CEM 142 FINAL EXAM 2026 ACTUAL QUESTIONS AND
SOLUTIONS SCORED A+
✔✔How to calculate pH from OH? - ✔✔The OH is complementary to H3O, so that
[H3O][OH]=10^-14
with the [H3O] you can find pH
✔✔Use pH of solutions of the same concentrations to determine relative strengths of
acids and bases. - ✔✔The smaller the pH = higher ionization, so stronger the acid.
Big pH = lower ionization, so stronger the base
When the pH is around the value of the hydrolysis of water, then it is neutral.
✔✔Identify and explain why acid base reactions are different from redox reactions. -
✔✔An acid base reaction may transfer electrons without altering the oxidation state of
the element, while the oxidation reactions involves transfer of electrons that changes in
oxidation state
✔✔How to look for charges in a redox reaction? - ✔✔Check for the formal charges,
based on the periodic table position.
- Oxygen = -2
- Elements = 0
- Hydrogen = +1.
If the formal charges changes, then it is a redox reaction.
✔✔How to use bond energies to predict and calculate enthalpy changes of reactions? -
✔✔1- look at the bonds being broken
2- Subtract by the bonds being formed
3- The result is the enthalpy changes.
✔✔Tell the steps to use molarity to calculate amounts of reactants and products for
reactions in solution. - ✔✔1- have a balanced equation
2- define the ratio between reactants and products
3- convert M to mols (multiply by volume)
4- Adapt to the ratio
✔✔8. Explain what rate mean when discussing a chemical reaction. - ✔✔Rate means a
change in quantity over time. (Ex: concentrations)
✔✔What are the factors that determine how fast and how far a reaction can go
(changes the reaction rates)? - ✔✔1- Concentration of acids and bases (probability of
collisions)
, 2- Temperature (increase kinetic of molecules - more probability of collisions with high
energy)
3- Presence of a Catalyst (decrease activation energy)
✔✔How to graphically determine the rate and order of a reaction from concentration vs.
time data? - ✔✔1- Collect Concentration vs. time table
2- do ln [A]
3- do 1/[A]
4- the one that gives a straight line is the order of the reaction
5- the slope of the line = k (constant)
✔✔How to experimentally determine the rate and order of a reaction? - ✔✔1- Choose
solutions of known and similar concentrations
2- React both with the same and compare the rate
3- Change one of the solutions and keep the other; compare the rate
4-the solutions that changes the rate (at a certain amount x2) will be present in the rate
law reaction.
✔✔Concentration changes depends on? - ✔✔The coefficients, and the amount of
reactants / products.
✔✔What is a half-life? - ✔✔It is the time for 1/2 a substance to decay.
✔✔How to use half-life for 1st order reaction? - ✔✔Normally use the carbon 14/12 ratio
and the time known for carbon to decay to find k. Then find half life.
ln ([A]/[Ai]) = -kt
t(1/2) = ln(2)/k
✔✔What the rate laws according with their order? - ✔✔0th order - rate = k
1st order - rate = k[A]
2nd order - rate =k [A][B]
3rd order - rate = k [A]^2[B]
✔✔What can be inferred from the rate law about the mechanisms? - ✔✔The rate law
shows which reactants play a role in the slowest reaction, if all the reactants are present
then we know it is a one mechanism, if there are reactants missing in the rate law than it
is a two step mechanism.
✔✔What a reaction mechanism? - ✔✔The order in which the bonds are broken and
formed. Example (image)
✔✔How to calculate an equilibrium constant (K) when given concentrations at
equilibrium? - ✔✔K = (products)^coeff / (reactants)^coeff
SOLUTIONS SCORED A+
✔✔How to calculate pH from OH? - ✔✔The OH is complementary to H3O, so that
[H3O][OH]=10^-14
with the [H3O] you can find pH
✔✔Use pH of solutions of the same concentrations to determine relative strengths of
acids and bases. - ✔✔The smaller the pH = higher ionization, so stronger the acid.
Big pH = lower ionization, so stronger the base
When the pH is around the value of the hydrolysis of water, then it is neutral.
✔✔Identify and explain why acid base reactions are different from redox reactions. -
✔✔An acid base reaction may transfer electrons without altering the oxidation state of
the element, while the oxidation reactions involves transfer of electrons that changes in
oxidation state
✔✔How to look for charges in a redox reaction? - ✔✔Check for the formal charges,
based on the periodic table position.
- Oxygen = -2
- Elements = 0
- Hydrogen = +1.
If the formal charges changes, then it is a redox reaction.
✔✔How to use bond energies to predict and calculate enthalpy changes of reactions? -
✔✔1- look at the bonds being broken
2- Subtract by the bonds being formed
3- The result is the enthalpy changes.
✔✔Tell the steps to use molarity to calculate amounts of reactants and products for
reactions in solution. - ✔✔1- have a balanced equation
2- define the ratio between reactants and products
3- convert M to mols (multiply by volume)
4- Adapt to the ratio
✔✔8. Explain what rate mean when discussing a chemical reaction. - ✔✔Rate means a
change in quantity over time. (Ex: concentrations)
✔✔What are the factors that determine how fast and how far a reaction can go
(changes the reaction rates)? - ✔✔1- Concentration of acids and bases (probability of
collisions)
, 2- Temperature (increase kinetic of molecules - more probability of collisions with high
energy)
3- Presence of a Catalyst (decrease activation energy)
✔✔How to graphically determine the rate and order of a reaction from concentration vs.
time data? - ✔✔1- Collect Concentration vs. time table
2- do ln [A]
3- do 1/[A]
4- the one that gives a straight line is the order of the reaction
5- the slope of the line = k (constant)
✔✔How to experimentally determine the rate and order of a reaction? - ✔✔1- Choose
solutions of known and similar concentrations
2- React both with the same and compare the rate
3- Change one of the solutions and keep the other; compare the rate
4-the solutions that changes the rate (at a certain amount x2) will be present in the rate
law reaction.
✔✔Concentration changes depends on? - ✔✔The coefficients, and the amount of
reactants / products.
✔✔What is a half-life? - ✔✔It is the time for 1/2 a substance to decay.
✔✔How to use half-life for 1st order reaction? - ✔✔Normally use the carbon 14/12 ratio
and the time known for carbon to decay to find k. Then find half life.
ln ([A]/[Ai]) = -kt
t(1/2) = ln(2)/k
✔✔What the rate laws according with their order? - ✔✔0th order - rate = k
1st order - rate = k[A]
2nd order - rate =k [A][B]
3rd order - rate = k [A]^2[B]
✔✔What can be inferred from the rate law about the mechanisms? - ✔✔The rate law
shows which reactants play a role in the slowest reaction, if all the reactants are present
then we know it is a one mechanism, if there are reactants missing in the rate law than it
is a two step mechanism.
✔✔What a reaction mechanism? - ✔✔The order in which the bonds are broken and
formed. Example (image)
✔✔How to calculate an equilibrium constant (K) when given concentrations at
equilibrium? - ✔✔K = (products)^coeff / (reactants)^coeff
