AP Chemistry - Reaction Rate Notes

Measuring Reaction Rate

Chemical Kinetics

• The speed of a chemical reaction is called its reaction rate.
• The rate of a reaction measures how fast the reaction makes products or uses reactants.
• Controlling the speed of a chemical reaction is important.

Defining Reaction Rate

• The rate of a chemical reaction is generally measured by how much the concentration of a reactant decreases or product concentration increases over time.
• For reactants, a negative sign is used.
  • For the reaction: $\text{H}<em>2(g) + \text{I}</em>2(g) \rightarrow 2\text{HI}(g)$

Reactant and Product Concentrations as a Function of Time

• Graph depicting concentration (M) vs. time (s) for the reaction $\text{H}<em>2(g) + \text{I}</em>2(g) \rightarrow 2\text{HI}(g)$
• At 40 seconds:
  • $[\text{HI}]$ increases by $0.56 \text{ M}$, denoted as $\Delta [\text{HI}]$
  • $[\text{H}<em>2]$ decreases by $0.28 \text{ M}$, denoted as $\Delta [\text{H}</em>2]$

The Rate of a Chemical Reaction

• The rate of reaction can be expressed as the change in concentration of a reactant over the change in time.
• Formula:
  • $\text{Rate} = -\frac{\Delta [\text{H}<em>2]}{\Delta t} = -\frac{[\text{H}</em>2]<em>{t</em>2} - [\text{H}<em>2]</em>{t<em>1}}{t</em>2 - t_1}$

Average Rate

• The average rate is the change in measured concentrations over a specific time period.
  • It's a linear approximation of a curve.
• The larger the time interval, the more the average rate deviates from the instantaneous rate.

The Average Rate of the Reaction

• Table of Time (s), $[\text{H}<em>2]$ (M), $\Delta [\text{H}</em>2]$ (M), $\Delta t$ (s), and Rate $\frac{-\Delta [\text{H}_2]}{\Delta t}$ (M/s)
• Shows how the rate changes over time, calculated every 10 seconds.

Instantaneous Rate

• The instantaneous rate is the change in concentration at one particular time.
  • Represented by the slope at one point on a curve.
• The instantaneous rate is determined by the slope of a line tangent to the curve at that point.
  • This is the first derivative of the function (calculus).

Reaction Rate Changes over Time

• As time increases, the rate of reaction generally slows down because the concentration of reactants decreases.
• Eventually, the reaction stops when either reactants run out or the system reaches equilibrium.

Reaction Rate and Stoichiometry

• In most reactions, the coefficients of the balanced equation are not all the same.
  • Example: $\text{H}<em>2 (g) + \text{I}</em>2 (g) \rightarrow 2 \text{HI}(g)$
• The change in the number of molecules of one substance is a multiple of the change in the number of molecules of another.
  • For every 1 mole of $\text{H}<em>2$ used, 1 mole of $\text{I}</em>2$ is also used, and 2 moles of $\text{HI}$ are made.
  • The rate of change will be different for each.
• To maintain consistency, the change in the concentration of each substance is multiplied by 1/coefficient.

Measuring Reaction Rate

• To measure the reaction rate, one must be able to measure the concentration of at least one component in the mixture at several points in time.
• Some reactions proceed slowly enough that samples can be periodically withdrawn from the reaction vessel and analyzed to determine the reaction's progress.
• Three common techniques to monitor a reaction mixture are:
  • polarimetry
  • spectroscopy
  • pressure measurement