Exam 3 Pt. 2 - Thermodynamics at Nonstandard Conditions

When are gases at standard conditions?

• Pure gas at 1 atm

When are solids/liquids at standard conditions?

• Pure substance at 1 atm and 25 degrees C

When are solutions at standard conditions?

• Concentrations of 1 M

What equation do you use when calculating ΔGrxn at nonstandard conditions?

• ΔGrxn = ΔG°rxn + RTlnQ

What does the variable R stand for?

• Gas constant: 8.314 J/mol x K

What does Q stand for?

• Reaction quotient: Q = (products^coefficients)/(reactants^coefficients)

Using the previous ΔGrxn equation, what occura to the equation under standard conditions?

• Q= 1

ΔGrxn = ΔG°rxn + RTln(1)

ΔGrxn = ΔG°rxn

Using the previous ΔGrxn equation, what occurs to the equation at equilibrium?

• ΔGrxn = 0 and Q=K

0= ΔG°rxn + RTlnK

ΔG°rxn = -RTlnK

When using the ΔGrxn equations, what units must we use for gibbs free energy?

• Joules

convert if easier to compare

How do you find out if a reaction is more or less spontaneous in nonstandard or standard conditions?

• Use Q; then compare values of ΔGrxn and ΔG°rxn

What equation do you use to calculate equilibrium constant of reaction?

• ΔG°rxn = -RTlnK

CONVERT TO JOUles, solve for K

How do you cancel out ln?

• e^()

What is the two point form equation of temperature dependence on equilibrium constant?

• ln (K2/K1) = (-ΔH°rxn/R) (1/T2 - 1/T1)

used to determine K at two diff Ts

How will you be asked to use the two point form equation?

• Given 4 variables, solve for 1

What happens to the k (equilibrium constant) when reactant conc. increases? Which way would reaction shift?

• k gets smaller

reaction shifting to left

What happens to the k (equilibrium constant) when product conc. increases? Which way would reaction shift?

• k gets larger

reaction shifting to right

What should you do to predict answer of two point form equation?

• Use equilibrium shifting concepts; to ensure answer makes sense