Chemistry Semester 2
Ms. Boon
Unit 9: Reaction Rates and Equilibrium
Homework #1 pp. 585 #1-13
1. What does the word “rate” mean in everyday life, and what do chemists mean
by “reaction rate”?
A: A rate is a change in some quantity divided by the length of time
over which the change occurred. A reaction rate is the decrease in the
concentration of a reactant (or the increase in the concentration of a
product) during a small time interval, divided by the duration of that
interval and by the corresponding coefficient in the chemical
equation.
2. What is the name given to the branch of chemistry dealing with reaction
rates? What are such studies important?
A: Chemical kinetics is the branch of chemistry dealing with reaction
rates. Kinetics helps chemists learn about the mechanism by which a
reaction occurs.
3. Why is a collision between molecules necessary in many reactions?
A: Atoms must be in contact with each other for new bonds to form.
4. How may reaction rates by measured?
A: Reaction rates may be measured by measuring the change in the
concentration of a reactant or product during a short time interval
and thereby calculating the rate.
5. Explain why reactant concentration influences the rate of a chemical
reaction.
A: The greater the concentration, the more molecules of reactant there are to
react and therefore the greater the reaction rate.
6. Give examples of the strong effect that temperature has on chemical
reactions.
A: Many reactions are temperature dependent. They speed up when heat is
added. For example, chemical reactions in the human body increase when
we get a temperature. That is why we need to maintain our core body
temperature. Food molds faster at higher temperatures. Organic substances
decompose faster at higher temperatures.
7. What is unique about surface reactions?
A: The rate of a surface reaction is proportional to the area of the surface.
8. Why must coefficient be included in the definition of reaction rate?
a. The rates of concentration change, for the species that participate in
the reaction, are proportional to that species’ coefficient in the
chemical equation. Therefore, in order that the rate of reaction be
defined in a way that is independent of the choice of species, the rate
of concentration change must be divided by the corresponding
coefficient.
9. Calculating the reaction rate from a product appeared to give an answer
different from that calculated from a reactant. Why?
a. One possibility is that the product might be involved in some other
reaction, a side reaction.
10. The usual unit for reaction rate is M/s. Can you think of another unit?
a. Moles per cubic centimeter per minute is one possible unit. Grams
per second could work too.
11. Explain why an increase in the frequency of collisions is not an adequate
explanation of the effect of temperature on reaction rate.
a. There is only a very mild increase in collision rate with temperature.
But, the reaction rate increases massively.
12. Would the factors that affect the rate of a chemical reaction influence a
physical change in the same way? Explain.
a. Generally, yes. For example, an increase in temperature greatly
increases the rate of evaporation of a liquid.
13. Why does pressure affect the rates of gas reactions?
a. Pressure affects the rate of gas reactions because it changes the
concentration of the reactants.