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Chemistry Alevel OCR YR 13 > Rates of reaction > Flashcards

Rates of reaction Flashcards

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1
Q

how do you calculate rate from concentration?

A

change in concentration/ time

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2
Q

what is the units for rate?

A

moldm-3s-1

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3
Q

describe a zero order reaction

A

-concentration of reactant has no effect on the rate, the reaction is zero order with respect to the reactant
-any number raised to the power of zero is one
-concentration does not affect the rate

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4
Q

describe the first order of reaction

A

-a reaction is first order with respect to the reactant when the rate depends on its concentration raised to the power of one
-e.g if the conc of reactant is doubled the rate will double

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5
Q

Describe second order reaction

A

-a reaction is second order with respect to a reactant when the rate depends on its concentration raised to the power of two
-e.g if the conc of reactant is doubled, the rate increases by four

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6
Q

what is the rate equation?

A

rate= k [A] to the power of m [B] to the power of n

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7
Q

what is the rate constant?

A

the proportionality constant, number that mathematically converts between the rate of reaction and concentrations and orders.

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8
Q

what is the overall order and how is it calculated?

A

-overall order of reaction gives the overall effect of the concentrations of all reactants on the rate of reaction.
-overall order= sum of orders with respect to reactants

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9
Q

what is the units for k in an overall reaction order of zero?

A

moldm-3s-1

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10
Q

what is the units for k in an overall reaction order of two?

A

dm3mol-1s-1

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11
Q

how do you continuously monitor the rate of reactants producing gases?

A

-monitor by gas collection
-monitor by mass loss

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12
Q

how do you monitor the rate of reactants forming a colour change?

A

colorimeter

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13
Q

how does a colorimeter work?

A

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

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14
Q

how do you calculate the rate of reaction from a concentration/ time graph?

A

calculate the gradient

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15
Q

what does a zero, first and second order look like on a concentration/ time graph?

A

-zero order: straight line with a negative gradient- the gradient is equal to the rate constant
-first order: downward curve with decreasing gradient- the rate constant can be determined using half life
-second order: also downward curve with steeper curve but trailing off more slowly

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16
Q

what is half life, t1/2?

A

time taken for half of a reactant to be used up

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17
Q

Describe half life and first order reactions?

A

first order reactions have a constant half life with the concentration halving every half life- exponential decay. If the successive half-lives are the same, the reaction is first order with respect to the reactant.

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18
Q

Describe the rate/ concentration graphs of a zero order reaction

A

1)zero order:
-intercept on the y-axis gives the rate constant, k
-horizontal straight line with zero gradient

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19
Q

Describe the rate/ concentration graphs of a first order reaction

A

First order:
-produces a straight line graph through origin
-rate is directly proportional to concentration
-rate constant determined by measuring the gradient

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20
Q

Describe the rate/ concentration graphs of a second order reaction

A

-produces upward curve with increasing gradient
-rate constant cannot be determined from this as its a curve
-by plotting a second graph of the rate against the concentration squared, the result is a straight line through the origin. The gradient of this straight line is equal to the rate constant, k.

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21
Q

how do you calculate rate constant from half-life?

A

k=In2/t1/2

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22
Q

how do you calculate initial rate from a graph?

A

extrapolate the graph, tangent and measure the gradient

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23
Q

what is initial rate proportional to?

A

1/t

24
Q

what is the iodine clock experiment and its equation?

A

A reaction where you can time how long it takes for a colour change too occur
H2O2(aq) + 2H+(aq) + 2I-(aq) -> 2H2O(l) + I2(aq)

25
Q

Describe how the iodine clock reaction takes place?

A

-add sodium thiosulfate and starch (indicator) to the reaction mixture
-sodium thiosulfate reacts with I2 produced in this reaction
-when there is no more sodium thiosulfate left then I2 reacts with starch forming a blue/black colour
-time is measure how how long it takes for the cross to disappear
-varying concentrations of I2 or/and H2O2 while keeping else constant will change the time taken therefore the order of reaction can be calculated

26
Q

Describe elementary steps?

A

-Basic steps in a reaction that cannot be broken down any further
-can take place at different rates

27
Q

what is a decomposition, substitution and synthesis elementary step?

A

decomposition- where one molecule breaks up into two
substitution- two particles collide to form two different particles
synthesis- two particles collide as a single particle

28
Q

what is a reaction mechanism?

A

-sequence of elementary steps
-can be multiple steps or a single step

29
Q

what are intermediates?

A

substances produced in one step of a multistep reaction and used up again in other step

30
Q

In a multistep reaction, will the overall rate of reaction change when the concentration of reactants involved before and in the rate determining step change?

A

Yes the overall rate will change. If the concentration increases the rate increases.

31
Q

In a multistep reaction, will the overall rate of reaction change when the concentration of reactants involved after the rate determining step change?

A

The overall rate will not change. The reactants involved after the rate determining step are in zeroth order.

32
Q

What is the rate determining step in a multistep mechanism?

A

the slowest elementary step which determines the overall rate of reaction

33
Q

What is an elementary reaction

A

reaction with one elementary step

34
Q

How do you find out the overall rate equation from knowing the elementary steps?

A

Combine the reactants involved before and in the rate determining step.

35
Q

How do you find out the order of reaction of reactants in an elementary step.

A

The rate order is the number of times the reactant appears before and during the rate determining step. If the reactant is involved after the rate determine step it is zeroth order.

36
Q

How can rate be measured in experiments?

A

-change in pH: pH of a reaction may change of time if H+ ions are used up or produced. pH meter can be used at regular intervals to calculate H+ ion concentration
-Amount of mass lost: If gas is lost
-Volume of gas produced: using a gas syringe and the ideal gas equation
-If a reaction has a colour change, the absorbency can be measured using a colorimeter. The calibration curve (absorbance is plotted against concentration) can be used to identify concentration

37
Q

How do you calculate rate from a straight lined graph?

A

-draw triangle
-calculate gradient

38
Q

How do you calculate rate from a curved graph?

A

-draw a tangent from the specific point on a graph
-calculate gradient from tangent

39
Q

How do you calculate the initial rate from a graph?

A

-extrapolate the tangent at 0 mins
-calculate gradient

40
Q

Describe clock experiments?

A

-a clock reaction is where you can time how long it takes for a reaction to occur
-we time how long it takes until we can’t see the cross through the beaker. There is a colour change known as the endpoint
-the quicker the clock reaction, the faster the initial rate

41
Q

What three assumptions are made in a clock experiment?

A

-the temperature of the reaction remains constant
-the concentration of reactants doesn’t change significantly during the time period of the reaction
-reaction has not proceeded too far when the end point is seen

42
Q

what is the equation for the iodine clock reaction?

A

H2O2(aq) + 2H+(aq) + 2I-(aq) -> 2H2O(l) + I2(aq)

43
Q

Describe the iodine clock reaction?

A

-add sodium thiosulfate and starch (which acts as an indicator) to excess hydrogen peroxide
-The sodium thiosulfate reacts immediately with the I2 that is produced in this reaction
2S2O3 2- + I2(aq) -> 2I-(aq) + S4O6 2-(aq)
-When there is no more sodium thiosulfate left then the I2 reacts with starch to gives a blue/black colour

44
Q

In the iodine clock experiment, how do you work out the order of reaction?

A

By varying the concentration of iodine (I2) and/or hydrogen peroxide (H2O2) and keeping everything else constant, we can use the time taken for the blue/black colour to appear changing.

45
Q

In the iodine clock experiment, how do you work out the order of reaction?

A

By varying the concentration of iodine (I2) and/or hydrogen peroxide (H2O2) and keeping everything else constant, we can use the time taken for the blue/blac colour to appear changing.

46
Q

Describe the rate constant

A

-K (rate constant) increases when temperature increases
-the larger the value of ‘k’, the faster the rate of reaction

47
Q

how do you work out the rate equation from a reaction?

A

1) Repeat the experiment several times but changing the concentrations of reactants in each experiment
2) calculate initial rate for each experiment by using graph
3) Record the concentrations of reactants used for each experiment and their initial rates in a table. From this we can work out the orders with respect to each reactant and write the rate equation

48
Q

what is a catalyst?

A

compound being used up and regenerated. It can be found in the rate equation but not the overall equation.

49
Q

what is the Arrhenius equation?

A

k=Ae (to the power of -Ea/RT)
k=rate constant
A=arrhenius constant
e=exponential
Ea= activation energy/J
R=gas constant/ 8.31KJ-1mol-1
T=temperature/K

50
Q

why does the rate constant(k) get bigger as the activation energy (Ea) gets smaller?

A

As activation energy drops the rate of reaction increases. More particles have enough energy to react when they collide

51
Q

Why does the rate constant (k) increase as the temperature (T) increases?

A

as the temperature increases, the particles have more kinetic energy and are more likely to collide as a result activation energy increases

52
Q

What is the logarithmic form of the Arrhenius equation?

A

lnk= lnA - Ea/RT

53
Q

Describe the linear form of the Arrhenius equation

A

lnk= y
-Ea/R= m (gradient)
1/T=x
lnA= c

54
Q

Describe the linear form of the Arrhenius equation

A

lnk= y
-Ea/R= m (gradient)
1/T=x
lnA= c

55
Q

how do you calculate activation energy (Ea) from gradient?

A

multiply by -(gas constant)

Chemistry Alevel OCR YR 13 (13 decks)

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