Reaction Rate — AP Chemistry
1. Core Definition of Reaction Rate ★☆☆☆☆ ⏱ 3 min
Reaction rate is the foundational concept of AP Chemistry Unit 5 Kinetics, which makes up 7-9% of the total AP Chemistry exam score. Reaction rate measures how quickly reactant concentrations are consumed or product concentrations are formed over a given period of time.
By universal convention, reaction rate is always reported as a positive value, regardless of whether you measure change in reactants or products. Unlike thermodynamics, which predicts if a reaction will occur spontaneously, kinetics describes how fast that reaction proceeds.
2. Average Reaction Rate & Stoichiometric Relationships ★★☆☆☆ ⏱ 4 min
Average reaction rate is the rate of a reaction averaged over a defined finite time interval, calculated as the total change in concentration divided by the total change in time. For any general balanced reaction $aA + bB \rightarrow cC + dD$, the standardized overall average rate is consistent for all species when normalized by stoichiometry:
\overline{r} = -\frac{1}{a}\frac{\Delta [A]}{\Delta t} = -\frac{1}{b}\frac{\Delta [B]}{\Delta t} = \frac{1}{c}\frac{\Delta [C]}{\Delta t} = \frac{1}{d}\frac{\Delta [D]}{\Delta t}
The negative sign for reactants accounts for the fact that reactant concentration decreases over time, so $\Delta [A]$ is negative. The negative sign converts this to a positive rate that follows convention. Dividing by stoichiometric coefficients ensures the overall rate is the same no matter which species you measure.
Exam tip: Always adjust for stoichiometry when asked to convert rate between different species in the reaction. Unadjusted values are almost always a trap option in multiple-choice questions.
3. Instantaneous and Initial Reaction Rate ★★★☆☆ ⏱ 3 min
Instantaneous reaction rate is the rate of the reaction at a single specific point in time, rather than averaged over an interval. Because reaction rate almost always decreases as reactant is consumed, the average rate over a large interval does not reflect how fast the reaction is going at any given moment.
AP Chemistry most commonly asks for instantaneous rate at $t=0$, called the **initial rate**, which is used for the method of initial rates to find reaction order and rate laws. To calculate instantaneous rate from a concentration vs time graph: (1) Draw a tangent line to the curve at the time of interest. (2) Calculate the slope of the tangent line. (3) For reactants, the instantaneous rate is the absolute value of the slope; for products, it equals the slope directly.
Exam tip: Extend your tangent line all the way to the axes of the graph to get two points far apart, which reduces error when calculating slope. Picking two points close together on the tangent will lead to calculation error that can cost you points on FRQs.
4. Units of Reaction Rate ★★☆☆☆ ⏱ 2 min
A very common point of confusion tested on the AP exam is the difference between units of reaction rate and units of the rate constant $k$. By definition, reaction rate is always change in concentration per unit time, so its units are always the same, no matter what the order of the reaction is.
Concentration in AP Chemistry is almost always measured in moles per liter (molar, $M$), and time is almost always measured in seconds ($s$), so the standard units of reaction rate are $M\ s^{-1}$. Stoichiometric coefficients and negative signs for reactants are unitless, so they do not change the units of rate. Only units of the rate constant $k$ change with reaction order.
Exam tip: Circle the noun in the question: if it asks for units of *rate*, it is always $M s^{-1}$. If it asks for units of *k*, adjust for reaction order. This simple step will save you from an easy trap.
Common Pitfalls
Why: Students remember to add a negative sign for reactants but forget that rate is standardized by stoichiometry, so they report the rate of consumption of the species as the overall reaction rate
Why: Students forget the convention that rate is always positive, and just copy the slope of the reactant concentration curve directly
Why: Students learn rate constant units immediately after reaction rate and mix the two concepts up, especially when the question mentions reaction order
Why: Students confuse average and instantaneous rate when rushed, and default to total change over total time
Why: Students are in a hurry and skip drawing a proper tangent, picking nearby points on the curve instead