KA Unit 6 Lesson 13: Principles of bioenergetics

What is ΔG?

ΔG is the symbol for Gibbs free energy. It indicates the thermodynamic favorability of a reaction.

What are the units of ΔG?

ΔG has units of Joules/mol reactant

What does it mean when ΔG is negative?

-ΔG indicates the reaction will proceed spontaneously. This is referred to as exergonic.

What does the spontaneity of a reaction tell you about the speed at which it will occur?

The spontaneity does not indicate the speed of a reaction, only that it will take place without the addition of external energy.

What is K_eq?

K_eq is the equilibrium constant. It describes the ratio of the concentration of products to reactants at equilibrium.

What is the equation for K_eq?

K_eq = [C]^c[D]^d/[A]^a[B]^b

What does it mean when K_eq is greater than 1? Less than 1? Equal to one?

If K_eq > 1, products are favored at equilibrium

If K_eq < 1, reactants are favored at equilibrium

If K_eq = 1, the concentrations of products and reactants are the same at equilibrium

What is ΔG^o?

ΔG^o is the symbol for standard Gibbs free energy change. This is the free energy change under stand conditions.

What is the equation relating ΔG^o and K_eq?

ΔG^o = -RTln(K_eq) where R is the gas constant (8.314 J/mol*K), T is the temperature in Kelvin, and K_eq is the equilibrium constant.

Based on the equation ΔG^o = -RTln(K_eq), what can you infer about ΔG^o when K_eq is positive?

ΔG^o will be negative and the reaction will be spontaneous.

The logarithm of a number greater than 1 is positive, making ΔG^o negative. A negative ΔG indicates that the reaction is spontaneous in the forward direction under standard conditions.

Based on the equation ΔG^o = -RTln(K_eq), what can you infer about ΔG^o when K_eq is less than 1?

ΔG^o will be positive and the reaction will be non-spontaneous.

The logarithm of a number less than 1 is negative, making ΔG^o positive. A positive ΔG indicates that the reaction is non-spontaneous in the forward direction under standard conditions.

Based on the equation ΔG^o = -RTln(K_eq), what can you infer about ΔG^o when K_eq is 1?

ΔG^o is 0 and the reaction is at equilibrium.

The logarithm of a 1 is 0, making ΔG^o = 0.

What is Le Chatelier’s principle?

A system at equilibrium disrupted by changes in concentration, pressure, or temperature will shift in the direction that counteracts the disturbance.

Based on Le Chatelier’s principle what will happen to a system if the pressure is increased in a gaseous reaction?

The system will shift toward the side with fewer moles of gas to reduce the pressure.

Based on Le Chatelier’s principle what will happen to a system if the temperature is increased for an exothermic reaction? What about an endothermic reaction?

For exothermic reactions, adding heat will shift the system towards the reactant side. Adding heat is like adding a product.

For endothermic reactions, adding heat will shift the system towards the product side. Adding heat is like adding a reactant.

Considering the body’s bicarbonate buffering system (CO₂ + H₂O ←→ H₂CO₃ ←→ HCO₃^- + H⁺). How does the body compensate with the increase in CO₂ produced by cells during exercise?

The increase production in CO₂ will shift the reaction to the right, increasing HCO₃^- + H⁺ concentration which would decrease the pH. The body compensates by increasing breath rate to expel more CO₂ and shift the reaction back to the left.

Fill in the blanks:

K_eq is affected by changes in _____________ but not by changes in _____________ and _____________.

A. Pressure

B. Concentration

C. Temperature

K_eq is affected by changes in temperature but not by changes in concentration or pressure.

Is ΔG affected by changes in temperature, pressure, and/or concentration?

Yes, ΔG is affected by changes to any of the three.

What is enthalpy for the purposes of a chemical reaction?

Enthalpy (H) can be thought of as the change in bond energy (heat) during a reaction.

What does it mean when ΔH is negative? Positive? Which is more favorable?

When ΔH is negative energy is released during a reaction (favorable). When ΔH is positive energy is absorbed during a reaction.

What is entropy for the purposes of a chemical reaction?

Entropy (S) is a measure of disorder in a system.

What does it mean when ΔS is negative? Positive? Which is more favorable?

When ΔS is negative there is a decrease in entropy (more order). When ΔS is positive there is an increase in entropy (more disorder, favored).

What is the equation for ΔG at constant pressure and temperature?

ΔG = ΔH - TΔS

What will ΔG be if ΔH is negative and ΔS is positive? Is this a thermodynamically favorable reaction?

ΔG will be negative (ex: -5 - 2*5 = -15), this is thermodynamically favorable.

What will ΔG be if ΔH is positive and ΔS is negative? Is this a thermodynamically favorable reaction?

ΔG will be positive (ex: 5 - 2*-5 = 15), this is thermodynamically unfavorable.

What will ΔG be if ΔH and ΔS are negative?

ΔG could be positive or negative, it will likely depend on the temperature.

What will ΔG be if ΔH and ΔS are positive?

ΔG could be negative or positive, it will likely depend on the temperature.

Fill in the blanks:

According to the equation ΔG = ΔH - TΔS, __________ can be thought of as the leftover energy available to do __________ after subtracting for the energy going to the __________ of the universe.

A. work

B. ΔG

C. disorder

According to the equation ΔG = ΔH - TΔS, ΔG can be thought of as the leftover energy available to do work after subtracting for the energy going to the disorder of the universe.

Do spontaneous reactions increase or decrease free energy?

Decrease free energy

What does -ΔH mean in terms of bond energy?

More thermal energy is released when new bonds in the products form than is needed to break bonds in the reactants.

More (thermal) energy is needed to overcome the forces of attraction between molecules and separate them from one another (the activation energy) than (thermal) energy released when new bonds are formed. What sign will ΔH have?

ΔH will be positive, this is an endothermic reaction.