WJEC AS Chemistry- Chemical Equilibrium

Complete notes from new WJEC text book on chemical equilibrium

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  • Created on: 10-04-13 14:44
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Chemical Equilibrium
Dynamic Equilibrium: a system is in dynamic equilibrium when the forward and reverse
reactions occur at the same rate.
Not all reactions proceed to completion
Some end up with a mixture of reactants and products
This is because some reactions are reversible, where products can be converted
back into reactants.
Equilibrium means the balance between the forward and reverse reactions. At equilibrium
there is no observable change, as the backward and forward reactions are equal and
opposite, however the system is in constant motion. As fast as the reactants are
converted into products, the products are converted back into reactants. So the
concentrations of both stay constant.
Position of Equilibrium
Equilibrium only applies as long as the system remains isolated. In an isolated system, no
materials are being added or taken away and no external conditions are being altered.
The proportion of products to reactants in an equilibrium mixture is known as the position
of equilibrium.
This can be altered by changing:
Concentration of reactants or products
Pressure in reactions involving gases.
The effect of a change can be predicted using Le Chateliers Principle.
Le Chateliers Principle states that if a system at equilibrium is subjected to a
change, the equilibrium tends to shift so as to minimise the effect of the change.

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Effect of concentration
Consider the equilibrium
2CrO42-(aq) + 2H+ (aq) Cr2O72-(aq) + H20(l)
Adding more acid increases the concentration of H+ ions, so the system will try and
minimise this effect but decreasing the concentration of H+ ions and the position of
equilibrium will move to the right, forming more products (colour change from yellow to
orange.)
Effect of pressure and temperature
Consider the equilibrium
N2(g) + 3H2(g) 2NH3(g)
H = -92 KJ mol-1.…read more

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Acids and alkalis
-An Acid is a proton (H+) donor.
-An alkali (base) is a proton (H+) acceptor.
When acid is added to water, it releases protons.
When aqueous bases/alkalis are added to water, a hydroxide ion forms (OH-.)
Acids and alkalis react in neutralisation reactions. In solution the hydroxide ions from
alkalis neutralise the protons, and form water.
The pH scale
The acidity of a solution is the measure of aqueous hydrogen
ions. These concentrations are very small and vary between 1
and 0.…read more

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Sea Water
About half of the carbon dioxide formed by burning fossil fuels dissolves in the oceans.
The pH of sea water remains fairly constant at a value between 7.5 and 8.5.. Its pH is
maintained by the buffering action of dissolved carbon dioxide, hydrogencarbonate and
carbonate ions.
An increase in carbon dioxide will affect the equilibrium of the carbon dioxide-water
reaction, and push it to the right. This will increase the concentration of
hydrogencarbonate and hydrogen ions. This will decrease the pH.…read more

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Titration Calculations
Example 1
20.0cm3 of sulphuric acid was exactly neutralised by 24.0cm3 of 0.950 mol dm-3 aqueous
sodium hydroxide. Calculate the concentration of the acid,
H2SO4 + 2NaOH Na2SO4 + 2H20
a) Calculate the amount, in moles, of NaOH that reacted
n= c x v = 0.95 x 0.024 = 0.0228 moles
b) Use the equation to deduce the amount, in moles, of H2SO4 used
From the equation: 2 moles NaOH require 1 mole H2SO4
Actual amounts used 0.0228 moles NaOH require 0.…read more

Comments

Sophie Thomas

Absolutely fantastic.

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