CEM 142 COMPREHENSIVE EXAM 2026 QUESTIONS AND
SOLUTIONS SCORED A+
✔✔Why can a buffer resist changes in pH? - ✔✔Because it can contains both a weak
acid and a weak conjugate base; thus when a strong base is added it reacts with the
weak acid forming a weak conjugate base. Therefore, the pH does not alter much as
the weak ones noes not ionizes much.
✔✔What is buffer capacity? - ✔✔the amount of acid or base that can be added to a
buffer solution before a significant change in pH occurs. (When ultra passes the #mols
of buffer)
✔✔How to calculate the pH of a buffer solution? - ✔✔1- have the pKa or Ka
2- if the concentrations of conjugate base and weak acid is the same then pH = pKa
3- If not, use the hasselbalch equation to calculate pH
✔✔How to calculate the pH of a buffer solution after adding a strong base/acid ? -
✔✔Do the ICE table, With the pKa, calculate by using the Henderson-Hasselbalch
equation.
pH= pKa + log[A]/[HA]
✔✔What processes cause a pH change when it dissolves in water (conjugate
bases/acid)? - ✔✔Conjugate base of a strong acid will not affect the pH, but conjugate
bases of weak acids are somewhat basic, so it will affect the pH.
Strong acids and bases (without buffer) changes a lot the pHf
✔✔Explain how and why reaction are coupled by a common intermediate and give
examples. - ✔✔How- the product of one reaction will be the reactant of the other thus
connecting both
Why - coupled reaction allows unfavorable reaction to happen by using free energy.
✔✔What it is the difference between an acid and its conjugate base. - ✔✔One proton
H+.
✔✔What happens when you add a strong acid/base to a buffer? - ✔✔If it os a acid, the
pH will decrease by a little bit
Base - the pH will increase for a little bit.
✔✔What are the three types of Lewis acid? - ✔✔1- The ones that can donate a proton
(same as Bronsted)
2- Electrophiles, carbon that becomes electron needed
, 3- Ions with empty orbitals such as B, Al, Transition metals that can create another bond
✔✔5. Define and give examples of open, closed and isolated systems. - ✔✔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. -
✔✔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. - ✔✔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. - ✔✔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. - ✔✔The higher probability the
higher the number of possible arrangements, so higher temperature.
✔✔Predict the sign of the entropy change for simple systems. - ✔✔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.
- ✔✔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. - ✔✔Entropy considers only the
SOLUTIONS SCORED A+
✔✔Why can a buffer resist changes in pH? - ✔✔Because it can contains both a weak
acid and a weak conjugate base; thus when a strong base is added it reacts with the
weak acid forming a weak conjugate base. Therefore, the pH does not alter much as
the weak ones noes not ionizes much.
✔✔What is buffer capacity? - ✔✔the amount of acid or base that can be added to a
buffer solution before a significant change in pH occurs. (When ultra passes the #mols
of buffer)
✔✔How to calculate the pH of a buffer solution? - ✔✔1- have the pKa or Ka
2- if the concentrations of conjugate base and weak acid is the same then pH = pKa
3- If not, use the hasselbalch equation to calculate pH
✔✔How to calculate the pH of a buffer solution after adding a strong base/acid ? -
✔✔Do the ICE table, With the pKa, calculate by using the Henderson-Hasselbalch
equation.
pH= pKa + log[A]/[HA]
✔✔What processes cause a pH change when it dissolves in water (conjugate
bases/acid)? - ✔✔Conjugate base of a strong acid will not affect the pH, but conjugate
bases of weak acids are somewhat basic, so it will affect the pH.
Strong acids and bases (without buffer) changes a lot the pHf
✔✔Explain how and why reaction are coupled by a common intermediate and give
examples. - ✔✔How- the product of one reaction will be the reactant of the other thus
connecting both
Why - coupled reaction allows unfavorable reaction to happen by using free energy.
✔✔What it is the difference between an acid and its conjugate base. - ✔✔One proton
H+.
✔✔What happens when you add a strong acid/base to a buffer? - ✔✔If it os a acid, the
pH will decrease by a little bit
Base - the pH will increase for a little bit.
✔✔What are the three types of Lewis acid? - ✔✔1- The ones that can donate a proton
(same as Bronsted)
2- Electrophiles, carbon that becomes electron needed
, 3- Ions with empty orbitals such as B, Al, Transition metals that can create another bond
✔✔5. Define and give examples of open, closed and isolated systems. - ✔✔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. -
✔✔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. - ✔✔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. - ✔✔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. - ✔✔The higher probability the
higher the number of possible arrangements, so higher temperature.
✔✔Predict the sign of the entropy change for simple systems. - ✔✔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.
- ✔✔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. - ✔✔Entropy considers only the
