CEM 142 QUESTIONS AND ANSWERS
WITH SOLUTIONS 2024
5. Define and give examples of open, closed and isolated systems. - ANSWER 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. - ANSWER 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. - ANSWER 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. -
ANSWER When phase changes, interactions are being broken (solid > liquid) so ΔH = +
When phase changes, interactions can be formed (liquid > solid) so ΔH = -
Explain the role of probability in entropy changes. - ANSWER The higher probability the higher the
number of possible arrangements, so higher temperature.
Predict the sign of the entropy change for simple systems. - ANSWER 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. - ANSWER 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. - ANSWER 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? - ANSWER ΔG - If the reaction
happens
ΔH - the heat transferred
ΔS - entropy
ΔG = ΔH - T ΔS (all refers to the system)
When is ΔH = 0 - ANSWER When treating with ideal gases.
When is ΔG = 0? - ANSWER When the reaction is in equilibrium.
ΔH = T ΔS
6. Define Solute - ANSWER Substance being dissolved
Define Solvent - ANSWER the substance that dissolves the solute
Define solvation - ANSWER The process of surrounding solute particles with solvent particles to form a
solution
Molarity - ANSWER the number of moles of solute per liter of solution. (mol/L)
WITH SOLUTIONS 2024
5. Define and give examples of open, closed and isolated systems. - ANSWER 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. - ANSWER 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. - ANSWER 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. -
ANSWER When phase changes, interactions are being broken (solid > liquid) so ΔH = +
When phase changes, interactions can be formed (liquid > solid) so ΔH = -
Explain the role of probability in entropy changes. - ANSWER The higher probability the higher the
number of possible arrangements, so higher temperature.
Predict the sign of the entropy change for simple systems. - ANSWER 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. - ANSWER 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. - ANSWER 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? - ANSWER ΔG - If the reaction
happens
ΔH - the heat transferred
ΔS - entropy
ΔG = ΔH - T ΔS (all refers to the system)
When is ΔH = 0 - ANSWER When treating with ideal gases.
When is ΔG = 0? - ANSWER When the reaction is in equilibrium.
ΔH = T ΔS
6. Define Solute - ANSWER Substance being dissolved
Define Solvent - ANSWER the substance that dissolves the solute
Define solvation - ANSWER The process of surrounding solute particles with solvent particles to form a
solution
Molarity - ANSWER the number of moles of solute per liter of solution. (mol/L)
