Unit 6: Chemical Kinetics ; (HL) Topic 16.1 Rate expression and Rate Mechanism

Question 1

What are Rate equations?

Answer 1

experimentally-determined equations

Question 2

what does the rate equations relate the rate of reaction to?

Answer 2

it relates the rate of reaction to the concentrations of substances in the mixture.

Question 3

what is Rate Constant(K)?

Answer 3

a constant of proportionality

Question 4

what does the rate constant relate the concentrations in the experimentally-determined rate expression to?

Answer 4

it relates the concentrations in the experimentally-determined rate expression to the rate of a chemical reaction.

Question 5

The rate constant is only a constant for…

Answer 5

a particular reaction at a particular temperature.

Question 6

what is the order of reaction?

Answer 6

the power of the reactant’s concentration in the experimentally-determined rate equation.

Question 7

what is the order of a reaction always with respect to?

Answer 7

a particular reactant

Question 8

what is the Individual order?

Answer 8

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

Question 9

what is the Overall order?

Answer 9

The sum of all the individual orders in the rate equation

Question 10

in the rate equation r = k[A][B]2
the order with respect to A is?
the order with respect to B is ?
the overall order is?

Answer 10

the order with respect to A is 1 1st Order
the order with respect to B is 2 2nd Order
the overall order is 3 3rd Order

Question 11

The value is zero if the rate is…

Answer 11

unaffected by how much of a substance is present.

Question 12

The units of k depend on what?

Answer 12

the overall order of reaction.

Question 13

r = k[A] k will have units of?
r = k[A]^2 k will have units of?
r = k[A][B]^2 k will have units of?

Answer 13

r = k[A] k will have units of sec^-1
r = k[A]2 k will have units of dm^3 mol^-1 sec^-1
r = k[A][B]2 k will have units of dm^6 mol^-2 sec^-1

Question 14

how can the rate equation be determined?

Answer 14

the rate equation can only be determined from experimental data.

Question 15

what is the connection between the chemical equation for a reaction and the rate equation?

Answer 15

there is none

Question 16

write a standard answer to the question: write the individual orders for A and B

Answer 16

When [B] is kept constant [A] is — (e.g. tripled). The rate also — (e.g. triples) (2 to 6). Therefore rate ∝ [A], so the order of reaction with respect to A is 1.

When [A] is kept constant [B] is — (e.g. doubled). The rate — (e.g. quadruples) (2 to 8). Therefore rate ∝ [B]^2, the order of reaction with respect to B is 2.

Question 17

write a standard answer to the question: what is the overall order of reaction?

Answer 17

The overall order is the sum of the individual orders
= (e.g.) 1 + 2 = 3

Question 18

write a standard answer to the question: what is the rate equation?

Answer 18

By combining the two proportionality relationships you can construct the overall rate equation;
therefor rate = k[A][B]

Question 19

how can the variation in rate be investigated using a graph?

Answer 19

by measuring the change in concentration of one of the reactants or products, plotting a graph, and then finding the gradients of the curve at different concentrations.

Question 20

the rate reaction at any moment can be found from the…

Answer 20

gradient of the tangent at that point.

Question 21

if the gradient is steeper, what can you conclude?

Answer 21

the rate of reaction is faster

Question 22

the order of reaction can be found by measuring what?

Answer 22

the rate at different times during the reaction and plotting the rate against either concentration or time.

Question 23

what is the shape of a curve for zero order? (concentration vs time)

Answer 23

a straight line showing a constant decline in concentration

Question 24

what is the shape of a curve for first order? (concentration vs time)

Answer 24

A slightly sloping curve that drops with a constant half-life

Question 25

what is the shape of a curve for second order? (concentration vs time)

Answer 25

the curve steeply declines at first then levels out.

Question 26

what is the shape of a curve for zero order? (rate of reaction vs concentration)

Answer 26

the rate does not depend on the concentration. The line is parallel to the x-axis.

Question 27

what is the shape of a curve for first order? (concentration vs time)

Answer 27

the rate is proportional to the concentration so you get a straight line of fixed gradient. The gradient of the line equals the rate constant for the reaction.

Question 28

what is the shape of a curve for second order? (rate of reaction vs concentration)

Answer 28

the rate is proportional to the square of the concentration. You get an upwardly sloping curve.