Question

Can you please analyze and break down why these 2 formulas change positive and negative signs to produce the answers to question 1. and 2.?

Do the words "absorb" and "release" have to do with the set up?

1. A gas absorbs 45 kJ of heat and does 29 kJ of work. ΔE = q+w= 45kJ+(-29kJ) = 16 kJ

2. A system releases 125 kJ as heat and 104 kJ of work are done on it. ΔE = -125kJ+104kJ = -21kJ

Answer 1

During reaction when energy releases, the negative sign (-) is used and when energy absorbs then positive sign (+) is used.

1. A gas absorbs 45 kJ of heat and does 29 kJ of work. ΔE = q+w= 45kJ+(-29kJ) = 16 kJ

In this example 45 kJ heat is absorbed, then positive sign (+) is shown i.e. +45 kJ. After absorption of heat, 29 kJ of work are done it means 29 kJ of energy releases, thus, negative sign (-29 kJ) is used.

2. A system releases 125 kJ as heat and 104 kJ of work are done on it. ΔE = -125kJ+104kJ = -21kJ

In this example 125 kJ heat is released, then negative sign (-) is shown i.e. -125kJ. After releasing heat, 104 kJ of work are done it means 104 kJ of energy absorbs, thus, positive sign (+104 kJ) is used.