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Chemical Equilibrium A Balancing Act
Equilibrium two opposing processes occurring at the same rate a system at equilibrium is in balance
During a game, players enter and leave.Always the same number of players on field.
Photochromic sunglasses AgCl + light Ag + Cl (transparent) (dark)
Reversible reaction a chemical reaction that can occur in both the forward and the reverse directions N2(g) + 3H2(g) NH3(g)
Chemical equilibrium a state in which the forward and reverse reactions balance each other because they take place at equal rates dynamic state; no net change
Law of chemical equilibriumAt a given temperature, a chemical system might reach a state in which a particular ratio of reactant and product concentrations has a constant value.
Equilibrium constant expressionratio of molar concentrations of products to reactants; each raised to a power equal to coefficient in balanced equation
aA + bB cC + dD
Equilibrium constant (Keq)numerical value of the ratio of product to reactant concentrations constant only at a specific temperature
Value of equilibrium constant (Keq) showsthe extent to which reactants are converted into products.
Keq > 1: Products are favored at equilibriumKeq < 1: Reactants are favored at equilibrium
Homogeneous equilibriumall reactants and products in same phase
Write an equilibrium constant expression for:N2(g) + 3H2(g) NH3(g) [NH3]2 Keq = ___________ [N2] [H2]3
Heterogeneous equilibriumreactants and products present in more than one physical state H2O(l) H2O(g)
Heterogeneous equilibriumSince concentrations of pure liquids and solids remain constant, these substances are omitted from the equilibrium constant expression.
Write an equilibrium constant expression for:H2O(l) H2O(g) Keq = [H2O(g)]
Equilibrium constant expression Keq = . . .Products over reactants raised to power of coefficient; leave out pure solids and liquids.
Equilibrium position Equilibrium concentrations can vary from trial to trial.
H2(g) +I2(g) 2HI(g)
Each set of equilibrium concentrations represent an equilibrium position.A system has only one value for Keq at a specific temperature, however there are unlimited number of equilibrium positions.
A system at equilibrium must:take place in closed system temperature remain constant all reactants and products are present (both reactions can occur)
Write the equilibrium constant expression forN2(g) + O2(g) 2NO(g)
Calculate the value of Keq if [N2] = 0.20 mol/L,[O2] = 0.15 mol/L, and [NO] = mol/L. Keq = 4.1 x 10-4 What does the value of Keq tell you about the equilibrium?
What happens when a system is at equilibrium and you upset the balance?
Le Chatelier’s PrincipleIf a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.
Le Chatelier’s PrincipleUsed to predict how a equilibrium system will react to changes in concentration, pressure (volume) and temperature.
Changes in ConcentrationAdd reactant 1. Forward rxn speeds up. 2. Over time, forward rxn slows down and reverse rxn speeds up.
3. At equilibrium, forward and reverse rxns occur at same rate, new equilibrium position established. Equilibrium has shifted right, value of Keq unchanged.
Changes in ConcentrationRemove reactant 1. Reverse rxn speeds up. 2. Over time, reverse rxn slows down and forward rxn speeds up.
3. At equilibrium, forward and reverse rxns occur at same rate, new equilibrium position established. Equilibrium has shifted left; value of Keq unchanged.
Changes in Pressure (Volume)For a gas, decreasing volume of container increases pressure; particles have less space, collide more frequently.
2. System will respond by trying to relieve stress (decrease pressure.) 3. Shifts to the side with fewer moles of gas.
4. Forward rxn speeds up. 5. Over time, forward rxn slows down and reverse rxn speeds up.
6. At equilibrium, forward and reverse rxns occur at same rate, new equilibrium position established. 7. Equilibrium has shifted right, value of Keq unchanged.
Changes in Pressure (Volume)If volume is decreased concentrations of all gaseous substances increases. Note: Side with fewer moles of gas increases more.
Changes in TemperatureReview: If DH is negative rxn is exothermic heat written as product If DH is positive rxn is endothermic heat written as reactant
Changes in TemperatureN2O4(g) 2NO2(g) DH = 55.3 kJ 55.3 kJ + N2O4(g) 2NO2(g) light dark
55.3 kJ + N2O4(g) 2NO2(g)
Le Chatelier’s PrincipleUsed to predict how a equilibrium system will react to changes in concentration, pressure (volume) and temperature. Chemists can change conditions to make rxn more productive.
Notes: Equilibrium: Le Châtelier’s Principle (18.1 & 18.2)
Equilibrium and Le Chatelier’s Principle
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OBJECTIVES Describe how the amounts of reactants and products change in a chemical system at equilibrium.
Equilibrium The condition of a system in which competing influences are balanced. The condition of a system in which competing influences are balanced.
Chemical Equilibrium. Reversible Reactions A reaction that can occur in both the forward and reverse directions. Forward: N 2 (g) + 3H 2 (g) 2NH 3 (g)
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Some reactions do not go to completion as we have assumed They may be reversible – a reaction in which the conversion of reactants to products and the.
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