Hess' Law

​Hess's Law

​Equation method for Hess’s law

Match

• A

• B

• C

• D

• E

2

2NH_{3 (g)} -> N_2(g) + 3H_2(g)

3

N_2(g) + O_2(g) -> 2NO(g)

2H₂ (g) + O₂ -> 2H₂O(l)

• 2

• 2NH_{3 (g)} -> N_2(g) + 3H_2(g)

• 3

• N_2(g) + O_2(g) -> 2NO(g)

• 2H₂ (g) + O₂ -> 2H₂O(l)

​2. method: Energy cycle

​2. method: Energy cycle

​Piemērs, formācijas entalpijas aprēķināšana, jeb entalpijas izmaiņa reakcijai:
6C + 3H₂ --> C₆H₆

​Piemērs, formācijas entalpijas aprēķināšana, jeb entalpijas izmaiņa reakcijai:
6C + 3H₂ --> C₆H₆

Poll

Can elements exist in their atomic form?

• Yes

• No

​Bond dissociation enthalpy and mean bond enthalpy

​The bond dissociation enthalpy is the energy needed to break one mole of the bond to give separated atoms - everything being in the gas state.

​Important! The point about everything being in the gas state is essential. You cannot use bond enthalpies to do calculations directly from substances starting in the liquid or solid state.

Multiple Choice

Cik atomi veido saišu entalpiju?

• 1

• 2

• 3

• Atkarīgs no situācijas

Multiple Choice

bond dissociation enthalpy, to break up 1 mole of gaseous hydrogen chloride molecules into separate gaseous hydrogen and chlorine atoms takes 432 kJ. The bond dissociation enthalpy for the H-Cl bond is:

• -432 kJ mol^-1.

• +432 kJ mol^-1.

​What happens if the molecule has several bonds, rather than just 1?

​Consider methane, CH4. It contains four identical C-H bonds, and it seems reasonable that they should all have the same bond enthalpy.

​However, if you took methane to pieces one hydrogen at a time, it needs a different amount of energy to break each of the four C-H bonds.


Every time you break a hydrogen off the carbon, the environment of those left behind changes. And the strength of a bond is affected by what else is around it.

In cases like this, the bond enthalpy quoted is an average value.

Poll

Is bond breaking exothermic?

• Yes

• No

​When you make a bond, as much energy is given out as you needed to break it, but the value is now negative - because we show exothermic changes with negative enthalpy changes.

When you are working out the enthalpy change when you are making bonds, you look up the bond enthalpy and reverse the sign.

​Another example:

Multiple Choice

Cik molus C-H saites satur 1 mols CH₄?

• 0.25

• 1

• 4

• 5

Multiple Choice

In the methane case, you can work out how much energy is needed to break a mole of methane gas into gaseous carbon and hydrogen atoms. That comes to +1660 kJ and involves breaking 4 moles of C-H bonds. Estimate the average bond energy of C-H bond!

• 103kJ/mol

• 415kJ/mol

• 1660kJ/mol

• 2320kJ/mol

​Mean (or average) bond enthalpies

​In fact, tables of bond enthalpies give average values in another sense as well, particularly in organic chemistry. The bond enthalpy of, say, the C-H bond varies depending on what is around it in the molecule. So data tables use average values which will work well enough in most cases

​That means that if you use the C-H value in some calculation, you can't be sure that it exactly fits the molecule you are working with. So don't expect calculations using mean bond enthalpies to give very reliable answers

​It is always a good idea to draw full structural formulae when you are doing bond enthalpy calculations. It makes it much easier to count up how many of each type of bond you have to break and make.

​A shorter way to calculate:

​All that has actually changed is that you have broken a C-H bond and a Cl-Cl bond, and made a new C-Cl bond and a new H-Cl bond. So you can just work those out.

​Cases where you have a liquid present

​If you have one or more liquids present, you need an extra energy term to work out the enthalpy change when you convert from liquid to gas, or vice versa. That term is the enthalpy change of vaporisation, and is given the symbol ΔH_vap or ΔH_v.