Chem 112 – Class Guide: Reaction Rates and Rate Laws (Chapter 14, Sections 1-3)
Chapter 14, Sections 1-3 Learning Goals:
Upon completion of Chapter 14, Sections 1-3, you should be able to determine the following:
Define reaction rate.
Identify four factors that change the rate of a reaction.
Write a reaction rate as either the disappearance of a reactant or appearance of a product.
Calculate instantaneous rates.
Compare the rate of appearance of one product to the rate of disappearance of a reactant.
Given appropriate data, calculate the rate law for a reaction.
Given appropriate data, calculate the rate constant k for a reaction.
Given appropriate data, calculate the rate of a reaction under specified conditions.
Determine the units of k for any reaction.
Given appropriate data, calculate the overall order of a reaction.
Chapter Reading Guide: Chapter 14, Sections 1 - 3
Section 1: FACTORS THAT AFFECT REACTION RATES:
Read Chapter 14.1
There are four factors that affect the rate at which a reaction occurs:
Physical State – if reactants are in different states (liquid and gas) the reaction can only
occur at the surface where the reactants meet. This can really slow down a reaction
rate.
Reactant Concentrations – most reactions speed up if the concentration of one reactant
or another is increased.
Reaction Temperature – most reactions speed up if the temperature is increased
Catalyst – a catalyst is something that speeds up the reaction, like an enzyme
Section 2: REACTION RATES:
Read Chapter 14.2
Reaction rate is defined as the change in the concentration of reactants or products over
time. We can calculate rate using:
Average Rate of Appearance of B =
Change in concentration of B
Change in Time
=
∆[𝐵]
∆𝑡
TIP! If a substance is in brackets, like [B], that means the concentration of that
substance in molarity
TIP! Rates are always expressed as positive numbers!
Rates of reactions decrease over time because the concentration of reactants decreases.
We can also calculate the instantaneous rate of a reaction – the rate at a particular instance
in time.
If we need to compare rates, we use a different equation than the one on the previous slide.
If we have a general reaction: a A + b B  c C  d D, we calculate rate using:
Rate = −
1 ∆[𝐴]
1 ∆[𝐵] 1 ∆[𝐶] 1 ∆[𝐷]
=−
=
=
𝑎 ∆𝑡
𝑏 ∆𝑡
𝑐 ∆𝑡
𝑑 ∆𝑡
Note that the term for A and B both have negative signs in front of them. This is because
they are reactants and their concentration is decreasing as the reaction progresses. C and D
are both positive because they are products and their concentration is increasing as the
reaction progresses.
Try Practice Exercises 14.1, 14.2, and 14.3.
REMEMBER!!! You only need to use equation 14.4 when COMPARING rates. When initially
calculating rates, you need only to use equation 14.1.
Section 3: CONCENTRATION AND RATE LAWS:
Read Chapter 14.3
Remember (from the first slide!) that reaction rates depend upon concentration of
reactants. We express this as a rate law – how the rate of a reaction depends upon reaction
concentrations.
For the general equation: a A + b B  c C  d D, the rate law is written as:
Rate = k[A]m[B]n
Where k = rate constant
m and n = whole numbers that show us how the reaction depends upon the
concentration of [A] and [B] and are called the reaction orders (which we say as “the
order with respect to A”).
TIP! We can calculate the rate, k, m and n given a series of experiments and information
about those experiments.
The sum of m and n (the orders of each reactant) give us the overall reaction order for the
total reaction. So if we knew that the reaction was first order with respect to A, and also
first order with respect to B (rate = k[A]1[B]1), we would say that the overall reaction order is
2 (1 + 1 = 2), and call this a second order reaction.
m and n are not the same as the coefficients in the balanced equation! They must be
determined experimentally.
The rate of a reaction depends on concentration (which is how we get the rate law) but the
rate constant k does not depend on concentration.
Try practice exercise 14.6.
Learning Resources
Chapter Learning Goals
Chapter 14, Sections 1 - 3 Learning Goals
Pre Class Assignment: This assignment must be completed prior to the next class. Check your syllabus
for the exact due date and time.
Complete the pre class assignment
(http://berks.psu.edu/clt/chem112/RatesandLaws_HW.docx)
Submit a copy to the dropbox located in ANGEL called “Pre Class Assignment Submission:
Reaction Rates and Rate Laws”
End of Chapter Problems:
Practice with these problems if you are having difficulty with any of the concepts covered in this
class guide AFTER we have met in class. If you cannot easily complete these problems, seek
help from your instructor, your mentor, or the learning center.
Chapter 14: 17, 19, 23, 27, 33, 35, 37