CHEMISTRY 12 REACTION RATES WORKSHEET

1. The following data were collected for the reaction: Zn(s) + 2HCl(aq)  H2(g) + ZnCl2(aq)

in which zinc metal was reacted with 0.200 M HCl(aq): Time Mass Zn
(seconds) (grams)
0.0 31.0
60.0 24.6
120.0 20.2
180.0 17.4

a) Calculate the average reaction rate, in g/s, from time 0 to 60 s.

b) Calculate the average reaction rate, in g/s, from time 120 to 180 s.
c) Calculate the average reaction rate in g/s, from time 0 to 180 s.
d) Explain why the average rate in (b) is less than in (a)

2. In each of the following pairs of reactions, which would have the faster reaction rate?

a) H2(g) + I2(g)  2HI(g) or Ag+(aq) + I–(aq)  AgI(s)

b) Fe(s) + 2H2O(l)  Fe(OH)2(s) or CH3COOH(aq) + H2O(l)  CH3COO–(aq) + H3O+(aq)

c) Cu(s) + S(s)  CuS(s) or CaO(s) + H2O(l)  Ca(OH)2(s)

d) C(s, powder) + O2(g)  CO2(g) or C(s, chunk) + O2(g)  CO2(g)

e) HCl(aq) + NaOH(aq)  H2O(l) + NaCl(aq) or 2H2O(aq) + 2H+(aq)  2H3O+(aq) + O2(g)

3. Which of the reactions in #2 are HOMOGENEOUS reactions?

4. One piece of magnesium is reacted with 3.00M hydrochloric acid at 25C. Another piece of magnesium of
equal size and shape is reacted with 1.00 M hydrochloric acid at 25C. Predict which reaction occurs at a
faster rate. Explain using collision theory.

5. A chunk of zinc is reacted with 3.00 M hydrochloric acid at 25C. An equal mass of powdered zinc is
reacted with 3.00 M hydrochloric acid at 25C. Compare the reaction rates.

6. State two ways that the number of effective collisions between reactants can be increased.

7. The following reaction occurs at constant temperature and constant volume in a closed system:

CaCO3(s) + 2H+(aq) + 2Cl-(aq)  CO2(g) + H2O(l) + Ca2+(aq) + 2Cl-(aq)

How could you experimentally measure the rate of this reaction?

1. Which of the following has the greatest reaction rate?
a. C(s) + O2(g)  CO2(g)
b. 2H2O2(l)  2H2O(l) + O2(g)
c. 2Al(s) + 3CuCl2(aq)  2AlCl3(aq) + 3Cu(s)
d. NaCl(aq) + AgNO3(aq)  AgCl(s) + NaNO3(aq)

2. Which of the following has the lowest rate of reaction?

a. Pb(s) + CuCl2(aq)  Cu(s) + PbCl2(aq)
b. HCl(aq) + NaOH(aq)  H2O(l) + NaCl(aq)
c. H2SO4(aq) + Ba(OH)2(aq)  2H2O(l) + BSO4(s)
d. Pb(NO3)2(aq) + 2NaI(aq)  PbI2(s) + 2NaNO3(aq)

3. Consider the following reaction: N2H4(l) + 2H2O2(l)  N2(g) + 4H2O(l)

In 5.0 seconds, 0.015 mol of H2O2 is consumed. The rate of production of N2 is:

a. 1.5 x 10-3 mol/s

b. 3.0 x 10-3 mol/s
c. 6.0 x 10-3 mol/s
d. 1.5 x 10-2 mol/s

4. At 25°C and considering only the nature of the reactants, which one of the following reactions most
probably has the highest rate?

a. Ca2+(aq) + CO32-(aq)  CaCO3(s)

b. CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
c. H2(g) + I2(g)  2HI(g)
d. C(s) + O2(g)  CO2(g)

5. The following reaction occurs at constant temperature and constant volume in a closed system: CaCO3(s)
+ 2H+(aq) + 2Cl-(aq)  CO2(g) + H2O(l) + Ca2+(aq) + 2Cl-(aq)

Changes in which one of the following would be useful in experimentally measuring the rate of this
reaction?
a. The mass of the system
b. The pressure of the system
c. The concentration of water
d. The concentration of the Cl-(aq)
6. Which one of the following reactions is most likely to have the highest reaction rate?
a. 2H2(g) +O2(g)  2H2O(g)
b. Mg2+(aq) + 2OH-(aq)  Mg(OH)2(s)
c. C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(g)
d. 2MnO4-(aq) + 16H+(aq) + 5C2O42-(aq)  2Mn2+(aq) + 10CO2(g) + 8H2O(g)

7. A 25.0 mL sample of hydrogen peroxide decomposes producing 50.0 mL of oxygen gas in 137 seconds.
The rate of formation of O2 in mL/min is
a. 0.182 mL/min
b. 0.365 mL/min
c. 10.9 mL/min
d. 21.9 mL/min

8. At 10°C, a small piece of Zn reacts with 2.0 M HCl to produce 15.0mL of H2 gas in 25 seconds. What
is the rate of this reaction?

a. 0.060 mol Zn per second

b. 0.4 C per second
c. 0.60 mL H2 per second
d. 0.80 mol/L HCl per second

9. A sample of magnesium having a mass of 0.360 g is dropped into dilute hydrochloric acid. At the end
of 4.00 minutes, the magnesium is removed and it is found to have a mass of 0.240 g. The average rate
at which the reaction took place was:

a. 0.003 mol/min
b. 0.120 mol/min
c. 2.06 x 10-5 mol/sec
d. 8.33 x 10-5 mol/sec
ANSWERS:

1. a) -0.11 g/s
b) -0.047 g/s
c) -0.076 g/s
d) In (b) there are less reactants, including HCl(aq), which decreases the rate of reaction due to
fewer collisions.

2. a) Ag+(aq) + I–(aq)  AgI(s) Aqueous ions react faster than gases because there is less
space between them and they actually attract one another rather than just having to collide randomly.
b) CH3COOH(aq) + H2O(l)  CH3COO–(aq) + H3O+(aq) The two liquids form a fast-reacting
homogenous mixture, whereas Fe(s) and water can only react at the surface of the iron.
c) CaO(s) + H2O(l)  Ca(OH)2(s) A liquid and sollid are expedted to react faster than two
solid reactants.
d) C(s, powder) + O2(g)  CO2(g) A powder has greater surface area than a solid chunk.
e) HCl(aq) + NaOH(aq)  H2O(l) + NaCl(aq) This is what we call a “formula equation” and we often
write it for simplicity’s sake. But in reality ionic substances dissolved in water are actually present as
separate ions. So we can rewrite the equation as follows:

H+(aq) + Cl–(aq) + Na+(aq) + OH–(aq)  H2O(l) + Na+(aq) + Cl–

(the “complete ionic equation”)

As you can see, only the H+ and the OH– are actually reacting.
So you could write the reaction as:

H+(aq) + OH–(aq)  H2O(l)

(the “net ionic equation”)

We are now looking at two oppositely charged aqueous ions which will react faster than a neutral
aqueous molecule and an aqueous ion because the two opposite charges will attract one another and will
not have to wait for a random collision.

3. Both reactions in (a) are homogeneous (the solid AgI is irrelevant since the REACTANTS are in the
same phase in each case).

The second reaction in (b) is homogeneous because H2O(l) is liquid water and CH3COOH(aq) refers to
acetic acid dissolved in water.

Both reactions in (e) are homogeneous.

Note: in (c) the reaction between Cu(s) and S(s) is NOT homogeneous because Cu and S are
different solid “phases” (have different properties depending on where one takes the sample).

4. In order for any reaction to occur, those particles must first collide. The reaction of zinc in 3.00 M HCl
will have the faster rate because with the greater number of particles there is a higher chance of
collisions occurring.

5. The reaction with the powdered zinc will be faster because there is significantly more surface area. By
increasing the surface area, there are more sites available for collision resulting in a higher chance of
collision and a faster reaction rate.
6. Collision theory is an aspect of the kinetic-molecular theory and is strongly connected with chemical
kinetics. The theory is used to explain how different variables affect the rate of reaction. This theory is
built on the idea that reactant particles must collide for a reaction to occur, but only a small percentage
of the total collisions have appropriate energies and "orientations" to effectively cause the reactants to
form into the products.

Effective (successful) collisions can be increased by:

- Increasing the total number of collisions (increase temperature, concentration, pressure, or surface
area)
- Increasing the energy of the collision (increase temperature of reaction)
- Optimize collision geometry (use a catalyst)

7. As the reaction is carried out at constant temperature and constant volume in a closed system, the easiest
property to measure to determine reaction rate would be the pressure change of the system. As CO2 gas
is being produced, the pressure of the system will be increasing as the reaction proceeds.

Multiple Choice Answers:

1. d
2. a
3. a
4. a
5. b
6. b
7. d
8. c
9. c