Exercises: Solutions/Answers
Exercises 1. What is the change in internal energy, ΔE, in joules, of a system that releases 675 J of thermal energy to the surroundings and has 530 calories of work done on it?
First convert 530 calories to J
\(\displaystyle 530\;cal\times\frac{4.184\;J}{1\;cal}\;=\;2218\;J\)If heat is released, q is negative. Work is doe on the system, and w is positive.
ΕE = q + w = -675 J + 2218 J = +1543 J
Exercises 2. Consider the following combustion reaction:
C2H2 (g) + 5/2 O2 (g) → H2O (g) + 2 CO2 (g)
If 1256 kJ of heat is released, what is the sign and value of q? Assuming the work done is 3716 J, what is the sign of w? What is the value and sign of ΔE? (5/2 is the coefficient of oxygen because we are determining q, w, and ΔE for one mole of C2H2.)
the sign of q is negative because heat is released from the system. For the sign of w, compare the number of moles of gas in both the reactants and products. There are 3 product moles of gas and 3.5 moles of reactant gas. The number of moles of gas is less for the products than for the reactants, therefore, work is done on the system–the volume is less for the products. The sign of w is positive. Convert w to kJ. ΕE = -1256 kJ + 3.716 kJ = -1252 kJ