Rate of chemical change

1. Rate of chemical change • Rate of chemical change is expressed in terms of DConcentration of reactants or products

2. 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.

3. 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 Activation energy

4. Will a Reaction take place? Only if the collisions have enough energy, and have the right orientation <= YES !! NO =>

6. 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

7. Large amounts of surface area Small amounts of surface area Particle size: small particle size increases collisions Movie Movie

8. The effect of temperature on rate • A high temperature means particles will collide more often • A high temperature means particles collide with more force

9. A catalyst provides an alternative route for the reaction with a lower activation energy Effect of a catalyst

10. Effect of a Catalyst

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

12. Reversible reactions reach a state of dynamic equilibrium N2 + 3H22 NH3 Keq= [NH 3]2 [H2]3 [ N2]

13. 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

14. 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

15. 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 & heat

16. 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

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

18. Is the reaction “spontaneous” This is a “tug of war” between entropy and enthalpy DG = DH - TDS (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