## Chem – College: Calculating Delta G (first way)

HOW DO WE CALCULATE Δ G (First Way)?

There are two different ways to calculate Δ G (Delta G). The first is look up the Δ G values on a Gibbs Free Energy Table (DELTA G) and then take the Δ G of the products minus the Δ G of the reactants. This is the same technique you used in the how to calculate Δ H section only you are going to use values found in a slightly different table (Gibbs Free Energy / Spontaneity Table). The equation for the Δ G is below:

Energy of products – Energy of reactants = Δ G

VIDEO Calculate Δ G (DELTA G) Demonstrated Example 1: Use the balanced chemical equation below and calculate its Δ G. (Use this link look up the Δ Gf values)

CH4(g) + 2 O2(g) —-> CO2(g) + 2 H2O(g)

What is the energy of the molecules of the products?

Answer: CO2(g) = -390 kJ/mol H2O(g) = -230 kJ/mol

Do any of the molecules in the products have any coefficients?

Answer: Yes, H2O(g) 2 H2O(g) = 2 * (-230 kJ/mol)

What is the energy of the molecules of the reactants?

Answer: CH4(g) = -50 kJ/mol O2(g) = 0

Do any of the molecules in the reactants have any coefficients?

Answer: Yes, O2(g) 2 O2(g) = 2 * (0)

What is the general formula for Δ G?

Answer: Energy of products – Energy of reactants = Δ G

Modify the Δ H equation for this chemical equation.

(CO2(g) + 2 H2O(g)) – (CH4(g) + 2 O2(g)) = Δ G

Fill in the numbers for this specific equation.

(-390 + 2 * (-230)) – (-50 + 2 * (0)) = Δ G

Solve for Δ H

COMPLETE ANSWER: Δ G = -800 kJ/mol

VIDEO Calculate Δ G (DELTA G) Demonstrated Example 2: Use the Δ G and balanced chemical equation below and calculate the Δ Gf of H2Ba(s). (Use this link look up the Δ Gf values)

2 NaH(g) + BaCl2(s) —-> H2Ba(s) + 2 NaCl(s)Δ G = -480 kJ/mol

What is the energy of the molecules of the products?

Answer: H2Ba(s) = X kJ/mol NaCl(s) = -384 kJ/mol

Do any of the molecules in the products have any coefficients?

Answer: Yes, NaCl(s) 2 NaCl(s) = 2 * (-384 kJ/mol)

What is the energy of the molecules of the reactants?

Answer: NaH(g) = -34 kJ/mol BaCl2(s) = -790 kJ/mol

Do any of the molecules in the reactants have any coefficients?

Answer: Yes, NaH(g) 2 NaH(g) = 2 * (-34 kJ/mol)

What is the general formula for Δ G?

Answer: Energy of products – Energy of reactants = Δ G

Modify the Δ G equation for this chemical equation.

(H2Ba(s) + 2 NaCl(s)) – (2 NaH(g) + BaCl2(s)) = Δ G

Fill in the numbers for this specific equation.

( X + 2 * (-380)) – (2 * (-34) + -790) = – 480

Solve for X

COMPLETE ANSWER: Δ Gf of H2Ba(s) = -578 kJ/mol

PRACTICE PROBLEMS: Calculate the Δ G or the Δ Gf as needed. (Use this link look up the Δ Gf)

Use the balanced chemical equation below and calculate its Δ G.

2 H2O(g) —-> 2 H2(g) + O2(g)

Answer: Δ G = -460 kJ/mol

Use the balanced chemical equation below and calculate its Δ G.

H+1(aq) + F -1(aq) —-> HF(g)

Answer: Δ G = +10 kJ/mol  