[AAMC 7] Rate Law

[sc4s2cg]

PS # 24

What is the value of c in Equation 2 as determined from the data in Table 1?
A) 1
B) 2
C) 3
D) 4

Equation 2: R = k[acetone]^a[Br2]^b[H+]^c
The reaction is zero order with respect to bromine.

The answer is A, 1.

The passage states that the reaction is zero order with respect to bromine, therefore bromine can be removed from Equation 2. A comparison of the results for Experiments 2, 4 and 6 (all with acetone concentrations of 0.80 M) shows that after dividing the rates of the reactions by the rate constants, the values obtained are directly proportional to the H+ concentration indicating the reaction is first order in H+.

If I'm interpreting the solution correctly, they said that because Br2 is zero order, and all the acetones are 0.8, that we can just compare the rate constants with [H+]. Then they said as constant increases, [H+] increases, and as k decreases [H+] decreases. So it must be proportional, so c must equal to 1. Is that the case?

Here's how I did it, but I did not get one:

3.52 = r2 = k[acetone]^a[H+]^c = k * 0.8^a * 0.1^c
4.00 = r4 = k[acetone]^a[H+]^c = k * 0.8^a * 0.4^c
3.70 = r6 = k[acetone]^a[H+]^c = k * 0.8^a * 0.2^c

r6/r4 = 3.7/4.0 = 0.2^c/0.4^c
0.925 = 0.5^c

Clearly, c =! 1. I used r6/r4 since those two had near identical Br concentrations.

What am I missing?

---

Members don't see this ad.

 

[Jepstein30]

PS # 24

What is the value of c in Equation 2 as determined from the data in Table 1?
A) 1
B) 2
C) 3
D) 4

Equation 2: R = k[acetone]^a[Br2]^b[H+]^c
The reaction is zero order with respect to bromine.

The answer is A, 1.
[quote = solution] The passage states that the reaction is zero order with respect to bromine, therefore bromine can be removed from Equation 2. A comparison of the results for Experiments 2, 4 and 6 (all with acetone concentrations of 0.80 M) shows that after dividing the rates of the reactions by the rate constants, the values obtained are directly proportional to the H+ concentration indicating the reaction is first order in H+.

If I'm interpreting the solution correctly, they said that because Br2 is zero order, and all the acetones are 0.8, that we can just compare the rate constants with [H+]. Then they said as constant increases, [H+] increases, and as k decreases [H+] decreases. So it must be proportional, so c must equal to 1. Is that the case?

Here's how I did it, but I did not get one:

3.52 = r2 = k[acetone]^a[H+]^c = k * 0.8^a * 0.1^c
4.00 = r4 = k[acetone]^a[H+]^c = k * 0.8^a * 0.4^c
3.70 = r6 = k[acetone]^a[H+]^c = k * 0.8^a * 0.2^c

r6/r4 = 3.7/4.0 = 0.2^c/0.4^c
0.925 = 0.5^c

Clearly, c =! 1. I used r6/r4 since those two had near identical Br concentrations.

What am I missing?

You set your equations equal to the rate constants, not the actual rate (which is given by the fifth column).

The question was pretty straightforwrad, the major trick being they didn't just state "rate" and instead headed the column with "change in concentration of a reactant over time"

 

[sc4s2cg]

Ridiculous. I spent a good 2 hours trying to figure what I did wrong and that was it.

So now I'm getting:

r6/r4 = 5.99x10^-6/12.9x10^-6 = 6x10^-6/13x10^-6
6x10^-6/13x10^-6 = 0.2^c/0.4^c
0.5 = 0.5^c
c = 1

Just a couple questions.

1. Did I interpret their answer solution right (I edited OP with quote just before you replied)? That we can just look at the pattern and say it's first order? It seems fishy to me..
2. Am I right in picking r6 and r4 for comparison because of Br2? Or can I ignore Br2 since it is zeroth order?

 

[Jepstein30]

Ridiculous. I spent a good 2 hours trying to figure what I did wrong and that was it.

So now I'm getting:

r6/r4 = 5.99x10^-6/12.9x10^-6 = 6x10^-6/13x10^-6
6x10^-6/13x10^-6 = 0.2^c/0.4^c
0.5 = 0.5^c
c = 1

Just a couple questions.

1. Did I interpret their answer solution right (I edited OP with quote just before you replied)? That we can just look at the pattern and say it's first order? It seems fishy to me..
2. Am I right in picking r6 and r4 for comparison because of Br2? Or can I ignore Br2 since it is zeroth order?

Well what's the reaction? You can just look at [H+] if that is the only other reactant outside of Br2 but looks like acetone is also a reactant so you can't just ignore that. If you do, you can be correlating an increased rate to increased [H+] when it is really from increased/decreased [acetone].

Also, don't bother doing math. Just look at how the rate reacts to different changes in concentration. If it doubles when a concentration doubles, it is first order. If it quadruples, 2nd order. If it remains the same, zero order.

You should compare experiments where acetone is kept constant and [H+] varies. So experiment 2, 4 and 6. Comparing 2 and 6 is simple, when [H+] is doubled, the rate doubles. Thus, first order and the exponent is one.