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A Level Chemistry > Kinetics 2 > Flashcards

Kinetics 2 Flashcards

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1
Q

What is order of reaction?

A

The sum of the powers to which the concentrations of reactants are raised in the experimentally determined rate equation

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2
Q

What is the partial order of one reactant?

A

It is the power to which the concentration of that reactant raised in the rate equation

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3
Q

What is rate constant, k?

A

The constant of proportionality that connects the rate equation with the concentration of the reactants

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4
Q

What would the rate equation be for a reaction:
A + 2B + 3C -> products
Where it is first order with respect to A and B, and second order with respect to C?

A

Rate = k[A][B][C]^2

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5
Q

What does the rate against concentration graph look like for a first order reaction?

A

A straight line graph passing through the origin, as rate is directly proportional to concentration

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6
Q

What units would k have in a first order reaction with rate equation rate = k[A]?

A

s^-1

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7
Q

What happens to the rate of a first order reaction when the concentration doubles?

A

Rate also doubles

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8
Q

What does the rate against concentration graph look like for a second order reaction/second order with respect to a particular reagent?

A

A curve, double concentration gives quadruple the rate

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9
Q

How does changing concentration of a reagent that is second order affect rate?

A 
It will square the number that the concentration has been multiplied by
e.g. 2x concentration = 4x rate
3x concentration = 9x rate
4x concentration = 16x rate
n x concentration = n^2 x rate
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10
Q

In the second order reaction:
H2 + I2 -> 2HI
Where it is first order with respect to hydrogen and iodine, how does concentration affect rate?

A

If [H2] doubles, but [I2] is constant, rate doubles
If [H2] trebles and [I2] trebles, rate is 9x faster
if [H2] increases n fold and [I2] increases m fold, rate increases by nm

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11
Q

What is the rate equation for H2 + I2 = 2HI, which is second order overall, but first order with respect to each reagent?

A

Rate = k[H2][I2]

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12
Q

What are the units for k in a second order reaction?

A

mol^-1 dm^3 s^-1

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13
Q

In the third order reaction:
2NO + O2 -> 2NO2
Where it is second order with respect to [NO] and first order with respect to [O2], how does concentration affect rate?

A

If [NO] doubles and [O2] is constant, rate becomes 4x faster
If [NO] trebles and [O2] doubles, rate becomes 8x faster
If [NO] halves and [O2] doubles, rate becomes half as fast
If [NO] increases n fold and [O2] increases m fold, rate increases me n^2m times

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14
Q

What are the units for k in a third order reaction?

A

mol^-2 dm^6 s^-1

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15
Q

What does the rate against concentration graph look like for a zero order reaction?

A

A straight horizontal line. Changing concentration has no effect on rate

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16
Q

What are the experimental methods for measuring rate?

A 
Quenching and titration
pH measurements
Colorimetry
Collecting gas evolved
Change in mass
Electrical conductivity
Polarimeter measurements
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17
Q

Outline the method for measuring rate of reaction using quenching and titrating

A

Measure out samples of the reactants with known concentrations
Mix them together, start a clock and stir the mixture thoroughly
At regular time intervals, withdraw samples using a pipette and quench the reaction. This can be done by adding the solution to ice-cold water or to a solution that reacts with one of the reactants or a catalyst to prevent further reaction from taking place
The quenched solution is then titrated against a suitable standard solution. The titre is proportional to the concentration of reactant or product being titrated

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18
Q

Outline the process of using pH measurements to measure rate of reaction

A

If H3O^+ ions are produced or removed in a reaction, the pH will change and this can be measured (pH = -log[H3O^+])
Therefore if a pH meter is placed in a solution, the change of pH with time will mirror the change of concentration of H3O^+ ions with time. If concentrations of the initial reactants are known then it is possible to find their concentrations at any time from the pH

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19
Q

Outline the process of using colorimetry to measure rate of reaction

A

If a reactant or product is coloured then a colorimeter is able to measure the concentration of the coloured species. The amount of light of a particular frequency that is absorbed depends on the concentration of the coloured substance.
See how the concentration of the coloured substance varies with time by following the variation of absorbance to find the order of reaction

20
Q

Outline the process of collecting gases evolved to measure rate of reaction

A

If a gas is produced in a reaction, the volume produced can be measured at regular intervals by attaching a gas syringe to the reaction vessel. Since volume of gas is proportional to the moles of gas, it can be used to measure the concentration of the product and then how it varies with time, therefore you can work out order of the reaction

21
Q

Outline the process of using change in mass to measure rate of reaction

A

The loss of mass can be measured as gas escapes from a reaction vessel. The reactants are mixed and placed on a top-pan balance and the mass is measured and recorded at regular time intervals.

22
Q

Suggest a limitation of using change in mass to work out the rate of reaction

A

The changes in mass are very small, particularly with light gases, so a very sensitive balance must be used

23
Q

Outline the process of using electrical conductivity to measure rate of reaction

A

The number and type of ions in a solution will affect how it conducts electricity. As some reactions in solution have ionic balance changes, the change in conductivity that results from the changing balance of ions will reflect the change in concentration of those ions.

24
Q

Outline the process of using polarimeter measurements to measure rate of reaction

A

If a reactant is optically active and the product has a different optical activity or is a racemic mixture, the reaction can be followed by the amount it will rotate the plane of plane polarised light.
The reaction mixture is placed in a cell and the angle of rotation is measured at regular time intervals, and the angle of rotation is proportional to the concentration of the optically active substance

25
Q

What methods are used to maintain the concentration of reactants we do not want to measure?

A

The large excess method

The initial rate method

26
Q

Outline the large excess method to keep the concentration of other reagents constant

A

By using a large excess of all reagents except the one we are measuring, we are able to assume the change in concentration of these reagents stays constant in the reaction, so the concentration time graph shows the effect of the only reagent that is not in excess

27
Q

Outline the initial rate method to keep concentration of other reagents constant

A

At the beginning of the reaction, the concentrations of all reactants are known. By measuring the initial rate of reaction for different concentrations of one reagent while all other concentrations are kept constant, the order of reaction with respect to this reagent can be found. This is tedious unless a clock reaction is used

28
Q

Do the substances in the slow step or the fast step in a mechanism appear in the rate equation?

A

Slow step

29
Q

In what circumstances may a substance appear in a rate equation but not in the balanced equation?

A

When it is a catalyst. It can be a part of the rate determining step, but will not appear as a reactant as it remains unchanged in the experiment

30
Q

Bromine and iodine react with propanone in similar ways. Explain why changing the halogen will not affect the rate of reaction. Both reactions have a rate equation of rate = k[CH3COCH3][H^+]

A

Neither halogen appears in the rate equation, so they do not appear in the rate determining step, meaning changing the halogen has no affect on rate

31
Q

How many species does SN2 have in the rate determining step?

A

2

32
Q

How many species does SN1 have in the rate determining step?

A

1

33
Q

1-bromopropane and sodium hydroxide react via SN2 (primary halogenoalkane). What is the rate equation?

A

Rate = k[CH3HCH2CH2Br][OH^-]

2 species in RDS

34
Q

2-bromo-2-methyl-propane and sodium hydroxide react via SN1 (tertiary halogenoalkane). What is the rate equation?

A

Rate = k[(CH3)3CBr]

1 specie in RDS

35
Q

What is a homogeneous catalyst?

A

A catalyst that is in the same phase as the reactants.

36
Q

How does a catalyst work?

A

They speed up chemical reactions by providing an alternative reaction mechanism for the reaction which has a lower activation energy. A greater proportion of particles will have sufficient energy to react so the frequency of successful collisions will increase

37
Q

What is a heterogeneous catalyst?

A

A catalyst that is in a different phase to the reactants. Generally it is a solid while the reactants are either gases or in solution

38
Q

What is the main advantage of heterogeneous catalysts?

A

They can be easily separated from the reaction mixture

39
Q

How do metal catalysts work?

A

They provide active sites onto which the reactants can bond (adsorb). This weakens or even breaks the bonds between the atoms in the reactant molecules.

40
Q

What is the sequence by which metal catalysts work?

A 
Step 1 (fast) - gaseous reactants + active site -> adsorbed reactants
Step 2 (slow) - adsorbed reactants -> adsorbed product
Step 3 (fast) - adsorbed product -> gaseous product + empty active site
41
Q

How can the effectiveness of a metal catalyst be improved?

A

By increasing surface area. Once all the active sites on the catalyst are used up, the rate cannot increase any further, but increasing the surface area will give more active sites to be used

42
Q

What is a reaction mechanism?

A

A reaction mechanism describes the one or more steps involved in the reaction in a way which makes it clear exactly how the various bonds are broken and made

43
Q

What does the energy level diagram for an SN1 reaction look like?

A

Reactants above products with an intermediate step

44
Q

What does the energy level diagram for an SN2 reaction look like?

A

Reactants above products. Transition state at the top of the curve

45
Q

How to determine rate equation from a mechanism
A + B ⇌ X (fast)
A + X -> C (slow)

A

It could be assumed that the rate equation is rate = k[A][X]
A and X appear in the slow step, but X is not something you start with.
X is made from 1 molecule of A and 1 of B, so its concentration will depend on the concentrations of A and B.
This means that [X] can be replaced with [A][B], so rate equation becomes rate = k[A][A][B] = k[A]^2[B]

46
Q

Using kinetics to determine activation energy for a reaction

A

Fraction of molecules having an energy of Ea (activation energy) is given by
fraction = e^(-Ea/RT)

So rate constant, k, is proportional to this

k = Ae^(-Ea/RT)
Where A, a constant of proportionality is often called the pre-exponential term or Arrhenius factor

This can then become
lnk = lnA - Ea/RT

47
Q

Using lnk = lnA - Ea/RT to plot a graph and what information it can give

A 
lnk = -Ea/R x 1/T + lnA
y    =   mx            + c 
So, for a graph of lnk against 1/T:
-Ea/R is the gradient, and activation energy can be found by Ea = (-m)R
lnA is the y intercept

A Level Chemistry (25 decks)

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