Question

A gas absorbs 45 kJ of heat and does 29 kJ of work. Calculate the change in energy.

The answer is 16 kJ, but I don't understand why.

If a gas absorbs heat and does 29 kJ of work. Wouldn't they both be positive?
Change in energy = q + w
45 kJ + 29 kJ = 74 kJ

Why is the work negative?

Thanks.

Answer 1

Work done BY the system is negative.

Here is the way to remember it.
You know delta H is + when heat is absorbed (receiving) and - when the heat is emitted(giving).
Work is + when receiving) and - when giving.

Answer 2

Ahhh, that's really helpful. Thanks for clearing it up.

Answer 3

This is wrong it should be 16 kJ as you subtract 45-29 which equals 16.

Answer 4

كول خرا