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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: XGen on March 11, 2012, 02:51:11 PM

Title: Rate Constants
Post by: XGen on March 11, 2012, 02:51:11 PM

A reaction has a forward rate constant of 2.3 x 10⁶ and an equilibrium constant of 4.0 x 10⁸. What is the rate constant for the reverse reaction?

Letting A, C, D, and E be arbitrary reactants and products, I have k_f[A][C] = k_b[D][E]. Solving for k_b, I get k_b = k_f([A][C]/[D][E]). This can also be simplified into k_b = k_f(1/K_eq), because K_eq is equal to ([D][E]/[A][C]). Plugging in the values, I get k_b = (2.3 x 10⁶)(1/4.0 x 10⁸), or 5.8 x 10^-3. However, the answer key says this is wrong.

Can anyone point out where I went wrong?

Title: Re: Rate Constants
Post by: sjb on March 11, 2012, 04:06:03 PM

Quote from: XGen on March 11, 2012, 02:51:11 PM

A reaction has a forward rate constant of 2.3 x 10⁶ and an equilibrium constant of 4.0 x 10⁸. What is the rate constant for the reverse reaction?

Letting A, C, D, and E be arbitrary reactants and products, I have k_f[A][C] = k_b[D][E]. Solving for k_b, I get k_b = k_f([A][C]/[D][E]). This can also be simplified into k_b = k_f(1/K_eq), because K_eq is equal to ([D][E]/[A][C]). Plugging in the values, I get k_b = (2.3 x 10⁶)(1/4.0 x 10⁸), or 5.8 x 10^-3. However, the answer key says this is wrong.

Can anyone point out where I went wrong?

I don't think you need the equilibrium constant, so that k_b k_f = 1

Title: Re: Rate Constants
Post by: Olympiad_Tutor on March 12, 2012, 04:23:24 AM

Quote from: XGen on March 11, 2012, 02:51:11 PM

A reaction has a forward rate constant of 2.3 x 10⁶ and an equilibrium constant of 4.0 x 10⁸. What is the rate constant for the reverse reaction?

Letting A, C, D, and E be arbitrary reactants and products, I have k_f[A][C] = k_b[D][E]. Solving for k_b, I get k_b = k_f([A][C]/[D][E]). This can also be simplified into k_b = k_f(1/K_eq), because K_eq is equal to ([D][E]/[A][C]). Plugging in the values, I get k_b = (2.3 x 10⁶)(1/4.0 x 10⁸), or 5.8 x 10^-3. However, the answer key says this is wrong.

Can anyone point out where I went wrong?

the answer key is wrong

unless additional details are given about the reaction

Title: Re: Rate Constants
Post by: qw098 on March 12, 2012, 06:15:49 AM

I believe the forward rate constant divided by the backward rate constant gives you the equilibrium constant so it is easy for you to solve for the equilibrium constant :)

Title: Re: Rate Constants
Post by: XGen on March 12, 2012, 10:03:16 PM

Quote from: qw098 on March 12, 2012, 06:15:49 AM

I believe the forward rate constant divided by the backward rate constant gives you the equilibrium constant so it is easy for you to solve for the equilibrium constant :)

Your method gives my answer, which is incorrect on the answer key.

Thank you for your input, Ch_Olympiad_Winner. There is no other information given.

Title: Re: Rate Constants
Post by: Olympiad_Tutor on March 13, 2012, 02:42:41 AM

sure.
when everything else fails we must accept the possibility of a typo.

as qw098 said the problem is easy.

if "can't be determined from the info given" is not the answer then it is definitely a typo.

Title: Re: Rate Constants
Post by: PIQgoogleme on March 15, 2012, 08:07:16 PM

I like this problem. Your solution looks good. Where'd you find it? any more like it?

Title: Re: Rate Constants
Post by: XGen on March 15, 2012, 08:50:46 PM

This is from the 1999 (or 2000? 2001? I forgot D:) USA National Chemistry Exam.

Title: Re: Rate Constants
Post by: PIQgoogleme on March 15, 2012, 10:28:12 PM

Oh cool. I'm studying for that also. Good luck

Title: Re: Rate Constants
Post by: juanrga on March 17, 2012, 07:49:28 AM

Quote from: XGen on March 11, 2012, 02:51:11 PM

A reaction has a forward rate constant of 2.3 x 10⁶ and an equilibrium constant of 4.0 x 10⁸. What is the rate constant for the reverse reaction?

Letting A, C, D, and E be arbitrary reactants and products, I have k_f[A][C] = k_b[D][E]. Solving for k_b, I get k_b = k_f([A][C]/[D][E]). This can also be simplified into k_b = k_f(1/K_eq), because K_eq is equal to ([D][E]/[A][C]). Plugging in the values, I get k_b = (2.3 x 10⁶)(1/4.0 x 10⁸), or 5.8 x 10^-3. However, the answer key says this is wrong.

Can anyone point out where I went wrong?

Could you give more info? units? answer key?...