Calculating energy changes

Question 1

What can you use to calculate the energy change in a reaction?

Answer 1

bond energies

Question 2

bond energy

Answer 2

energy needed to break 1 mol of a particular covalent bond

(e.g. 413 kJ is needed to break 1 mol of C-H bonds
413 kJ is released when 1 mol of C-H bonds forms)

(a mol is just a unit)

Question 3

What do the different bond energies depend on?

Answer 3

factors such as:

• elements involved
• length of the bond

Question 4

Hydrogen burns in oxygen to form water.
The equation shows the structures of the substances involved in this balanced reaction.

2 x (H - H) + O==O -> 2 x ( H–O–H)

Bond energies (kJ mol-1):
H-H = 436 
O-H = 464
O=O = 498

a) Calculate the energy needed to break all the bonds in the reactants
b) Calculate the energy released when new bonds form in the products
c) Calculate the overall energy change for the reaction

Answer 4

a) 2 x (H-H) = 2 x 436 = 872 kJ mol-1
O=O = 498 kJ mol-1

872 + 498 = 1370 kJ mol-1

b) 4 x (O-H) = 4 x 464 = 1856 kJ mol-1
c) energy change = 1370 - 1856 = -486 kJ mol-1

Question 5

total energy change

Answer 5

total energy in - total energy out

Question 6

Use these values to answer the questions:

Bond energies (kJ mol-1):
C-H = 413
H-H = 436 
O-H = 464
O=O = 498
C=O = 805

During electrolysis, water decomposes to form hydrogen and oxygen.

a) Calculate the overall energy change in the reaction.
b) Explain whether the process is exothermic or endothermic.

2 x (H-O-H) -> 2 x (H-H) + O=O

Answer 6

a) energy to break bonds:
4 x (O-H) = 4 x 464 = 1856 kJ mol-1

energy out when bonds form:
2 x (H-H) + O=O = (2 x 436) + 498 = 872 + 498 = 1370 kJ mol-1

energy change = 1856 - 1370 = +486 kJ mol-1

b) process is endothermic
energy change is postive/energy taken in to break bonds is greater than energy given out when bonds form

Question 7

Use these values to answer the questions:

Bond energies (kJ mol-1):
C-H = 413
H-H = 436 
O-H = 464
O=O = 498
C=O = 805

Methane burns in oxygen to form carbon dioxide and water.

Calculate the overall energy change in the reaction.

(CH4) + 2 x (O=O) -> 2 x (H-O-H) + O=C=O

Answer 7

energy in:
4 x (C-H) + 2 x (O=O) = (4 x 413) + (2 x 498) = 2648 kJ mol-1

energy out:
4 x (O-H) + 2 x (C=O) = (4 x 464) + (2 x 805) = 3466 kJ mol-1

energy change = 2648 - 3466 = -818 kJ mol-1