[qjones]

I need help with these Kinetics questions, I've looked everywhere and I still havent been able to find the answers:

The decomposition reaction, X --> products, is zeroth-order with rate constant k=0.0921 mol L-1 s-1. How long will it take for the decomposition to be 25% complete, if the initial concentration of X is 1.35 mol/L?

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The reaction X + Y --> products was studied using the method of initial rates. The initial rate of consumption of X was measured in three different experiments. Data are provided below.

The first column: Expt #
Second column: [X] o (in mol/L)
Third column: [Y] o(in mol/L)
Last column: Initial Rate (in mol L -1 s - 1 )

1         0.200          0.200          1.310
2         0.600          0.200          0.4367
3         0.200          0.500          8.188

What is the value of the rate constant, k? (Use concentrations in mol/L and time in seconds. Enter your answer accurate to three significant figures.)

Hint: First determine the rate law, and then evaluate k.


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Thanks for the help in advance!

[sjb]

1) What does zeroth order mean, in terms of things like half-lives and the like?

2) If the concentration of A triples, as in experiments 1 and 2, what happens to the rate? If the concentration of B increases by 2.5 what happens to the rate? Can you write an expression on this basis?

[qjones]

For #1, I know 25% decomposition means that the sample is at 1.0125 and the zero order formula would look like this:

(1.0125 - 1.35) / 0.0921    =   3.66 s

I'm just not sure if thats right in terms of what they mean by 25%.

For the second question I'm stumped on finding the order for either of the substances.

[sjb]

First one sounds OK to me, unless there's some nuance I'm missing. Check your signs, though

The reaction X + Y --> products was studied using the method of initial rates. The initial rate of consumption of X was measured in three different experiments. Data are provided below.

The first column: Expt #
Second column: [X] o (in mol/L)
Third column: [Y] o(in mol/L)
Last column: Initial Rate (in mol L -1 s - 1 )

1         0.200          0.200          1.310
2         0.600          0.200          0.4367

Here the concentration of X has increased by a factor of 3, what's happened to the rate of the reaction? Gone up? Down? Stayed the same?

1         0.200          0.200          1.310
3         0.200          0.500          8.188

Here [Y] has increased by a factor of 2.5, what's happened to the rate?