When a solid dissolves in a liquid:
Solubility is defined in terms of the maximum mass of a solute that dissolves in 100g of solvent. The mass depends on the temperature.
For example, the solubility of sodium chloride (NaCl) in water at 25⁰C is about 36g per 100g of water.
The solubility of solids changes as temperature changes. This can be plotted on a solubility curve.
The salts shown on this graph are typical: the solubility increases as temperature increases.
For example, the graph above shows that in 100g of water at 50⁰C the maximum mass of potassium nitrate (KNO₃) which will dissolve is 80g.
However, if the temperature were 80⁰C a mass of 160g of potassium nitrate (KNO₃) would dissolve in 100g of water.
At a chosen temperature (e.g. 40⁰C) a saturated solution is created of potassium nitrate (KNO₃) for example.
Some of this solution (not any residual solid) is poured off and weighed. The water is then evaporated from this solution to leave a residue of potassium nitrate which is then weighed.
The difference between the two measured masses is the mass of evaporated water.
The solubility, in grams per 100g of water, is equal to 100 times the mass of potassium nitrate residue divided by the mass of evaporated water.
Pure substances, such as an element or a compound, melt and boil at fixed temperatures.
However, mixtures melt and boil over a range of temperatures.
Example: although pure water boils at 100⁰C, the addition of 10g of sodium chloride (NaCl) to 1000cm³ of water will raise the boiling point to 100.2⁰C.
Example: although pure water melts at 0⁰C, the addition of 10g of sodium chloride (NaCl) to 1000cm³ of water will lower the melting point to -0.6⁰C.
The higher up we go in group 7 (halogens) of the periodic table, the more reactive the element. The explanation concerns how readily these elements form ions, by attracting a passing electron to fill the outer shell.
In fluorine the outer electron shell is very close to the positively charged nucleus, so the attraction between this nucleus and the negatively charged electrons is very strong. This means fluorine is very reactive indeed.
However, for iodine the outer electron shell is much further from the nucleus so the attraction is weaker. This means iodine is less reactive.
Most metals are found in the Earth’s crust combined with other elements. Such compounds are found in rocks called ore, rocks from which it is worthwhile to extract a metal.
A few very unreactive metals, such as gold, are found native which means they are found in the Earth’s crust as the uncombined element.
An alloy is a mixture of a metal with, usually, other metals or carbon.
For example, brass is a alloy of copper and zinc, and steel is an alloy of iron and carbon.
Alloys are harder than the individual pure metals from which they are made.
In an alloy, the different elements have slightly different sized atoms. This breaks up the regular lattice arrangement and makes it more difficult for layers of ions to slide over each other.
An acid is a proton (H⁺) donor.
A base is a proton (H⁺) acceptor.
A proton is the same as a hydrogen ion. A good way to think about that is to realise that a hydrogen atom is just one proton and zero neutrons surrounded by only one electron. If that atom becomes an ion by the removal of the electron, then only one proton is left.
When sulfuric acid reacts with copper (II) oxide (CuO):
Cu²⁺O²⁻ (s) + H₂SO₄ (aq) → Cu²⁺ (aq) + SO₄²⁻ (aq) + H₂O (l)
H₂SO₄ is an acid. It donates protons (H⁺) to CuO, the base.
An acid is a proton donor.
A base is a proton acceptor.
A proton is the same as a hydrogen ion. A good way to think about that is to realise that a hydrogen atom is just one proton and zero neutrons surrounded by only one electron. If that atom becomes an ion by the removal of the electron, then only one proton is left.
The member of the homologous series called Alcohols have names which end in “ol”. Examples are methanol, ethanol and propanol.
Alcohols all contain an -OH functional group attached to a hydrocarbon chain.
Structural formula and displayed formula for methanol:
CH₃-OH
Structural formula and displayed formula for ethanol:
CH₃-CH₂-OH (or simply C₂H₅OH)
Structural formula and displayed formula for propan-1-ol:
CH₃-CH₂-CH₂-OH
Structural formula and displayed formula for butan-1-ol:
CH₃-CH₂-CH₂-CH₂-OH
This video introduces alcohols:
1) Ethanol can be oxidised by complete combustion. With excess oxygen the complete combustion of ethanol (C₂H₅OH) in air produces carbon dioxide and water:
C₂H₅OH (l) + 3O₂ (g) → 2CO₂ (g) + 3H₂O (l)
2) Ethanol can be oxidised in air in the presence of microorganisms (‘microbial oxidation’) to form ethanoic acid (CH₃COOH).
3) Ethanol can be oxidised by heating with the oxidising agent potassium dichromate(VI) (K₂Cr₂O₇) in dilute sulfuric acid (H₂SO₄).
In the equation below, [O] means oxygen from an oxidising agent.
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
This mixture starts orange but when the reaction happens turns green which indicates the presence of Cr³⁺ ions which are formed when the potassium dichromate(VI) is reduced.
In the production of ethanol the process of fermentation is carried out at a low temperature (30⁰-40⁰).
Above 40⁰ the enzymes would permanently lose their structure (denature).
At a temperature lower than 30⁰ the process would be too slow.
Fermentation is conducted in the absence of air. In the presence of air (aerobic conditions), enzymes in the yeast produce carbon dioxide and water instead of ethanol.
Also, in the presence of air, the ethanol can oxidise to ethanoic acid.
Here’s a great video which shows the 2 ways to make ethanol.
Carboxylic acids contain the functional group -COOH
An example of a carboxylic acid is butanoic acid:
The four simplest carboxylic acids are methanoic acid, ethanoic acid, propanoic acid and butanoic acid.
Methanoic acid
Displayed formula:
Molecular formula: CH₂O₂
Structural formula: HCOOH
Ethanoic acid
Displayed formula:
Molecular formula: C₂H₄O₂
Structural formula: CH₃COOH
Propanoic acid
Displayed formula:
Molecular formula: C₃H₆O₂
Structural formula: CH₃CH₂COOH
Butanoic acid
Displayed formula:
Molecular formula: C₄H₈O₂
Structural formula: CH₃CH₂CH₂COOH
This video introduces carboxylic acids:
Dilute carboxylic acids react with metals in the same way as other dilute acids (e.g. hydrochloric acid) only more slowly.
For example, dilute ethanoic acid reacts with magnesium with a lot of fizzing to produce a salt and hydrogen, leaving a colourless solution of magnesium ethanoate:
magnesium + ethanoic acid → magnesium ethanoate + hydrogen
Mg (s) + 2CH₃COOH (aq) → (CH₃COO)₂Mg (aq) + H₂ (g)
Dilute carboxylic acids react with metal carbonates as they do with other acids, to give a salt, carbon dioxide and water.
For example, dilute ethanoic acid reacts with sodium carbonate with a lot of fizzing to produce a salt, carbon dioxide and water, leaving a colourless solution of sodium ethanoate:
sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water
Na₂CO₃ (s) + 2CH₃COOH (aq) → 2CH₃COONa (aq) + CO₂ (g) + H₂O (l)
As can be seen in the examples above the charge on the ethanoate ion is -1.
Vinegar is an aqueous solution containing ethanoic acid (CH₃COOH).
Esters contain the functional group -COO-
An example of an ester is ethyl ethanoate:
Ethyl ethanoate is the ester produced when ethanol and ethanoic acid react in the presence of an acid catalyst.
ethanoic acid + ethanol ⇋ ethyl ethanoate + water
CH₃COOH (l) + CH₃CH₂OH (l) ⇋ CH₃COOCH₂CH₃ (l) + H₂O (l)
The ethyl ethanoate produced is an ester.
The reaction is called esterification.
The reaction can also be described as a condensation reaction because water is made when two molecules join together.
Ethyl ethanoate is an ester.
Structural formula: CH₃COOCH₂CH₃
Displayed formula:
To work out the structure of the ester formed when an alcohol reacts with a carboxylic acid, it is easiest to first draw the structures of alcohol and acid and then remove the H₂O to see what is left when the molecules join.
For example, propan-1-ol and ethanoic acid react together to form propyl ethanoate and water.
propan-1-ol + ethanoic acid → propyl ethanoate + water
CH₃CH₂CH₂OH (l) + CH₃COOH (l) → CH₃COOCH₂CH₂CH₃ (l) + H₂O (l)
The displayed formula for esters is typically written to show the part which came from the carboxylic acid on the left:
This is still called propyl ethanoate: the alcohol bit of the name (propyl) comes first and then the carboxylic acid bit (ethanoate), even though the displayed formula is typically written the other way around. Notice that the structural formulae for esters is also typically written with the carboxylic acid bit first, so propyl ethanoate is CH₃COOCH₂CH₂CH₃.
Esters are volatile compounds with distinctive smells. A volatile liquid is one that turns into to a vapour easily.
Esters are used as food flavourings and in perfumes.
Esters are often described as having a sweet, fruity smell. They typically smell of bananas, raspberries, pears or other fruit because esters occur in all these natural products.
Heating a mixture of ethanoic acid and ethanol produces a liquid called ethyl ethanoate. A few drops of concentrated sulfuric acid must be added for the reaction to work. The sulfuric acid acts as a catalyst.
ethanoic acid + ethanol ⇋ ethyl ethanoate + water
CH₃COOH (l) + CH₃CH₂OH (l) ⇋ CH₃COOCH₂CH₃ (l) + H₂O (l)
The ethyl ethanoate produced is an ester.
The reaction is called esterification.
The reaction can also be described as a condensation reaction because water is made when two molecules join together.
Notice that the reaction is reversible. Pure reactants are used to maximise the yield of ethyl ethanoate. Pure ethanoic acid is called glacial ethanoic acid.
This video introduces addition polymers:
Condensation polymers are formed by a condensation reaction.
These polymers are formed by the combination of two different monomers, such as a dicarboxylic acid and a diol.
When these particular monomers join in an alternating pattern they form a long polymer called a polyester. Where each monomer joins to the next, a separate molecule of water is also produced.
Polyesters are polymers formed when two types of monomer join together alternately. Where each joins to the next a small molecule, such as water or hydrogen chloride, is lost. This is called a condensation polymerisation reaction.
One of the monomers is a diol, an alcohol with a -OH functional group at each end. An example is hexane-1,6-diol which has the structural formula CH₂OHCH₂CH₂CH₂CH₂CH₂OH and the displayed formula:
Since it is only the -OH functional groups which are important for polymerisation, this can we re-written with the central block of carbons represented as a block:
The other monomer is a dicarboxylic acid, a molecule with a -COOH functional group at each end. An example is hexane-1,6-dioic acid which has the structural formula HOOCCH₂CH₂CH₂CH₂COOH and the displayed formula:
Since it is only the -COOH functional groups which are important for polymerisation, this can we re-written with the central block of 4 carbons represented as a block:
These two different types of monomer (the diol and the dicarboxylic acid) can join to form a polymer with the loss of a water molecule at every bond. As above, this can be simplified by only looking at the functional groups and representing the other carbons as blocks, so the whole process looks like:
A simple example of this is the condensation polymerisation reaction between ethanedioic acid and ethandiol:
This video introduces the idea of condensation polymers:
There are environmental issues with the disposal of condensation polymers, though because of their ester linkage the issues are not as severe as with addition polymers. Normally condensation polymers can take hundreds of years to break down, but chemists has developed biopolyesters which break down much more quickly.