Chemical Equilibrium

If a chemical equation is modified in some way, the equilibrium constant for the equation changes because of the modification. There are three modifications. What are they?

1. If we reverse the equation, invert the equilibrium constant. For example, consider this equilibrium equation: A + 2B <=> 3C. The expression for the equilibrium constant of this reaction is Kforward = [C]^3 / [A][B]^2 If we reverse the reaction: 3C <=> A + 2B then, according to the law of mass action, the expression for the equilibrium constant becomes Kreverse = [A][B]^2 / [C^3] = 1/Kforward 2. If we multiply the coefficients in the equation by a factor, we raise the equilibrium constant to the same factor. If we multiply the equation above by n, we get nA + 2nB <=> 3nC. Applying the law of mass action, the expression for the equilibrium constant becomes ([C]^3/[A][B]^2)^n = K^n 3. If we add two or more individual chemical equations to obtain an overall equation, we multiply the corresponding equilibrium constants by each other to obtain the overall equilibrium constant. Consider these two chemical equations and their corresponding equilibrium constant expressions: A <=> 2B K1= [B]^2/[A] 2B <=> 3C K2= [C]^3/[B]^2 The two equations sum as follows: A <=> 3C According to the law of mass action, the equilibrium constant for this overall equation is then: Koverall= [C]^3 / [A] Notice that Koverall is the product of K1 and K2 Koverall = K1 x K2 = [B]^2/[A] x [C]^3/[B]^2 = [C]^3 / [A]

What does a large equilibrium constant (K>1) imply about a reaction?

A large equilibrium constant indicates that the numerator (which specifies the amounts of products at equilibrium) is larger than the denominator (which specifies the amounts of reactants at equilibrium). Therefore, when the equilibrium constant is large, the forward reaction is favored-high concentrations of products, low concentrations of reactants.

KEY CONCEPT

As concentration of product increases, and concentrations of reactants decrease, rate of forward reaction slows down and rate of reverse reaction speeds up. Dynamic equilibrium: rate of forward reaction = rate of reverse reaction. Concentrations of reactants and products no longer change.

What is the difference between reaction quotient and equilibrium constant?

At a given temperature, the equilibrium constant has only one value and it specifies the relative amounts of reactants and products at EQUILIBRIUM. The reaction quotient depends on the current state of the reaction and has many different values at it proceeds.

Why is the reaction quotient useful?

Because the value of Q relative to K is a measure of the progress of the reaction toward equilibrium. At equilibrium the reaction quotient is equal to the equilibrium constant.

What effect does a volume (or pressure) change on equilibrium?

Decreasing the volume increases the pressure, causing the reaction to shift to the right (fewer moles of gas, lower pressure). Increasing the volume reduces the pressure, causing the reaction to shift to the left (more moles of gas, higher pressure).

What is the difference between equilibrium constant and reaction quotient?

Equilibrium constant, K, allows us to see how far a reaction has proceeded at equilibrium. Reaction quotient, Q, is also the ratio of the concentration of product to the concentration of reactant but unlike K, Q does not refer to equilibrium concentrations. It refers to concentrations at some point in time. If Q=K, reaction is at equilibrium If Q>K, reaction is reactant favored If Q<K, reaction is product favored

The concentrations of reactants and products are equal at equilibrium. T or F?

False. Although the concentrations of reactants and products no longer change at equilibrium, the concentrations of reactants and products are NOT EQUAL to one another at equilibrium. Some reactants reach equilibrium only after most of the reactants have formed products; others reach equilibrium only after most of the reactants have formed products; other reach equilibrium when only a small fraction of the reactants have formed products. It depends on the reaction.

K>1

Forward reaction is favored; forward reaction proceeds essentially to completion

How does oxygen exchange occur between the maternal and fetal circulation?

In the placenta, the blood of the fetus comes into close proximity with that of the mother, although the two do not mix directly. Because the reaction of HbF with O2 has a larger equilibrium constant than the reaction of maternal Hb with O2, the fetus receives O2 from the mother's blood.

Reaction rates generally increase with increasing concentration of the reactants (unless the reaction rate is zero) and decrease with decreasing concentration of reactants. With this in mind, explain dynamic equilibrium using the following reaction: H2(g) + I2(g) <-> 2HI (g)

Initially H2 and I2 begin to react to form 2 HI molecules, but the 2 HI molecules also can react to form re-form H2 and I2. As H2 and I2 react, their concentrations decrease, which in turn decreases the rate of the forward reaction. At the same time, HI begins to form. As the concentration of HI increases, the reverse reaction begins to occur at a faster and faster rate. Eventually, the rate of the reverse reaction (which has been increasing) equals the rate of the of the forward reaction (which has been decreasing). At that point dynamic equilibrium is reached.

Why is dynamic equilibrium "dynamic"?

It is "dynamic" because the forward and reverse reactions are still occurring; however, they are occurring at the same rate. When dynamic equilibrium is reached, the concentrations of the reactants and products no longer change (as long as the temperature is constant). The concentration remains constant because the reactants and products form at the same rate they are depleted.

What does a small equilibrium constant (K<1) mean?

It means that the reverse reaction is favored and that there will be more reactants than products when equilibrium is reached. It means that the equilibrium constant is very small, indicating that the equilibrium point for the reaction lies far to the left-high concentrations of reactants, low concentrations of products.

What is the equilibrium constant for the following equation: aA + bB <-> cC + dD?

K = [C]^c [D]^d / [A}^a [B]^b [A] represents the molar concentration of A. The equilibrium constant quantities the relative concentration of reactants and products at equilibrium. The relationship between the balanced chemical equation and the expression of the equilibrium constant is known as the law of mass action.

Express the equilibrium constant for the following chemical equation: 2N2O5(g) <=> 4NO2(g) + O2(g)

K = [NO2]^4[O2] / [N2O5]^2

What is the equilibrium constant?

Keq; The concentrations of the reactant and products in a reaction at equilibrium are described the equilibrium constant, K. A large value of K means that the reactions lies far to the right at equilibrium0a high concentration of products and a low concentration of reactants. A small value of K means that the reaction lies far to the left at equilibrium

What happens when a chemical system at equilibrium is disturbed?

Le Chatelier's principle states that the chemical system responds to minimize the disturbance. Le Chatelier's Principle: When a chemical system at equilibrium is disturbed, the system shifts in a direction that minimizes the disturbance.

K=1

Neither direction is favored; forward reaction proceeds about halfway

reaction quotient

Ratio of the concentrations of the products to the concentrations of the reactants at any point during the reaction aside from equilibrium, where each reactant and product in the expression is raised to the power of its stoichiometric coefficient. Commonly denoted by Q.

Many chemical reactions involve pure solids or pure liquids as reactants or products. Consider the reaction: 2CO(g) <=> CO2(g) + C(s) Do we include C(s) in the equilibrium expression? Why?

Since carbon is a solid, its concentration is constant (if we double the amount of carbon, its concentration remains the same). The concentration of a solid does not change because a solid does not expand to fill its container. Its concentration, therefore, depends only on its density, which as constant as long as some solid is present. The concentration of solid carbon (the number of atoms per unit volume) is constant as long as some solid is present. The same is true for pure liquids. For this reason, the concentration of solids and pure liquids are not included in equilibrium constant expressions.

Law of Mass Action

The rate of a chemical reaction is proportional to the product of the concentrations of the reactants

Why does a developing fetus get oxygen from the mother's blood?

The reaction between O2 and fetal Hb has a larger equilibrium constant than the reaction between O2 and maternal Hb. After we are born, we inhale air into our lungs and that air diffuses into capillaries, where it comes into contact with our blood. Within our RBCs, a protein called Hb reacts with O2 according to the following equations: Hb+O2 <-> HbO2. The double arrows in this equation indicate that the reaction can occur in both the forward and reverse directions and can reach chemical equilibrium. The equilibrium constant for the reaction between Hb and O2 is such that Hb efficiently binds O2 at typical lung O2 concentrations, but Hb can also release O2 under the appropriate conditions. Any system at equilibrium, including the Hb-O2 system responds to changes in ways that maintain equilibrium. If any of the concentrations of the reactants or products change, the reaction shifts to counteract that change. For the Hb system, as blood flows through the lungs where O2 concentrations are high, the reaction shifts to the right-Hb binds O2. As blood flows out of the lungs and into muscles and organs where O2 concentrations have been depleted (because muscles and organs use O2), the reaction shifts to the left-Hb releases oxygen. In order to maintain equilibrium, Hb binds O2 when the surrounding O2 concentration is high, but it releases O2 when the surrounding O2 concentration is low. In this way, Hb transports O2 from the lungs to all parts of the body that use oxygen. A fetus has its own circulatory system. The mother's blood never flows into the fetus's body, and the fetus cannot get any air in the womb. How, then does the fetus get oxygen? The answer lies in the properties of fetal hemoglobin HbF. Like adult hemoglobin, HbF is in equilibrium with O2: HbF + O2 <--> HbFO2. However, the equilibrium constant for fetal hemoglobin is larger than the equilibrium constant for adult Hb, meaning that the reaction tends to go farther in the direction of the product. Consequently, HbF loads O2 at a lower O2 concentration that does adult Hb. In the placenta, fetal blood flows in close proximity to maternal blood. Although the two never mix, because of the different equilibrium constants, the maternal hemoglobin releases O2 that the HbF then binds and carries into its won circulatory system.

Nearly all chemical reactions are theoretically reversible. T or F?

True. In may cases, however, the reversibility is so small that it can be ignored.

The ratio defined by the law of mass of mass action is always the same, regardless of the initial concentrations of reactants. T or F?

True. Whether we start with only reactants or only products, the reaction reaches equilibrium at concentrations in which the equilibrium constant is the same. No matter what the initial concentrations are, the reaction always goes in a direction that ensures that equilibrium concentrations-when substituted into the equilibrium expression-result in same constant, K (at constant temperature).

How do we quantify the concentrations of reactants and products at equilibrium?

We use the equilibrium constant (K). It is the ratio-at equilibrium-of the concentrations of the products raised to the stoichiometric coefficients divided by the concentrations of the reactants raised to their stoichiometric coefficients.

If a reaction mixture not at equilibrium contains both reactants and products, can we predict the direction of change for such a mixture?

Yes. We can use reaction quotient. The definition of the reaction quotient takes the same form as the definition of the equilibrium constant, except that the reaction doesn't need to be at equilibrium. So for the general reaction: aA + bB <-> cC + dD the reaction quotient (Qc) is the ratio, at any point in the reaction-of the concentrations of the products raised to their stoichiometric coefficients divided by the concentrations of the reactants raised to their stoichiometric coefficients. For gases with amounts measured in atmospheres, the reaction quotient uses the partial pressures in place of concentrations and is called Qp

What is a reversible reaction?

a reaction where the products can react together to form the original reactants

In an endothermic chemical reaction, heat is a reactant. Decreasing the temperature causes...

an endothermic reaction to shift left (in the direction of the reactants); the equilibrium constant decreases

In an endothermic chemical reaction, heat is a reactant. Increasing the temperature causes...

an endothermic reaction to shift right (in the direction of the products); the equilibrium constant increases

In an exothermic chemical reaction, heat is a product. Decreasing the temperature causes...

an exothermic reaction to shift right (in the direction of the products); the value of the equilibrium constant increases.

In an exothermic chemical reaction, heat is a product. Increasing the temperature causes...

an exothermic reaction to shift the left (in the direction of he reactants); the value of the equilibrium constant decreases.

If a chemical system is at equilibrium, decreasing the volume...

causes the reaction to shift in the direction that has the fewer moles of gas particles

If a chemical system is at equilibrium, increasing the volume...

causes the reaction to shift in the direction that has the greater number of moles of gas particles

If a chemical system is at equilibrium, adding an inert gas to the mixture at a fixed volume...

has no effect on the equilibrium

If a chemical system is at equilibrium, decreasing the concentration of one or more of the products...

makes Q < K and causes the reaction to shift to the right (in the direction of the products)

If a chemical system is at equilibrium, increasing the concentration of one or more of the reactants...

makes Q < K and causes the reaction to shift to the right (in the direction of the products)

If a chemical system is at equilibrium, decreasing the concentration of one or more of the reactants...

makes Q > K and causes the reaction to shift to the left (in the direction of the reactants)

If a chemical system is at equilibrium, increasing the concentration of one or more the products...

makes Q > K and causes the reaction to shift to the left in the (in the direction of the reactants)

If a chemical system is at equilibrium, when a reaction has an equal number of moles of gas on both sides of the chemical equation, a change in volume...

produces no effect on the equilibrium

Q > K

reaction goes to the left (toward reactants)

Q < K

reaction goes to the right (toward products)

Q=K

reaction is at equilibrium

ΔG=0

reaction is at equilibrium Q=K

ΔG>0

reaction is nonspontaneous

ΔG<0

reaction is spontaneous

K<1

reverse reaction is favored; forward reaction does not proceed very far

What is dynamic equilibrium?

the condition of a reaction in which the rate of the forward reaction equals the rate of the reverse reactions

What equation is used to relate change in free energy and reaction quotient?

ΔG=ΔG* + RTlnQ ΔG=Gibb's Free Energy ΔG*=Standard Gibb's Free Energy R=Gas Constant (8.314 J/mol*K) Q=Reaction Quotient