Rate of chemical change

• Rate of chemical change is expressed in terms of

DConcentration of reactants or products

Collision Theory

Reaction Rate depends on:

1) The number of collision per unit time between the reacting species.

2) The fraction of these collisions that are successful in producing a new molecule.

Activation energy• The reactants must

gain enough energy to climb an energy hill

• They form an activated complex or a “transition” state before they cascade down becoming products

Will a Reaction take place? Only if the collisions have enough energy, and have the right orientation

Concentration & Reaction Rate

• The more concentrated the solution, the more chance of a collision

• The more collisions, the more likely an “activated complex” will form

Click on picture to see movie

Particle size: small particle size increases collisions

• Large amounts of surface area

• Small amounts of surface area

Movie Movie

The effect of temperature on rate

• A high temperature means particles will collide more often

• A high temperature means particles collide with more force

Effect of a catalyst• A catalyst

provides an alternative route for the reaction with a lower activation energy

Effect of a Catalyst

Reversable Reactions and Keq

• Keq = equilibrium constant

• Keq = [productC]c[productD]d [reactantA]a[reactantB]b

• Keq changes only when the temperature changes.

aA + bB cC + dD

Reversible reactions reach a state of dynamic equilibrium

N2 + 3H2 2 NH3

Keq = [NH 3]2

[H2]3 [ N2]

LeChatelier’s principle

• A system at equilibrium resists change. If it is exposed to stress (change) it will shift the equilibrium point to try to reduce the stress (change)

• [Co(H2O)6]+2 + 4Cl- <==> [CoCl4]-2 + 6H2O

• Pink blue

LeChatelier’s principle

• If you add a reactant – the equilibrium shifts toward the product

• If you add a product – the equilibrium shifts toward the reactant

• If you take away a reactant – the equilibrium shifts toward the reactants

• If you take away a product – the equilibrium shifts toward the product side

LeChatelier’s principle & heat

• If the reaction is exothermic (heat is a product), So, adding heat shifts the reaction to the reactant side

• If the reaction is endothermic (heat is a reactant) – adding heat shifts the reaction to the product side

LeChatelier’s principle & Pressure

• If the reaction contains gas particles, pressure is considered a “stress” condition

• 2SO2 + O2 2SO3

• Adding pressure shift the equilibrium to the side with fewer gas particles will have less pressure

Entropy: chaos

DS = Sp-Sr

IfDS

DS = Sp-Sr

• Parts of a whole

• Unmix

• Unexplode

• unmelt

The universe favors chaos

Is the reaction “spontaneous”This is a “tug of war” between entropy and enthalpy

DG = DH - T DS (T is in kelvin)

•If Gibbs free energy is a negative number, the reaction is spontaneous

•If Enthalpy is negative, (exothermic), and entropy is + (chaos favored) the reaction is always spontaneous

•If one of the conditions is not favored – it becomes a tug of war