## Half Life

$\frac{d[R]}{dt}=-k[R]; \ln [R]=-kt + \ln [R]_{0}; t_{\frac{1}{2}}=\frac{0.693}{k}$

Charisse Vu 1H
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Joined: Thu Jul 25, 2019 12:17 am

### Half Life

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

Eesha Sohail 1D
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Joined: Sat Aug 17, 2019 12:16 am

### Re: Half Life

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.

Helen Struble 2F
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Joined: Sat Aug 24, 2019 12:17 am

### Re: Half Life

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 alone—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.

805312064
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Joined: Thu Jul 11, 2019 12:16 am

### Re: Half Life

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

Maika Ngoie 1B
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### Re: Half Life

It would probably only explicitly ask for half-life

William Chan 1D
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Joined: Sat Sep 14, 2019 12:15 am

### Re: Half Life

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.

Dina Marchenko 2J
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Joined: Thu Jul 25, 2019 12:16 am

### Re: Half Life

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.

Abby Soriano 1J
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Joined: Sat Aug 24, 2019 12:16 am

### Re: Half Life

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.

Michelle Xie 2B
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### Re: Half Life

Half lives are used more for real world applications.

Anthony Hatashita 4H
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### Re: Half Life

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.

Julia Holsinger_1A
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Joined: Tue Feb 26, 2019 12:16 am

### Re: Half Life

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

Posts: 102
Joined: Tue Oct 02, 2018 12:16 am

### Re: Half Life

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.

Posts: 102
Joined: Tue Oct 02, 2018 12:16 am

### Re: Half Life

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.

Julia Holsinger_1A
Posts: 50
Joined: Tue Feb 26, 2019 12:16 am

### Re: Half Life

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?