Chemical Equilibrium Chemistry. Ms. Siddall Standard 9b: equilibrium conditions Reversible Reactions Most reactions are ‘reversible’ Forward reaction: reactants make products e.x. 3O2(g) 2O3(g) Reverse reaction: products make reactants e.x. 2O3(g) 3O2(g) In a reversible reaction the forward and reverse reactions occur at the same time e.x. 3O2(g) 2 O3(g) Summary 1 What is a reversible reaction? Reversible reactions reach equilibrium: a balance between reactants and products Conditions of Equilibrium: 1. rate of forward reaction = rate of reverse reaction 2. Concentration of reactants and products is constant (does not change) NOTE: Rate = speed Concentration = number of particles or moles example: [HCl] = concentration of HCl 6M HCl = 6mole/L HCl = 6 moles of HCl per liter of solution Summary 2 What is equilibrium? Reversible reaction: X Y [X] XY concentration rate [Y] YX time time Concentrations Reaction rates are equal are constant equilibrium Summary 3 Describe the part of each graph that illustrates equilibrium conditions. 9a: Le Chatelier’s Principle Le Chatelier’s Principle A system in equilibrium will react to relieve stress (change) and re-establish equilibrium Stress: Adding reactants or products Removing reactants or products Changing temperature Changing pressure (for gases only) Summary 4 Accordingto Le Chatelier’s Principle, what will happen to a system at equilibrium if more reactants or products are added? Example: N2(g) + 3H2(g) 2NH3(g) + heat Stress: Add N2 Stress relief: Forward reaction (→) to get rid of N2 H2 is used up (↓) NH3 and Heat are produced (↑) Summary 5: N2(g) + 3H2(g) 2NH3(g) + heat Stress: remove N2 Stress relief: Which way does equilibrium shift? What happens to [H2]? What happens to [NH3]? What happens to heat? Stress relief. Adding products or reactants Equilibrium shifts to remove addition Removing products or reactants Equilibrium shifts to replace what has been removed Gasses Equilibrium shifts to produce: • more gas at low pressure • Less gas at high pressure Summary 6 Why would a gas equilibrium system produce more gas at low pressure and less gas at high pressure? Haber Process: N2(g) + 3H2(g) 2NH3(g) + heat Change Equilib. [N2] [H2] [NH3] ↑ [N2] ↓ [N2] ↑ [H2] ↓ [H2] ↑ [NH3] ↓ [NH3] Haber Process: N2(g) + 3H2(g) 2NH3(g) + heat Change Equilib. [N2] [H2] [NH3] ↑ heat ↓ heat ↑pressure ↓pressure Summary 7 According to Le Chatelier’s Principle: Increasing reactant concentration will cause: other reactants to __________? products to __________? Decreasing reactant concentration will cause: other reactants to __________? products to __________? A(g) + B(g) AB(g) + heat Summary 8 A(g) + B(g) AB(g) + heat Complete the table of equilibrium changes Change Equilib. [A] [B] [AB] ↑ heat ↓ [A] ↑pressure ↓[AB] A(g) + B(g) AB(g) + heat Change Equilib. [A] [B] [AB] ↑ [A] ↓ [A] ↑ [B] ↓ [B] ↑ [AB] ↓ [AB] A(g) + B(g) AB(g) + heat Change Equilib. [A] [B] [AB] ↑ heat ↓ heat ↑pressure ↓pressure HONORS Standard 9c: equilibrium constant Equilibrium Constant: Keq Atequilibrium concentrations are constant Keq represents concentrations of reactants and products at equilibrium Example: aA + bB cC + dD Keq = [C]c[D]d [A]a[B]b Summary 9 Keq expression for the Haber-Bosch write Process: N2(g) + 3H2(g) 2NH3(g) Concentrations calculated in mol/L (M) Only solutions(aq) & gases(g) are considered No solids (s) No liquids (l) Example: 2H2O(l) 2H2(g) + O2(g) Keq = [H2]2[O2] Summary 10 Fe(OH)2(aq) + 2HSO3(aq) Fe(SO3)2(aq) + 2H2O(l) Find Keq What Keq tells us If Keq ≤ 1 There are more reactants than products at equilibrium If Keq ≤ 1/100 There are mostly reactants at equilibrium If Keq ≥ 1 There are more products than reactants at equilibrium If Keq ≥ 100 There are mostly products at equilibrium Summary 11 CO(g) + 2H2(g) CH3OH(g) Keq=290 at 430°C Write the expression for Keq Reaction is… (mostly products or reactants?) Solubility Ksp is the equilibrium constant for solubility Example: AgCl(s) Ag+(aq) + Cl-(aq) Ksp AgCl = 1.77 x 10-10 Does not really dissolve, mostly solid Example: AgNO3(s) Ag+(aq) + NO3-(aq) Ksp AgNO3 ~ 1 x 1010 Very soluble Summary 13 Write the balanced equation for the dissolving of sodium sulfate. Write a Ksp expression for the reaction.
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