3.5 Chemical kinetics

Question 1

Different ways of measuring rate

Answer 1

1) Measuring gas volume at constant pressure by using a gas syringe
2) Mass pan balance
3)Colorimetry
4) measuring gas pressure at constant volume
5) sampling + quenching

Question 2

Gas syringe?

Answer 2

labelling
delivery tube
conical flask
HCl
Mg
wire
change in volume at constant pressure
Mg+2Hcl—- Mgcl2+H2

Question 3

Mass pan balance?

Answer 3

Cotton wool
conical flask
mass pan balance
change in mass
cotton wool prevents entry of gases

Question 4

Colorimetry

Answer 4

colour change
turbidity (colourness)
Ns2S2O3(aq) +2HCl(aq)
—- S2(g) + 2 NaCl (aq) + H2O(l) + S(s)

Sodium thiosulfate
Smell of rotten egg
Cloudy

Question 5

Manometer?

Answer 5

measuring gas pressure at constant volume
machine used for measuring pressure reactants + products
N2 (g) + 3 H2(g) ——– 2 NH3(g)

Question 6

Sampling + quenching?

Answer 6

used when no other method is suitable
involves removing small samples of reaction mixture at regular time intervals
the sample is placed in ice cold water - to lower the concentration of the reactants
als to stop the reaction continuing
the sample is tested using titration

Question 7

Equation for the rate of reaction?

Answer 7

change in concentration / time
draw a tangent to the curve close to the line as possible

Question 8

Rate definition?

Answer 8

rate of change of concentration of a reactant or product

Question 9

Rate constant?

Answer 9

the constant in the rate equation which is not affected by changing the concentration of the reactants at a specific temperature

Question 10

Order of reaction?

Answer 10

the power to which the concentration is raised in the rate equation

Question 11

0 order?

Answer 11

the concentration of the reactant does not affect the rate of reactant + will not appear in the rate equation

Question 12

1st order?

Answer 12

if the concentration doubles, the rate also doubles
if the concentration triples, the rate also triples

Question 13

What is the unit for 0 order?

Answer 13

Rate = mol 1 dm -3 s-1
concentration = mol dm-3
K = mol 1 dm -3 s -1

Question 14

what is the unit for 1st order?

Answer 14

Rate = mol 1 dm -3 s -1
concentration = mol 1 dm -3
K = S -1

Question 15

what is the unit for second order?

Answer 15

Rate = mol 1 dm -3 s -1
concentration = mol 1 dm3
k = mol -1 dm -3 s -1

Question 16

Rate determining step?

Answer 16

the slowest step in a reaction mechanism
each step in the mechanism occurs at a different rate with its own rate equation
slowest step which prevents the overall reaction from taking place

Question 17

Mechanism?

Answer 17

a series of steps which occur in a chemical reaction

Question 18

Collision theory?

Answer 18

for a reaction to occur, the reacting particles must collide
the particles must have sufficient energy for the reaction to proceed
In a 2nd order reaction, 2 particles must collide
In a third order reaction, 3 particles must collide
In a first order reaction, only 1 particle in a rate determining step

Question 19

Arrhenius equation?

Answer 19

Increasing temp of a reaction increases the rate explained by collision theory
Increasing temp means there are more collisions with sufficient energy to react.
This is known as activation energy

Question 20

Boltzmann’s distribution curve?

Answer 20

higher temp, lower peak

Question 21

Equation?

Answer 21

K = Ae ^ - Ea/RT
K = rate constant
A = frequency factor
e = Button on calc
Ea = activation energy
R = molar gas
T = temp

Question 22

A?

Answer 22

frequency factor - frequency of the collisions between the particles
( only a constant over a limited temp range)

Question 23

• Ea / RT?

Answer 23

shows the fraction of collisions that possess energy above the activation energy

Question 24

Effects of catalysts on rate?

Answer 24

Catalysts increase the rate of a chemical reaction by providing an alternative route with a lower activation energy
This does not affect the concentrations n rate equation
So it is the rate constant that changes
Reducing activation energy will increase the value of
-Ea/RT and increase the rate constant