DF4 - Bond enthalpies

Question 1

What is energy density?

Answer 1

How much energy you get per kilogram of fuel

Question 2

What is a chemical bond?

Answer 2

Electrical attraction between atoms/ions. (Breaking them involves overcoming these attractive forces)

Question 3

What needs to happen to break a chemical bond completely?

Answer 3

The atoms/ions need (theoretically) to be an infinite distance apart

Question 4

What is bond enthalpy

Answer 4

The quantity of energy needed to break one mole of a bond in a molecule to give separate atoms all in the gaseous state. (also known as bond dissociation enthalpy)

Question 5

Is bond-breaking an endothermic or exothermic process?

Answer 5

Bond-breaking is endothermic - so bond enthalpies are always positive

Question 6

Is bond-making endothermic or exothermic?

Answer 6

Bond-making is exothermic

Question 7

The stronger the bond, the higher its bond enthalpy. True or false? Explain why.

Answer 7

True. More energy is needed to break it, so it has a higher bond enthalpy.

Question 8

Describe and explain the attraction between e.g two H atoms (H2) and bond length

Answer 8

• When a covalent bond between the two H atoms form, the atoms move towards one another because of the attractive forces between nuclei + electrons
• There are also repulsive forces between the nuclei of the two atoms
• These get bigger as atoms approach until the atoms stop moving together
• Distance between them is now equilibrium bond length. The shorter the bond length, the stronger the attraction between the atoms

Question 9

Why are average bond enthalpies usually used in calculations?

Answer 9

The exact value of bond enthalpy depends on particular compound in which the bond is found

Question 10

What is the difference between the bond enthalpies of single, double and triple bonds?

Answer 10

• Double bonds have much higher bond enthalpies than single bonds
• Triple bond enthalpies are even higher

Question 11

What is relationship between bond enthalpy + bond length?

Answer 11

• In general, the higher the bond enthalpy, the shorter the bond
• This is because there are more electrons between the atoms being attracted to the positive nuclei - more attraction makes shorter bonds

Question 12

How and why are bond enthalpies measured indirectly?

Answer 12

Why?
* It isn’t easy to measure bond enthalpies as there is often more than one type of bond in a compound
* It’s also very difficult when everything is in the gaseous state
How?
* They are measured indirectly using enthalpy cycles

Question 13

Why do small atoms form strong covalent bonds

Answer 13

This is because the electrons are closer to the nucleus

Question 14

Bond enthalpies are stated at standard conditions. True or false?

Answer 14

True

Question 15

When methane burns, does the reaction involve the breaking or making of bonds?

Answer 15

It involves both making + breaking bonds

Question 16

You have to break all the old bonds before you can make new ones. True or false?

Answer 16

False. New bonds start forming as soon as the first of the old bonds have broken

Question 17

All reactions initially need energy to stretch + break bonds. True or false?

Answer 17

True

Question 18

Describe what bond breaking + making look like on an enthalpy cycle

Answer 18

• Reactants —> Products
• Bond-breaking arrow points from reactants (in top line) to elements that make up reactants/products
• Bond-making arrow point from elements that make up reactants/products to the products (in top line)
• Remember that all of the compounds/atoms are in their gaseous states (include g state symbol)

Question 19

Why is the value calculated for the bond enthalpy different from the value given in the data sheets? (Two main reasons)

Answer 19

• Value of bond enthalpy value is not actually the standard value - e.g in the equation for combustion of methane the water product is gaseous, not liquid, as it would be liquid under standard conditions - gaseous form of water is used as when using bond enthalpies you have to work it out in gaseous state
• Bond enthalpies are given often as averages - this means the values are not always precise

Question 20

What are common errors that you should be aware of when calculating bond enthalpies?

Answer 20

• Forgetting that bonds in O2 are broken
• Not noticing that there are two C=O bonds per CO2
• Not noticing that there are two O-H bonds per H2O

Question 21

In exothermic reactions, describe how energy is involved in the bonds made and broken

Answer 21

More energy is released when bonds are formed in the products than is used to break the bonds in the reactants

Question 22

In endothermic reactions, describe how energy is involved in the bonds made and broken

Answer 22

More energy is absorbed when bonds in reactants are broken than is released when new bonds are formed in the products