5.1 Reaction Rates

What is reaction rate?

Reaction rate measures how fast reactant concentration decreases or product concentration increases over time, expressed as Δ[concentration]/Δt. Rates are typically reported as positive values in units of M/s. Because reactants and products change at rates tied to their stoichiometric coefficients, different species in the same reaction can have different numerical rates.

Instantaneous vs. average rate

Average rate is calculated over a time interval using the slope between two points on a concentration-vs-time graph. Instantaneous rate is the rate at one specific moment, found by taking the slope of the tangent line at that point on the curve. Instantaneous rate at t=0 is called the initial rate, often used in kinetics experiments.

Graphical representation

On a concentration-vs-time graph, reactant curves decrease and product curves increase, both leveling off as the reaction nears completion. The steepness (slope) of the curve reflects the rate — steeper slopes mean faster rates. Rates generally slow down over time as reactant concentration decreases, which is why the curve flattens.

Key terms

Reaction rate

The change in concentration of a reactant or product per unit time, Δ[concentration]/Δt.

Average rate

Rate calculated over a time interval using the slope between two points on a concentration-vs-time graph.

Instantaneous rate

The rate at a specific moment, found from the slope of the tangent line at that point on the curve.

Initial rate

The instantaneous rate at t = 0, often used to study reaction kinetics.

Why rate decreases over time

Reactant concentration drops as the reaction proceeds, reducing collision frequency and slowing the rate.

Try an AP-style question

In 2N₂O₅ → 4NO₂ + O₂, N₂O₅ is consumed at 0.020 M/s. How fast is NO₂ formed?

1. 0.020 M/s
2. 0.010 M/s
3. 0.040 M/s
4. 0.080 M/s