4d) Equilibria

Some reaction are reversible which means that they have the same rate backwards and forwards
That means that there is no overall change: reactants --> products    products --> reactants

This can only take place in a CLOSED system or it would escape. Initially there are no products when you first start to mix the reactants. After a while there will be products; but still more reactants. With time the reactants go down and the products go up and we reach the SAME RATE, this is called DYNAMIC EQUILIBRIUM.
The position of equilibrium is not always at the half way point therefore there could be a position where there is more reactants than products.

Dehydration of hydrated copper (II) sulfate:
hydrated copper(II) sulfate + heat   anhydrous copper(II) sulfate + water

Heat makes the reaction go forwards (endothermic), whilst cold water makes it go backwards (exothermic) 

Effect of heat on ammonium chloride:

ammonium chlorideammonia + hydrogen chloride

Ammonium chloride decomposes when it is heated so the forward reaction is endothermic, whilst the backwards reaction is exothermic 

Factors which influence equilibrium:

• Temperature increase
• moves the equilibrium to the right hand side, in the direction that produces the fewer molecules/moles of gas on the RHS  
• Pressure increase
• moves equilibrium to the right hand side, moves into the direction that absorbs heat energy e.g. endothermic reactions 
• Concentration 
• Catalyst to get products or reactants faster 

 

LHS              RHS 

A + B  C + D 

4c) Rates of Reaction

RATE - ^{AMOUNT REACTION}⁄<sub>TIME

Depends on four things:</sub>

1. Temperature 
2. Concentration
3. Catalyst 
4. Size of particles (Surface area) 

The Collision Theory:
Higher Temperature: particles move faster, collide more often and with more force. If the temp. is high there will be enough energy to overcome Ea.

Higher Concentration: more particles in more conc., so they collide more frequently

Large surface area: smaller pieces increase sa, more area to work on, collisions more frequent

Catalyst: increase the number of successful collisions by lowering the Ea.

Activation energy or Ea is the amount of energy needed to start a reaction
(higher temp. increases reaction)

4b) Energetics

Exothermic reactions:
Energy is transferred from the reacting mixture to the surroundings and temperature of surroundings increases

Endothermic:
Energy is transferred from the surroundings to the reaction mixture and temperature of surroundings decreases

Copper cup experiment is a simple calorimetry experiment:

Paper Two - Molar Enthalpy:
Measure temperature change of the experiment
Then use this calculation - Energy, Q = mc∆T                  m= mass of surroundings (water or solution) ∆T = change in temp.
                                           or

                                           ∆H = Q/n                                  n = number of moles    sign - exo and + endo 

∆H is the change in heat energy 

Q is measured in Joules 

Simple Energy Level Diagram:

Exothermic

Endothermic 

Ea or activation energy is needed to break or make bonds

Bonds:

Breaking bonds puts energy in - endothermic, 

whilst making new bonds releases energy - exothermic

Paper Two - Using average bond energies to calculate the enthalpy change during a simple chemical equation:

You are given the reaction and table of data 

Then with the information work out the broken bonds (A) and the bonds you make (B)  

Then subtract them to get the ∆H 

4a) Acids and Alkalis

Indicators:

1. Universal indicator is a very useful combination of dyes which give one of the colours on the pH scale
2. Litmus paper tests whether a solution is acidic or alkaline because it changes colour at pH 7.  
1. Red in acidic solutions 
2. Blue in alkaline solutions 
3. Purple in neutral solutions 
3. Phenolphthalein will change from colourless in acidic solutions to bright pink in alkaline solutions 
4. Methyl Orange changes from red in acidic solutions to yellow in alkaline solutions. 

pH Scale:

+----+---------------------------+---------+--------------------------------+
|    |           Acids           | Neutral |            Alkaline            |
+----+---------------------------+---------+--------------------------------+
| pH | 0 | 1 | 2 | 3 | 4 | 5 | 6 |    7    | 8 | 9 | 10 | 11 | 12 | 13 | 14 |
+----+---+---+---+---+---+---+---+---------+---+---+----+----+----+----+----+

Acids;

• below pH scale 
• produce hydrogen ions, H+ when dissolved in water 
• acids + metal ----> salt and hydrogen 
• acids + metal oxides ----> salt and water 
• acids + metal carbonates ----> salt and water and carbon dioxide 

Common acids and salts produced

-hydrochloric acid to chloride

-nitric acid to nitrates 

-ethanoic acid to ethanoates 

Alkalis:

• above pH 7 
• produce OH- (hydroxide) ion when dissolved into water
• alkalis + acids ----> salt and water 

Common alkalis 

-sodium hydroxide 

-ammonia solution 

General rules for predicting the solubility of salt in water:

• All common sodium, potassium and ammonium salts are soluble 
• All nitrates are soluble 
• Common chlorides are soluble, except silver chloride 
• Common sulfates are soluble, except those of barium and calcium
• Common carbonates are insoluble, except those of sodium, potassium and ammonium 

Preparing soluble salts from acids and insoluble bases:

• Add excess solid to ensure that all the acid has reacted 
• Filter it 
• Evaporate half water 
• Leave to crystallise 

Insoluble salts using precipitation reactions:

• Mix the two solutions which contain the ions you need 
• filter precipitate
• wash 
• dry

Acid-Alkais titration:

• Using a pipette and pipette filler, add some alkali to a conical flask, along with two or three drops of indicator 
• Fill a burette with acid 
• Using the burette, add the acid to the alkali a bit at a time- giving the conical flask a regular swirl. Go especially slowly when you reach the end point of the colour change of the indiactor
• The indicator changes colour when all the alkali has been neutralise 
• Record the volume of acid used to neutralise the alkali, it's best to repeat the process several times to make sure you have reliable results. 

Section 4

Physical Chemistry