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Chemical equilibrium. Chapter 18 I will discover that many reactions and processes reach a state of chemical equilibrium. I will use Le Chatelier’s Principle to explain how various factors affect chemical equilibrium.
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Chemical equilibrium Chapter 18 I will discover that many reactions and processes reach a state of chemical equilibrium. I will use Le Chatelier’s Principle to explain how various factors affect chemical equilibrium. I will calculate equilibrium concentrations of reactants and products using the equilibrium constant expression. I will determine the solubilities of sparingly soluble ionic compounds.
Equilibrium: A state of dynamic balance • I will recognize the characteristics of chemical equilibrium. • I will write equilibrium expressions for systems that are at equilibrium. • I will calculate equilibrium constants from concentration data.
completion • Reaction goes to completion • When a reaction results in almost complete conversion of reactants to products • Rarely happens
Reversible reaction • Most reactions • Do NOT go to completion • Appear to stop • Are reversible • Reversible reaction • One that can occur in both the forward and the reverse directions • Denoted with a double arrow to show that both reactions occur • Forward reaction = reactants on left • Reverse reaction = reactants on right
Rate of reactionDepends on concentration of the reactants N2 (g) + 3 H2 (g) < -- > 2NH3 (g) • The concentrations of reactants decrease at first. • The concentrations of the product increases at first. • Then, before all the reactants are used up, all concentrations become constant.
Chemical equilibrium • A state in which the forward and reverse reactions balance each other because they take place at equal rates • Rate forward reaction = Rate reverse reaction • Concentrations of reactants and products are constant • HOWEVER! • The amounts or concentrations of reactants and products • Are NOT usually equal • MAY even differ by a factor of a million or more!
Equilibrium expressions and constants • Majority of reactions reach equilibrium with varying amounts of reactants unconsumed • NOT all our predicted moles of product gets produced • Law of Chemical Equilibrium • At a given temperature, a chemical system may reach a state in which a particular ratio of a reactant and product concentrations has a constant value • aA + bB<--> cC + dD
Equilibrium Constant • The numerical value of the ratio of product concentrations to reactant concentrations • Constant only at a SPECIFIC TEMPERATURE • Products on top, reactants on bottom • Keq > 1: MORE products than reactants at equilibrium • Keq < 1: LESS products than reactants at equilibrium • Keq = [C]c[D]d [A]a[B]b
HOMOgeneousequilibrum • All the products and reactants are in the same physical state • Must use ALL CONCENTRATIONS for Keq
Heterogeneous Equilibrium • Reactants and products of a reaction are present in more than one physical state • Do NOT count concentrations of solids or liquids when calculating Keq • Can be OMITTED from the Keq expressions • If a solid or liquid state of a substance is present in addition to the gas state….LABEL the gas concentration in your expression to distinguish between the two
Factors affecting chemical equilibrium • I will describe how various factors affect chemical equilibrium. • I will explain how Le Chatelier’s Principle applies to equilibrium systems.
Le Chatelier’s Principle • Apply stress to a system at equilibrium • System will shift in the direction that relieves the stress • Stress • Any kind of change in a system at equilibrium that UPSETS equilibrium • Types: • Change in concentration • Change in volume (pressure) • Change in temperature
Change in Concentration • Changes equilibrium POSITION • Shifts left or right • Does NOT change equilibrium constant (Keq) • Add reactant = shift right • Remove reactant = shift left • Add product = shift left • Remove product = shift right
Change in Volume (Pressure) • Changes equilibrium POSITION • Shifts left or right • ONLY if # moles of gaseous reactants is DIFFERENT than # moles gaseous products • Does NOT change equilibrium constant (Keq) • Volume Pressure
Change in Volume (Pressure) Decrease volume (increase pressure) • Situation 1: more moles gas reactants & less moles gas products • Shift right • Situation 2: moles gas reactants = moles gas products • NO shift • Situation 3: less moles gas reactants & more moles gas products • Shift left
Change in Temperature • Changes equilibrium POSITION • Shifts left or right • CHANGES equilibrium constant (Keq) • Large Keq = more product in equilibrium mixture • Small Keq = less product in equilibrium mixture
Change in Temperature Endothermic- absorbs heat Reactants + heat <--> Products Exothermic- releases heat Reactants <--> Products + heat • Hot • -∆H° (lose heat) • Forward reaction = exo, backward = endo • ↑temp = shift left, ↓Keq • ↓temp = shift right, ↑Keq • Cold • +∆H° (gain heat) • Forward reaction = endo, backward = exo • ↑temp = shift right, ↓Keq • ↓temp = shift left, ↑Keq
Catalysts • Speeds up a reaction • Speeds it up EQUALLY in BOTH directions (Right & Left) • Helps a reaction reach equilibrium quickly • But NO CHANGE in the AMOUNT of PRODUCT formed
Using equilibrium constants • I will determine equilibrium concentrations of reactants and products. • I will calculate the solubility of a compound from its solubility product constant. • I will explain the common ion effect.
Using Equilibrium constants • Review: • Large Keq = Products favored • Small Keq = Reactants favored • Knowing the size of the Keq helps a chemist • Decide whether a reaction is practical for making a particular product • Calculate the equilibrium concentration of ANY substance involved in the reaction • (if the concentrations of all other reactants/products are known)
Calculating Equilibrium Concentrations • Write the equilibrium constant (Keq) expression • Solve the equation for the unknown (using algebra skills) • Substitute in all known concentrations and the Keq value • Use calculator to find unknown concentration • Chemists would then use this concentration to determine if enough of their desired unknown could be produced in the reaction
Solubility Equilibria Ionic Compounds Some dissolve readily in water • Ex NaCl(s) • High solubility Some barely dissolve at all • Ex BaSO4(s) • low solubility
Solubility Product Constant • An equilibrium constant for the dissolving of a sparingly soluble ionic compound in water • Ksp = the product of the concentrations of the ions each raised to the power equal to the coefficient of the ion in the chemical equation • Small Ksp = Products NOT favored at equilibrium
Solubility Product Constant • Example: (Remember it depends ONLY on [ IONs])
Review: Solubility • Solubility in water • The amount of the substance (moles) that will dissolve in a given volume of water (Liter)
Molar Solubility • To determine solubility of a sparingly soluble compound X X 2X S = = (x)(2x)2
Calculating ion concentration from Ksp • Write the Ksp expression in terms of X • Solve for X • Once you know what X is you can find the concentration of ions