46. Rates of Reaction: Measuring and Graphing

1. Key Definitions & Formulas

• Rate of Reaction: The speed at which reactants are turned into products.

• Formulas:

  • Rate of Reaction = (Quantity of reactant used) / (Time taken)

  • Rate of Reaction = (Quantity of product formed) / (Time taken)

• Units: Typically grams per second (g/s), cubic centimeters per second (cm³/s), or moles per second (mol/s).

2. Measuring the Rate

To find the rate, you must measure how quickly a reactant disappears or how quickly a product is created.

• Example 1 (Product): If a reaction produces 180 cm³ of gas in 2 minutes (120 seconds):

  • Rate = 180 / 120 = 1.5 cm³/s

• Example 2 (Reactant): If 3 g of magnesium is used up in 4 minutes (240 seconds):

  • Rate = 3 / 240 = 0.0125 g/s

3. Reaction Rate Over Time

The rate of a reaction is rarely constant throughout the process:

• Start: The reaction is fastest at the beginning because there is a high concentration of reactant particles available to react.

• Middle: The reaction slows down as reactants are used up and collisions become less frequent.

• End: The reaction stops (the graph plateaus) once one of the reactants is completely exhausted.

4. Interpreting Graphs

Graphs typically plot Time on the x-axis and Quantity on the y-axis.

• Volume of Product Graph: Starts at (0,0). The curve is steepest at the beginning and levels off at the top when the reaction finishes.

• Mass of Reactant Graph: Starts at the initial mass (e.g., 3 g). The curve falls quickly at first and then flattens out at the bottom as the reactant is consumed.

• Steepness: The gradient (slope) of the line represents the rate. A steeper line indicates a faster reaction.

5. Summary Checklist

• Always check the units requested in the question (e.g., convert minutes to seconds if the answer needs to be in g/s).

• Remember that the "mean rate" is the average over a period of time, while the "initial rate" is the speed at the very start of the reaction.