Self Test 4A.3A

[Hasmik Dis 2F]

Potassium perchlorate, KClO4, is used as an oxidizer in fireworks. Calculate the heat required to raise the temperature of 10.0 g of KClO4 from 25°C to an ignition temperature of 900°C. The specific heat capacity of KClO 4 is 0 .8111 J⋅K−1⋅g − 1.

So in this problem, I know that we have to do the heat capacity equation, which includes the change in temperature. However, I noticed the book didn't convert the Celsius to Kelvin, even though the units for the other numbers are in Kelvin. Why is that so?

[Becca Nelson 3F]

I believe this is because you are looking at delta T, not T. A change from 25 celsius to 900 celsius is 875 Celsius and a change from 298 Kelvin to 1173 Kelvin is 875 Kelvin. An increase in temp of one degree celsius is equal to an increase in temp of one degree Kelvin. You always can convert to Kelvin, your final answer just will end up in Kelvin and you may have to convert back to Celsius. I hope this helps!

[Pratika Nagpal]

How are we supposed to know whether work done on a system is positive or negative

[Trevor_Ramsey_3H]

How are we supposed to know whether work done on a system is positive or negative

If the work you calculate is positive then work is being done ON the system. If the work is negative, then work is being done by the system. If you think about it conceptually, when work is positive then work is being added so an external surrounding is giving that work on to the system, while if it is negative then that means work is being released by the system so the system itself is doing work. This is at least how I think of it.

[Pratika Nagpal]

How are we supposed to know whether work done on a system is positive or negative

If the work you calculate is positive then work is being done ON the system. If the work is negative, then work is being done by the system. If you think about it conceptually, when work is positive then work is being added so an external surrounding is giving that work on to the system, while if it is negative then that means work is being released by the system so the system itself is doing work. This is at least how I think of it.

Thank you !