Overview
This lecture covers the factors affecting the rate of chemical reactions, methods to measure reaction rates, equilibrium in reversible reactions, and Le Chatelier's principle.
Rate of Reaction
- Rate of reaction is how quickly a reaction happens, calculated as change in quantity divided by time.
- The quantity measured can be the mass of reactant used or volume/mass of product formed.
- Rate measured over a time interval gives the mean rate.
- Example experiment: Reacting hydrochloric acid with sodium thiosulfate turns a solution cloudy (increased turbidity), timing until a cross underneath becomes invisible.
- Another experiment: Measure volume of gas produced using a gas syringe connected to the reaction vessel.
Measuring and Graphing Rate
- Graphs plot quantity (y-axis) vs. time (x-axis), typically showing a curve that levels off as the reaction finishes.
- To find the rate at a specific point, draw a tangent to the curve and calculate the gradient (change in quantity/change in time).
Factors Affecting Rate of Reaction
- Increasing concentration (in solution), pressure (for gases), or surface area (for solids) increases the rate by causing more frequent collisions between particles.
- Increasing temperature causes particles to move faster and collide with more energy, increasing the likelihood of successful reactions.
- Adding a catalyst lowers the activation energy required, making successful collisions more likely; catalysts are not used up in the reaction.
Reversible Reactions and Equilibrium
- Reversible reactions can go both forward and backward; products can revert to reactants.
- In a closed system, both forward and reverse reactions occur until the rates are equal, reaching equilibrium.
- At equilibrium, quantities stay constant but reactions continue; changes in conditions can shift equilibrium.
Le Chatelier's Principle
- If a system at equilibrium experiences a change, it adjusts to counteract that change.
- Increasing pressure favors the reaction with fewer gas molecules.
- Increasing concentration or removing products/reactants shifts equilibrium to produce more of the depleted substance.
- Increasing temperature favors the endothermic direction; decreasing favors the exothermic.
Key Terms & Definitions
- Rate of reaction — How quickly reactants are used up or products formed per unit time.
- Mean rate — Average rate over a chosen time interval.
- Turbidity — Cloudiness of a solution due to formation of insoluble products.
- Equilibrium — Point at which forward and reverse reaction rates are equal in a closed system.
- Le Chatelier's Principle — Rule predicting how equilibrium shifts in response to changes in conditions.
- Catalyst — Substance that speeds up a reaction by lowering activation energy, not consumed in the process.
Action Items / Next Steps
- Review energy diagrams from previous lessons.
- Practice drawing and analyzing rate graphs.
- Prepare for experiments measuring reaction rates under different conditions.