Kinetics I & II

Question 1

Effect of Temperature on Rate of Reaction

Answer 1

When temperature is increased, the particles gain kinetic energy

1. With more kinetic energy, the particles will move faster and move around more, meaning there is a higher chance of collision
2. With more kinetic energy, more particles will have energy greater than or equal to the activation energy

Question 2

Effect of a Catalyst on Rate of Reaction

Answer 2

A catalyst lowers the activation energy of the reaction by providing an alternative pathway for the reaction to take so that more particles will have sufficient energy to overcome the energy barrier.

This means that there is a higher chance of succesful collisions

Question 3

Effect of a Surface Area on Rate of Reaction

Answer 3

When surface area is increased,there are more exposed particles meaning there is ahigher chanceofcollision

Question 4

Effect of Concentration on Rate of Reaction

Answer 4

When concentration is increased, there are more particles within the same volume, meaning there is a higher chance of collisions

Question 5

Maxwell Boltzmann Curve

Answer 5

Shows the distribution of energy among different molecules at a particular temperature

Area under curve = number of molecules at that temperature

Question 6

Measuring the Rate of Reaction

Answer 6

Initial Rates: used when only one measurement is taken

Continuous: used when taking measurements at timed intervals

1. Draw a tangent to the curve at the particular time
2. Calculate the gradient of the tangent
3. The gradient is equal to the rate of reaction at that particular time
4. To find the initial rate, the tangent should be drawn at time = 0s

Rate of Reaction = ∆Concentration(mol dm^-3)/Time(s)

Question 7

Order of Reaction and Half Life

Answer 7

The half life is the time taken for the concentration of a reactant to reduce by half, they can be calculated from a graph of concentration against time.

1. Draw lines from the graph to find the time corresponding to a concentration
2. Do the same for half this concentration and find its corresponding time
3. The half-life is the difference between these two times

If the half life is constant, the reaction is first order with respect to the reactant being measured

K = ln2/(1/2 life)

Rate = k[A]^m[B]ⁿ ⇒ order of reaction = m + n

Question 8

Zero Order of Reaction

Answer 8

Reaction rate = k

Half life decreases as time increases

Question 9

First Order of Reaction

Answer 9

Rate = k [A]

Constant half life

Question 10

Second Order of Reaction

Answer 10

Rate = k [A]²

Half life increases as time increases

Question 11

Catalysed Reactions

Answer 11

Homogenous Catalysts: the reactants and the catalyst in the same physical state (often aqueous solutions or gases)

Heterogenous Catalysts: the reactants and the catalysts are in a different physical state (often reactants are gases and catalysts are solids)

Question 12

Heterogeneous Catalysis In Industry

Answer 12

Heterogenous Catalysts: the reactants and the catalysts are in a different physical state (often reactants are gases and catalysts are solids)

Used in:

1. Haber Process to make ammonia using iron
2. Contact Process to make sulfuric acid used vanadium oxide
3. Hydrogenation of margarine uses nickel

Catalysts helping the environment:

• lowers activation energy so less energy needed
• lower temperature can be used
• can avoid harmful waste products

Economic Importance of Catalysts:

• less energy
• lower temperature required
• can encourage specific reactions to avoid unwanted side products
• changes atom economy
• hetergeneous catalysis makes it easy to carry out separations
• catalysts are expensive - increase surface are

Question 13

Titration

Answer 13

Titration

• Used when you want to find out how the concentration of one of the reagents is changing
• It must be possible to neutralise the reactant or product
• if iodine is used, sodium thiosulfate must be present

1. A sample of a known volume is removed from the reaction mixture at a certain time
2. The sample is quenched (cooled or diluted) to slow the reaction
3. The sample is titrated with an appropriate reagent

Question 14

Collecting Gas

Answer 14

Collecting Gas

• Only used if product is a gas

1. Measure the oxygen given off using a gas syringe
2. Record the volume of oxygen collected at regular intervals
3. In order to find the concentration, use the ideal gas equation

Question 15

Colorimetry

Answer 15

Colorimetry

• Used if there is a significant change in colour or a liquid or a solution
• Used if a precipitate is formed

1. The colorimeter detects the amount of light which passes through the reaction vessel
2. If a darker solid or liquid is produced, less light will be detected as the concentration of product increases
3. Use a calibration curve to convert absorbance to concentration

Question 16

Mass Change

Answer 16

Mass Change

• Used if the product is a gas

1. An open reaction vessel is placed on the mass balance
2. Change in mass may be small is the gas has a low density

Question 17

pH Measurements

Answer 17

pH Measurements

• If there is a change in pH (either an acid or alkali is consumed or produced)
  1. A pH probe can monitor the pH of a mixture as the reaction proceeds

Question 18

Reaction Mechanisms

Answer 18

The overall rate of reaction is controlled by the rate of the slowest step known as the rate determining step

Question 19

The Arrhenius Equation

Answer 19

Rate = k[A]^a[B]^b

The effect of temperature or a catalyst on the rate is determined by the rate constant. Altering the temperature or adding a catalyst will change the rate constant.

k = A exp (-E_A/RT)

ln (rate) = ln (A) - E_A/RT

This equation is in the form of a straight line: y = mx + c

ln (rate) = y axis

1/T = x axis

-E_A/R = gradient (should be negative)

ln (A) = y intercept

To find the activation energy:

1. Measure the rate of reaction at a range of different temperatures
2. Calculate the natural logarithm of each
3. Convert temperatures to Kelving and find 1/T
4. Plot a graph of ln (rate) against 1/T and draw a line of best fit

Activation energy (J mol^-1) = - R x Gradient

Question 20

Mechanisms of Reaction

Answer 20

SN1

• This reaction takes place with two or more steps
• The slow step being the rate determining step and often being first order with respect to the organic product
• The reaction is likely to be zero order with respect to the other reactant, meaning it does not have any role in determining the rate, and takes part in steps after the rate determining step

SN2

• This reaction can take place in one step as both the elimination and substitution occur in one step
• The reaction is first order with respect to both reactants

Question 21

Acid-catalysed iodination of propanone

Answer 21

Propanone reacts with iodine in acidic solution (acid catalyst)

1. The reaction is followed by removing small samples from the reaction mixture with a volumetric pipette
2. The sample is then quenched by adding excess sodium hydrogencarbonate to neutralize the acid catalyst which stops the reaction
3. The sample is then titrated against sodium thiosulfate using a starch catalyst
4. yellow brown solid –> colourless

CH₃COCH₃ (aq) + I₂ (aq) → CH₃COCH₂ I(aq) + H⁺ (aq) + I-^-(aq)

2S₂O₃ ^2- (aq) + I₂ (aq) → 2I^- (aq) + S₄O₆^2-(aq)

Finding the rate determining step

The reaction is zero order with respect to I2

The reaction is 1st order with respect to propanone and the catalyst

Rate = k[CH3COCH3(aq)][H+(aq)]

Reaction Mechanism

If there is a zero order reactant, there must be at least two steps in the mechanism, because the RDS will not involve the zero order reactant

• The RDS must contain one propanone molecule and one H+ ion forming and intermediate
• The iodine will be involved in a subsequent faster step