Advanced Chemical Principles and Molecular Dynamics · 6th Year · Rates of Reaction · Summer Term

Effect of Concentration and Pressure

Investigate how changing the concentration of reactants in a solution, or the pressure of reacting gases, affects the rate of reaction.

TL;DR:Let's explore why some reactions are over in a flash while others take their time. This topic delves into how crowding particles together, either in a solution or as a gas, can dramatically speed things up.

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

About This Topic

This topic is a cornerstone of the Chemical Kinetics section within the Leaving Certificate Chemistry syllabus. It builds directly upon students' initial understanding of reaction rates and collision theory, typically introduced in the context of temperature and surface area. The focus here shifts to two crucial variables: concentration for reactions in solution, and pressure for gaseous reactions. For the Irish curriculum, this topic is not just theoretical; it is intrinsically linked to Mandatory Experiment 2, which involves studying the effect of concentration on the rate of reaction between sodium thiosulfate and hydrochloric acid. A deep understanding of this topic is essential for students to explain macroscopic observations (e.g., a reaction speeding up) by describing the microscopic behaviour of particles. It provides a quantitative dimension to kinetics, requiring students to interpret graphs of concentration versus time and calculate initial rates, a key skill for the Leaving Cert examination. The principles explored here also lay the groundwork for understanding chemical equilibrium, particularly Le Châtelier's principle, where changes in concentration and pressure can shift the position of equilibrium.

Key Questions

1. Explain, using collision theory, why increasing concentration increases reaction rate.
2. Analyse the relationship between gas pressure and the frequency of collisions.
3. Compare the initial reaction rates at two different reactant concentrations.

Learning Objectives

• Explain, using the principles of collision theory, how increasing reactant concentration or gas pressure leads to an increased reaction rate.
• Design and carry out an experiment to investigate the effect of concentration on the rate of a reaction.
• Analyse data from a rate experiment by plotting a graph and determining the initial rate of reaction by drawing a tangent.
• Relate the pressure of a gas to the frequency of collisions between its particles.
• Solve problems involving the calculation of reaction rates from experimental data.

Key Vocabulary

Rate of Reaction	The change in concentration of a reactant or product per unit of time.
Collision Theory	A theory stating that for a chemical reaction to occur, reactant particles must collide with sufficient energy (activation energy) and with the correct orientation.
Concentration	The amount of a substance (solute) dissolved in a given volume of solvent, typically expressed in moles per litre (mol/L or M).
Pressure	The force exerted per unit area. In gases, it is caused by the collisions of gas particles with the walls of their container.
Initial Rate	The instantaneous rate of reaction at the very beginning of the reaction (time t=0).

Watch Out for These Misconceptions

Common Misconception

Increasing concentration makes the reactant particles move faster.

What to Teach Instead

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.

Common Misconception

Pressure and concentration are the same thing.

What to Teach Instead

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.

Common Misconception

If you double the concentration, the reaction rate will always double.

What to Teach Instead

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.

Active Learning Ideas

Simulation Game

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.

40 min·Pairs

Simulation Game

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.

50 min·Small Groups

Simulation Game

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.

20 min·Individual

Real-World Connections

• The Haber-Bosch process for making ammonia uses very high pressures to increase the rate of reaction between nitrogen and hydrogen gas.
• Food preservation techniques, like vacuum packing, work by reducing the concentration of oxygen, which slows down the rate of oxidative spoilage.
• In medicine, the concentration of a drug in the bloodstream determines how quickly it takes effect and its overall efficacy.
• Catalytic converters in cars operate on hot exhaust gases at pressure, converting pollutants like nitrogen oxides into less harmful substances more quickly.
• The fizzing of a fizzy drink is faster when it's first opened because the high pressure of dissolved carbon dioxide is released, causing a rapid decomposition of carbonic acid.

Assessment Ideas

Peer Assessment

A 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.

Exit Ticket

An 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.

Discussion Prompt

Give 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.

Frequently Asked Questions

Why does pressure only significantly affect the rate of reactions involving gases?

Liquids and solids are considered largely incompressible. This means that applying external pressure does not significantly change their volume or bring their particles much closer together. Therefore, the concentration of reactants in liquids and solids is not really affected by changes in pressure.

How do we measure the initial rate of reaction from a graph?

To find the initial rate, you draw a tangent to the curve at time t=0. The gradient (slope) of this tangent, calculated as the change in concentration divided by the change in time, gives you the initial rate of the reaction.

Can a reaction rate become infinitely fast if you keep increasing the concentration?

No, there is a practical limit. At very high concentrations, other factors can become the rate-limiting step, such as the speed at which the reactants can mix or, in catalysed reactions, the availability of the catalyst's active sites.