Equilibrium
Le Chatelier's Principle: Pressure
*Only systems that contain gases are affected.* The total # of moles of gas on the left side must be different from the total # of moles of gas on the right side (look at coefficients). -An increase in pressure favors the side w/ less moles of gas, and vice-versa. Gases do not thrive in high pressure environments. -Pressure changes are typically induced by changing the volume of the container. low volume = higher pressure high volume = lower pressure
solubility equilibrium
-a saturated solution contains the maximum amount of solute possible at a given temp. in equilibrium with an undissolved excess of substance -a saturated solution is not necessarily a concentrated solution; it all depends on the solubility of the solute Soluble - solubility is > 1 g per 100 g H2O Insoluble - solubility is < .1g per 100 g H2O (not entirely incapable of dissolving, only a small amount of it does) Slightly or Sparingly soluble - solubility is between .1 g & 1 g -Equilibrium principles are based on sparingly soluble & insoluble solutes.
What does a high Ksp represent?
A high concentration of ions at equilibrium & a high degree of solubility.
Le Chatelier's Principle: Concentration
A+B <-------> C+D -An increase in A or B shifts the equilibrium position to the right. -An increase in C or D shifts the equilibrium position to the left. -The removal of a substance causes the equilibrium position to move towards the removed substance.
Le Chatelier's Principle: Temperature (example)
C(s)+O2(g)<----->CO2(g)+393.51kJ The forward reaction is exothermic because the energy term is on the right side. The reverse reaction is endothermic. A temperature increase favors the reverse reaction in this case. Temperature increases favor endothermic reactions, which in this example is the reverse reaction. A temperature decrease favors the forward reaction. Temperature decreases favor exothermic reactions, which in this example is the forward reaction.
list 2 processes that are irreversible
C3H8 + O2 ----> CO2 + H2O (combustion/exothermic - could determine this for certain by looking at an energy diagram) When you say something hurtful to me :(
What is Le Chatelier's principle?
Equilibrium shifts occur to relieve an applied stress to the system.
What are 3 stresses that can upset the equilibrium of a chemical system?
Temperature changes, pressure changes, concentration changes
How do the forward and reverse reaction rates compare for a system at equilibrium?
They are equal.
How do the amounts of products and reactants compare for a system at equilibrium?
They are produced at equal rates, though they are not necessarily equal. The [product] and [reactant] remain unchanged or constant at equilibrium.
How do the amounts of reactants and products change after a reaction has reached chemical equilibrium?
Chemical equilibrium is a dynamic process. The forward and reverse reactions continue to occur even after equilibrium has been reached. However, because the rates of the reactions are the same, there is no change in the relative concentrations of reactants and products for a reaction that is at equilibrium (no net changes occur in the actual amounts of the components of the system). They remain unchanged. This is the basis for why we can establish Keq values.
How can a balanced equation be used to write an equilibrium constant expression?
Equilibrium expression: Keq = [C]^c [D}^d/[A]^a [B]^b
Le Chatelier's Principle: Pressure (example)
H2O(l) + CO2(g) ---->H2CO3(aq) <---- a. increase in pressure shifts RIGHT -moves in the direction of lesser # moles gas, thus more carbonic acid is formed b. increase in volume shifts LEFT -increase in volume is a decrease in pressure, the reaction moves in the direction of higher # moles gas Water, CO2 under pressure forms carbonic acid. CO2 leaves when pressure is lowered (aka can is opened).
Shifting Equilibrium: Le Chatelier's Principle
If a system is subjected to stress, the equilibrium is shifted in the direction that relieves the stress.
How do both ion product & Ksp relate to precipitation?
If the ion product is > than Ksp, precipitation occurs!
Precipitation Calculations:
Insoluble can still dissolve; ion product is less (low) - not enough to saturate it and cause it to precipitate out; higher = more precipitate. -Ksp is calculated based on a saturated solution (1 in which an excess of undissolved solute is present). -If the [ions] in a solution do not reach or exceed the concentration necessary for Ksp, an unsaturated solution exists. Therefore, no precipitate or solid is/can be present. This analysis of concentrations of ions is known as the determination of the ION PRODUCT. *If the ion product > Ksp for the potential precipitation, precipitation occurs!
What does the value of the equilibrium constant tell you about the amounts of reactants and products present at equilibrium?
Keq>1, products are favored at equilibrium (a higher concentration of products at equilibrium) Keq<1, reactants are favored at equilibrium Keq = 1, relatively equal amounts of products and reactants at equilibrium
What is the difference between Keq and Ksp? How are their expressions different?
Ksp is a type of Keq. Keq represents all equilibrium systems; Ksp only represents how something dissolves. Keq = [products]^coefficient/[reactants]^coefficient Ksp = [cation]^coefficient x [anion]^coefficient
Can a pressure change shift the equilibrium position in every reversible reaction? Explain.
No, the system must contain gases and the gases must be in different mole amounts on each side of the equation.
How can pressure variation affect an equilibrium system?
Pressure affects systems containing gases in different mole amounts on each side of the rxn. See previous.
Reactions that are truly reversible can favor a direction.
Reaction to the right - product formation is favored; there is a higher concentration of product at equilibrium. A+B----->AB <-- Reaction to the right - reactant formation is favored. there is a higher concentration of reactant at equilibrium. A+B-->AB <------
Le Chatelier's Principle: Temperature
The effect of changing the temperature on an equilibrium system depends on which of the opposing reactions is endothermic and which is exothermic. -The addition of heat/increase in temperature favors the endothermic reaction. -The removal of heat/decrease in temperature favors the exothermic reaction. *forward = endo, then reverse = exo and vice-versa*
Compare and contrast the ion product & Ksp.
The expression used to solve for the ion product & Ksp is the same. The difference is the concentration used for Ksp represents the ions of a just saturated solution. The ion product uses concentrations of ions at any time.
What actual variable is typically changed to cause pressure variations in a system?
Volume
reversible reaction
a chemical reaction in which the products re-form the original reactants; a reaction in which the conversion of reactants into products and the conversion of products into reactants occur simultaneously reactants form products, and the products can EASILY reform those reactants *In theory, almost all reactions are reversible given the right conditions.*
irreversible reaction
a chemical reaction where products cannot revert back to reactants; involves reactants converted to product at a rate that yields a net loss of reactants and a net gain in product more difficult for products to reform reactants, but it is not impossible (less common) *Irreversibility is presumed as the result of formation of a gas, formation of a precipitate (insoluble), formation of a slightly ionized product.*
common-ion effect
a decrease in the solubility of an ionic compound caused by the addition of a common ion An equilibrium reaction may be driven in the desired direction by applying Le Chatelier's Principle. Suppose hydrogen chloride gas is bubbled into a saturated solution of sodium chloride. Hydrogen chloride is extremely soluble in water, and it is almost completely ionized. HCl(g)+H2O(l)----->H3O+(aq)+Cl-(aq) The equilibrium for a saturated solution of sodium chloride is described by the following equation: NaCl(s)<----->Na+(aq)+Cl-(aq) As the hydrogen chloride dissolves in sufficient quantity, it increases the concentration of Cl- ions in the solution, which is a stress on the equilibrium system. The system can compensate, according to Le Chatelier's Principle, by combining some of the added Cl- ions with an equivalent amount of Na+ ions. This causes some solid NaCl to precipitate out, relieving the stress of added chloride. The new equilibrium has a greater concentration of Cl- ions but a decreased concentration of Na+ ions. However, the product of [Na+] and [Cl-] still has the same value as before. This phenomenon, in which the addition of an ion common to two solutes brings about precipitation or reduced ionization, is called the common-ion effect. The common ion effect is also observed when one ion species of a weak electrolyte is added in excess to a solution. The solution of CH3COOH combined with the solution of NaCH3COO produces a slightly acidic solution due to the decreased ionization of the acid. The common ion in this case is CH3OO-, which both solutions contain.
Equilibrium
a state in which opposing forces or influences are balanced.
Predict the effect that decreasing pressure would have on each of the following reaction systems:
a. H2(g)+Cl2(g) <-----> 2HCl(g) -no effect b. NH4Cl(s) <-----> NH3(g)+HCl(g) -shift to the right/forward c. 2H2O2(aq) <------> 2H2O(l)+O2(g) -shift to the right/forward d. 3O2(g) <------> 2O3(g) -shift to the left/reverse
Using the following equilibrium constants for several reactions, determine in which reactions the products are favored:
a. Keq = 1x10^2 - products favored b. Keq = 0.003 - reactants favored c. Keq = 3.5 - products favored 1x10^2 favors products more than 3.5 (reaction to the right, more stable, more products because bigger number)
Equilibrium systems can be expressed in terms of numerical values. These values relate the amounts of reactants to products at equilibrium.
aA+bB<---->cC+dD Equilibrium expression: Keq = [C]^c [D}^d/[A]^a [B]^b *Liquids and solids do not appear in the equilibrium expression. The reason is that they are pure substances and do not have concentration values. The brackets used in the expression represent Molar Concentrations.* SEE VIDEOS/TUTORIALS
list 2 processes that are reversible
ice to water, water to ice typing and deleting
More examples with Le Chatelier's Principle:
see notes 1. N2(g)+2O2(g)<---->N2O4(g) What direction does an increase in system volume favor? =decrease in pressure LEFT (amount of reactants will increase, amount of products will decrease) 2. What type of pressure (high or low) would result in the maximum amount of CO2 production in the following system? 2CO(g)+O2(g)<---->2CO2(g) -shift to the right HIGH 3. What conditions of pressure and temperature would result in higher concentrations of the underlined substances? a. 2CO(g)+O2(g)<----->2CO2(g)+167kJ 2CO(g) - lower pressure, higher temperature b. Cu2+(aq)+4NH3(aq)<---->Cu(NH3)42(aq)+42kJ Cu(NH3)42+(aq) - pressure = no influence, lower temperature c. 2HI(g)+126kJ<---->H2(g)+I2(g) H2(g) - pressure = no influence, higher temperature d. 4HCl(g)+O2(g)<--->2H20(g)+2Cl2(g)+113kJ 4HCl(g) - lower pressure, higher temperature e. H2O(l)+42kJ<----->H2O(g) H2O(g) - lower pressure, higher temperature
Ksp
solubility product constant Ksp=[A+]^m[B-]^n do not include concentration of pure solid/liquids -The product of the concentration of ions in a dissolution. Ksp can be derived from direct concentration measurements or from solubility values. *aligns with solubility*
equilibrium position
the relative concentrations of reactants and products of a reaction that has reached equilibrium; indicates whether the reactants or products are favored in the reversible reaction look at the product concentration and the reactant concentration when a system is at equilibrium
equilibrium constant
value that expresses how far the reaction proceeds before reaching equilibrium. A small number means that equilibrium is towards the reactants side while a large number means that the equilibrium is towards the product side the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation
chemical equilibrium
when the forward reaction rate equals the reverse reaction rate because the reactions are happening simultaneously -once established, it looks like nothing is happening even though so much is actually happening
Keq
where equilibrium lies, equilibrium constant, [products]/[reactants] The value of Keq varies w/ temperature. Keq>1, products are favored at equilibrium Keq<1, reactants are favored at equilibrium Keq = 1, relatively equal amounts of products and reactants at equilibrium *Keq is reported w/o units.* *Favorability (most of the time) is based on stability.* See notes for examples.