### Half Life

Posted:

**Sun Mar 08, 2020 11:26 pm**Why do we need to know how to calculate half lives? In what context would a half life question be given?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=148&t=61711

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Posted: **Sun Mar 08, 2020 11:26 pm**

Why do we need to know how to calculate half lives? In what context would a half life question be given?

Posted: **Sun Mar 08, 2020 11:45 pm**

The half life of a reaction is a useful concept for relating time passed to amounts of substance in the real world. Not sure exactly what context may show up on the test, but it is frequently referenced in radioactive dating.

Posted: **Sun Mar 08, 2020 11:58 pm**

Since half-lives are not dependent on initial concentration for first order reaction, they allow you to easily figure out certain questions using time and reaction constant aloneâ€”how long will it take to decompose by 2x, 4x, etc. So if you're given a time period but no initial concentration, it may be easier to use the half-life equation.

Posted: **Mon Mar 09, 2020 11:09 pm**

If only the rate constant is given or can be found, or the question mentions something about half of the substance remaining, then you would probably need to find the half life

Posted: **Wed Mar 11, 2020 4:51 pm**

It would probably only explicitly ask for half-life

Posted: **Wed Mar 11, 2020 10:02 pm**

Half-life reaction rates are just a convenient way of measuring the time it takes for half the substance to decay. We could as easily have "quarter-life reaction rates," but this is more conventional. Most likely, it will just ask us to calculate the half-life of a certain substance of a given reaction.

Posted: **Fri Mar 13, 2020 2:57 pm**

Half-lives in general are also useful ways of measuring how old a substance is. Real-world applications include figuring out how old a fossil is or how long it will take a substance to decay.

Posted: **Fri Mar 13, 2020 3:29 pm**

You'll most likely be asked to find the half-life of something you want to know the age of. By comparing how much of the reactant there was initially versus how much of it exists now, and by knowing the half-life, you should be able to calculate how old a substance is.

Posted: **Fri Mar 13, 2020 5:28 pm**

Half lives are used more for real world applications.

Posted: **Fri Mar 13, 2020 10:34 pm**

Half lives are useful for lots of real world applications. If asked for in a question it would ask for it explicitly and the equations should be on the equation sheet.

Posted: **Sat Mar 14, 2020 10:20 am**

How do you determine the order of the reaction when needing to calculate half life? Will it always be given?

Posted: **Sat Mar 14, 2020 10:25 am**

Half lives can be used to determine how much product is left after any amount of time. if you know the half life, you set the equation:

(1/2)^x =1/A

where a is the value of how much of the original product remains. For example, 1/9 of the initial amount. Solve for x to see how many half lives there are. Then, multiply x by the half life to get the amount of time needed to reach the desired amount.

(1/2)^x =1/A

where a is the value of how much of the original product remains. For example, 1/9 of the initial amount. Solve for x to see how many half lives there are. Then, multiply x by the half life to get the amount of time needed to reach the desired amount.

Posted: **Sat Mar 14, 2020 10:26 am**

Julia Holsinger_1A wrote:How do you determine the order of the reaction when needing to calculate half life? Will it always be given?

To determine the order, you will likely use scientific data, are told, or can assume pseudo-first reaction if the information permits.

Posted: **Sat Mar 14, 2020 10:35 am**

Madelyn Romberg 1H wrote:Julia Holsinger_1A wrote:How do you determine the order of the reaction when needing to calculate half life? Will it always be given?

To determine the order, you will likely use scientific data, are told, or can assume pseudo-first reaction if the information permits.

I am a bit confused about psuedo-first reaction, could someone explain it?