(5) Heat is transferred from the warm hand (system/surroundings) to the cold hand (surroundings/system). Cold hand: > 0, warm hand: < 0
1.3
Is the earth an isolated, closed, or open system?
Answer: The earth is an open system because energy and matter can be exchanged between earth and space.
State functions and the laws of thermodynamics
2.1
A triathlete performs 1000 kJ of work and loses 400 kJ of heat while swimming in cold water.
How does the internal energy of the triathlete change? How much does the pool (50 m length, 20 m width, 2 m depth) warm up? Cp (H2O) = 4.18 kJ kg-1 K-1, (H2O) = 1 g cm-3. Answer: The work performed by the triathlete (system) on the surroundings is w = -1000 kJ. The heat transferred from the triathlete (system) to the surroundings (water) is q = -400 kJ. The overall change in internal energy is U = w + q = -1400 kJ. The pool volume is V = 50∙20∙2 m3 = 2000 m3 = 2∙109 cm3. This volume corresponds to a water mass of mwater = 2∙106 kg. The transfer of 1400 kJ leads to a temperature change T of ∆ = 2.2
=
400 ∙ ∙ 4.18 ∙ 2 ∙ 10
= 4.8 ∙ 10
K
Isothermal expansion of an ideal gas does not change its internal energy. What is the
associated change in enthalpy? Answer: The enthalpy is defined as H = U + pV. U is constant. pV is also constant (Boyle-Mariotte law), and the enthalpy does not change: H = const.
3