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Chemistry 12
Tutorial #1 – Reaction Kinetics
Tutorial #1 - Unit 1 – Reaction Kinetics
Multiple Choice Explanations
The answer is B
Reaction “A” looks simple but the diatomic bonds in H2 & I2 must be broken before the reaction
can take place. This means there is a higher energy requirement (activation energy), so the
reaction is fairly slow at room temp. The “at room temp” is important because this reaction
could be very fast at a high temp.
Reaction “B” is the answer because there are no bonds broken, the Ag+ and the CrO42-(aq)
simply join together (precipitation reactions are always fast!)
Reaction “C” is slow because of the solid (it is heterogeneous). Reactions can take place only on
the surface of the lead.
Reaction “D” is slow because covalent C-H bonds in CH4 must be broken. This requires more
energy.
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Chemistry 12
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The answer is C
The higher the Temp. and the higher the [HCl], the faster the rate, so Rx. 3 is fastest (high
temp. and high [HCl]), followed by Rx. 1 (high temp. and low [HCl]) and slowest is
Rx. 2 (low temp. and low [HCl])
The answer is D
“D” is the definition of Activation Energy. “A” is just Kinetic Energy, “B” is the actual energy
of the AC, not an energy difference on the PE Graph, “C” of course is the ∆H.
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Chemistry 12
Tutorial #1 – Reaction Kinetics
Ea (rev) = +10 kJ
H (rev) = -20 kJ
The answer is A
The EA for the reverse reaction is shown by the blue arrow placed on the diagram above and the
ΔH for the reverse reaction is shown by the red arrow. It is –20 because it’s going down from
130 to 110.
The EA for the forward reaction would be 140 – 110 = 30 and the ∆H for the forward reaction
would be 130 – 110 = +20. Look at the graph and make sure you know how these are obtained!
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Chemistry 12
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The answer is B
Increasing the temperature increases the average kinetic energy of the molecules so there are
more collisions with a high kinetic energy (II) and if they are moving faster, there is also a
greater probability (chance) of collisions (I). The potential energy barrier for a successful
collision is called the activation energy, which cannot be altered by changing the temp. It can
only be altered by using a suitable catalyst.
The answer is D
A catalyst can change only the activation energy (the energy difference between the reactants or
products and the activated complex). It cannot change the energy difference between the
reactants and products () or the potential energy of the reactants or products. Looking at
“B”, only a change in temperature can change the kinetic energy.
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Tutorial #1 – Reaction Kinetics
Chemistry 12
The answer is A
Since N2 & O2 are the only things added, the concentrations of these are both high. Since these
are the reactants, the rate of the forward reaction starts out fast. Since there is no product to
begin with, the rate of the reverse reaction is initially zero. As time goes on and the system
approaches equilibrium, N2 & O2 are used up, their concentrations decrease and the rate of the
forward reaction decreases. In the meantime, and as N2 & O2 were reacting, NO2 was being
formed, so the [NO2] would be increasing and the chance that one NO2 molecule could collide
with another is increasing, therefore the rate of the reverse reaction would be increasing as the
system approaches equilibrium. Remember the graph of forward and reverse reaction rate vs.
time:
Forward rate
Reverse rate
equilibrium
time
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Chemistry 12
Tutorial #1 – Reaction Kinetics
The answer is C
“Dynamic” means things are continuing to happen. In other words the forward and reverse
reactions continue to occur. Remember “A” is not true because maximum randomness
(entropy) is NOT always achieved at equilibrium. “B” and “D” are both characteristics of
equilibrium but these do not explain why it’s called dynamic.
The answer is A
In reaction “A”, there is 1 mole of gas on the left side and 2 moles of gas on the right, therefore
entropy is increasing as the reaction proceeds. In “B”, entropy is decreasing because gases and
forming a liquid, in “C”, 3 moles of gas are forming 2 moles of gas and in “D”, (aq) is going to
solid.
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Chemistry 12
Tutorial #1 – Reaction Kinetics
The Answer is B
“B” is the fastest reaction because aqueous ions are highly mobile and more concentrated than
molecules in a gas. The aqueous ions have a high probability of colliding. Also, when Ag+ and Ireact, there are no bonds to break. They simply collide and bond to form the solid. (The
activation energy is very low.)
“A” is slow at room temperature because the covalent bonds in the diatomic molecules of H2
and I2 must be broken before they can react to form HI. This takes energy (fairly high
activation energy), and is therefore slow at room temperature.
In answer “C”, many C—C covalent bonds (in the sugar) must be broken (and also the diatomic
bond in O2 ). Covalent bonds require a high activation energy and are therefore slow to break.
Answer “D” is slow at room temperature because there are 23 reacting particles, therefore the
reaction would require numerous steps and hence is quite slow. (“B” is a 2 particle collision
which takes place in a single step).
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Chemistry 12
Tutorial #1 – Reaction Kinetics
The slope is
steepest in this
interval
The Answer is A
Rate is defined as amount/time, which in this case would be Volume CO2/time. Looking
at the graph above, so can see that Volume CO2/time is just the slope of the graph. The
slope of the graph is steepest in the time interval between 0 and 1 minute. Therefore the rate of
the reaction is fastest during this interval. This makes sense because generally as a reaction
proceeds (starting with reactants), the rate starts out fast and then slows down as the
reactants are used up and their concentration(s) decrease.
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Chemistry 12
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The Answer is B
The only thing that can change the average kinetic energy of molecules is a change in the
temperature. Adding a catalyst simply decreases the activation energy (minimum energy
required for a successful collision) by providing an alternate mechanism (pathway) for the
reaction. Increasing the surface area allows for more possible collisions between reactant
molecules in two different phases (heterogeneous). Increasing the concentration, again
increases the probability of collisions but does not affect the kinetic energy.
Reactants
The Answer is D
Looking at the red markers on the graph above, the H for the forward reaction is the net
energy difference between the products and the reactants (arrow IV). Because PE or enthalpy is
decreasing as the reaction proceeds from left to right, the value of Hforward would be a
negative number (exothermic). The Hreverse would be the same amount but a positive value
(endothermic). Just to review, arrow I is Ea(forward), arrow II is really nothing because the
starting point of the graph is arbitrary. (Only the energy differences among reactants, products
and activated complex is relevant in these PE graphs.) Arrow III is Ea(reverse).
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Chemistry 12
Tutorial #1 – Reaction Kinetics
The Answer is A
A catalyst only provides an alternate reaction mechanism which has a lower activation
energy. The heat of reaction (H) is the net energy difference between the reactants and the
products. The energies of the reactants and the products cannot be changed with a catalyst, so
H cannot be changed with a catalyst. Since a catalyst lowers both the Ea(forward) and the
Ea(reverse) by the same amount, both the forward and the reverse reactions will be speed up by
the catalyst. Therefore the equilibrium will not shift and the equilibrium constant (Keq) will
remain the same. Remember, the only thing that can change the value of Keq is the
temperature
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Tutorial #1 – Reaction Kinetics
Chemistry 12
HOOBr is an intermediate
HOBr is an intermediate
Overall reaction: 4HBr + O2
 2 H2O + 2 Br2
HBr and O2
are reactants
H2O and Br2
are products
The Answer is D
Remember that a reaction intermediate is produced in one step and used up in a later step. (So
it is on the right in a step and on the left in a step somewhere below). If you look at the question
above you will see that HOOBr and HOBr are both reaction intermediates. When the reaction
intermediates are cancelled out and the rest of the equations are added, you obtain the overall
reaction which shows the reactants and the products. Remember that a catalyst is used up (on
the left) of one step and regenerated (on the right) in a later step.
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Chemistry 12
Tutorial #1 – Reaction Kinetics
The Answer is D
According to the Chemistry 12 PLO’s from the Ministry:
It is expected that students will:
D4. describe chemical equilibrium as a closed system at constant temperature:
1. whose macroscopic properties are constant
2. where the forward and reverse reaction rates are equal
3. that can be achieved from either direction
4. where the concentrations of reactants and products are constant
You can see that I, II and III in the box above are contained in the characteristics of
equilibrium as described by the Ministry. Recall that concentrations of reactants and products
are constant but NOT usually equal to each other. Macroscopic means observable.
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Chemistry 12
Tutorial #1 – Reaction Kinetics
The Answer is C
For the answers “A” and “C” in which H is negative, the reactions are exothermic and the
tendency toward minimum enthalpy favors the
products:
An exothermic reaction
Reaction “A” has 1 mole of gas (mog) in the reactants
and 2 mog’s in the products, so products have
maximum entropy.
reactants
PE
products
Products have
minimum enthalpy
Reaction proceeds
Reaction “C” has 3 mog’s in the reactants and 2 mog’s in the products, so maximum entropy
favors the reactants. So in reaction “C” minimum enthalpy favors the products and
maximum entropy favors the reactants so these two driving forces are opposing each
other.
If you check reaction “B” you will see that because it is endothermic, minimum enthalpy favors
the reactants, and because it goes from 3 mog’s to 2 mog’s, maximum entropy also favors the
reactants.
In reaction “D”, minimum enthalpy favors the reactants (endothermic) and maximum
entropy also favors the reactants. (10 mog’s in reactants and 9 mog’s in products.)
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Chemistry 12
Tutorial #1 – Reaction Kinetics
The Answer is C
The ratios of all of the species present in the reaction is constant. Therefore you are able to
calculate the RATE of production EXACTLY like calculating MOLES of one species given the
moles of another species.
4.0x10-4 moles x 3 mol H2 = 6.0 x10-4 moles
2 mol NH3
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Chemistry 12
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The Answer is B
The key to this question is the OPEN SYSTEM.
I.
Balance - can measure weight – the weight of the system will decrease because a gas
is formed and will leave the system because it is open.
II.
Pressure Gauge – will measure pressure. The P will NOT change because it is an
open system. The gas is escaping.
III.
pH meter - DO NOT look at H2 Gas. This will not affect pH. The [HCl] will decrease
as the reaction proceeds forward, so the pH will increase.
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Chemistry 12
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The Answer is C
The heat of reaction (H) is the net energy difference between the reactants and the products
and CAN NOT be changed by temperature. Increasing the temperature increases the average
kinetic energy of the molecules so there are more collisions with a high kinetic energy (III) and
if they are moving faster, there is also a greater probability (chance) of collisions (II).
The Answer is D
The activated complex is the INTERMEDIATE complex that is TEMPORARILY formed from
the combination of the reactants (unstable) . As the reactants approach each other they slow
down because the e-‘s repel so their kinetic energy decreases. As the KE decreases the PE
increases (high PE) .
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Chemistry 12
Tutorial #1 – Reaction Kinetics
The Answer is B
As stated many times before, as the T is increased the KE is increased, if the KE is increased
then the speed at which the molecules are traveling is increased which will increased the
number of collisions. If the number of collisions is increased then the reaction rate will
increase and the time for the reaction to occur will decrease.
The Answer is D
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Chemistry 12
Tutorial #1 – Reaction Kinetics
The Answer is D
The Answer is D
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Chemistry 12
Tutorial #1 – Reaction Kinetics
The Answer is B
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Chemistry 12
Tutorial #1 – Reaction Kinetics
Tutorial #1 - Unit 1 – Reaction Kinetics
Multiple Choice Explanations
Student handout
The answer is
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Chemistry 12
Tutorial #1 – Reaction Kinetics
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Chemistry 12
Tutorial #1 – Reaction Kinetics
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Chemistry 12
Tutorial #1 – Reaction Kinetics
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Chemistry 12
Tutorial #1 – Reaction Kinetics
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Chemistry 12
Tutorial #1 – Reaction Kinetics
The slope is
steepest in this
interval
The Answer is
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Chemistry 12
Tutorial #1 – Reaction Kinetics
The Answer is
The Answer is
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Chemistry 12
Tutorial #1 – Reaction Kinetics
The Answer is
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Tutorial #1 – Reaction Kinetics
Chemistry 12
Overall reaction: 4HBr + O2
 2 H2O + 2 Br2
The Answer is
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Tutorial #1 – Reaction Kinetics
The Answer is
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Chemistry 12
Tutorial #1 – Reaction Kinetics
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Tutorial #1 – Reaction Kinetics
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Chemistry 12
Tutorial #1 – Reaction Kinetics
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Chemistry 12
Tutorial #1 – Reaction Kinetics
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Chemistry 12
Tutorial #1 – Reaction Kinetics
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