AQA GCSE FOUNDATION Chemistry: Measuring Rates of Reaction

AQA GCSE Foundation Chemistry: Measuring Rates of Reaction

Introduction

In this tutorial, we'll explore how to measure the rate of a chemical reaction. Understanding the rate of a reaction is crucial for many applications, from designing efficient industrial processes to studying the complex mechanisms within living organisms.

What is a Reaction Rate?

The rate of a chemical reaction tells us how quickly the reactants are used up and the products are formed. In simpler terms, it's a measure of how fast the reaction is happening.

Methods for Measuring Reaction Rate

There are several ways to measure the rate of a reaction, and the best method depends on the specific reaction being studied. Here are some common techniques:

1. Measuring the disappearance of a reactant:

• Example: The reaction between magnesium ribbon and hydrochloric acid.

• Method: Measure the volume of hydrogen gas produced over time using a gas syringe. As the reaction proceeds, the volume of hydrogen gas increases. The rate can be calculated by dividing the change in volume by the time taken.

2. Measuring the appearance of a product:

• Example: The reaction between sodium thiosulfate and hydrochloric acid.

• Method: Measure the time it takes for the solution to become cloudy. The cloudiness is caused by the formation of a sulfur precipitate. The faster the solution becomes cloudy, the faster the reaction rate.

3. Measuring the change in mass:

• Example: The reaction between calcium carbonate and hydrochloric acid.

• Method: Measure the mass of the reaction mixture at regular intervals. As the reaction proceeds, the mass will decrease due to the formation of carbon dioxide gas, which escapes from the reaction mixture. The rate can be calculated by dividing the change in mass by the time taken.

4. Measuring the change in colour:

• Example: The reaction between potassium permanganate and oxalic acid.

• Method: Measure the time it takes for the purple potassium permanganate solution to decolourize. The faster the colour disappears, the faster the reaction rate.

Units for Measuring Reaction Rate

Reaction rates are typically expressed in units of concentration per unit time.

• Concentration can be measured in mol/dm3 (moles per decimeter cubed) or g/dm3 (grams per decimeter cubed).
• Time can be measured in seconds (s), minutes (min), or hours (h).

Therefore, the units for reaction rate can be mol/dm3/s, g/dm3/min, etc.

Factors Affecting Reaction Rate

Several factors can affect the rate of a chemical reaction:

• Temperature: Increasing the temperature generally speeds up a reaction.
• Concentration: Increasing the concentration of reactants generally speeds up a reaction.
• Surface area: Increasing the surface area of a solid reactant generally speeds up a reaction.
• Catalyst: A catalyst speeds up a reaction without being used up in the process.

Example Calculation

Problem:

A reaction between magnesium ribbon and hydrochloric acid produces 20 cm3 of hydrogen gas in 30 seconds. Calculate the rate of the reaction.

Solution:

• Change in volume: 20 cm3
• Time: 30 s
• Rate: (20 cm3) / (30 s) = 0.67 cm3/s

Conclusion

Measuring the rate of a reaction is essential for understanding and controlling chemical processes. By using appropriate methods and units, we can gain valuable insights into how reactions occur and what factors influence their speed.