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### Half-life

Posted: **Mon Mar 11, 2019 2:33 pm**

by **Rogelio Bazan 1D**

As I am going through Kinetics in the textbook I am still having trouble understanding Half-life. If someone can help me clarify the concepts to Half-life and the equation(s) involved it would be of great help. Thank you.

### Re: Half-life

Posted: **Mon Mar 11, 2019 2:55 pm**

by **aisteles1G**

Half life is referring the time it takes for half of the current sample to die, so its not a set amount of substance that will die since 1/2 of 200 is different than 1/2 of 300 but the time for the half of the material to 'die' is the same regardless of the initial amount so t1/2 (half life) is a constant for a specific substance. Therefore we can use it to figure out the initial or final concentrations.

Use t1/2=Ln(2)/kr for zero order reactions so either find the half life or Kr, then you can plug the kr or t1/2, in for time, into the other equations to get [A], hope this helps!

### Re: Half-life

Posted: **Mon Mar 11, 2019 2:57 pm**

by **MadelynNguyen1F**

These are the equations I found for Half Lives:

For a zero order reaction A products , rate = k:

t½ = [Ao] / 2k

For a first order reaction A products , rate = k[A]:

t½ = 0.693 / k

For a second order reaction 2A products or A + B products (when [A] = [B]), rate = k[A]2:

t½ = 1 / k [Ao]

### Re: Half-life

Posted: **Mon Mar 11, 2019 3:01 pm**

by **Rogelio Bazan 1D**

Thank you so much, this is really helpful. So just to clarify, according to which order reaction we are dealing with we use the corresponding equation.

For example) zero order; rate = k: t½ = [Ao] / 2k, first order; rate = k[A]: t½ = 0.693 / k, second order; rate = k[A]2: t½ = 1 / k [Ao]

### Re: Half-life

Posted: **Mon Mar 11, 2019 3:34 pm**

by **JT Wechsler 2B**

The half life is the amount of time that it takes for half of a sample to die off. This can be taken with respect to radioactive isotopes or even (like in our case) the amount of time for a certain reactant to decrease by half.