Why does charles law have to be in kelvin

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It is partially about definitions and partially an about making the formulae simpler

Consider how you work out the relationship between volume and temperature of a gas. You measure the volume of a fixed amount of gas at different temperatures and plot the results on a chart (volume on Y, temperature on X). The points show a linear relationship (for gases whose behaviour is close to ideal at the range of chosen temperatures).

Where does the line cross the Y-axis (temperature)? It isn't at 0 °C, is it? It is at about -273 °C. So the formula could look like this: V=x(T+273) with temperature in Celsius. Or, you could redefine the temperature scale and make it simpler: V=xT with T in Kelvin. This is far simpler in practice and keeps the maths neater.

But this is, essentially, how we know what absolute zero is. This is the very definition of the term (which works on other temperature scales as well but only Americans still care about Fahrenheit). We chose 0 as the original base for the Celsius scale because that is the freezing point of water not because of anything to do with gas laws. So that is an arbitrary point that happens to give a more complicated gas law. Hence the Kelvin scale where the zero relates to the gas law and makes it simpler.

So volume is proportional to temperature in Celsius only if you adjust the temperature because the scale has an arbitrary point chosen for zero. Kelvin keeps things simpler and neater.

NOTE** Charles Law problems must have the PRESSURE CONSTANT.  Try using Charles’s law to solve the following problem.

Example

A sample of gas at 15ºC and 1 atm has a volume of 2.50 L. What volume will this gas occupy at 30ºC and 1 atm?

OK, Here is our formula,

NO  

As a Chemistry teacher I am REQUIRED to give you the temperature in Celsius and make you convert to Kelvin. It is the Law of all Chemistry Teachers.

So convert the temperatures to Kelvin.  T1= 15C +273=288K  and T2 =30C +273=303K

Now Plug in and Solve

This makes sense—the temperature is increasing slightly, so the volume should increase slightly. Again be careful of questions like this. It’s tempting to just use the Celsius temperature, but you must first convert to Kelvin temperature (by adding 273) to get the correct relationships!

Why must the temperature be absolute?  If temperature is measured on a Celsius (non absolute) scale, T can be negative. If we plug negative values of T into the equation, we get back negative volumes, which cannot exist. In order to ensure that only values of V= 0 occur, we have to use an absolute temperature scale where T= 0. The standard absolute scale is the Kelvin (K) scale. The temperature in Kelvin can be calculated via Tk = TC + 273.15. A plot of the temperature in Kelvin. Charles' law predicts that volume will be zero at 0 K. 0 K is the absolutely lowest temperature possible, and is called absolute zero. from-www.sparknotes.com/.../ ideal/section2.rhtml

Charles' law can be summarized like this:

#V_1#/#T_1# = #V_2#/#T_2#

Imagine you used temperatures in Celcius, it would be possible to have a gas at a temp of 0 degrees Celcius. What would happen to the volume if you divide it by 0?

Is this a problem for a gas at 0K? Not really, because at this temp all particle movement stops so the substance could not be in the gaseous state, it would be a solid. The gas laws are only applicable in the range of T and P where substances will exist in the gas state.

Another reason is that Kelvin is an absolute scale for temperature. A gas at 10K has only half the heat energy of a gas which has a temp of 20K. This is not true for a gas at 10 degrees Celcius compared to the gas at 20 degrees Celcius.

Noel P.