Topic 16: Kinetics II (chemical kinetics)

Question 1

Why does the rate of reaction decrease over time?

Answer 1

The concentration of reactants decreases

Question 2

Units for rate of reaction

Answer 2

moldm^-3s^-1

Question 3

How to find the rate of reaction at a particular time

Answer 3

Draw a tangent - t=0 for initial rate

Question 4

Experimental methods to measure reaction rates

Answer 4

Colorimetry - change in colour or precipitate formed
Titration - if acids/alkalis are used which can be neutralised or if iodine is used/produced
Loss in mass with balance - gas formed
Volume of gas produced with syringe - gas formed
pH measurements with pH probe - change in pH if acid/alkali is produced/consumed

Question 5

Issues with measuring gas produced

Answer 5

There is gas in syringe - higher value than expected
Bung is not placed quickly enough after adding reactant - lower value than expected
Uncertainty in syringe

Question 6

Issues with measuring loss of mass

Answer 6

Place on open vessel on mass balance
Delay on adding cotton wool - acid escapes - lower value than expected
Loss of mass may be small if mass eg gas is small eg hydrogen
Uncertainty on balance

Question 7

When does the rate constant (k) not remain constant

Answer 7

If temperature is changed or a catalyst is used

Question 8

What do the orders determine

Answer 8

How the concentrations of reactants affects the rate of reactions

Question 9

What is the effect of zero order on the rate of reactions

Answer 9

It has no effect
If A doubles, the rate of reaction doesn’t change
Doesnt appear in rate equations as (A)^0=1

Question 10

What is the effect of first order on rate of reaction

Answer 10

The rate of reaction is proportional to conc. A^1
If A doubles, rate doubles

Question 11

What is the effect of second order on rate of reaction

Answer 11

The rate of reaction is proportional to conc. A^2
If A doubles, rate quadruples
If A were multiplies by 4, reactions would multiply by 4^2 (16)

Question 12

How can orders of reactions be found

Answer 12

By experiment

Question 13

Units of k

Answer 13

Depend on rate equation - have to cancel out

Question 14

Effect on raising temperature on value of k

Answer 14

Increases

Question 15

Effect of adding a catalyst on value of k

Answer 15

Increases

Question 16

Effect of increasing concentration of value of k

Answer 16

No change

Question 17

Rate equation includes reactants/products?

Answer 17

reactants only

Question 18

Half life

Answer 18

The time taken for the concentration of a reactant to half

Question 19

How to find reaction orders from experiments

Answer 19

1. Continuous monitoring - measure the quantity of a reactant at interval (every 30 secs) during the course of a reaction
2. Using initial rates - change conc of one reactant at a time: find from graph or clock reaction (how long it takes for a certain amount of product to be made)

Question 20

Concentration-time graph for first order

Answer 20

Straight line of best fit

Question 21

Concentration-time graph for first order

Answer 21

Curve - constant half life

Question 22

Concentration-time graph for second order

Answer 22

Curve - half life is not constant (increases significantly)

Question 23

Rate-concentration graph for zero order

Answer 23

Straight line across - no effect

Question 24

Rate-concentration graph for first order

Answer 24

Directly proportional

Question 25

Rate-concentration graph for second order

Answer 25

Curve - exponential

Question 26

How to obtain a rate-concentration graph

Answer 26

Make tangents of the time-concentration graph

Question 27

How to plot a log graph

Answer 27

log(rate) on y axis log(conc) on x axis gradient is order log(rate)= logk + nlog[A]

Question 28

How to write answers to deducing order from experiments

Answer 28

Order wrt [A] from experiments 1 and 2 [A] 0.5 --> 1.0 doubles [B] is unchanged Rate 0.2 --> 0.4 doubles Rate (initial) is proportional to [A] Therefore order wrt [A] is first

Question 29

Initial rate of reaction =

Answer 29

1/time when a fixed concentration (C1) is used (same time)

Question 30

Why is initial rate=1/t an approximation

Answer 30

The rate could be changing as we can't calculate rate at t=0 exactly

Question 31

What does the Arhenius equation show

Answer 31

The effect of changing temperature/catalyst on the rate constant Can work out the activation energy from it

Question 32

Arrhenius equation

Answer 32

ln(k) = ln(A) - Ea/RT can use ln(rate)

Question 33

Graph to find activation energy

Answer 33

ln(rate) on y axis 1/T in Kelvin on x axis - (often 10^-3)

Question 34

What is the reaction mechanism

Answer 34

The sequence of elementary steps that leads to the formation of a product

Question 35

What types of elementary steps are there

Answer 35

Unimolecular Bimolecular Termolecular

Question 36

What order is unimolecular elementary step

Answer 36

A --> product rate = k[A] first

Question 37

What order is bimolecular elementary step

Answer 37

A+A or A+B --> product rate = k[A]^2 or k[A][B]

Question 38

What order is termolecular elementary step

Answer 38

A+A+A or A+A+B --> product rate = k[A]^3 or k[A]^2[B]

Question 39

The slowest step

Answer 39

The Rate Determining Step - determines the overall rate of reaction

Question 40

What species appear in the rate equation

Answer 40

The ones in the RDS

Question 41

What is the overall order of an SN1 reaction

Answer 41

First as there is only one species in the RDS

Question 42

What is the overall order of an SN2 reaction

Answer 42

Second as there are two species in the RDS

Question 43

What are four examples of clock reactions

Answer 43

Iodine clock reaction Bromine clock reaction Sulphur clock reaction Reaction of iodide ions with peroxodisulphate (VI) ions

Question 44

Describe the sulphur clock reaction

Answer 44

Add a dilute solution of sodium thiosulphate to a beaker with water, making it 25cm in total Add 25cm of HCl and stir and record the time it takes for enough sulphur to be produced to conceal the X below the beaker Repeat with different amount of sodium thiosulphate S2O3^2- + 2H+ --> S(s) + H2O + SO2

Question 45

Describe the iodine clock reaction

Answer 45

Oxidation of I- by H2O2 in acid solution 50cm of H2O2 and water (50:50) Mix with 25cm of KI (aq) and 5cm of sodium thiosulphate H2O2 + 2I- + 2H+ --> 2H20 + I2 I2 reacts instantaneously with sodium thiosulphate until it us used up I2 + 2S2O3^2- --> 2I- + S4O6^2- I- is colourless When all sodium thiosulphate is used up, I2 reacts with starch indicator to produce a blue/black colour Repeat with different relative amounts of H2O2 (still 50cm total)

Question 46

Describe the bromine clock reaction

Answer 46

Beaker with bromate (V) ions and water and methyl red indicator Another beaker with Br- ions and a little bit of phenol Mix beakers and record time for colour change to occur 5Br- + BrO3- + 6H+ --> 3H2O + 3Br2 Br2 reacts instantaneously with phenol 3Br2 + C6H5OH --> C6H2OHBr3 + 3H+ + Br- (colourless) When phenol is used up, Br2 reacts with methyl red indicator (pink in acidic solution) and bleaches it

Question 47

Describe the reaction of iodide ions with peroxodisulphate (VI) ions

Answer 47

S2O8^2- + I- --> 2SO4^2- + I2 I2 + S2O3^2- --> 2I- + S4O6^2- I2 + starch --> blue/black complex

Question 48

Why does H+ appear in the rate equation for the reaction between iodine and propanone

Answer 48

It is a catalyst for the reaction, known as autocatalysis