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### Student Question

I have a hard time understanding how why ° C is used in the thermal energy equation Q = m * c * delta T. In many chemistry exercises, we have to use Kelvin. Thank you

Chemistry ## Explanations (1)

• Explanation from Alloprof  Explanation from Alloprof

This Explanation was submitted by a member of the Alloprof team.

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Team Alloprof • 1yr. edited September 2021

Hello

A temperature variation can be calculated in Kelvin or in degrees Celsius. To convince yourself of this, let's take for example two temperatures $$T_1$$ and $$T_2$$ given in ° C. To convert them to Kelvin, we must add 273.15. The temperature variation $$\triangle T$$ therefore gives:

$$\triangle T = T_1-T_2$$

$$\triangle T = (T_1+273,15)-(T_2+273,15)$$

By distributing the negative sign in the second part of the equation, we obtain:

$$\triangle T = T_1\color{red} {+273,15} \color{black} {-T_2} \color{red} {-273,15}$$

We therefore notice that the 273.15 cancel each other out, which amounts to our starting equation.

When the formula includes  $$\triangle T$$ , you can use either temperatures in Kelvin or in degrees Celsius since we have just shown that:

$$\triangle T = T_1-T_2 = (T_1+273,15)-(T_2+273,15)$$

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