CEM 142 EVALUATION EXAM 2026 QUESTIONS AND
SOLUTIONS SCORED A+
✔✔What needs to happen for a reaction to occur? - ✔✔-Collisions.
*Molecules have to bump not each other.
*Therefore the more collisions, the more likely the reaction.
✔✔The molecules have to have enough energy-(What for?)
As the temperature goes up we can expect the molecules to bump into each other with
more energy. - ✔✔-To break the bonds between the reactants.
✔✔For the reaction A---><----B, how does the rate of
1) The forward reaction change over time?
2) The reverse reaction change over time?
*Explain why
A. Increase
B. Decrease
C. Stays the same
D. It depends - ✔✔- B
*Less reactants and more products as the reaction progresses in the forward reaction.
The concentration decreases and so does the likelihood that there will be collisions.
-A
*As forward reaction progresses, more products formed, the concentration increases
and so does the likelihood of products colliding into one another thus increasing the rate
of the reverse reaction.
✔✔Eventually, the the rate of the forward reaction will ___________ the rate of the
reverse reaction. At this point, the reaction has reached ____________________. -
✔✔-Equal.
-Equilibrium.
✔✔Dynamic Equilibrium: - ✔✔-The reactions are still going on- you just can't detect
changed in the concentrations anymore.
✔✔2AB--><--A2 +B2
If rate = Delta [A2]/Delta t, how would that compare to the rate in terms of [B2]?
A. Delta [A2]/Delta t = -Delta [B2]/Delta t
B. Delta [A2]/Delta t = Delta [B2]/Delta/Delta t
C. Delta [A2]/Delta t = -2 Delta [B2]/Delta t
D. Delta [A2]/Delta t = -1/2 Delta [B2]/Delta t - ✔✔-B
*Balanced equation = coefficients are all 1 = 1:1 Ratio.
✔✔2AB--><--A2+B2
How is Delta [A2]/Delta t related to Delta [AB]/Delta t?
What would it look like graphically?
,A. Delta [A2]/Delta t = -Delta [AB]/Delta t
B. Delta [A2]/Delta t = -2 Delta [AB]/Delta t
C. Delta [A2]/Delta t = -1/2 Delta [AB]/Delta t - ✔✔-C
*Inversely proportional. Ab is consumed trice as fast as A2 is produced.
✔✔Kinetics: - ✔✔-Study of rates of reactions.
✔✔Why do we measure kinetics? - ✔✔-Not because we are really interested in how fast
they go-but because...
-Information about rates and how they change with conditions (temperature and
concentration_ tells us about the REACTION MECHANISM.
✔✔Reaction mechanism: - ✔✔-The pathway that leads from reactants to products.
-The series of events(bond breaking and formation) and intermediates that are formed.
✔✔Rate depends on ______________ of reactants.
For reactions to happen-reactants have to ______________.
The more molecules-the higher the probability of a collisions- the ______________ the
rate. - ✔✔-Concentrations.
-Collide.
-Faster.
✔✔Rate Law: - ✔✔-Shows that the relationship of the rate of a reaction to the rate
constant and the concentration of the reactants.
✔✔Formula for the general reaction: aA+bB-->cC+dD - ✔✔-Rate=k[A]^x[B]^y.
*x and y are Not the stoichiometric coefficients.
*k=the rate constant. (little k).
*The units of k depend on the order of the reaction.
✔✔Reaction "order" MUST be determined by _________________? - ✔✔-Experiment.
✔✔For the reaction A + B--> the rate reaction could be: - ✔✔-Rate = k[A] First Order.
-Rate = k[A][B] Second Order (first order in A and first order in B).
-Rate = k[A]^2 Second order (in A).
-Rate = k[A]^2[B] Third Order (second order in A and first order in B).
✔✔What are the units of K?
1) For a first order reaction
*Delta[A]/Delta t = k[A]
2) For a second order reaction
*Delta[A]/Delta t = k[A]^2
A. 1/s
B. mol/L.s
, C. L/mol.s
D. s.L/mol - ✔✔1) A
2) C
✔✔First-Order: - ✔✔-Reaction rate depends on the reactant concentration raised to the
first power.
Rate = k[A].
✔✔For a First-Order Reaction: plot of ln[ ] vs t is _______________. - ✔✔-Linear.
✔✔Second-Order Reactions: - ✔✔-A--> Products.
-Rate = k[A]^2.
✔✔For Second-Order Reactions: plot of 1/[A] vs t is _____________. - ✔✔-Linear with
a slope k.
✔✔Generally reaction rates ______________ with temperature. Some reactions need
an initial "__________" to make them go (e.g. H2+O2). WHY? - ✔✔-Increase
-"Spark"
✔✔Since T is proportional to KE- we can conclude the molecules are colliding with
more __________- and this increases the rate- but WHY? - ✔✔-Energy
✔✔The activation energy affects the...? - ✔✔-Rate of the reaction.
✔✔The higher the activation energy, the __________ the reaction. - ✔✔-Slower.
✔✔Does Activation Energy affect Delta H? - ✔✔-No, ONLY affects reaction rate.
✔✔Transition state: - ✔✔-The highest point on the energy diagram.
*We do not have reactants and products at this point, rather we have something in
between due to the breaking and forming of bonds.
✔✔What effect does increasing the temperature have?
A. Increasing the temperature increases the average kinetic energy of the molecules.
B. They collide with more energy.
C. This provides energy to get over the activation energy barrier.
D. All of the above. - ✔✔-D
✔✔Recall the Boltzmann distribution. An increase in T provides more molecules that
have ___________ ___________ to get over the activation energy barrier. - ✔✔-
Enough Energy.
✔✔Typical Combustion Reaction:
SOLUTIONS SCORED A+
✔✔What needs to happen for a reaction to occur? - ✔✔-Collisions.
*Molecules have to bump not each other.
*Therefore the more collisions, the more likely the reaction.
✔✔The molecules have to have enough energy-(What for?)
As the temperature goes up we can expect the molecules to bump into each other with
more energy. - ✔✔-To break the bonds between the reactants.
✔✔For the reaction A---><----B, how does the rate of
1) The forward reaction change over time?
2) The reverse reaction change over time?
*Explain why
A. Increase
B. Decrease
C. Stays the same
D. It depends - ✔✔- B
*Less reactants and more products as the reaction progresses in the forward reaction.
The concentration decreases and so does the likelihood that there will be collisions.
-A
*As forward reaction progresses, more products formed, the concentration increases
and so does the likelihood of products colliding into one another thus increasing the rate
of the reverse reaction.
✔✔Eventually, the the rate of the forward reaction will ___________ the rate of the
reverse reaction. At this point, the reaction has reached ____________________. -
✔✔-Equal.
-Equilibrium.
✔✔Dynamic Equilibrium: - ✔✔-The reactions are still going on- you just can't detect
changed in the concentrations anymore.
✔✔2AB--><--A2 +B2
If rate = Delta [A2]/Delta t, how would that compare to the rate in terms of [B2]?
A. Delta [A2]/Delta t = -Delta [B2]/Delta t
B. Delta [A2]/Delta t = Delta [B2]/Delta/Delta t
C. Delta [A2]/Delta t = -2 Delta [B2]/Delta t
D. Delta [A2]/Delta t = -1/2 Delta [B2]/Delta t - ✔✔-B
*Balanced equation = coefficients are all 1 = 1:1 Ratio.
✔✔2AB--><--A2+B2
How is Delta [A2]/Delta t related to Delta [AB]/Delta t?
What would it look like graphically?
,A. Delta [A2]/Delta t = -Delta [AB]/Delta t
B. Delta [A2]/Delta t = -2 Delta [AB]/Delta t
C. Delta [A2]/Delta t = -1/2 Delta [AB]/Delta t - ✔✔-C
*Inversely proportional. Ab is consumed trice as fast as A2 is produced.
✔✔Kinetics: - ✔✔-Study of rates of reactions.
✔✔Why do we measure kinetics? - ✔✔-Not because we are really interested in how fast
they go-but because...
-Information about rates and how they change with conditions (temperature and
concentration_ tells us about the REACTION MECHANISM.
✔✔Reaction mechanism: - ✔✔-The pathway that leads from reactants to products.
-The series of events(bond breaking and formation) and intermediates that are formed.
✔✔Rate depends on ______________ of reactants.
For reactions to happen-reactants have to ______________.
The more molecules-the higher the probability of a collisions- the ______________ the
rate. - ✔✔-Concentrations.
-Collide.
-Faster.
✔✔Rate Law: - ✔✔-Shows that the relationship of the rate of a reaction to the rate
constant and the concentration of the reactants.
✔✔Formula for the general reaction: aA+bB-->cC+dD - ✔✔-Rate=k[A]^x[B]^y.
*x and y are Not the stoichiometric coefficients.
*k=the rate constant. (little k).
*The units of k depend on the order of the reaction.
✔✔Reaction "order" MUST be determined by _________________? - ✔✔-Experiment.
✔✔For the reaction A + B--> the rate reaction could be: - ✔✔-Rate = k[A] First Order.
-Rate = k[A][B] Second Order (first order in A and first order in B).
-Rate = k[A]^2 Second order (in A).
-Rate = k[A]^2[B] Third Order (second order in A and first order in B).
✔✔What are the units of K?
1) For a first order reaction
*Delta[A]/Delta t = k[A]
2) For a second order reaction
*Delta[A]/Delta t = k[A]^2
A. 1/s
B. mol/L.s
, C. L/mol.s
D. s.L/mol - ✔✔1) A
2) C
✔✔First-Order: - ✔✔-Reaction rate depends on the reactant concentration raised to the
first power.
Rate = k[A].
✔✔For a First-Order Reaction: plot of ln[ ] vs t is _______________. - ✔✔-Linear.
✔✔Second-Order Reactions: - ✔✔-A--> Products.
-Rate = k[A]^2.
✔✔For Second-Order Reactions: plot of 1/[A] vs t is _____________. - ✔✔-Linear with
a slope k.
✔✔Generally reaction rates ______________ with temperature. Some reactions need
an initial "__________" to make them go (e.g. H2+O2). WHY? - ✔✔-Increase
-"Spark"
✔✔Since T is proportional to KE- we can conclude the molecules are colliding with
more __________- and this increases the rate- but WHY? - ✔✔-Energy
✔✔The activation energy affects the...? - ✔✔-Rate of the reaction.
✔✔The higher the activation energy, the __________ the reaction. - ✔✔-Slower.
✔✔Does Activation Energy affect Delta H? - ✔✔-No, ONLY affects reaction rate.
✔✔Transition state: - ✔✔-The highest point on the energy diagram.
*We do not have reactants and products at this point, rather we have something in
between due to the breaking and forming of bonds.
✔✔What effect does increasing the temperature have?
A. Increasing the temperature increases the average kinetic energy of the molecules.
B. They collide with more energy.
C. This provides energy to get over the activation energy barrier.
D. All of the above. - ✔✔-D
✔✔Recall the Boltzmann distribution. An increase in T provides more molecules that
have ___________ ___________ to get over the activation energy barrier. - ✔✔-
Enough Energy.
✔✔Typical Combustion Reaction:
