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

In a chemical reaction, bonds in the reactants first break and then bonds form to make the products

e.g.      Calculate the energy change for the following reaction: N2(g) + 3H2(g) ⇌ 2NH3(g)

            (the bond energies are N≡N 945 kJ/mol, H-H 436 kJ/mol, N-H 391 kJ/mol)

∆H                   = sum of all bonds broken - sum of all bonds formed

Bonds broken   = 1 x N≡N + 3 x H-H

= (1 x 945) + (3 x 436)

= 2253 kJ

Bonds formed   = 6 x N-H (each NH3 has 3 N-H bonds and there are two molecules in the equation)

= 6 x 391

= 2346 kJ

∆H                    = 2253 - 2346

= -93 kJ/mol

4.19      understand the term activation energy and represent it on a reaction profile

Particles will not necessarily react when they collide. They need to have enough energy to be able to react on collision (i.e. they need to hit one another hard enough).  The minimum energy required for particles to react on collision is called the activation energy.  Such collisions are called “successful collisions”.  The higher the activation energy the slower the reaction.

The diagrams below show the activation energy on both an endothermic and an exothermic reaction reaction profile. Note also the enthalpy change (∆H).

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

Learn it!   Breaking bonds requires energy (takes energy in) and forming bonds releases energy.

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

Energy level diagrams show the energy of reactants relative to products. The height difference between them is ∆H for the reaction.

If you draw these for a particular reaction, label an arrow for ∆H with its sign and value on the dotted line, and put in the formulae of the reactants and products on the appropriate horizontal lines.

endothermic energy level diagram

exothermic energy level diagram

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

∆H is the symbol for enthalpy change.  This means more or less the same thing as a change in heat energy, but it is usually used to represent the change for one mole of a substance (molar enthalpy change) AND it has a sign to show whether the reaction is exothermic or endothermic.  A negative sign means the reaction is exothermic, and a positive sign means the reaction is endothermic.

4.12 calculate molar enthalpy change from heat energy change

The example above showed how much energy this particular mass of ethanol released.  To find out how much a mole of ethanol would release, we need to divide it by the number of moles of ethanol burned. 

Ethanol has a relative formula mass of 46, so 1 mole of ethanol is 46 grams.

Moles of ethanol           =  mass ÷ molar mass

= (145.34 - 144.82) g  ÷ 46 g/mol

= 0.0113 mol

Energy of combustion of 1 mole of ethanol         = 5.292 kJ ÷ 0.0113 mol

= 468 kJ/mol

The ENTHALPY change, ∆H, has a sign to show whether the reaction was exothermic or endothermic (see next section).  Combustion reactions are exothermic, so we must write our final answer with a negative sign, so:  ∆H for this reaction is –468 kJ/mol.

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

Reactions such as displacements and neutralisation or the dissolving of solids can be done in a polystyrene cup with a lid. The initial temperature of the liquid(s) is measured. The reaction is then started and the mixture is stirred with a thermometer. The maximum (or minimum) temperature reached is noted.

The heat energy change (q) can be calculated using the relationship below: q = m c ∆T

NOTE: You will not be expected to remember this relationship.  It will be given to you if you need to use it in the exam.

q = m c ∆T

m = mass of liquid in the “cup” (aqueous solutions are taken to weigh 1 g per cm3)

c = specific heat capacity of water = 4.2 J/g/°C

∆T = temperature change  =  final temperature - initial temperature

e.g.      1.24 g of Mg was added to 25 cm3 of 1.0 mol/dm3 hydrochloric acid. The temperature rose from 18.5 °C to 30.8 °C. What was the energy change?

q          = 25 g x 4.2 J/g/°C x (30.8 - 18.5) °C

= 1292 J  =  1.292 kJ

(This can be converted into kJ/mol if you divide it by the number of moles of the substance reacting that is not present in excess. See the next example. This step is extension material and is not required for the Core paper 1.  It could appear on Paper 2.)

To measure the energy change of combustion, we can heat water in a copper calorimeter as shown below. We need to know the mass of water heated and the temperature rise of the water. If we want to be able to work out the energy of combustion in kJ/mol, we also need the mass of fuel burned.

e.g. 200 cm3 of water is heated by burning ethanol in a spirit burner.  The initial mass of the burner is

145.34 g and the final mass is 144.82 g.  The initial temperature of the water is 20.5 °C, and the final temperature is 26.8 °C.  How much heat energy is given out in this reaction?

q        = m c ∆T

q           = 200 g x 4.2 J/g/°C x (26.8 - 20.5) °C

= 5292 J   =  5.292 kJ

Energetics

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

Learn it! Note that endothermic reactions get cold because when they take energy in, it is removed from the surroundings in the form of heat and turned into chemical energy.

4.9        describe how to carry out acid-alkali titrations.

Titrations are used to work out exactly how much acid is need to neutralise a certain amount of alkali. If the concentration of one is known, the concentration of the other can be calculated (see Section 1.27).  Accurate glassware is used for titrations.  A burette measures volumes to the nearest 0.05cm3.  Pipettes come in various sizes and deliver a precise and accurate volume each time they are used.

Method

1)   Use a pipette to put 25 cm3 of the alkali in a conical flask.

2)   Fill a burette with the acid.

3)   Put a few drops of indicator in the conical flask.

4)   Note the initial volume of acid in the burette.

5)   Add the acid a little at a time to the conical flask whilst swirling it.

6)   Stop when the indicator just changes colour and note the final volume of acid in the burette.  For example, if using methyl orange, the indicator would start off yellow in the alkali in the flask.  You need to stop when it JUST turns orange.  If you go to red, you have added too much acid.

Notes on Method

1)   The acid and the alkali can be either way around.

2)   Putting a white tile under the conical flask can help spot colour changes.

3)   Before they are used, it is good practice to rinse the burette and the pipette firstly with water, and then with the solution they are going to be used to measure.

4)   Readings (for burette and pipette) are taken at eye level from the bottom of the meniscus.

Concordant Titres

1)   It is usual to repeat titrations to get several titres (titre = the volume added from the burette)

2)   The aim is to get two titres within 0.10 cm3 of each other.

3)   Such results are called concordant titres.

4)   When the average titre is calculated, only concordant titres are used. The others are ignored.

	Run 1	Run 2	Run 3	Run 4
Final Burette Reading / cm3	21.30	21.25	23.85	21.90
Initial Burette Reading / cm3	1.30	0.80	3.25	1.80
Titre / cm3	20.00	20.45	20.60	20.10

The concordant values are from Run 1 and Run 4. Therefore the average titre for this titration is:

(20.00 cm3 + 20.10 cm3) ÷ 2   =  20.05 cm3

(Run 1 can be done as a quick rough titration to check the indicator colour change and approximate  volumes. In such cases it should be labelled Rough Titration, and its value is not used in calculations). 

 

Take readings from the bottom of the meniscus. Note that burettes read in the opposite direction to measuring cylinders.

This reads 21.30 cm3 and not 22.70 cm3

Pipette

These measure fixed volumes with accuracy. They come in different sizes. (Burettes measure variable volumes)