# Rate of chemical change

## Presentation on theme: "Rate of chemical change"— Presentation transcript:

Rate of chemical change
Rate of chemical change is expressed in terms of D Concentration 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
Will a Reaction take place? Only if the collisions have enough energy, and have the right orientation <= YES !! NO =>

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
aA + bB  cC + dD Keq = equilibrium constant Keq = [productC]c[productD]d [reactantA]a[reactantB]b Keq changes only when the temperature changes.

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 The universe favors chaos Parts of a whole DS = Sp-Sr
Unmix Unexplode unmelt DS = Sp-Sr If DS is + , entropy change is favored

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