H9
Chemical Equilibrium and Sulfuric acid
Many of the industrial processes used to produce new substances or to replace natural
substances involve equilibrium reactions. This chapter looks at how Le Chatelier’s
principle and our understanding of equilibria are used to develop more efficient
industrial processes.
Synthetic and natural
products
Synthetic products are produced to replace natural products
because of
 Increasing demand for the product
 Inability of the natural sources to keep up with demand
 Depletion of the natural resources
 Competition for the natural resource from another use.
 Escalating price for the natural resource
 Increasing availability and/or decreasing cost of starting
materials for the synthetic product
 Decreasing price for the synthetic product
 Greater reliability of supply and stability of price for the
synthetic product.
Some natural products replaced by synthetic products include
 Rubber
 Soap
 Wool
 Silk
Chemical equilibrium
Qualitative features of a chemical equilibrium include
 Reversible reaction
 The rate of the forward reaction equals the rate of the
back reaction.
 The final equilibrium state is the same regardless of the
direction from which it is approached.
 When an equilibrium is disturbed the equilibrium tends to
move in a direction that minimises the disturbance. (Le
Chatelier’s principle). The disturbances include changes
in
o Concentration
o Pressure
o Volume
o Temperature
Equilibrium
expression
For any chemical reaction at equilibrium
aA + bB
cC + dD
The equilibrium constant (K) is
K = [C]c [D]d
[A]a [B]b
Q = [C]c [D]d
[A]a [B]b
Reaction quotient
If Q< K, the reaction goes from left to right until Q = K
If Q> K, the reaction goes from right to left until Q = K
If Q = K, the reaction is at equilibrium.
Writing equilibrium
expressions

The reaction quotient (and hence the equilibrium
expression) is always written with products (right-hand
side) in the numerator (top line) and the reactants (lefthand side) in the denominator (bottom line)
Q or K = [products]
[reactants]

The equilibrium expression always uses the coefficients
of the reaction as written in the balanced chemical
equation.
Units are not required for equilibrium constants but unless
stated concentrations are always in mol L-1.
Measuring
equilibrium
concentrations
The equilibrium concentrations and therefore the equilibrium
constant can be determined if the initial concentrations of the
reactants are known and the equilibrium concentration of one
reactant or product.
Using the equilibrium
constant



Uses of sulfuric acid
K can be used to decide whether a reaction is at
equilibrium
K can be used to calculate equilibrium concentrations
Changes in K with temperature changes can be used to
indicate if the reaction is exothermic or endothermic.
Sulfuric acid is produced more than any other chemical and
some of its uses include the manufacture of:
 Fertiliser
 Viscose rayon and other synthetic fibres
 Ethanol from ethene
 Pigments for paints, plastics and paper
 Detergents
 Explosives, drugs, dyes and pesticides
 Lead-acid batteries for cars
Sulfuric acid is also used in
 Steel processing
 Oil refining
 Extraction of metals from their ores
Frasch extraction of
sulfur
Sulfur is extracted from underground deposits of sulfur by the
Frasch process which relies on sulfur’s:
 Low melting point
 Low density
The industrial
synthesis of sulfuric
acid
This industrial process involves three steps
 Conversion of sulfur to sulfur dioxide
 Conversion of sulfur dioxide to sulfur trioxide
 Absorption of sulfur trioxide to form sulfuric acid
Properties of sulfuric
acid









Colourless
Viscose
Dense
Non-volatile
Boiling point 338oC
Density 1.84g mL-1
Dilution with water is a very exothermic reaction (-90kJ
mol-1)
Dilute sulfuric acid is a very strong acid (completely
dissociated)
Concentrated sulfuric acid (98%) is almost completely
associated.
Moderately strong oxidising agent
Cheapest of the common acids to produce.
Reactions of sulfuric
acid




Acid-base reactions
Precipitation reactions (barium and lead ions).
Dehydration
Oxidation
Safety precautions
when using sulfuric
acid




Always wear safety glasses
Wear protective gloves and a laboratory coat or apron
Work near a ready supply of running water
Have a supply of sodium carbonate or sodium hydrogen
carbonate nearby
Store the acid in glass bottles no more than one litre.
Avoid dribbling the acid down the side of the bottle


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Transport and storage


Concentrated sulfuric acid can be transported and store in
steel tankers but care must be taken to ensure that it does
not become contaminated with water.
Dilute sulfuric acid reacts vigorously with iron and steel
and should be transported in glass or plastic containers.
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