Chemical Equilibrium Concepts: Pretest
Chem 1B
Fall 2009
As a part of a study of conceptual learning, we ask that you complete this short test of
your basic perceptions of the behavior of chemical reactions as it applies to the first unit
in Chem 1B, Chemical Equilibrium. We request that you include your name only to
facilitate the data analysis for this study; the results of the exam will in no way effect
your overall performance in this class nor will they be included in the calculation of
your final grade. Results of this exam will be kept confidential throughout the class.
Please circle the answers you feel to be most correct for each question. Note that more
than one of the answers may be correct. Be sure to circle the reason or reasons behind
your answers.
Thank you for your help with this project.
_______________________________________________________
Name
Chemical Equilibrium Diagnostic Test 1
Circle the correct answer(s). (Note that more than one answer per problem may be correct!)
Name________________________________
I: CHEMICAL REACTIONS: RATE AND CONCENTRATION (Chem 1A Review)
===============================
The questions below refer to the following reaction: 2AB + C2 => 2AC + B2
===============================
(Reaction 1)
1) List the reactants and products of this reaction.
Reactants:
Products:
2) Is this reaction balanced? Why or why not?
3) Why do we require equations to be balanced?
4) For the chart below, indicate which statement is true:
a) Curves 1 and 2 are reactants; curves 3 and 4 are products
b) Curves 1 and 2 are products; curves 3 and 4 are reactants
Concentration
1
2
3
4
Reaction coordinate (time)
5) As this reaction proceeds, which reactant element will be consumed most slowly?
a) A
b) B
c) C
d) All will be consumed at the same rate
6) Which product molecule will be generated most quickly?
a) AC
b) B2
c) Both will be generated at the same rate
Reaction coordinate (time)
a)
0.8
0.6
0.4
C2
0
AB
AB
0.2
Concentration (M)
0.8
0.6
0.4
0.2
C2
0
AB
Concentration (M)
0.6
0.4
0.2
C2
0
Concentration (M)
0.8
7) Assume that this reaction goes to completion. If the initial concentrations of AB and C 2 are
0.6M, which of these charts are possibly correct?
Reaction coordinate (time)
b)
Reaction coordinate (time)
c)
2
--------------------------------------------------------------The rate of a reaction is the “speed” of the reaction, and can be measured by how fast reactants
are consumed (how fast reactant concentration changes) or how fast products are generated.
rate of reaction =
(change in reactant concentration)
(change in product concentration)
- ---------------------------------------------- = --------------------------------------------change in time
change in time
Given this equation, it can be seen that the reaction rate is proportional to the concentrations of
reactants and products in a reaction.
--------------------------------------------------------------9) Why is one of these terms positive and one negative?
Reactants are consumed, so the rate of reactant concentration change is negative
10) What will happen to the rate of a reaction if [reactant] is increased? If it is decreased?
The reaction rate will increase with increased reactant concentration and vice versa
II: REVERSIBLE REACTIONS
---------------------------------------------------------------Two gases, nitric oxide (NO) and chlorine (Cl2) are mixed in equal concentrations. An exothermic
reversible reaction takes place:
2NO(g) + Cl2 (g)
⇌
2NOCl(g) + heat
(Reaction 2)
This is a “reversible” reaction, consisting of two related reactions (a “forward” reaction,
corresponding to upper arrow) and a “reverse” reaction (corresponding to the bottom arrow), and
we can speak of a “forward rate” and a “reverse rate”. The questions below refer to this reaction.
------------------------------------------------------------------11) The gases are mixed and the reaction begins. Which of the following statements are true?
a)
b)
c)
d)
The reverse reaction will not begin until the forward reaction is complete
The forward and reverse reactions both take place at the same time
The forward reaction will be faster than the reverse reaction
The forward and reverse reaction rates will always be equal
Reason
a) The forward reaction is driven by the reactants and always goes first
b) Heat drives the reverse reaction while the forward reaction is running
c) Reaction rates are a function of concentration, so both forward and reverse reactions
will run simultaneously
12) After the reactants (NO and Cl2) are mixed and the reaction begins,
a)
b)
c)
d)
The concentration of NO(g) will increase
The concentration of Cl2 (g) will increase
The concentration of NOCl(g) will increase
Cannot make a prediction
3
Reason
a) An exothermic reaction will produce a higher concentration of products
b) The concentration of Cl2 (g) will decrease relative to NO(g) and NOCl(g) because of its
lower coefficient
c) As the reaction proceeds, NO(g) and Cl2 (g) will be consumed to generate NOCl(g)
d) Cannot answer; all concentrations are determined by a chemical equation
e) Don’t have a good reason
f) Other
13) As the reaction continues and proceed toward the right (the forward reaction),
a) The rate of the forward reaction (proceeding to the right, from products to reactants)
decreases with time and is greater than that of the reverse reaction (proceeding to
the left, from reactants to products)
b) The rate of the reverse reaction increases with time and is greater than that of the
forward reaction
c) The forward reaction progresses until completion, then the reverse reaction starts
d) The forward and reverse reaction rates are equal
Reason
a) There are two reactants and only one product
b) This is an exothermic reaction, so the forward reaction will be faster
c) As the reaction starts and proceeds to equilibrium, reactants are consumed, reducing
the rate of the forward reaction
d) Forward and reverse reaction rates of reversible reactions are always equal
e) Don’t have a good reason
f) Other:
==========================================
At some point in time along the reaction coordinate of a reverse reaction the reaction will reach a
“steady state”, at which time the concentrations of both reactants and products and the rates of
the forward and reverse reactions will stop changing and become constant. This is the point of
“chemical equilibrium”.
==========================================
III: CHEMICAL EQUILIBRIUM
14) At the equilibrium point,
a)
b)
c)
d)
The concentration of all products and reactants will be equal
The concentration of reactants will be greater than the concentration of products
The concentration of products will be greater than the concentration of reactants
Cannot predict the relative concentrations of products and reactants
Reason
a) Equality of concentration is the definition of equilibrium
b) Concentrations of reactants are higher due to the stoichiometric relationships of
reactants and products
c) This is an exothermic reaction so concentrations of products are higher
d) Cannot answer; all concentrations at equilibrium are determined by a chemical
equation
e) Don’t have a good reason
f) Other:
4
15) When equilibrium is reached,
a)
b)
c)
d)
e)
The forward and reverse reaction rates will go to zero
The forward reaction rate will be greater than the reverse reaction rate
The reverse reaction rate will be greater than the forward reaction rate
The forward and reverse reaction rates will oscillate around the equilibrium point
The forward and reverse reaction rates will be equal and non-zero
Reason
a) At equilibrium, both forward and reverse reactions are complete
b) The forward reaction is always greater for an exothermic reaction
c) Once equilibrium is reached, the forward reaction is complete and the reverse reaction
begins
d) Both forward and reverse reactions will continue as long as reactants and products exist
in the mixture
e) Forward and reverse rates must oscillate as concentrations increase and decrease
f) Don’t have a good reason
g) Other:
16) An equimolar solution of NO and Cl2 is prepared and allowed go to equilibrium (Reaction 2).
Which of the following statements are true?
a)
b)
c)
d)
e)
Concentrations of NO and NOCl in the equilibrium solution are equal
Only NOCl and Cl2 will be found in the equilibrium solution
The concentration of NO will be half that of Cl2 in the equilibrium solution
The concentration of Cl2 will be greater than that of NO in the equilibrium solution
None of the above
Reason
a) NO is the limiting reactant, so it will go to zero in the final solution
b) NO is consumed twice as fast as Cl2, so its concentration will be lower than Cl2
c) NO and NOCl concentrations will be equal as they have the same stoichiometry
d) The reaction uses two NO molecules for every molecule, so there will only be half as
many NO molecules left at equilibrium
e) Cannot make a statement about relative concentrations
17) Which of the following could NOT be a representation of the equilibrium state for the reaction
described in Question 16?
- NO;
- Cl2;
- NOCl
a)
b)
c)
5