Measuring Reaction Rate (College Board AP® Chemistry): Study Guide

Calculate the rate for the reaction

A → B

The concentration of A decreases from 2.5 M to 0.6 M in 20 seconds.

Answer:

• Step 1: Identify the correct formula

  • Since A is a reactant, we must use the formula that includes the negative sign:

rate of reaction = − $\frac{\mathrm{final}\mathrm{concentration}\mathrm{of}\mathrm{reactants}\left(M\right)-\mathrm{initial}\mathrm{concentration}\mathrm{of}\mathrm{reactants}\left(M\right)}{\mathrm{time}\left(\mathrm{s}\right)}$

• Step 2: Replace the values in the formula and calculate

rate of reaction = $\frac{-\left(0.6M-2.5M\right)}{20\mathrm{s}}$

rate of reaction = $\frac{-\left(-1.9M\right)}{20\mathrm{s}}$

rate of reaction = 0.095 M s^-1

Iodine and methanoic acid react in aqueous solution.

I₂ (aq) + HCOOH (aq) → 2I⁻ (aq) + 2H⁺ (aq) + CO₂ (g)

The progress of the reaction can be followed by measuring the volume of carbon dioxide produced over time and plotting a graph, as shown:

Calculate the rate of production of CO₂, in mL s^-1, at 20 seconds

Answer:

• Step 1: Draw a tangent to the curve at 20 seconds

• Step 2: Complete the triangle and use the values of x and y to calculate the slope. The slope is the rate of CO₂ production

slope = $\frac{\mathrm{∆}\mathrm{y}}{\mathrm{∆}\mathrm{x}}$

slope = $\frac{27\mathrm{mL}-3\mathrm{mL}}{40\mathrm{s}-0\mathrm{s}}$

slope = $\frac{24\mathrm{mL}}{40\mathrm{s}}$

slope = 0.60 mL s^-1

When drawing the tangent to a curve, you must:

• Make the triangle as large as possible

• Intersect with grid lines if they are given

When you describe rates:

• Use ‘faster’ and ‘slower’ for rate, and ‘longer’ and ‘shorter’ for time. A reaction that takes longer is not necessarily slower: it may just have more reactant to use up

• Rate depends on concentration, not on the number of moles present