Overview
This lecture covers the rates of chemical reactions, factors affecting rate, experiments to measure rate, reversible reactions, equilibrium, and Le Chatelier’s principle.
Rate of Reaction
- The rate of reaction is how quickly a chemical reaction happens.
- Rate is calculated as change in quantity (mass or gas volume) divided by time.
- This usually gives a mean (average) rate since rate can change during the reaction.
Measuring Rate of Reaction
- React hydrochloric acid with sodium thiosulfate and measure how long it takes for a cross under the flask to disappear (cloudiness indicates turbidity).
- Repeat at different temperatures to see that higher temperatures decrease the reaction time.
- Measure gas produced using a gas syringe; plot a graph with quantity (y-axis) versus time (x-axis).
- The graph typically curves and plateaus, indicating the reaction’s endpoint.
- To find the rate at a specific moment, draw a tangent to the curve at that point and calculate the slope.
Factors Affecting Rate
- Increasing concentration, pressure (for gases), or surface area (by crushing solids) increases collision frequency, thus increasing reaction rate.
- Higher temperature increases both the frequency and energy of particle collisions, making reactions faster by overcoming activation energy more easily.
- Adding a catalyst lowers the activation energy, increasing rate; catalysts are not used up in the reaction.
Reversible Reactions and Equilibrium
- Reversible reactions can go both ways, producing reactants from products (e.g., Haber process: hydrogen + nitrogen ⇌ ammonia).
- In a closed system, both forward and reverse reactions happen until their rates are equal—this is equilibrium.
- At equilibrium, quantities of reactants and products remain constant, but reactions still occur.
Le Chatelier’s Principle
- Changing concentration, pressure, or temperature shifts the position of equilibrium to counteract the change.
- Increasing pressure favors the side with fewer gas molecules (less space).
- Removing molecules from one side shifts equilibrium toward that side.
- Increasing temperature favors the endothermic direction; decreasing temperature favors the exothermic direction.
- In reversible reactions, if the forward reaction is exothermic, the reverse is endothermic (and vice versa).
Key Terms & Definitions
- Rate of reaction — The speed at which reactants are converted into products.
- Turbidity — Cloudiness in a solution due to particles formed.
- Activation energy — Minimum energy needed for a reaction to occur.
- Catalyst — Substance that increases rate by lowering activation energy; not consumed.
- Equilibrium — State where forward and reverse reaction rates are equal in a closed system.
- Le Chatelier’s Principle — If a system at equilibrium is disturbed, it shifts to counteract the disturbance.
- Endothermic reaction — Reaction that absorbs energy.
- Exothermic reaction — Reaction that releases energy.
Action Items / Next Steps
- Practice drawing rate curves and tangents to analyze reaction rates.
- Review energy diagrams and examples of catalysts.
- Understand and apply Le Chatelier’s principle to various reaction scenarios.