15.19 Part C

$\frac{d[R]}{dt}=-k[R]^{2}; \frac{1}{[R]}=kt + \frac{1}{[R]_{0}}; t_{\frac{1}{2}}=\frac{1}{k[R]_{0}}$

Wilson Yeh 1L
Posts: 42
Joined: Fri Sep 29, 2017 7:06 am

15.19 Part C

Determine the value of the rate constant given the following values:

A0: 1.25
B0: 1.25
C0: 1.25
Initial Rate: 8.7

Rate law: k[A][B]^2[C]^2

Using the concentrations given in mmol*L^-1 and rates given in mmol A*L^-1*s^-1, I got the value 2.85. However, in the answer the value is 2.85x10^12. Can someone explain to me where the 10^12 comes from? Thanks!

Janine Chan 2K
Posts: 71
Joined: Fri Sep 29, 2017 7:04 am

Re: 15.19 Part C

So we have rate=k[A][B]2[C]2. The initial rate is given in units (mmolA)*L-1*s-1. Each of the concentrations are given in units mmol*L-1. When we multiply all the concentrations of reactants together, we end up with units mmol*L-5. By dividing (mmolA)*L-1*s-1 by mmol5*L-5 to find units of k, we have units L4*mmol-4*s-1. After converting mmol to mol we have units L4*mol-4*s-1 and the value is 1012.

Wilson Yeh 1L
Posts: 42
Joined: Fri Sep 29, 2017 7:06 am

Re: 15.19 Part C

Janine Chan 2K wrote:So we have rate=k[A][B]2[C]2. The initial rate is given in units (mmolA)*L-1*s-1. Each of the concentrations are given in units mmol*L-1. When we multiply all the concentrations of reactants together, we end up with units mmol*L-5. By dividing (mmolA)*L-1*s-1 by mmol5*L-5 to find units of k, we have units L4*mmol-4*s-1. After converting mmol to mol we have units L4*mol-4*s-1 and the value is 1012.

Thank you so much! I didn't account for this in part A of number 19 and I still got the right answer. Was I supposed to watch out for this when writing the rate law?

Wilson Yeh 1L
Posts: 42
Joined: Fri Sep 29, 2017 7:06 am

Re: 15.19 Part C

Janine Chan 2K wrote:So we have rate=k[A][B]2[C]2. The initial rate is given in units (mmolA)*L-1*s-1. Each of the concentrations are given in units mmol*L-1. When we multiply all the concentrations of reactants together, we end up with units mmol*L-5. By dividing (mmolA)*L-1*s-1 by mmol5*L-5 to find units of k, we have units L4*mmol-4*s-1. After converting mmol to mol we have units L4*mol-4*s-1 and the value is 1012.

Okay I reread what you told me and I realized what you're saying is I didn't account for the conversion of mmol to mol. However, the final answer given in the textbook was 2.85*1012 given in L4*mmol-4*s-1, not mol. Or am I misunderstanding something?

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