Scientific Tutor

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 G_f values)

CH_4(g) + 2 O_2(g) —-> CO_2(g) + 2 H₂O_(g)

 

What is the energy of the molecules of the products?

Answer: CO_2(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: CH_4(g) = -50 kJ/mol O_2(g) = 0

 

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

Answer: Yes, O_2(g) 2 O_2(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.

(CO_2(g) + 2 H₂O_(g)) – (CH_4(g) + 2 O_2(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 G_f of H₂Ba_(s). (Use this link look up the Δ G_f values)

2 NaH_(g) + BaCl_2(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 BaCl_2(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) + BaCl_2(s)) = Δ G

 

Fill in the numbers for this specific equation.

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

 

Solve for X

COMPLETE ANSWER: Δ G_f of H₂Ba_(s) = -578 kJ/mol

 

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

 

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

2 H₂O_(g) —-> 2 H_2(g) + O_2(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