You perform a series of experiments for the reaction A ? B + C and find that the

Question

You perform a series of experiments for the reaction A ? B + C and find that the rate law has the form rate = k[A] x. Determine the value of x in each of the following cases:

1. There is no rate change when [A]0 is tripled.

2. The rate increases by a factor of 9 when [A]0 is tripled.

3. When [A]0 is doubled, the rate increases by a factor of 8.

You must also answer the following questions:

1. What are the units usually used to express the rates of reactions occurring in solution?

2. From your everyday experience, give two examples of the effects of temperature on the rates of reactions.

3. What is the difference between average rate and instantaneous rate?

Explanation / Answer

Rate of a chemical reaction

For a reaction A ---> B + C

rate = k[A]^x

values of x when,

1. no change on rate when [A]o is tripled : x = 0 [zero-order reaction]

2. rate increases by a factor of 9, when [A]o is tripled : x = 2 [second-order reaction]

3. rate increases by a factor of 8, when [A]o is doubled : x = 3 [third-order reaction]

Also,

1. units of rate in a solution = Molarity/time units

2. Two examples of of effect of temperature on rate of reaction : In general rate increases with increase in temperature.

You can cook something faster (shorter time) at higher temperature than at lower temperature (longer time) for example cooking chicken at 50 oC to 90 oC.

Drying of clothes is slower at lower temperature (say at 25 oC) than at say 35 oC.

3. average rate is total rate change for a process say going from point A to point B. So rate change is (B-A)/time units.

Instantaneous rate is rate change at a particular time, such that at point A or point B. So rate is A(or B)/time units.

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