Orders from rate-concentration graphs 120-121

Question 1

Draw the rate vs concentration graph of a zero order reaction.

Answer 1

• The rate is unaffected by changes in concentration.

Question 2

Draw the rate vs concentration graph of a first order reaction.

Answer 2

• If the order is 1 with respect to a reactant B, then rate = K x [B]¹.

Question 3

Draw the rate vs concentration graph of a second order reaction.

Answer 3

• If the order is 2 with respect to a reactant C, then rate = k x [C]².

Question 4

Explain the two ways of calculating the initial rate of a reaction ( just mention them don’t go into detail.).

Answer 4

• Initial rate can be determined from a concentration-time graph. A tangent is draw at t = 0 and the rate is the gradient of this tangent.
• Another method is to measure the time for a certain amount of product to be formed. During a clock reation, you are measuring the time,t, for the iniital part of the reaction to take place. To a good approximation, the initial rate is proportional to 1/t.

Question 5

Explain how clock reactions allow us to calculate the initial rate of a reaction.

Answer 5

A clock reaction measures the time from the start of the reaction until there is a visual change(preferably sudden) such as:

• appearance of a precipitate.
• disappearance of a solid
• A change in colour.

During a clock reaction, you are measuring the time, t, for the initial part of the reaction to take place. To a good approximation, the initial rate is proportional to 1/t.

You carry out a series of clock reactions, varying each reactant in turn. You then plot a graph of initial rate against initial concetration for each reactant.