Activity 01
The Disappearing Cross Experiment
Students react sodium thiosulfate with hydrochloric acid at various concentrations in a conical flask placed over a paper marked with a cross. They time how long it takes for the cross to be obscured by the sulfur precipitate, allowing them to relate concentration to reaction rate.
Explain, using collision theory, why increasing concentration increases reaction rate.
Facilitation TipEnsure all students view the cross from the same angle and height to maintain consistency in results.
What to look forA formal lab report on an experiment investigating the effect of concentration on reaction rate, including data processing, graph plotting, and a conclusion based on collision theory.
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Activity 02
Gas Collection Race
Students react a standard length of magnesium ribbon with different concentrations of hydrochloric acid in a flask connected to a gas syringe. They measure the volume of hydrogen gas produced at regular intervals, then plot graphs to compare the initial rates.
Analyse the relationship between gas pressure and the frequency of collisions.
Facilitation TipRemind students to start the timer at the exact moment the magnesium is added to the acid for accurate rate calculations.
What to look forAn exit ticket where students must draw and annotate two diagrams, one with low concentration and one with high, to explain the effect on collision frequency.
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Activity 03
Pressure & Collisions Simulation
Using a PhET interactive simulation, students can manipulate the pressure of a gas in a container and observe the direct effect on the number of particle collisions per second. This provides a clear visual model for a concept that is difficult to observe directly.
Compare the initial reaction rates at two different reactant concentrations.
Facilitation TipAsk students to predict what will happen to the collision frequency before they increase the pressure to engage their prior knowledge.
What to look forGive students a set of data (time vs. product concentration) and ask them to plot a graph and calculate the initial rate. This can be done as a think-pair-share activity.
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Generate Complete Lesson→A few notes on teaching this unit
Begin with a dramatic visual demonstration, like the 'Disappearing Cross' experiment, to hook students. Follow this with a particle-level explanation using diagrams or animations to solidify their understanding of collision theory. Ensure you explicitly link the macroscopic observation (the cross vanishing faster) with the microscopic explanation (more frequent collisions). Use scaffolded worksheets for graph analysis, starting with a pre-drawn tangent before asking them to draw their own.
By the end of this topic, your students will be able to explain reaction rates using collision theory and analyse graphs to prove how concentration and pressure are key factors.
Watch Out for These Misconceptions
Increasing concentration makes the reactant particles move faster.
Particle speed is related to temperature, not concentration. Increasing concentration means there are more reactant particles crowded into the same volume, which increases the frequency of collisions, not the speed of the particles themselves.
Pressure and concentration are the same thing.
While related, they are distinct concepts. Concentration (mol/dm³) is used for solutions, describing the amount of solute in a given volume. Pressure is used for gases, describing the force exerted by gas particles on the container walls. Increasing the pressure of a gas does increase its effective concentration, as the particles are forced closer together.
If you double the concentration, the reaction rate will always double.
This is only true for a reaction that is 'first order' with respect to that reactant. The exact relationship between concentration and rate is determined by the reaction's specific rate law, which can be more complex. For Leaving Cert level, the key principle is that increasing concentration increases the rate; the relationship is not always a simple doubling.
Methods used in this brief