Le Chatelier's Principle

What if the conditions of the equilibrium changed? Le Chatelier’s principle states that if a closed system at equilibrium is subjected to a change(stress), things will happen to counteract that change (regain back balance).

The effect of Temperature changes Temperature affects heat terms in a reaction. If you increase temperature, you are increasing the heat term(energy). Therefore, the reaction will shift to use up this “extra” heat/energy and go back to original state.

The Effect of Temperature Changes A + B ⇌ C + Energy Change: increase Temp reaction- shifts left

The Effect of Temperature Changes A + B + Energy ⇌ C Change: increase Temp reaction- shifts right

The Effect of Temperature Changes A + B + Energy ⇌ C Change: decrease Temp reaction- shifts left

The Effect of Concentration Changes

Adding a reactant or product shifts the reaction in the direction that removes them/uses them up. A + B ⇌ C + Energy change- increase [A] reaction- shifts right

Adding a reactant or product shifts in the direction that removes them. A + B ⇌ C + Energy stress- increase [C] reaction- shifts left

Removing a reactant or product shifts reaction to make MORE. A + B ⇌ C + Energy change- decrease [A] reaction- shifts left

Removing a reactant or product shifts reaction to make more. A + B ⇌ C + Energy change- decrease [C] reaction- shifts right

Effect of pressure changes

A(g) + B(g) ⇄ AB(g) 2 mols 1 mol more pressure less pressure Usually we refer only to gases(g) when we talk about pressure changes. Those pressure changes we control by changing the volume.

As you know… If you decrease the volume, you increase the pressure. If you increase the volume, you decrease the pressure. Also, increasing pressure increases the concentrations of ALL gaseous species found.

Where to start? First identify where there is more pressure or where there is less pressure by counting up the molecules of gas you have on each side.

4HCl(g) + 2O2(g) ⇄ 2H2O(l) + 2Cl2(g) 6 mols 2 mols more pressure less pressure If we decreased the volume, pressure will ________ and so the reaction will go to low pressure side.

increasing pressure again and goes to more pressure side 2SO3(g) ⇄ 2SO2(g) + O2(g) 2 gas 3 gas more pressure less pressure If we increased the volume, pressure will _________, so the reaction responds by… increasing pressure again and goes to more pressure side

4HCl (g) + 2O2 (g) ⇄ 2H2O (l) + 2Cl2 (g) + 98kJ Increasing the pressure by decreasing the volume shifts to the eqlm fewest gas molecules. Only (g) count for gas particles. 4HCl (g) + 2O2 (g) ⇄ 2H2O (l) + 2Cl2 (g) + 98kJ   6 2  We decrease the volume, we increase pressure! The reaction responds by shifting to right to decrease the pressure! All concentrations go up!

7 9 5A(g) + 2B(g) ⇌ 4C(g) + 5D(g) We ⇓ V We ⇑ P 7 9 5A(g) + 2B(g) ⇌ 4C(g) + 5D(g) We ⇓ V We ⇑ P The Reaction opposes and goes to low pressure side⇓ P Shifts left

6 5 4A(g) + 2B(g) ⇌ 3C(g) + 2D(g) What volume change shifts right? 6 5 4A(g) + 2B(g) ⇌ 3C(g) + 2D(g) What volume change shifts right? We ⇑ P We ⇓ V

4 7 2A(g) + 2B(g) ⇌ 3C(g) + 4D(g) What volume change shifts right? 4 7 2A(g) + 2B(g) ⇌ 3C(g) + 4D(g) What volume change shifts right? We ⇓ P We ⇑ V

4HCl (g) + 2O2 (g) ⇄ 2H2O (l) + 2Cl2 (g) + 98kJ Describe the shift in the above equilibrium system for each stress indicated. Stress Shift [HCl] [O2] [Cl2] ↑ [HCl] → ↑ ↓ ↑ ↓ [O2] ← ↑ ↓ ↓ Add water ― ― ― ― ↑ [Cl2] ← ↑ ↑ ↑ ↓ ↓ ↓ ↑ V ← When the volume increases, pressure decreases and all concentration go down

Adding a catalyst does not shift the equilibrium Forward and reverse rates are increased by the same amount Equilibrium is attained faster reactants products

Adding an inert (non-reactive) gas does not shift the equilibrium. 2NH3(g) ⇌ N2(g) + 3H2(g) Adding Ne(g) No shift!

Homework Exercises 17-23 on page 54. Exercises 24-26, do NOT graph, only state what each change/stress would do to the equilibrium (ie; where it shifts it)