C5 Bond Energies (page 140)

Question 1

Energy must always be supplied to make or break bonds?

Answer 1

Break Bonds

Question 2

During a chemical reaction what happens to old bonds?

Answer 2

old bonds are broken and new bonds are formed

Question 3

Energy must be SUPPLIED to break existing bonds - so bond breaking is a endorthermic or exothermic process?

Answer 3

it is an endothermic process

Question 4

Energy is RELEASED when new bonds are formed - so bond formation is an Endothermic or Exothermic process?

Answer 4

Exothermis process

Question 5

What is BOND BREAKING, - Endothermic or Exothermic?

Answer 5

Bond breaking - Endothermic

H CL (strong bond)&raquo_space;»» H + Cl - (bond broken) see diagram on page 140

Question 6

Is BOND FORMING - Endothermic or Exothermic?

Answer 6

C + O &raquo_space;»> C O (strong bond formed) + Energy released (see page 140 for diagram)

Question 7

In Exothermis reactions how is the energy released?

Answer 7

by forming bonds greater than the energy used to break them

Question 8

In Endothermic reactions how is the energy used?

Answer 8

it is used to break bonds that is greater than energy released by forming them

Question 9

Every chemical bond has a particular bond energy associaed with it. This bond energy varies slightly depending on what?

Answer 9

the compound the bond occurs in.

You can use these known bond energies to calculate the overall energy change for a reaction. The overall energy change is the sum of the energies needed to break bonds in the reactans minus the energy released when the new bonds are formed in the products. (need to practice a few calculations)

Question 10

Using the bond energies, calculate the energy change for the reaction between

H2 and Cl2 forming HCL?

                                                                            H   -   Cl H  -  H  +  Cl  -  Cl    >>>>>>>>>>>>>>>>>>>>
                                                                            H   -   Cl

Answer 10

The bond energies you need are:

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

1) find the energy required to break the original bonds:

(1 x H-H) + (1 x Cl-Cl) = 436 kj/mol + 242 kj/mol = 678 kj/mol

2) Find the energy released by forming the new bonds:

2 x H-Cl = 2 x 431 kj/mol = 862 kj/mol

3) Find the overall energy change for the reaction using this equation:

Overall energy change = energy required to break bonds energy released by forming bonds = 678 kj/mol - 862 kj/mol = -184 kj/mol

(see example on page 140)

Question 11

You cant compare the overall energy changes of reactions unless you know what?

Answer 11

the numerical differences in the bond energies.

Question 12

Chlorine and Bromine react with Hydrogen in a similar way. Br-Br bonds are weaker than Cl-Cl bonds and H-Br bonds are weaker than H-Cl bonds, so what do this tell you?

Answer 12

Less energy is needed to break the bonds in the reaction with Bromine, but less energy is released when the new bonds form.

So unless you know the exact differences, you can’t say which reaction releases more energy

Question 13

N2 reacts with H2 in the following reaction: N2 + 3H2 &raquo_space;> 2NH3

The bond energies for these molecules are: N☰N: 941 kj/mol; H-H: 436 kJ/mol; N-H: 391 kJ/mol

              H-H                    H                H
                                         l                   l N☰☰N +    H-H  >>>>         N      +         N
                                        /  \                /  \
               H-H                 H   H          H    H

calculate the overall energy change for this reaction? (3 marks)

Answer 13

Energy required to break original bonds:
(1 x N☰N) + (3 x H-H) = 941 kJ/mol + 1308 kJ/mol = 2249 kJ/mol (1 mark)

Energy released by forming new bonds:
(6 x N-H) = 2346 kJ/mol (1 mark)

Overall energy change:
= 2249 kJmol - 2346 kJ/mol = -97 kJ/mol (1 mark)

Question 14

What do kJ/mol mean?

Answer 14

kJ/mol is the abbreviation of kilojoules per mole, which is used to describe the amount of energy in kilojoules that is put into a number of moles of atoms.

(kilojoules are the metric measurement o find the energy content, a system of unit energy units equial to one thousand joules)