C5
Electrolysis
Equilibrium
Ionic Substances
• Ionic substances contain ions which are in fixed
positions in the solid but can move in solution or
when melted
Electrolysis
• Electrolysis is the decomposition of a liquid by
passing an electric current through it.
• Positive ions discharge at the negative electrode
(cathode)
• Negative ions discharge at the positive electrode
(anode)
• Electrolytic decomposition of:
– K2SO4 =
– KNO3 =
• The ionic equations are:
– K2SO4
• Cathode =
• Anode =
– KNO3
• Cathode =
• Anode =
• In the electrolysis of aqueous solutions it may be
easier to discharge ions from the water rather
than from the solute.
Copper II sulphate
• This uses copper electrodes
• The negative electrode gets plated with copper
• The positive electrode gets smaller and dissolves
• Change in mass of the copper electrodes:
– The negative electrode gains mass
– The positive electrode loses mass
– The gain in mass of the negative
electrode is equal to the loss in mass of
the positive electrode
• Ionic equations :
– Cathode =
– Anode =
Charge Transfer, Current and Time
• Q=It
• Example = A current of 0.5A passed around the
circuit for 3860 sec. The cathode has gained
0.64g. How many coulombs of charge are
needed to deposit 1 mole of copper?
– Q = Charge (coulombs)
– I = Current (amps)
– T = Time (sec)
Factors
• Factors that affect the amount of substance
produced during electrolysis are:
– Time = amount of substance increases
as time increases
– Current = amount of substance
increases as current increases
Reversible reactions
• In a reversible reaction there is a forward and
backward reaction
• They can occur at the same time
• The symbol is ………
• Some reversible reactions may reach an
equilibrium
• At equilibrium the rate of the forward reaction
equals the rate of the backward reaction
• The concentrations of the reactants and the
products do not change
• When the position of the equilibrium is on the
right the concentration of product is greater than
the concentration of reactant
• When the position of the equilibrium is on the
left the concentration of reactant is greater than
the concentration of product
• At equilibrium
– It is a closed system
– Initially rate of forward reaction
decreases
– Initially rate of backward reaction
increases
– Eventually rate of forward equals rate
of backward reaction
• Change in temperature, pressure or concentration
of reactant or product may change the position of
equilibrium
• Removing a product moves the position of
equilibrium to the right
• Adding extra reactant moves the position of
equilibrium to the right
• Increasing the pressure moves the position of
equilibrium to the side with the least number of
moles of gas molecules
Contact Process
• This process is used to make sulphuric acid
• Equation =
• Raw materials needed are
– Sulphur
– Air
– Water
– This involves a reversible reaction
between sulphur dioxide and oxygen
The sulphur dioxide comes from burning sulphur
Conditions needed
 Catalyst …..increases rate but does not change
position of equilibrium
 Temperature …….high temp decreases yield and
increases rate of reaction so an equilibrium is
used
 Pressure ……..position of equilibrium is already
on the right so high pressure is expensive and is
not needed
Write down the symbol equations for the three stages in
the manufacture of sulphuric acid
C5
Gas Volumes
Moles and Empirical Formulae
Measurement
• The measurement of change of mass
may be used to monitor the amount of
gas made in a reaction.
• A reaction stops when one of the
reactants is all used up
• The greater amount of reactant the
greater amount of gas is made
Moles
• The unit for the amount of a substance is
the MOLE.
• The molar mass of a substance is its
relative atomic mass in grams
Limiting reactants
• The limiting reactant is the one that is
used up first of all
• The amount of gas produced is directly
proportional to the amount of the
limiting reactant present.
Mole of gas
• One mole of gas occupies a volume of
24dm3 at room temperature and pressure
Calculations
• How many moles are there in 24dm3 of
Hydrogen
• How many moles are there in 6 dm3 of
Hydrogen
• How many moles are there in 24 dm3 of
Chlorine
• How many moles are there in 120 cm3 of
Oxygen
• How many moles are there in 48 cm3 of
Nitrogen
• What volume does 0.1 mole of oxygen
occupy?
• What volume does 3 mole of hydrogen
occupy?
• What volume does 5 mole of chlorine
occupy?
• What volume does 0.1 mole of nitrogen
occupy?
• What volume does 8g of oxygen gas
occupy?
• What volume does 4g of hydrogen gas
occupy?
• What volume does 3.55g of chlorine gas
occupy?
• What volume does 8.8g of carbon
dioxide gas occupy?
Molar Mass
• Calculate the molar mass of:
– NaOH
– CaCO3
– K2SO4
– MgCl2
– Ca(OH)2
Mass
Mass is conserved during a chemical reaction.
Equations
• Number of moles = mass / molar mass
• How many moles of atoms are there in:
– 2.4g of carbon
– 2g of hydrogen
– 36g of carbon
– 160g of oxygen
– 1.4g of nitrogen
– 0.19g of fluorine
– 8g of copper oxide
– 36g of water
– 170g of ammonia
• What is the mass of
– 0.1moles of carbon
– 2 moles of hydrogen
– 5 moles of nitrogen
– 20molesof oxygen
– 0.5 moles of fluorine
– 0.01 moles of carbon
Empirical Formulae
• An empirical formulae gives the
simplest whole number ratio of each
type of atom in a compound
Carbon
• The relative atomic mass of an element
is the average mass of an atom of the
element compared to the mass of 1/12th
of an atom of carbon -12.
C5
Quantitative Analysis
Strong and Weak Acids
► Concentration of solutions may be
 An acid is a substance that ionises in water to
measured in g/dm3 (g per dm3)
produce hydrogen ions
► Concentration of solutions may be
3
3
 A strong acid ionises completely in water
measured in mol/dm (mol per dm )
 A weak acid only partially ionises in water
► Volume is measured in dm3 or cm3
3
3
► 1000 cm equals 1dm
Acids
► Dilution is needed in areas such as food
 A strong acid produces more H+ ions than a weak
preparation, medicine and baby milk.
acid of the same concentration
► Suggest some dangers of getting the
 HCl → H+ + Cldilution wrong.
 Ethanoic acid partially ionisies:
► The more concentrated a solution the more
CH3COOH
CH3COO + H+
crowded the solute particles
 Strong acids such as HCl, nitric acid and sulphuric
► Convert
3
3
acid have a lower pH than weak acids such as
 1000 cm into dm
3
3
ethanoic acid.
 20dm into cm
 Weak acids can be used to flavour food (e.g.
 600 cm3 into dm3
vinegar) and to de scale kettles and boilers
 20000 cm3 into dm3
 5dm3 into cm3
Reactions with Acid
► Convert
 Ethanoic acid or HCl react with
 10 g/dm3 into mol/dm3
3
3
 Magnesium to produce……..
 5 mol/dm into g/dm
 Calcium carbonate to produce …………
 200 g/dm3 into mol/dm3
3
3
 The volume of gas made is determined by the
 65 mol/dm into g/dm
amount of reactants used not by the acid’s strength
 3000 g/dm3 into mol/dm3

But the reaction with ethanoic acid is slower as
► How many moles are there in 8g of copper
there are fewer hydrogen ions in it than in the same
oxide?
concentration of HCl so there are fewer collisions
► How many moles are there in 16g of
 The strength of an acid is determined by how much
NH4NO3?
it ionises.
► How many moles of NaCl are there in
3
3
 HCL reacts quicker than ethanoic acid because:
22cm of a 2 mol/dm solution?
 HCl is a stronger acid than ethanoic acid
► What is the mass of 100cm3 of 2 mol/dm3
 HCl contains a greater concentration of
KOH solution?
hydrogen ions than ethanoic acid does
 The greater concentration of hydrogen ions
in HCl leads to a higher collision frequency
of hydrogen ions
Electrolysis of acids
 Acids conduct electricity.
 A strong acid e.g. HCl is a better conductor than the
same concentration of a weak acid e.g. ethanoic
acid. This is because there are fewer hydrogen ions
in ethanoic acid to carry the charge.
 When HCl or ethanoic acid are used in electrolysis,
the hydrogen gas is made at the negative electrode.
This is because when electricity is past through the
acid, the hydrogen ions are attracted to the negative
electrode where they are changed into hydrogen
C5
Titrations
Acids and Alkalis
• pH ________ when acid is added to an alkali
• pH ________ when alkali is added to a acid
• _______ _______ can be use to estimate the pH of a solution
Apparatus
• Label the apparatus used in an acid base titration
– Burette
– Conical flask
Method
• Acid in burette
• Alkali in conical flask
• (or vice versa)
• Acid slowly added to alkali (or vice versa) until end point is reached
• End point detected by the change in colour of an indicator
Indicators
• Universal indicator – acid – red
– Acid = red
– Alkali = purple/blue
• Litmus =
– blue litmus turns pink in acid
– Red litmus turns blue in alkali
• Phenolphthalein
– Acid = colourless
– Alkali = pink-purple
• Methyl orange
– Acid = red
– Alkali = yellow
• Single indicator such as litmus produces a sudden colour change during titration
whereas a mixed indicator such as universal produces a continuous colour
change. Which is better?
• There is a sudden change in pH at the endpoint of a titration
pH Curves
• www.avogadro.co.uk/chemeqm/acidbase/titration/phcurves.htm
Calculation
Moles = concentration * volume (dm3)