S4 - Bond energy calculations

Question 1

How can you calculate the enthalpy change for a reaction?

Answer 1

Look at the bonds that are made and broken

Question 2

What is bond energy?

Answer 2

The amount of energy in a bond

Question 3

What does each type of chemical bond have?

Answer 3

A particular bond energy associated with it

Question 4

What bonds do you add up?

Answer 4

The energy of the bonds that are broken

Question 5

What bonds do you subtract?

Answer 5

The energy of the bonds that are made

Question 6

Using the bond energies for the following reaction H2 + Cl2 —> 2HC

H-H: +436 kj/mol
Cl-Cl: +242 kj/mol
H-Cl: +431 kj/mol

Work out the energy needed for breaking the bonds

Answer 6

436 + 242 = 678 kj

Question 7

Using the bond energies for the following reaction H2 + Cl2 —> 2HC

H-H: +436 kj/mol
Cl-Cl: +242 kj/mol
H-Cl: +431 kj/mol

Work out the energy needed to form the bonds

Answer 7

2 x 431 = 862kj

Question 8

What formula can you use to calculate the differences in the energy calculated in the formation of HCl?

Answer 8

Enthalpy change = Total energy absorbed to break bonds - total energy released in making bonds

Question 9

What is the enthalpy change in the formation of HCl (breaking=678 and forming=862)?

Answer 9

678 - 862 = -184 kj/mol

Question 10

What does the enthalpy change of the formation of HCl show?

Answer 10

The value is negative so it in an exothermic reaction