Advanced Chemical Principles and Molecular Dynamics · 6th Year

Active learning ideas

Measuring Reaction Rates

Let's explore the dynamics of chemical change and figure out how to measure the speed of a reaction, from the initial burst of activity to its final slowdown.

• NCCA Curriculum Specifications
• Leaving Certificate Chemistry Syllabus: Physical Chemistry - Rates of Reaction

• 30–50 min
• Pairs → Whole Class
• 3 activities

Activity 01

• Experiential Learning
• 40 min · Pairs

The Disappearing Cross Reaction

Students mix sodium thiosulfate and hydrochloric acid in a flask placed over a paper with a cross drawn on it. They time how long it takes for the precipitate of sulfur to obscure the cross, repeating with different reactant concentrations to investigate its effect on the rate.

Explain how you would measure the rate of reaction between marble chips and hydrochloric acid.

Facilitation Tip

Ensure the lab is well-ventilated as a small amount of sulfur dioxide gas is produced.

What to look for

During the practical, circulate and use targeted questions to check students' understanding of why they are taking measurements at intervals and what they expect the graph to look like.

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Activity 02

• Experiential Learning
• 50 min · Small Groups

Gas Collection from Marble Chips and Acid

Students add hydrochloric acid to marble chips (calcium carbonate) in a flask connected to a gas syringe or an inverted measuring cylinder in a trough of water. They record the volume of carbon dioxide gas collected at regular time intervals, then plot a graph of volume vs. time.

Analyse a graph of product concentration versus time to determine the initial rate.

Facilitation Tip

Remind students to start the timer at the exact moment the acid is added to the chips for accurate results.

What to look for

Provide a Leaving Cert-style question with experimental data. Students must plot the data, draw a curve, and calculate the initial rate and the rate at a specified later time.

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Activity 03

• Experiential Learning
• 30 min · Individual

Rate Graph Analysis Workshop

Provide students with pre-prepared data sets from various reactions. In pairs, they plot the data on graph paper, draw the curve of best fit, and then practice drawing tangents at t=0 and other specified times to calculate and compare the initial and instantaneous rates.

Compare instantaneous rate with average rate of reaction.

Facilitation Tip

Model how to draw an accurate tangent on the board, emphasising it should only touch the curve at one point.

What to look for

Students use a peer-marked checklist to evaluate each other's graphs for correct axis labelling, scale, plotting accuracy, and the quality of their drawn tangent.

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

A few notes on teaching this unit

Start by demonstrating a fast and a slow reaction side-by-side to create a memorable contrast. Before the main practical, ensure all students can confidently read the scale on a gas syringe or measuring cylinder. When teaching graphical analysis, model the process of drawing a tangent and calculating its gradient step-by-step on the board, as this is often a point of difficulty.

Your students will gain the practical skills to collect rate data in the lab and the analytical skills to interpret the story that a reaction rate graph tells us.

---

Watch Out for These Misconceptions

• The rate of a reaction is constant until it suddenly stops.

  The reaction rate is typically fastest at the very beginning because the concentration of reactants is at its highest. As reactants are used up, the rate slows down gradually and eventually becomes zero when one of the reactants is completely consumed.

• A faster reaction always produces more product.

  The rate of reaction describes how quickly the product is formed, not the total amount. The final amount of product, or yield, is determined by the amount of the limiting reactant you start with, not the speed of the reaction.

• The average rate and the instantaneous rate are the same.

  The average rate is the change in concentration over a period of time, like the average speed of a car over a whole journey. The instantaneous rate is the rate at one specific moment, like looking at the car's speedometer at a particular instant.

---

Methods used in this brief

• Experiential Learning