09-214 Physical Chemistry Name_______KEY__[15 pts total]__

Quiz 9 April 13, 2001

A certain reaction of molecular species "S" to form product "P" is known to behave with time according to the equation below. The original amount of S present is S₀.

S = S₀e^-kt

The half-life for the reaction is one hour. What is the rate constant, k? [2 pts]

k=(ln_e2)/t_1/2 = 0.693/(1 hr) = 0.693 hr^-1

What is the expression for the production of P as a function of time if no P is present originally? [3 pts]

[P] = [S₀] - [S]= [S₀] - [S₀]e^-kt = [S₀](1-e^-kt)

The temperature is raised causing the rate constant to double. Will the production rate of P remain unchanged, increase, or decrease? [2 pts]

increase

DISREGARD THE BELOW ERROR-STREWN GARBAGE!

A certain reaction of molecular species "X" with catalyst "C" forms product "Y" according to the following elementary step: C + X --> C + Y. There is no Y originally present, and the concentration of X is originally 0.50 M. One hour later, the concentration of Y is measured to be 0.10 M. What is the rate constant? [2 pts]

Zeroth-order reaction (Y production is linear with time); proportionality constant = k; d[Y]/dt = k

d[Y]/dt = 0.10 M/1 hr = k

What is the half-life in this reaction? [2 pts]

[Y] = kt = 0.10 (M/hr)t; for half of Y (0.25 M) to be produced = half of X consumed takes 2.5 hrs

What is the concentration of Y after two half-lives? [2 pts]

Since [Y] is proportional to t, two half-lives produces [Y] = 2(0.25 M) = 0.50 M

What is the concentration of Y after three half-lives? [2 pts]

It's still 0.50 M because all of the starting material X has been used up.