CEM 142 EXAM QUESTIONS AND
ANSWERS 2026 VERIFIED.
5. Define and give examples of open, closed and isolated systems. - ANS Open- both matter
and energy can travel around without barriers (all biological systems)
Closed- where only energy can travel around, matter remains stable (temperature change)
Isolated system- a thermodynamic system enclosed by rigid immovable walls through which
neither mass nor energy can pass
Explain the difference between state and path functions and give examples. - ANS State
functions only depend on these parameters and not on how they were reached. Examples of
state functions include density, internal energy, enthalpy, entropy. Path functions depend on the
route taken between two states. Two examples of path functions are heat and work
Identify the direction of the thermal energy change and the sign of q or ΔH for exothermic and
endothermic processes. - ANS Endothermic (+) - when thermal energy is input in the system
from the surroundings making it colder
Exothermic (-) - when thermal energy is released from the system to the surroundings making it
hotter
Identify the direction of the thermal energy change and the sign of q or ΔH for a phase change. -
ANS When phase changes, interactions are being broken (solid > liquid) so ΔH = +
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, When phase changes, interactions can be formed (liquid > solid) so ΔH = -
Explain the role of probability in entropy changes. - ANS The higher probability the higher the
number of possible arrangements, so higher temperature.
Predict the sign of the entropy change for simple systems. - ANS Entropy can be predicted by
looking at the phases of the reactants and products. Whenever there is an increase in gas
moles, entropy will increase.
Explain the second law of thermodynamics in terms of the system and surroundings. -
ANS The second law of thermodynamics states that the entropy of the universe is always
increasing. Thus it indicates that for a reaction to happen ΔG needs to be negative.
Explain why we usually use ΔG instead of the total entropy change to predict whether a process
is thermodynamically favorable. - ANS Entropy considers only the number of arrangements it
happens in a system, determining through the bigger number of arrangements when it is
favorable or not. On the other hand, Gibbs can be a simple method that can correlates both
enthalpy and entropy in an equation that determines when the system is favorable or not.
Determine the function of sign of ΔG, ΔH, or ΔS and how to calculate it? - ANS ΔG - If the
reaction happens
ΔH - the heat transferred
ΔS - entropy
ΔG = ΔH - T ΔS (all refers to the system)
When is ΔH = 0 - ANS When treating with ideal gases.
When is ΔG = 0? - ANS When the reaction is in equilibrium.
ΔH = T ΔS
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ANSWERS 2026 VERIFIED.
5. Define and give examples of open, closed and isolated systems. - ANS Open- both matter
and energy can travel around without barriers (all biological systems)
Closed- where only energy can travel around, matter remains stable (temperature change)
Isolated system- a thermodynamic system enclosed by rigid immovable walls through which
neither mass nor energy can pass
Explain the difference between state and path functions and give examples. - ANS State
functions only depend on these parameters and not on how they were reached. Examples of
state functions include density, internal energy, enthalpy, entropy. Path functions depend on the
route taken between two states. Two examples of path functions are heat and work
Identify the direction of the thermal energy change and the sign of q or ΔH for exothermic and
endothermic processes. - ANS Endothermic (+) - when thermal energy is input in the system
from the surroundings making it colder
Exothermic (-) - when thermal energy is released from the system to the surroundings making it
hotter
Identify the direction of the thermal energy change and the sign of q or ΔH for a phase change. -
ANS When phase changes, interactions are being broken (solid > liquid) so ΔH = +
@COPYRIGHT ALL RIGHTS RESERVED PAGE 1 OF 11
, When phase changes, interactions can be formed (liquid > solid) so ΔH = -
Explain the role of probability in entropy changes. - ANS The higher probability the higher the
number of possible arrangements, so higher temperature.
Predict the sign of the entropy change for simple systems. - ANS Entropy can be predicted by
looking at the phases of the reactants and products. Whenever there is an increase in gas
moles, entropy will increase.
Explain the second law of thermodynamics in terms of the system and surroundings. -
ANS The second law of thermodynamics states that the entropy of the universe is always
increasing. Thus it indicates that for a reaction to happen ΔG needs to be negative.
Explain why we usually use ΔG instead of the total entropy change to predict whether a process
is thermodynamically favorable. - ANS Entropy considers only the number of arrangements it
happens in a system, determining through the bigger number of arrangements when it is
favorable or not. On the other hand, Gibbs can be a simple method that can correlates both
enthalpy and entropy in an equation that determines when the system is favorable or not.
Determine the function of sign of ΔG, ΔH, or ΔS and how to calculate it? - ANS ΔG - If the
reaction happens
ΔH - the heat transferred
ΔS - entropy
ΔG = ΔH - T ΔS (all refers to the system)
When is ΔH = 0 - ANS When treating with ideal gases.
When is ΔG = 0? - ANS When the reaction is in equilibrium.
ΔH = T ΔS
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