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Aqa a level chemistry > Topic 5: Kinetics > Flashcards

Topic 5: Kinetics Flashcards

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1
Q

Explain how rate of reaction could be increased and how it can be measured

A

By crushing the solid into a powder surface area increases which increases rate of reaction. More particles are available so more frequent and successful collisions.

or increase conc of solution then increase temperature

Measure gas produced in a given time

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2
Q

true or false - pressure only affects gaseous reactions

A

true - no change for liquids

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3
Q

rate of reaction

A

-Reactions take place over a certain amount of time. Some reactions occur more quickly than others ctions occur more qui

-Rate of reaction –> the speed in which reactants are turned into products

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4
Q

equation for rate of reaction

A

change in conc / time

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5
Q

Activation energy

A

Activation energy = the minimum amount of energy required for a reaction to take place

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6
Q

Collision theory

A

-For a reaction to occur –> particles must collide with the correct orientation and particles must have sufficient energy

-Reactants need kinetic energy

-collisions and activation energy are required for a successful reaction to occur

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7
Q

Factors affecting rate

A

-pressure

-concentration

-surface area to volume ratio

-temperature

-catalyst

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8
Q

how to make a reaction faster

A

-increase number of collisions

-lower activation energy

-increase energy of particles

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9
Q

concentration

A

A concentrated solution would have a faster rate of reaction than a diluted solution because it has a higher number of particles due to higher chance of collisions occuring

-decrease rate of reaction using a more dilute solution

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10
Q

explain how using pure oxygen instead of air would affect the rate of burning

A

-rate of burning would increase using pure oxygen

-greater concentration of O2 particles in a given volume

-more successful and frequent collisions

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11
Q

pressure

A

-increasing pressure increases the number of particles in a given volume which increases the amount of frequent and successful collisions

-gas = greater pressure = faster rate of reaction

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12
Q

surface area

A

-increasing surface area will increase rate of reaction as the surface area to volume ratio is higher. This increases the amount of collisions between particles and are more frequent in nature

-Larger surface area to volume ratio means size of particles are smaller so greater rate of reaction due to more successful collisions

Increase the surface area of a solid by crushing it down into a powder/smaller pieces

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13
Q

temperature

A

-rate of reaction is greater at a higher temperature as particles have more kinetic energy

-At higher temperatures a higher proportion of molecules have sufficient energy = more successful collisions

-Energy –> frequent collisions –> successful collisions

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14
Q

calculating rate of reaction

A

Gradient = rate of reaction

Rate of reaction = change in concentration (reactant used or product formed) / change in time

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15
Q

measuring rate of reactions experimentally

A

-In order to measure rate of reaction, we need to measure the amount of reactant being used or the amount of product being produced

-For example, you can measure the volume of hydrogen gas formed in a reaction

-When talking about measuring products we cannot say amount –> we have to say mass, volume or concentration

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16
Q

what is rate of reaction proportional to

A

Rate of reaction is proportional to 1/time

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17
Q

initial rate experiments

A

-Silver nitrate was added to three test tubes containing different haloalkanes

1 – 20mins and 50 seconds

2 – 9 mins and 15 seconds

3- 5 seconds

Rate of reaction is proportional to 1/time

1/1250 : 1/555 : 1/5 = 1:2:25:250 (divide by smallest) = 1/1250

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18
Q

graphs/curves for powder

A

volume = half
slope = shallow

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19
Q

what is correct about gas molecules

A

at a fixed temperature, their average kinetic energy is constant

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20
Q

explain why the reaction is fastest at the start

A

-larger concentration of reactants at the start
-this means that there were more frequent and successful collisions

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21
Q

explain how a catalyst increases the rate of reaction

A

-catalyst provides a different pathway with a lower activation energy

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22
Q

true or false - the lower the temp the higher the equillbrium yield

A

true

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23
Q

explain why an increase in temperature increases the rate in which gas decomposes

A

-more molecules with activation energy
-higher rate of successful collisions

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24
Q

y and x axis

A

x = energy
y = number of molecules

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25
Q

State what happens to the rate of reaction when temperature is increased

A

-average kinetic energy of molecules increases
-more molecules have sufficient energy greater than or equal to the activation energy
-this means there are a greater proportion of more frequent and successful collisions

26
Q

true or false - as temperature increases more particles have sufficient energy

A

true

27
Q

How does the curve change when temperature is increased

A

the peak decreases in height and moves to the right

28
Q

What would be the effect to the curve if reactant concentrations were increased

A

there would be no change

29
Q

what would the effect on the curve if a catalyst was added to the activation energy

A

activation energy would move to the left and the peak would be unchanged

30
Q

if the temperature were lowered what would be the effect on the shape of the curve

A

the peak would shift to the left and be higher

31
Q

what would shift the activation energy line to the right

A

the removal of a catalyst

32
Q

Use your understanding of collision theory to explain why the student did not obtain a straight line

A

-as concentration increases, rate of reaction increases as there are more particles in a given volume so more frequent and successful collisions occur so gradient becomes steeper.
-however, the reaction is not a straight line as the amount of pure magnesium remains unchanged

33
Q

reversible reaction

A

two directions each with different outcomes to a reaction

-If the forward reaction has an enthalpy change of –92KJ/mol the enthalpy change of the backwards reaction will be +92KJ/mol

34
Q

thermodynamically feasible

A

a reaction which is most likely to occur based on the enthalpy changes of reaction

An exothermic reaction is more thermodynamically feasible as it does not require energy overall to occur

35
Q

kinetically stable

A

Some reactions do not occur unless high temperatures are provided. It is therefore said to be kinetically stable and has a high activation energy

36
Q

what does a maxwell boltzmann distribution graph show

A

A maxwell-boltzmann distribution graph shows us the energy and speed different particles have bc not all particles have the same amount of kinetic energy

37
Q

maxwell boltzmann curve

A

-Area under the curve represents the total number of particles in the sample

-The line to the right represents the activation energy. The number of particles with at least the activation energy to the right of the line is shaded (these are the reacting particles)

38
Q

effect of temperature on maxwell boltzmann curve

A

-changing the temperature of the reaction will change the shape of the energy distribution curve without changing the activation energy

-When we increase the temperature, the proportion of particles with higher energies increase and so the graph shifts to the right and the peak is lower

-The area under the curve remains the same and therefore the number of molecules remains the same

39
Q

temp on curve

A

As we increase temp, the curve shifts right

As we decrease temp, the curve shifts left

As temperature increases the area to the right of the activation energy increases

40
Q

catalysts

A

-not used up during the reaction but increases rate of reaction

-starting mass of a catalyst equals the final mass of a catalyst

41
Q

definition of catalyst

A

Catalysts provide an alternative reaction pathway with a lower activation energy so a higher proportion of molecules have an energy greater than the activation energy

42
Q

what is catalysis

A

One way a catalyst can work is by changing during the reaction and then changing back to reform the catalyst (alternative reaction pathway) -present in reactants and products

43
Q

types of catalysts

A

Heterogeneous –> the catalyst and reactants are in different phases

Homogenous –> catalyst and the reactants are in the same phase

44
Q

heteregenous catalysts

A

-different phase to reactants

-A catalytic converter is a catalyst which reduces toxic emissions from a car engine by converting harmful exhaust products into harmless ones

-reactants = gas catalyst = solid (catalytic converter)

45
Q

steps for heterogeneous catalysts

A

1) During heterogenous catalysis the reactants adsorb on the surface of the catalyst onto an active site

2) A reaction occurs when another reactant molecule collides with the adsorbed molecule. This forms the products

3) The products desorb from the surface of the catalyst

46
Q

adsorption, reaction, desorption

A

Adsorption –> a molecule forms weak bonds with the surface of the catalyst

Desorption –> the weak bonds with the surface of the catalyst break and the molecule leaves the surface

1) adsorption of reactants

2) reaction occurs

3) desorption of products

47
Q

effect of adsorption on activation energy

A

Effect of adsorption of activation energy –> adsorbing reactant in a particular orientation and weaking bonds in the reactant

48
Q

steps - catalysts

A

-adsorption increases the chance of collision by holding one or more of the reactants in a stationary position

-by holding the reactants in the best geometry for reaction or by weaking bonds in the reactants, this process creates an alternative reaction pathway with lower activation energy

-once the reaction has taken place on the surface of the catalyst the product desorbs

Step 1 (fast) –> gasesous reactants + active site –> adsorbed reactants

Step 2 (slow) –> adsorbed reactants –> adsorbed product

Step 3 (fast) –> adsorbed product –> gaseous product + empty active site

49
Q

role of catalytic coverters

A

Catalytic converters are designed to remove CO and NO produced by internal combustion engines.

50
Q

Explain why a small decrease in temperature can lead to a large decrease in the rate of reaction

A

fewer molecules have sufficient energy greater than or equal to the activation energy. Less frequent and successful collisions occur

51
Q

State two ways of speeding up a gas phase reaction other than by changing the temperature

A

-increase pressure
-add a catalyst

52
Q

suggest two factors that suggest hydrogen bromide is behaving as a catalyst

A

-unchanged
-offers and alternative reaction route

53
Q

explain why most collisions do not result in a reaction

A

not sufficient energy

54
Q

maxwell boltzmann distribution
(spread of energies at a specific temperature)

A

-only few particles have energy greater than Ea

-Emp = most probable energy (peak)

-area under curve = total number of molecules present

-mean energy is past the peak

-starts at origin (0) because there are no molecules with no energy

55
Q

higher temp

A

at higher temperatures most probable and mean energy shift higher but less molecules have this much energy (right but lower)

-total number of particles however is constant but their energy is different

56
Q

gradient of curve

A

=rate of reaction

higher concentration = faster rate = steeper gradient

57
Q

as temperature increases

A

a larger proportion of molecules have energy greater than or equal to the activation energy

58
Q

catalyst

A

Definition: Catalysts increase reaction rates without getting used up.
Explanation: They do this by providing an alternative route or mechanism with a lower
activation

with catalyst activation energy on Maxwell boltzmann distribution shifts left

59
Q

using the reactants suggest why small amounts of the reactants are used in this experiment

A

SO2 is toxic

60
Q

suggest why the student chose not the carry out experiments at low temperatures

A

reaction would be too slow

Aqa a level chemistry (19 decks)

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