AP Chemistry Unit 5: Kinetics Notes

Introduction

Kinetics explains how fast concentrations change. Experimental slopes and initial-rate comparisons establish empirical rate laws; balanced overall coefficients do not supply their exponents.

Collision models and energy profiles explain why concentration, temperature, orientation, and catalysts affect rate. These models support causal reasoning but do not replace measured rate evidence.

Mechanisms must reproduce the net equation and the observed rate law. Topic 5.8 uses a stated irreversible slow first step; Topic 5.9 is the separate case where a rapid pre-equilibrium eliminates an intermediate.

Current exam guidance: Aligned to the current 2026–27 AP Chemistry course framework and exam scope.

5.1

Reaction Rates

How can several species concentration slopes describe one reaction rate?

Short answer

Give reactant slopes a negative sign and divide every species slope by its stoichiometric coefficient so all report the same positive rate.

Know firstBalanced coefficients · Concentration units · Graph slope

Normalize slope before comparing species

Concentration-time graphs show reactants decreasing and products increasing. Their raw slopes need not have equal magnitudes because the balanced equation may consume or form several units of one species per reaction event.

For $2A\rightarrow B$ , a 0.40 M decrease in A over 20.0 s is a slope of −0.020 M s $^{-1}$ . Dividing its magnitude by 2 gives a reaction rate of 0.010 M s $^{-1}$ , matching a 0.20 M increase in B over the same interval.

Average rate uses a secant over a finite interval; instantaneous rate uses a tangent. Concentration, temperature, exposed surface area, catalysts, and the surrounding conditions can change measured rate. Compare one parameter at a time and do not confuse a factor that changes rate with the coefficient normalization used to report that rate.

Chemistry model

Reaction rate is a normalized slope

Move time and compare reactant disappearance with product formation after stoichiometric normalization.

Time3.0 s

Time: 3.0 s
[A]: 0.517 M
[P]: 0.966 M
Normalized rate: −d[A]/dt = ½d[P]/dt

rate = −(1/a)d[A]/dt = (1/p)d[P]/dt

Read this figure without the graphic

Reaction rate is a normalized slope. The figure connects reactant slope, product slope, one reaction rate so the relationship can be read without relying on color alone.

Representation	What to read
reactant slope	starting evidence or quantity
product slope	intermediate relationship
one reaction rate	result or conclusion

Formula

One rate from several species

r=-\frac{1}{a}\frac{\Delta[A]}{\Delta t}=\frac{1}{c}\frac{\Delta[C]}{\Delta t}

Stoichiometric normalization makes consumption and formation slopes describe the same reaction progress.

Use when: The balanced equation coefficients and concentration interval refer to the same reaction and volume basis.
Do not assume: Using a raw reactant slope gives a negative value and can differ by a coefficient factor.
Variables: $r$ : reaction rate (mol L^-1 s^-1)
Common misuse: Ignoring the negative sign or coefficient normalization.

Slope ledger for $2A\rightarrow B$

Species	Change in 20 s	Raw slope	Normalized rate
A	−0.40 M	−0.020 M s^-1	0.010 M s^-1
B	+0.20 M	+0.010 M s^-1	0.010 M s^-1

Keep these three ideas

Reactants have negative concentration slopes.
Divide by coefficient to obtain one positive rate.
Average and instantaneous rates use different slopes.

Kinetics