Chemical Equilibrium

1. Chemical Equilibrium Types of Equilibrium and Le Chateliers Principle Mr. Shields Regents Chemistry U13 L02

2. Equilibrium reactions can be categorized as falling into 1 of 3 • types: • Phase Equilibrium • Solution Equilibrium • Chemical Equilibrium Chemical Equilibrium reactions can be further categorized as either 1) Homogeneous or 2) Heterogeneous

3. Types of Equilibrium • A homogeneous equilibrium is one in which the states of • Matter for all reactants and products are the same • Example: H2 (g) + I2 (g)  2HI (g) • A heterogeneous equilibrium is one in which the states of • Matter vary between reactants and products • Example: C (s) + O2 (g)  CO2 (g)

4. DYNAMIC EQUILIBRIUM Phase Equilibrium Phase equilibrium exists between two phases of matter (S, L, G) usually in a closed vessel. They are easily recognizable: 1) H2O(s) ↔ H2O(l) ice – water equilibrium (def. of MP) 2) H2O (l) ↔ H2O(g) Water – Vapor equilibrium 3) I2 (s) ↔ I2 (g) Iodine sublimation 1 above occurs at 0 deg 2 & 3 above must be Sealed systems! Why?

5. (Saturated sol’n) Solution Equilibrium Besides The dynamic equilibrium of a liquid vapor (or Gas) and a liquid, solution equilibriums are created … - When additional solid is added to an already saturated solution For a solid in a saturated solution at equilibrium: The rate at which solids re-crystallize from sol’n & the rate at which Crystals dissolve must be equal

6. Solution Equilibrium In our example using NaCl we would write the equilibrium as: NaCL (s) + H20 ↔ Na+ (aq) + Cl- (aq) If the solid is molecular (for example sugar) then the Following equation would define the equilibrium condition: C12H23O11 (s) + H20 ↔ C12H23O11 (aq) Notice there are No IONS formed for molecular compounds.

7. Solution Equilibrium For GASES dissolved in Liquids in a SEALED system The rate at which the gas leaves the liquid and the Rate at which the gas re-dissolvesmust be equal to be in equilibrium. O2 (aq) ↔ O2 (g) Why a sealed system? But this equilibrium can be affected by shifts in T and/or P - if T increases the equilibrium shifts to the right  and - If P increases the equilibrium shifts to the left  WHY ?

8. Le Chatelier’s Principle Henri Le Chatlier 1850-1936) This principle describes how a system at equilibrium reacts to changes (stresses) in system conditions. Le Chatelier’s principle: If a change is imposed on a system at equilibrium, then the system will shift the direction of the reaction to reduce the change in conditions. In other words, When a stress is imposed on a system the system will respond to counteract the stress

9. Le Chatelier’s Principle For example: consider the rxn … A + B ↔ C + D + Heat If the above reaction proceeds to the right, we produce more C, D and heat. If the reaction proceeds to the left we consume heat, and Produce more A and B If we consume heat what does Le Chatelier’s principle predict? Le Chatelier’s Principle says if we remove heat the system will Try to return to equilibrium conditions by producing heat to Replace the heat consumed. The Rxn will be Driven to the right!

10. Factors affecting Equilibrium When considering Le Chatelier’s principle there are Several Factors that will affect equilibrium: 1) Adding or Removing HEAT 2) Increasing or decreasing PRESSURE 3) Changing the CONCENTRATION … By adding, decreasing OR totally removing one or one or more of the reactants/products

11. Head to Tail Method We can use Le Chatelier’s principle to predict how Chemical Reactions will respond to external forces. It sounds like this might be difficult to do but there’s an Easy way to determine how an equilibrium will react to stress. It’s called the “Head to Tail” methodand its used to determine the Direction of the shift as a stress is applied To a system in equilibrium. There are 3 simple rules … 2 1 3

12. Head to Tail Method Lets say we increase the heat in a system in which there Is an equilibrium chemical reaction occurring A + B ↔ C + D + Heat Rule 1: Draw the 1st ARROW next to the component that is changing. Draw it upwards if it is increasing and downward If it is decreasing A + B ↔ C + D + Heat Draw a second arrow from the 1st arrow going across the TOP of “↔”. Each arrow must go from “Head to Tail” Rule 2: A + B ↔ C + D + Heat

13. Head to Tail Method Rule 3: The direction of the second arrow indicates the Direction the equilibrium reaction shifts when the stress is Applied. Rxn Shifts in this direction A + B ↔ C + D + Heat Of course we could also have predicted this using Le Chatelier’s principle. If we add stress to the system by increasing heat the system responds by trying to oppose the Stress. Reaction moves to the left because that direction Consumes heat.

14. A + B ↔ C + D + Heat Problem: In the following reaction the concentration of C is decreased. Which way will the equilibrium reaction shift? A + B ↔ C + D + Heat Ans: Reaction shifts towards the right in Response to the stress (decreasing concentration of C) to try to increase the concentration of C back to it’s Equilibrium value

15. We’ve discussed how to determine the affects of • Changing 2 of the three factors that affect Equilibrium • Namely using le Chatlier’s principle: • Changing temperature • Changing concentration • The effect of the last factor, PRESSURE, we’ll leave • For discussion in the next lesson.