4.16 use average bond energies to calculate the enthalpy change during a simple chemical reaction

Each type of bond (for example O-O or C-H) has a particular bond energy... we will be given these in the exam so don't worry about learning them. These energies can be used to calculate enthalpy change, for example...

Using bond energies, calculate the enthalpy change for the following reaction
H2 + Cl2 ---> 2HCl

Bond energies...
H-H: +436 kJ/mol
Cl-Cl: +242 kJ/mol
H-Cl: +431kJ/mol

1. Work out what bonds are broken & the energy made... 

1 mole of H-H is broken and 1 mole of Cl-Cl is broken. Therefore, +436 + +242 = +678 kJ/mol is required to break the bonds in this reaction

2. Work out what bonds are being made & the energy released...

Forming 2 moles of H-Cl bonds. This released 2 x +431 = 862kJ/mol


3. Use the formula 'ΔH = total energy absorbed to break bonds - total energy released in making bonds' to find out the enthalpy change

ΔH  = 678 - 862 = -184 kJ/mol


4. ΔH  is negative which means the reaction must be exothermic

4.12 Calculate molar enthalpy change from heat energy change

Okay so you have calculated the amount of energy produced , this can be used to work out the molar enthalpy change (this is basically the enthalpy change given out by one mole of the reactant).

NOTE:

To calculate the molar enthalpy change, you need to know the equations moles = mass / Mr (if unsure of this equation, click here) and molar enthalpy change = energy produced / moles. For example...


0.9g of methylated spirit produces 6510J of heat energy, work out the heat produced per mole. (The Mr of methylated spirit is 44.6)

- First, work out the amount of energy transferred...

We know that 6510J of heat energy was produced, this means 6510J of energy was transferred... this needs to be converted into kJ (as the unit or enthalpy change is kJ/mol)... 6.510kJ

- Next, find out how many moles of fuel produced this heat...

moles = mass / Mr

= 0.9 / 44.6

= 0.02 moles

- Now, divide the amount of heat energy produced by the amount of moles...


6.510 / 0.02 = 325.5 kJ/mol

The end (:

1.20 understand the term molar volume of a gas and use its values (24 dm3 and 24,000 cm3) at room temperature and pressure in calculations.

The space that one mole of gas takes up is called its molar volume.

At room temperature and pressure, one mole of any gas always occupies 24 dm3 (equivalent to 24,000 cm3) - room temperature is 25 ºc and room pressure is 1 atmosphere (atm).

In calculations, use the equations below to convert the number of moles or mass of any gas to a volume…

Volume (dm3) = moles of gas x 24

Volume (dm3) = (mass of gas / Mr of gas) x 24

For example: Whats the volume of 4.5 moles of chlorine at RTP?

Volume (dm3) = 4.5 x 24 = 108 dm3

Example two: How many moles are there in 8280cm3 of hydrogen gas at RTP?

Volume of gas = moles of gas x 24 therefore, moles of gas = volume of gas / 24

One more thing, we need to convert 8280cm3 into dm3. To do this, just divide by 1000.

8280 / 1000 = 8.28

Now substitute into the rearranged equation…

moles of gas = volume of gas / 24 = 8.28 / 24 = 0.345 moles

NOTE: RTP just means room temperature and pressure

1.19 carry out mole calculations using relative atomic mass (Ar) and relative formula mass (Mr)

This equations is used to find the number of moles of a substance...

Number of moles = Mass in g (of element or compound) / Mr (or Ar, of element or compound)

For example: How many moles are there in 60g of carbon dioxide

Mr of CO2 = 12 + 16 + 16 = 44

Number of moles = 66 / 44 = 1.5 moles of CO2