Lecture 27: Reaction Rates

Reaction Rates: Chem 101

Quiz on Hypotonic Solutions

  • What happens to blood cells in a hypotonic solution?

    • They swell up and explode (hemolysis)

Demo: Fireworks on Ice

  • Initial Reaction:

    • Mix powders, nothing happens initially.

    • Adding water triggers a reaction.

  • Key Questions:

    • Why do reactions occur under some conditions, but not others?

      • the water dissolves some of the ions which allowed the ions to find other ions and react

      • the water was a catalyst

    • Why are some fast, while others are slow?

    • Why do some reactions go 100% while others have very low yields?

    • Can we influence the reaction rate or yield?

Collision Theory of Reactions

  • Kinetic theory of reactions:

    • molecules are constantly moving

    • molecules must collide to react

    • collisions must have sufficient energy

    • collision must have correct orientation

Activation Energy

  • Definition:

    • Minimum energy needed for a reaction to occur upon proper collision of reactants.

Reaction Rate

  • Definition:

    • the speed at which reactant is used up, or amount of reactants consumed per unit time

    • is the speed at which product forms or amount of products formed per unit time

Factors Affecting Reaction Rate

  1. Concentration:

    • Increasing the concentration of reactants increases the number of collisions which increases the reaction rate

  2. Temperature:

    • an increase in kinetic energy —> more collisions

    • an increase in ave. K. E. —> more molecules with enough energy to break bonds

    • therefore an increase in reaction rate

  3. Catalyst:

    • lowers the energy of activation

    • often improves the orientation of the collision

    • speeds up the tare of a reaction

    • is not used up during the reaction

Summary: Increasing Reaction Rate

  • Requirements:

    • Higher concentration.

    • Higher temperature.

    • Use of a catalyst.

  • To slow down a reaction:

    • Lower concentration.

    • Lower temperature.

    • Remove the catalyst.

Quiz on Reaction Rate Over Time

  • What happens to the reaction rate over time?

    • a) Remains constant

    • b) Speeds up

    • c) Slows down

    • d) Not enough information

    • Answer: Using up reactant = decreasing reactant concentration = slower reaction rate.

Demo: Reversible Reactions

  • Many reactions are reversible.

  • Notation:

    • Forward reaction: 22()—> 24()

    • Reverse reaction: 24()→22()

Characteristics of Reversible Reactions

  1. Proceed in either direction depending on conditions.

  2. Represented by half arrows.

  3. Reach equilibrium in a closed system at constant temperature

    What happens at equilibrium?

    • Rate of forward reaction = rate of reverse reaction.

    • Concentrations of species remain constant.

Quiz on Reversible Reactions

  • Write the forward and reverse reactions for the following reversible reaction:

    • Forward: CH4(g) + 2H2S(g) → CS2(g) + 4H2(g)

    • Reverse: CS2(g) + 4H2(g) → CH4(g) + 2H2S(g)

Chemical Equilibrium

  • A reversible reaction is in equilibrium when:

    • Rate of forward reaction = Rate of reverse reaction

    • the concentrations of reactants and products stop changing

Quiz on Equilibrium

  • Why do reactant and product concentrations stop changing at equilibrium?

    • a) Forward and reverse reactions stop occurring

    • b) Molecules no longer have enough energy to overcome activation barrier

    • c) Rate at which reactants and products are formed is the same

    • d) Reactant and product concentrations are now the same

Equilibrium Examples

  • Examples of equilibrium:

    • Vapor pressure above a liquid.

    • Phase changes.

    • Osmosis.

Rates of Forward and Reverse Reactions

  • Example Reaction:

    • H2(g) + I2(g) ⇌ 2HI(g)

    • 2 SO2 (g) + O2(g) ⇌ 2SO3

  • Reaction progression from initial state to equilibrium, with concentration changes over time.