Electrolysis Calculations
We remember that electrolysis is decomposition brought
about by an electric current.
Electrolysis reactions are unusual because different parts of the reaction
take part in different places.
Electrolysis reactions are REDOX reactions - oxidation takes place at the
anode and reduction at the cathode.
During electrolysis calculations, we need to keep an eye on the number
of electrons transferred.
Electrode reactions are endothermic - they take in heat from their surroundings.
e.g. Magnesium is produced by the electrolysis of molten magnesium chloride.
a) Mg2+ 2e- = Mg 2Cl- - 2e- = Cl2
Moles Mg = 1000 = 41.7
MMMMMMI24
The same number of electrons are transferred to make 1 mole Cl2 as 1 mole Mg.
So moles Cl2 = 41.7
Mass Cl2 = 41.7 x 71 = 2960g (3 s.f.) = 2.96 Kg.
b) Volume of Cl2 = 41.7 x 24 litres = 1000 litres.
Energy Transfer in Chemical reactions
When Chemical reactions occur, energy is almost always transferred to or from the surroundings. When energy is transferred to its surroundings, we say the reaction is exothermic. Reactions which take in heat from the surroundings are endothermic.
Exothermic: D H is negative
Endothermic: D H is positive
D = Change H = Heat
Although most energy transfer is in the form of heat,
sound, light and movement can be given out. Almost all reactions are exothermic.
The amount of heat given out in a reaction does not necessarily show how
much the reaction will be: rate of reaction depends on ACTIVATION ENERGY
(Ea) barrier.
During a chemical reaction: Energy must be supplied to break bonds.
Energy is released when bonds are formed.
In an exothermic reaction, the energy released from forming new
bonds is greater than that needed to break existing bonds.
In an endothermic reaction the energy needed to break existing bonds
is greater than that needed to make new bonds.
We can use these energies to work out the heat change in the gas phase
e.g. Burning Natural Gas
Methane + Oxygen = Carbon Dioxide + Water vapour
CH4 + 2O2 = CO2 + 2H2O
Bonds Broken MMMMDH/kJmol-1 MMMMBonds Made MMMMDH/kJmol-1
4C-HMMMMMMMM +413x4 = +1652 MM2C=O MMMMMMMM-805x2 = -1610
2O=O MMMMMMMII+498x2 = +996 MMII4O-H MMMMMMMM-464x4 = -1856
Positive = Bonds broken
Negative = Bonds made
Reaction = 1652 + 996 + (-1610) + (-1856) = -818 kJ mol-1
The previous example can be displayed on an energy level diagram.
Activation Energy (Ea) and the use of Catalysts
The activation Energy of a reaction is the minimum energy which a collision between reacting particles must have if a reaction is to occur - energy must be supplied to break bonds before energy can be released before bonds can be made.
Particles must also collide at the right orientation (i.e. the right angle), if they are to produce a reaction.
We can represent this on an energy profile diagram for a reaction.
Most collisions don’t produce a reaction. The higher the activation barrier, the fewer the number of successful collisions per second, and the slower the reaction. When the temperature of the reaction is raised, the energy of the remaining reacting particles is raised, so more collisions are sufficiently energetic to produce a reaction, and so the reaction rate increases; the reaction speeds up. The value of DH has no influence or rate - very exothermic reactions can be too slow to be observed because of a high activation barrier, and some endothermic reactions can be vigorous.
Catalysts work by changing the reaction path (They often increase the number of steps), so that the activation energy is lowered.
The use of a catalyst does not affect the value of DH or the equilibrium position of the reaction, although it will increase the speed at which equilibrium is reached.
Rates of Reaction (Kinetics)
The speed (rate) of a chemical reaction increases :
Definition
A catalyst increases the rate of a chemical reaction, but is not used up during the reaction. It is used over and over again to speed up the conversion of reactants to products
N.B. Different reactions need different catalysts
Reaction Catalyst
CH4 + H2O «
CO + 3H2 Ni
N2 + 3H2 « 2NH3
Fe
C2H4 + H2 «
C2H6 Ni
2SO2 + O2 « 2SO3
V2SO5
4NH3 + 5O2 à
4NO + 6H2O Pt/Rh
N.B.2 We remember that a catalyst makes a new reaction path available which has lower activation energy
Increasing the rates of chemical reactions in industry is important because it helps to reduce costs. The rate of a chemical reaction can be followed EITHER by measuring the rate at which products are formed OR the rate at which reactants are used up. This allows a comparison to be made of the changing rate of a chemical reaction under different conditions.
Collision Theory
Chemical reactions can only occur if reacting particles collide with each other at the correct orientation and with sufficient energy. We have seen that activation energy is the minimum amount of energy which a collision between reacting particles must have to be successful, I.e. produce a reaction.
Increasing the temperature, increases the speed at which
the particles travel, so they collide more frequently and energetically.
So probability of a successful collision increases AND collision are more
frequent, thus the rate of the reactions increase for two reasons.
In general, a 10oC rise in temperature will cause a doubling in rate.
Increasing the concentration of reactants in reactions OR increasing the pressure of reacting gasses OR increasing the surface area of a solid reacting with a solution or a gas, leaves the probability of a successful collision unaltered, but does increase the frequency of collisions and so the frequency of successful collisions increases and the rate increases.
Reactions involving enzymes
Living cells use chemical reactions to produce new materials. Yeast cells convert sugar into carbon dioxide and ethanol. This is known as fermentation and is used:
a) to produce alcohol in beer and wine
b) to produce bubbles of CO2 which makes bread rise.
A simple lab test for CO2 is lime water which will turn milky if present.
Ca(OH)2 + CO2 = CaCO3 + H2O
White Precipitate
Bacteria is used to produce yoghurt from milk. The bacteria converts the sugar in milk (Lactose) into lactic acid.
The chemical reactions brought about by living cells are quite fast, in conditions that are warm rather than hot. This is because the cells use catalysts called enzymes. Enzymes are protein molecules which are usually damaged by temperatures above 45oC (Denatured). They are also highly efficient and very reaction specific.
Rections which are suitable are:
Acid + Carbonate
CaCO3 + 2HCl = CaCl2 + CO2 + H2O
Metal + Acid
Zn + H2SO4 = ZnSO4 + H2
Catalytic decomposition of hydrogen peroxide.
2H2O2 = 2H2O + O2
The reaction mixture is put together, the bond is placed
inside the flask, the stopwatch is started, and the volume of gas is collected
and recorded using a syringe.
We notice that if the reaction was plotted on a graph, then the volume
of gas collected would be rather steep at the start as the reaction is
at its quickest.
Volume and Concentration notes
1m3 = (100cm)3 = 1000000cm3
1m3 = 1000l
1l = 1000cm3
1cm3 = 1ml
1dm = 10cm
1dm3 = (10cm)3 = 1000 cm3
The symbol M means "Molar" concentration units.
1M = 1mol solute / litre of solution, = 1mol/l or 1mol-1 or 1mol/dm-3
Concentration = moles
volume
CaCo3 +2HCl = H2O + CO2 +CaCl3
Excess
Makes Hcl = C x V
= 0.1 mol/litre x 0.1 litre
= 0.01 moles
Moles CO2 = 0.01 = 0.005
MMMMMMM2
= 0.005 x 24 litres at room temperature.
= 0.120 litres
= 120 cm3
Curve A
Hcl = 0.2 mol/litre x 50 litres = 0.01
MMMMMMMM1000
Volume = 120cm3 as CaCO3 is in excess
Rate is quicker as the concentration is double.
Curve B
Hcl = 0.05l x 0.1 mol/litre = 0.005 mol
Moles CO2 = 0.0025
Volume = 60cm3
Rate is the same as the acid concentration is the same.
Curve C
Volume of CO2 = 60cm3
Rate much quicker as surface area CaCO3 has increased; more frequent collision between H+ and CO32-.
An alternative way of monitoring rate of a reaction which produces a gas
We remember that no chemical reaction can create or destroy a mass. The same atoms are always present before and after, although they have been rearranged. However, the contents of a container in which a reaction producing a gas will lose mass with time because the gas defuses out of the flask.
Volumetric Calculations
These are mainly about titrations, and they link the concentration of a solution with the number of moles present. We remember that:
Concentration (mol-1) = Number of moles
Volume (Litres)
N.B. i) Write an equation ii) Use It iii) Work in moles.
e.g. 25.0 cm3 of a sodium hydroxide solution was neutralised by 29.2 cm3 of a 0.6M nitric acid solution. Find the concentration of the sodium hydroxide solution.
a) in moles per litre and
b) in grams per litre.
HNO3 +NaOH = NaNO3 + H2O
Moles HNO3 =
0.6 x 292 mol/l x l = 0.01752
MMMMMMMMII1000
Moles NaOH reacting = 0.01752
Concentration NaOH = Moles = 0.01752 = 0.701M (3 s.f.)
Volume 0.0250
Moles = 0.701 x Mr(NaOH) g/litre
= 0.701 x 40 g/litre
= 28.0 g/litre (3 s.f)