HOW DO YOU CALCULATE ΔH (DELTA H)?
Once you memorized the relationship between the side the energy is on in a chemical equation, the Δ H and endothermic or exothermic, you are probably curious about how Δ H is calculated. It is calculated by the total energy contained in the molecules of products minus the total energy of the molecules in the reactants. Any time you are calculating the Δ of something it is always FINAL – INITIAL. Another way to say that is FINISH – START or PRODUCTS – REACTANTS. The formula is below.
Energy of products – Energy of reactants = Δ H
But how do we count the energy of the products or the energy of the reactants? It turns out that each chemical has an energy associated with it. Just like you would read the amount of calories off the back of a food label. You can also find tables where you can look up the energy of different chemicals. Where do we get the energy of the individual chemicals? Here is a link to the place where you find the enthalpy (energy) of each chemical. It is referred to in chemistry as the Δ Hf or the ENTHALPY OF FORMATION. REMEMBER to make sure the chemicals you look up also have the correct states of matter. If you want to find it in a chemistry text book, then it will usually be in the very back of the book in the index section.
VIDEO Calculate Δ H (DELTA H) Demonstrated Example 1: Use the balanced chemical equation below and calculate its Δ H. (Use this link look up the Δ Hf values)
CH4(g) + 2 O2(g) —-> CO2(g) + 2 H2O(g)
What is the energy of the molecules of the products?
Answer: CO2(g) = -394 kJ/mol H2O(g) = -242 kJ/mol
Do any of the molecules in the products have any coefficients?
Answer: Yes, H2O(g) 2 H2O(g) = 2 * (-242 kJ/mol)
What is the energy of the molecules of the reactants?
Answer: CH4(g) = -75 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 H?
Answer: Energy of products – Energy of reactants = Δ H
Modify the Δ H equation for this chemical equation.
(CO2(g) + 2 H2O(g)) – (CH4(g) + 2 O2(g)) = Δ H
Fill in the numbers for this specific equation.
(-394 + 2 * (-242)) – (-75 + 2 * (0)) = Δ H
Solve for Δ H
COMPLETE ANSWER: Δ H = -803 kJ/mol
VIDEO Calculate Δ H (DELTA H) Demonstrated Example 2: Use the Δ H and balanced chemical equation below and calculate the Δ Hf of H2Ba(s). (Use this link look up the Δ Hf values)
2 NaH(g) + BaCl2(s) —-> H2Ba(s) + 2 NaCl(s)Δ H = -536 kJ/mol
What is the energy of the molecules of the products?
Answer: H2Ba(s) = X kJ/mol NaCl(s) = -411 kJ/mol
Do any of the molecules in the products have any coefficients?
Answer: Yes, NaCl(s) 2 NaCl(s) = 2 * (-411 kJ/mol)
What is the energy of the molecules of the reactants?
Answer: NaH(g) = -55 kJ/mol BaCl2(s) = -858 kJ/mol
Do any of the molecules in the reactants have any coefficients?
Answer: Yes, NaH(g) 2 NaH(g) = 2 * (-55 kJ/mol)
What is the general formula for Δ H?
Answer: Energy of products – Energy of reactants = Δ H
Modify the Δ H equation for this chemical equation.
(H2Ba(s) + 2 NaCl(s)) – (2 NaH(g) + BaCl2(s)) = Δ H
Fill in the numbers for this specific equation.
( X + 2 * (-411)) – (2 * (-55) + -858) = -536
Solve for X
COMPLETE ANSWER: Δ Hf of H2Ba(s) = -682 kJ/mol
PRACTICE PROBLEMS: Calculate the Δ H or the Δ Hf as needed. (Use this link look up the Δ Hf)
Use the balanced chemical equation below and calculate its Δ H.
2 H2O(g) —-> 2 H2(g) + O2(g)
Answer: Δ H = +484 kJ/mol
Use the balanced chemical equation below and calculate its Δ H.
3 CaSO4(s) + 2 Al(OH)3(s) —-> 3 Ca(OH)2(s) + Al2(SO4)3(s)
Answer: Δ H = 462 kJ/mol
Use the Δ H and the balanced chemical equation below and calculate the Δ Hf of F–(g).
H+(g) + F–(g) —-> HF(g)Δ H = – 150 kJ/mol
Answer: Δ Hf of F–(g) = -120 kJ/mol
Use the Δ H and balanced chemical equation below and calculate the Δ Hf of CN(s).
2 HCN(g) + AgCN(s) —-> Ag(s) + H2(g) + 3 CN(s)Δ H = -113 kJ/mol
Answer: Δ Hf of CN(s) = 101 kJ/mol (do not forget to divide by the 3 coefficient)
