Average rate spans a time interval; instantaneous rate is the slope of the concentration curve at one moment.
Practise Kinetics & Equilibrium All Kinetics & Equilibrium topics
Average rate is the change in concentration divided by the elapsed time over a chosen interval, which gives an overall picture of how fast a reaction proceeded. Instantaneous rate is the slope of the concentration-time curve at one particular moment and is found by drawing a tangent to the curve. Because most reactions slow down as reactants are consumed, the average rate over a long interval is smaller in magnitude than the instantaneous rate at its start. The initial rate, the instantaneous rate at time zero, is especially useful because the concentrations are then known exactly and product effects are negligible.
Key ideas: To get an instantaneous rate from a graph, draw a tangent line at the chosen time and compute rise over run, taking the magnitude. For an interval, use abs(delta[A]) divided by delta t. As the interval shrinks to zero the average rate approaches the instantaneous rate, which is the derivative d[A]/dt.
Common mistakes and connections: Students often read the slope of the chord between two points as the rate at one moment. Another slip is using a tangent that cuts the curve, which gives a wrong slope. For zero order reactions the two rates agree everywhere because the plot is straight. The initial rate is the key input for the method of initial rates.
The tangent slope at any point is the instantaneous rate, steepest at t = 0; the chord between two times gives the smaller average rate over that interval. The interactive lesson shows this as a live animation or chart.
If [A] falls from 0.80 M to 0.60 M between 0 and 10 s, the average rate is 0.020 M per s over that interval.
Average rate spans a time interval; instantaneous rate is the slope of the concentration curve at one moment.
Average and Instantaneous Rate is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Kinetics & Equilibrium first: Chemical Kinetics, Reaction Rate.
The next topics in Kinetics & Equilibrium are Rate Law and Rate Constant.
Yes. The interactive lesson includes a self-check question on Average and Instantaneous Rate, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Average and Instantaneous Rate is topic 3 of 91 in Kinetics & Equilibrium.