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Chemistry preparation Cap1 > Rate Equations > Flashcards

Rate Equations Flashcards

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1
Q

Define the rate of reaction

A

Change in concentration per unit of time

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2
Q

Define activation energy

A

The minimum energy required for a reaction to occur

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3
Q

Why does a higher temperature increase the rate of reaction

A

More particles have more energy than activation energy, there are more frequent successful collisions so particles react

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4
Q

Why does a higher concentration/pressure increase the rate of reaction

A

More particles in the same volume, more frequent successful collisions

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5
Q

Why does breaking a solid into smaller powder particles increase the rate?

A

More surface area, more frequent successful conditions

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6
Q

What is a catalyst

A

A substance which increases the rate of a reaction
Without being used up

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7
Q

How does a catalyst work

A

Provides an alternative reaction route, with a lower activation energy

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8
Q

Key points for the maxwell-Boltzmann distributions

A

Must start at the origin-> no particles have 0 energy
The highest peak is the most probable energy
Never touches the X axis at the end
Area under the curve tells you the number of particles

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9
Q

What is the calculation of rate

A

Rate= change in concentration/ time

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10
Q

Why do chemical reactions start off with a fast rate and then slow down

A

high conc of reactants means a high rate of collisions. As reactants get used up successful collisions become less frequent, once one or more reactants are used up there are 0 successful collisions

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11
Q

Zero order

A

If changing the concentration of a reactant has no effect on the rate of reaction it is said to be zero order with respect to the rate of reaction.
For example if the concentration of A doubles, the rate remains the same
As a graph this is a horizontal line

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12
Q

1st order

A

Changing concentration is directly proportional to rate of reaction
For example if the concentration of A is doubled, the rate is also doubled
As a graph this is a vertical line

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13
Q

Second order

A

Changing the concentration increases the rate of the reaction by the square of the concentration
For example if concentration A is doubled, the rate is squared.
As a graph this starts of steep and curves

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14
Q

How could you express rate for X+Y—> Z

A

Rate= K[X][Y]2

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15
Q

How do you calculate the units for an order reaction

A

1-the overall order
Moldm-3s-1

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16
Q

Why would you use a large excess of reactant

A

Means the order of reaction is zero with respect to that reactant as the reactants concentration is effectively constant

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17
Q

What are the two types of experiment done to determine the order of a reaction

A

Continious monitoring- following the course of a single reaction
Initial rates method- doing multiple experiments

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18
Q

Continious monitoring method

A

Take samples at regular intervals by using a visible indicator such as gas volumes

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19
Q

What is the problem with the continious monitoring method

A

The reaction is still reacting- quench the reaction, add a large volume of cold distilled water, it will cool and dilute the reaction, decreasing the rate.

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20
Q

Monitoring using a physical property and its problem

A

The volume of gas produced could be measured at regular intervals
Problem- it records the product produced and the order of reaction is about reactants. Volume recorded would need converting to reactant concentration

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21
Q

How to measure a reaction that changes colour

A

Colourimetry

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22
Q

Draw graphs for zero, first and second order for continious monitoring method

A

Reference to picture on phone

23
Q

How to measure the initial rate using a graph produced from the continuous monitoring method

A

Draw a tangent, find the gradient- this will give the rate for that specific time.

24
Q

How to work out an order from a CTT graph- use the example:
Concentration went from 0.1 to 0.03. The rate changed from 2x10-4 to 6x10-5

A

Take two tangents at different concentrations and compare the factor of change in concentration to rates.
Conc and rate have decided by 3
Rate is proportional to change so 1st order

25
Q

How does an initial rate reaction occur

A

The time it takes to get to a specific point in the reaction is recorded for an experiment. E.g- time is recorded when 20cm3 gas is produced or when mixture changes or a ppt is formed

26
Q

Assumptions made in initial rates

A

When changing volume of A, concentration is proportional to volume of sample because total volume is kept the same using water
Rate is 1/ts-1

27
Q

The thiosulfate- acid reaction

A

Na2S2O3 + 2HCl —> 2NaCl + SO2 + H2O + S(makes it cloudy)
Calculate the time it takes for the solution to go cloudy, the volume of S2O32- is changed every time to measure order

28
Q

Why does the concentration not need to be known in this experiment

A

Total volume must be constant. [reagent] is proportional to volume

29
Q

What is the rate of reaction in initial rates

A

1/time. Mass of sulfur formed is not known. However, it is the same each time you repeat it.

30
Q

iodine clock initial rates reaction

A

Measure known volumes of some reagent
Measure known amounts of X or use a calorimeter
In separate containers add A B and C
Start timer at the point of mixing
Time recorded for appearance of blue colour
Use the same conc of B and C and same volume of X
Keep temperature same and constant
Repeat with different concentrations of A
1/Time taken to measure the rate
Plot graph of 1/t against volume of A
Straight line of best fit through origin and find order

31
Q

Zero order from initial rates

A

Change in concentration X has no effect on the rate so x is zero order
Horizontal line

32
Q

First order for initial rates

A

Rate is proportional to change in [H2O2] so H2O2 is zero order

33
Q

Second order for initial rates

A

Rate is proportional to [NH3]2- so is second order

34
Q

Rate and temperatures

A

The higher the temp the faster the rate of reaction, particles have more kinetic energy so a greater proportion of particles have activation energy so greater frequency of successful collisions

35
Q

Arrhenius equation for rate

A

k= Ae^-Ea/RT

36
Q

Equation to find activation energy

A

Find the gradient and multiply by 8.31. Then divide by 1000

37
Q

rearrangement to form A

A

A= K/e^Ea/Rt

38
Q

Rearrangement for Ea

A

Ea= RT(LnA-lnK)

39
Q

Rearrangement for T

A

T= Ea/R x (LnA-lnK)

40
Q

How do you determine which step is the rate determining step

A

Must match the species in the rate equation.

41
Q

Example of the rate determining step
(CH3)3CBr + OH- —> (CH3)3COH + Br-
Step 1- (CH3)3CBr —> (CH3)3C+ + Br-
Step 2- (CH3)3C+ + OH- —> (CH3)3COH

A

Step one only depends on [(CH3)3CBr] whereas step two depends on [(CH3)3C+] + [OH-]
As step one is dependant on the same species at the rate equation, step 1 must be the rate determining step

42
Q

By considering both rate and yield, suggest why the reaction is carried out at 300 and not any higher

A

Would increase rate and decrease yield, chosen temperature compromises both factors

43
Q

The effect of less molecules in a reaction

A

Particles spread apart so less frequent successful collisions between molecules

44
Q

Explain why doubling the temperature has a greater effect on the rate of reaction than doubling the concentration of E

A

Reaction occurs when molecules have energy greater than activation energy- doubling T causes many more molecules to have this whereas doubling E only causes a change in the conc of E

45
Q

State one method that you would use to distinguish these solutions other than smell

A

Measure pH with a meter, methylamine has a higher pH

46
Q

Suggest the role of the cyanide ion and explain your answer

A

Catalyst- appears in the rate equation but not the written

47
Q

State and explain why different volumes of water are added to mixtures in the experiment

A

To make volume constant for all mixtures so that volume of propanone is proportional to concentration.

48
Q

Suggest the purpose of adding sodium Hydrogencarbonate to the reaction

A

To quench the reaction by neutralisation- reaction is ongoing and adding excess will decrease the rate of reaction

49
Q

Suggest why the order with respect to iodine is 0

A

iodine is not involved in the rate determining step

50
Q

Explain why using excess of a concentration is useful

A

Large excess means that concentration of reactant is effectively constant

51
Q

Suggest why initial rates of reaction are used to determine these orders rather than rates of reaction

A

Concentrations are known at 0

52
Q

State the significance of this zero order for the mechanism of the reaction

A

No significance- doesn’t occur in a rate determining step, only A does.

53
Q

State one way this method could be improved over than repeating

A

Establish a constant temp using a water bath, increases accuracy of time for colour change

Chemistry preparation Cap1 (16 decks)

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