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# Rate constant units?

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1. Hi,

I'm doing my Chemistry coursework and it's driving me INSANE. I'm trying to work out the units for a rate constant where k = rate in cm3s-1
concentration in moldm3

Could anybody tell me what the units of k are? I'm losing the will to live...

Thanks
2. (Original post by Scope_o_steth)
Hi,

I'm doing my Chemistry coursework and it's driving me INSANE. I'm trying to work out the units for a rate constant where k = rate in cm3s-1
concentration in moldm3

Could anybody tell me what the units of k are? I'm losing the will to live...

Thanks
The units of the rate constant depend on the order that appears in the rate equation
3. Rate is always in mol per dm3 per second. The units of the constant can be found by writing out the units of all the species in the equation and they can be multiplied/cancelled just like in a regular equation.

The units of the constant may be different in every calculation, depending on the number of species and the order of each.
4. (Original post by charco)
The units of the rate constant depend on the order that appears in the rate equation
Sorry, forgot to mention that it'sa first order reaction!
5. For a 1st order reaction, the unit of k is s^-1.
6. If it's first order it will be s-1 as the concentrations cancel out, however as rate is in cm3 you will need to divide the number you have for it by 1000.
7. (Original post by Waitingname)
Rate is always in mol per dm3 per second. The units of the constant can be found by writing out the units of all the species in the equation and they can be multiplied/cancelled just like in a regular equation.

The units of the constant may be different in every calculation, depending on the number of species and the order of each.
My coursework project is to do with the decomposition of hydrogen peroxide and the rate in this equation is for production of oxygen, which is why I've put it in cm3s-1. I'm trying to form a rate equation for the decomposition of hydrogen peroxide in terms of the production of oxygen, so the rate equation should still be rate=k[H2O2 shouldn't it?
8. (Original post by Scope_o_steth)
My coursework project is to do with the decomposition of hydrogen peroxide and the rate in this equation is for production of oxygen, which is why I've put it in cm3s-1. I'm trying to form a rate equation for the decomposition of hydrogen peroxide in terms of the production of oxygen, so the rate equation should still be rate=k[H2O2 shouldn't it?
To use a rate equation you have to use initial concentration of a reactant, so you have to find the initial concentration of peroxide and use that in the equation. I'd suggest a graph. Also, always always work in dm3.

Also, you seem to be using change in volume as rate, but you have to use change in concentration.
9. (Original post by Waitingname)
To use a rate equation you have to use initial concentration of a reactant, so you have to find the initial concentration of peroxide and use that in the equation. I'd suggest a graph. Also, always always work in dm3.

Also, you seem to be using change in volume as rate, but you have to use change in concentration.
This coursework has become the bane of my life recently!
10. (Original post by Scope_o_steth)
My coursework project is to do with the decomposition of hydrogen peroxide and the rate in this equation is for production of oxygen, which is why I've put it in cm3s-1. I'm trying to form a rate equation for the decomposition of hydrogen peroxide in terms of the production of oxygen, so the rate equation should still be rate=k[H2O2 shouldn't it?
You would be better off using the volume of oxygen produced to find the moles of hydrogen peroxide decomposed (using the equation stoichiometry) and hence the new concentration of hydrogen peroxide.

You can then plot change of concentration against time, or simply calculate average rates.
11. (Original post by charco)
You would be better off using the volume of oxygen produced to find the moles of hydrogen peroxide decomposed (using the equation stoichiometry) and hence the new concentration of hydrogen peroxide.

You can then plot change of concentration against time, or simply calculate average rates.
I've already formed a rate equation in terms of loss of hydrogen peroxide; I just wanted to try and form one in terms of production of oxygen as well. Think I might just be overcomplicating things though...
12. Rate equations are only normally formed for reactants. So I wouldn't bother with the other one.

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