Rates and Equilibrium

Question 1

what is the rate of reaction equal to? (graph)

Answer 1

the change in concentration over time

- the gradient of a conc/time graph

Question 2

what is the rate equation?

Answer 2

where A + B –> products
rate = k[A]^m[B]^n
[A] is the initial conc of A
[B] is the initial conc. of B
k is the rate constant
m is the order of reaction with respect to A (0, 1 or 2)
n is the order of reaction with respect to B (0, 1 or 2)

Question 3

What is meant by the term rate constant, or k?

Answer 3

relates to the rate of a chemical reaction with the reaction concentrations. The larger the value of k, the FASTER the reaction goes.

Question 4

What is the order of a reaction?

Answer 4

the power to which its concentration is raised in the rate equation
total order = the sum of the orders for each reactant

Question 5

how are the units for k written?

e.g. for rate = k[A]^1

Answer 5

dm3mol-1s-1

Question 6

what is the rate determining step?

Answer 6

A step in the reaction mechanism that occurs much more slowly than the others and thus determines how fast a reaction goes - the slowest step in a sequence of steps.

Question 7

If the order with respect to a particular reactant is 1 what is the relationship between its concentration and the rate of the reaction?

Answer 7

Directly proportional: as the concentration doubles, the rate doubles

Question 8

If the order with respect to a particular reactant is 2 what is the relationship between its concentration and the rate of the reaction?

Answer 8

The rate will change by a factor of the square of the concentration - if the concentration doubles the rate quadruples.

Question 9

What effect do temp. and catalysts have on k?

Answer 9

An increase in temperature increases rate and therefore k must increase.
The lower the activation energy the faster the reaction and the higher the rate constant will be.

Question 10

How can you deduce the rate equation given a table of data showing the initial rate of reaction and the initial concentrations of the reactants where in separate experiments the initial conc. of one reactant is changed whilst the other is kept constant??

Answer 10

If effect of doubling [B] but keeping [A] constant doubles the initial rate, the order wrt B is 1.
If doubling [A] and keeping [B] constant quadruples the initial rate the order wrt A is 2.

k = [A]^2[B]1

Question 11

Why must other reactants be kept in excess when working out order of reaction wrt a chosen reactant?

Answer 11

So that changes in their concentrations are less significant

Question 12

What order is a reactant whose concentration/time graph shows a descending straight line?

Answer 12

Rate remains the same as the conc. reactant decreases - order wrt reactant is 0

Question 13

What order is a reactant whose concentration/time graph shows a curve?

Answer 13

IF two successive half lives are constant the order is 1,

If the half life doubles the order is 2

Question 14

How do you find the order of a reactant using a rate-concentration graph?

Answer 14

1. A horizontal line shows that the rate remains constant as the concentration changes and therefore the order is 0
2. An ascending straight line through the origin shows that rate is proportional to concentration and the order is 1
3. An ascending curve (half parabola) shows that the order is 2 - to prove, plotting rate against the square of the concentration of that reactant will give a straight line through the origin.

Question 15

What order are reactants that are not in the rate equation?

Answer 15

0

Question 16

what is the rate determining step for a reaction where rate = k[A][B]

Answer 16

A + B –> AB

Question 17

propose steps in a reaction mechanism from the rate equation rate = k[A][B] and the balanced equation 2A + B –> A2B for the overall reaction

Answer 17

Rate determining step:
A + B --> AB
 & another fast step:
A + AB --> A2B
Combining the 2 equations gives the overall balanced equation.

Question 18

explain the effect of changing temperature on the value of Kc for exothermic and endothermic reactions;

Answer 18

Exothermic: increase in temp. means a decrease in value of Kc because position of equilibrium shifts left
Endothermic: increase in temp means increase in value of Kc because the position of equilibrium shifts right

Question 19

What is the effect of changes in concentration on the value of Kc?
Pressure?
Presence of a catalyst?

Answer 19

Kc is unaffected