be automated
  • sampling is only appropriate when a reaction mixture is homogeneous, such as reactions that are all in solution.
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12
Q

Why can’t you sample a reaction mixture if it’s not homogenous?

A

The sample take may not be representative of the overall mixture

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13
Q

What should you do to take a sample of a reaction that uses a heterogenous catalyst? Why?

A

Sampling can be undertaken without quenching. When a solid catalyst is used in a gas or liquid mixture the catalyst speeds up the reaction significantly :: removing a sample of the gas or liquid takes it away from the catalyst so the reaction is immediately reduced.

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14
Q

What should you do to take a sample of a reaction that uses a homogenous catalyst? Why? (2)

A

1) Taking a sample also takes a sample of the catalyst with the reactants, so the reaction will continue :: quenching is needed to stop the reaction. This can be done by cooling and dilution like any other reaction.
2) It can also be quenched by destroying the catalyst, for instance an acid catalyst can be neutralised using an alkali.

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15
Q

What is the equation for the rate of reaction?

A

Rate= change in concentration/time, this produces the average rate over the time period.

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16
Q

Why is it likely the rate of reaction will change as the reaction progresses?

A

Because the concentrations of reactants will change, for instance the rate decreases as the concentration of reactants decreases

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17
Q

How do you find the initial rate?

A

Results for each sample should be plotted on a graph and a ranger draw to the curve of best fit at time= 0, then plug figures into rate equation
(Example on pg 51)

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18
Q

How do you measure the rate of reaction at different times?

A

Plot tangents at different places on the curve.

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19
Q

When plotting time vs concentration of a product will the concentration increase or decrease?

A

Concentration of product will increase (then plateau if no more reactant is added)

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20
Q

When plotting time vs concentration of a reactant will the concentration increase or decrease?

A

The concentration of reactant will decrease as it’s formed into products

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21
Q

What must you do when explaining changes in rate?

A

Use collision theory where the rate should be linked to the frequency of successful collisions.

22
Q

When would the rate of reaction decrease?

A

When the frequency of successful collisions decreases or when fewer collisions have more than the activation energy

23
Q

What you note when calculating the rate of a reaction?

A

Should ensure the units of the rate match the units of the measurements for example time can be given in second or minutes and therefore units could be to the s-1 or min-1

24
Q

What does the rate of a chemical reaction in solution depend on?

A

The concentration of the reactants

25
Q

An example: when the concentration of one reactant [A] is doubled, show the link to rate in terms of order

A

1- Rate of reaction stays the same: rate is not proportional to concentration: rate is [A]0

2-Rate is reaction doubles: rate is proportional to concentration: rate is [A]1

3- Rate if reaction increases by 4 times: rate is proportional to concentration squared: rate is [A]2

26
Q

Give the rate equation for a general reaction:

A

A+B —> products

27
Q

Give the rate equation for a specific reaction, include a key.

A

m n
Rate = K [A] [B]

Where:
K= the rate constant
M= the order of reaction with respect to A
N= the order of the reaction with respect to B

28
Q

Define the term rate:

A

The rate of change of the concentration, or the amount of a particular reactant or product

29
Q

Define the term rate constant:

A

The constant in the rate equation. It is constant for a given reaction as a particular temperature and is not affected by changing the concentrations of the reactants.

30
Q

When is the rate constant not constant?

A

When you change the temperature of the reaction

31
Q

Define the term order of reaction:

A

The order of reaction with respect to a particular reactant is the power to which the concentration is raised in the rate equations (m or n in the specific rate equation)

32
Q

What is the overall order of reaction?

A

The sum of all these orders of reactions I.e m+n for the specific rate equation

33
Q

What is the overall order of reaction when we describe reactions as being zeroth order?

A

The total of m+n = 0

34
Q

What is the overall order of reaction when we describe reactions as being first order?

A

When the total of m+n = 1

35
Q

What is the overall order of reaction when we describe reactions as being second order?

A

When the total of m+n = 2

36
Q

What is the overall order of reaction when we describe reactions as being third order?

A

When the total of m+n is 3

37
Q

What is the units for a typical rate of reaction?

A

Mol dm-3 s-1

The change in concentration (mol dm-3) per second (s-1

38
Q

For a zero order reaction, what are the units of k?

A

In a zero order reaction Rate=K

:: units of rate= mol dm-3 s-1 :: units of K= mol dm-3 s-1

39
Q

For a first order reaction what are the units of K?

A

For a first order reaction Rate= K[A]
The units for rate = mol dm-3 s-1
The units for [A] = mol dm-3
:: the units of K = s-1

40
Q

For a Second order reaction what are the units of K?

A

For a second order reaction Rate= K[A]2 (squared)
The units of rate = mol dm-3 s-1
The units of concentration squares = mol-2 dm-6
:: K= mol-1 dm3 s-1

41
Q

How can you find rate equation data?

A

Only via experimentation, by studying the effects changing the concentration of each individual reactant.

42
Q

What is important to note about obtaining the rate equation

A

THERE IS NO WAY OF OBTAINING THIS RATE EQUATION FROM THE OVERALK EQUATION OF THE REACTION

43
Q

How do you derive a rate equation from experimental data?

A
  1. Look at the given table and find two experiments that differ only in the concentration of one reactant
  2. If doubling this reactant concentration does not affect the rate of reaction the order with respect to this reactant is zero
  3. If doubling this reactant concentration doubles the rate of reaction, the order with respect to this reactant is one
  4. I’d doubling this reactant concentration increases the reaction rate by a factor of four, the order with respect to this reactant is two
  5. Repeat this process for each reactant, to find the order with respect to each one
  6. The order of the reaction is the sum of each of these orders.
44
Q

LOOK AT EXAMPLES ON PG 54 😊

A

LOOK AT EXAMPLES ON PG 54 😊

45
Q

Define the term rate determining step

A

The rate determining step is the slowest step in a reaction mechanism

46
Q

What is a mechanism?

A

A mechanism is a description of the series of steps that occur during a chemical reaction.

47
Q

Will each step in a mechanism occur at the same or different rates?

A

Each step will occur at different rates with its own rate equation

48
Q

What is important about the rate determining step?

A

The rate of the slowest step limits the rate of the overall reaction, called the rate determining step (RDS)

49
Q

What does the rate equation tell us about how many particles must collide in the rate determining step?

A

In a first order reaction there is only one particle in the rate determining step

In a second order reaction, two particles must collide

In a third order reaction, three particles must collide

50
Q

If the Rate= K[C3H7I] [Br-]

What reactants does the rate determine step have?

A

C3H7I + Br- —> products

51
Q

If the Rate= K[C4H9I]

What reactants does the rate determine step have?

A

C4H9I —> products

52
Q

What is important to note about the reactants in the rate determining step?

A

It is not possible to identify conclusively the products of the rate-determining step in every case; however it is possible to suggest the products, they must balance like any other equation e.g:

C3H7I +Br- —> C3H7Br + I-

Chemistry Unit 3 (6 decks)

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