Unit 9 – Reaction Rates and Equilibrium

The area of chemistry that concerns reaction rates (how fast a reaction occurs)

Collision ModelKey Idea: Molecules must collide to react.

However, only a small fraction of collisions produces a reaction. Why?

Not all “swings” are successful.

Why is it not burning?•Activation energy must be supplied (in the form of friction between friction strip and match tip)

Collision ModelCollisions must have sufficient energy to produce the reaction

Must equal or exceed the activation energy.

1.

Activation energy

A = Energy of reactants

D = Energy of products

B = Activation

energy(Highest Point) C =

Energy released by

reaction

Collision ModelColliding particles must be correctly oriented to one another in order to produce a reaction.

HCl with C2H4

Reaction rate•A change in concentration of a reactant or product over time • In other words, speed a chemical reaction occurs

Factors Affecting Rate1. Temperature As temperature INCREASES , reaction rate INCREASES. Because…

Particles collide more FREQUENTLY

Particles collide more ENERGETICALLY

2. Surface area as the surface area INCREASES, reaction rate INCREASES.

3. Concentration As the concentration INCREASES, reaction rate USUALLY INCREASES.

4. Presence of Catalysts, which lower the activation energy by providing alternate pathways

Collision ModelRemember…Collisions must have sufficient energy AND a correct orientation to produce a reaction

Increasing temp, concentration, or particle size contributes to more energy and/or more chances at colliding with a correct orientation

1.

Effect of temperature – more chances of successful collision

Effect of surface area – More chances of successful collision

Effect of concentration – More chances of successful collisions

CATALYST

•A substance that speeds up a reaction without being consumed

Lowering of Activation Energy by a Catalyst

* You don’t get more product, you just get it faster

Catalysis•Enzymes: A large molecule (usually a protein) that catalyzes biological reactions• Example: Digestion• Substrate (s) ------------------- > product(s)

Catalysis•Heterogeneous catalyst: Present in a different phase than the reacting molecules.

•Example: Catalytic converters in automobiles

•NO (g) N2 + O2

•CO (g) CO2

•Fuel + O2 CO2 + H2O

* Rh, Pt, metal oxides

***

Catalysis•Homogeneous catalyst: Present in the same phase as the reacting molecules.

•Example: “Elephant’s Toothpaste”

•2 H2O2 (aq) ---- > 2 H2O (l) + O2 (g)

•Intermediate steps: • H2O2(aq) + I-(aq) → OI-(aq) + H2O(l)• H2O2(aq) + OI-(aq) → I-(aq) + H2O(l) + O2(g)• I- is not consumed in the reaction.

KI