Determining orders of reaction

Question 1

orders of reaction can only be determined by

Answer 1

experiment

Question 2

the two methods for determining order of reaction are

Answer 2

• continuous method

- initial-rate method

Question 3

the continuous method is when

Answer 3

• one reaction miture is made up and samples of the mixture (aliquots) are withdrawn at timed intervals
• the aliquot reactions are stopped, usually through quenching
• the aliquots are then titrated to determine the concentration of the reactant
• a concentration-time graph is drawn
• half lives are obtained from the graph
• the series of half lives determines the order

Question 4

if the half lives are consistent, then the reaction is ………..order

Answer 4

1st

Question 5

if the half life doubles as the reaction proceeds, the reaction is….order

Answer 5

2nd

Question 6

if the concentration-time graph is a straight line with a negative gradient, then the reaction is….order because

Answer 6

0 order

because the reaction is constant no matter what the concentration of the reactant

Question 7

the half-life of a reaction is

Answer 7

the time taken for the concentration of the reactant to fall to one-half of its initial value

Question 8

the instantaneous reaction rate is

Answer 8

the gradient of a tangent drawn to the line of the graph of concentration against time. the instantaneous rate varies as the reaction proceeds (except for a zero order reaction)

Question 9

the rate of reaction at any given time can be determined from a concentration-time or volume-time graph by

Answer 9

drawing a tangent to the curve at the given time and calculating the gradient of the tangent

Question 10

gradient=

Answer 10

change in y / change in x

Question 11

the initial-rate method is when

Answer 11

• several reaction mixtures are made up and the initial rate is measured for the time taken for a fixed amount of reactant to be used up or a fixed amount of product to be made
• from these time, we can see how different concentrations of reactant then affect the rate of reaction

Question 12

for the initial rates method, the rate of reaction is directly proportional to

Answer 12

the reciprocal of the time
1
rate ∝ —–
t

Question 13

a zero order rate-concentration graph with respect to a reactant or product will look like

Answer 13

a flat lstraight line

Question 14

a f irst order rate-concentration graph with respect to a reactant or product will look like

Answer 14

a straight line with a positive gradient

Question 15

a second order rate-concentration graph with respect to a reactant or product will look like

Answer 15

a curved line going upwards

Question 16

when a curved rate-concentration graph is obtained, it is necessary to……………..to prove that the reaction is indeed second order

Answer 16

• plot the rate (1/t) against the square of the concentration of the reactant or product ([A]2)
• this should be a straight line graph with the line going through the origin