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

4.15 understand that the breaking of bonds is endothermic and hat the making of bonds is exothermic

Not much to say here... when you make a bond it is an exothermic reaction, when you break a bond it is an endothermic reaction.

Energy must be supplied to break existing bonds, so bond breaking is endothermic. However, energy is released when new bonds are formed, so bond formation is exothermic.

4.14 represent exothermic and endothermic reactions on a simple energy level diagram

In exothermic reactions
This energy level diagram shows an exothermic reaction, we can tell this because the products are at a lower energy level than the reactants. The difference in height (ΔH) represents the energy given out in the reaction. ΔH is negative here because the reaction is giving out energy (as it is an exothermic reaction). The activation energy represents the energy needed to break the old bonds.


In endothermic reactions

This energy level diagram shown an endothermic reaction, we can tell this because the products are at a higher energy level than the reactants. The difference in height (ΔH) represents the energy taken. ΔH is positive because the reaction is taking in energy (because is an endothermic reaction).

Basis of notes source: CGP

4.13 understand the use of ΔH to represent enthalpy change for exothermic and endothermic reactions

The enthalpy change is the overall change in energy in a reaction, it is symbolised by ΔH and its unit is kJ/mol, as it is the amount of energy in kilojoules per mole of reactant. It can be positive or negative, if the reaction is exothermic, the enthalpy change is negative because the reaction gives out energy, if the reaction is endothermic, the enthalpy change value is positive because the reaction takes in energy.

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 (:

4.11 describe simple calorimetry experiments for reactions such as combustion, displacement, dissolving and neutralisation in which heat energy changes can be calculated from measured temperature changes

NOTE: THIS IS SO CONFUSING I DON'T UNDERSTAND WHATS GOING ON the only example/explanation for this I could find was in the CGP revision guide but I still don't understand it so will defiantly be coming back to this post to edit it after i have finished the spec, if anyone knows how to do it or has any info or anything please comment it and I will add it to the post!

Combustion
This is basically the same method as 2.32 of the biology spec but with fuel not food... to measure the amount of energy produced when a fuel is burnt, just burn the fuel and use the flame to heat up some water...

- Put 50g of water into a copper can (because copper is a very good conductor of heat)
- Record the temperature of the water
- Weigh the spirit burner and lid (the spirit burner contains the fuel)
- Place the spirit burner underneath the copper can and light its wick.
- Stir constantly until the water reaches about 50ºC
- Put the flame out using the burner lid
- Record the final temperature of the water
- Weigh the spirit burner and lid again
- calculate the enthalpy change

Your setup should look something like this...

NOTE: the draught shield is just there to ensure no/little heat escapes

Dissolving, displacement and neutralisation reactions

4.10 understand that chemical reactions in which heat energy is given out are described as exothermic and those in which heat energy is taken in are endothermic

NOTE: before you get your head round endothermic and exothermic, it might be a good idea to understand that energy is given out when bonds are made, and taken in when bonds are destroyed...


If a reaction gives out heat, it is said to be exothermic and the energy released in bond formation is greater than the energy used in breaking bonds.

Definition for exams: An exothermic reaction is one which gives out energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature.


If a reaction is endothermic, it takes in heat from its surroundings during the reaction process. This is because the energy required to break old bonds is greater than the energy released when new bonds are formed.

Definition for exams: An endothermic reaction is one which takes in energy from the surroundings, usually in the form of heat and usually shown by a fall in temperature.