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 Rarely happens When a reaction results in almost complete conversion of reactants to products Rarely happens

Reversible reaction Most reactions Reversible reaction 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

Reversible Reactions

Rate of reaction Depends 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

HOMOgeneous equilibrum 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 4 moles gas reactants > 2 moles gas products 2 moles gas reactants = 2 moles gas products 2 moles gas reactants < 3 moles gas products

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 Exothermic- releases heat Reactants <--> Products + heat Endothermic- absorbs heat Reactants + heat <--> Products 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 ADDING HEAT to an exothermic reaction causes STRESS REMOVING HEAT to an endothermic reaction causes STRESS

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

Calculating Equilibrium Concentrations

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 𝑆= 𝑀𝑜𝑙𝑒𝑠 𝐿 Solubility in water The amount of the substance (moles) that will dissolve in a given volume of water (Liter) 𝑆= 𝑀𝑜𝑙𝑒𝑠 𝐿

Molar Solubility To determine solubility of a sparingly soluble compound X X 2X S = = (x)(2x)2

Molar Solubility

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

Calculating ion concentration from Ksp

Predicting Precipitates