- Created by: Nadine Norton
- Created on: 19-03-14 22:21
Qualitative analysis shows whether a particular substance is present, but does not give an indication of how much is present. Examples of qualitative analysis for metal ions include flame and precipitate tests. Examples for non-metal ions include halide and ammonium ion tests. These are used in medicine and the water industry.
Qualitative and quantitative analysis
Many different types of scientific analysis are used for a variety of reasons or in different situations. Doctors often complete analytical tests to determine the causes of illness, while forensic scientists do so to solve crimes.
Scientific analyses can be grouped into two categories:
Qualitative analysis - this type of test identifies whether a substance is present, but not how much of it exists. A common example would be using a pH indicator to see if an acid is present.
Quantitative analysis - this type of test identifies the amount of a substance that is present. A common example would be using an acid-base titration to determine exactly how much acid is present.
Testing for metal ions
Ionic substances can be identified by testing for the separate ions present. Compounds made from metals all have a metal ion (the ‘cation’) bonded with either a non-metal ion or compound ion (the ‘anion’). The test for any ion needs to be unique. Metal ions are identified using flame tests and precipitate tests.
When metals are placed into a flame they change colour. Different metals give different colours to the flame, so flame tests can be used to identify the presence of a particular metal. To carry out a flame test you would:
Clean a wire loop in dilute hydrochloric acid.
Dip it into the sample solution or solid.
Hold the loop at the edge of a Bunsen burner flame.
Observe the colour of the flame and use this to determine which metal ion is present.
Testing for metal ions
Barium - Pale green Calcium - Yellow / red Copper - Green / Blue Lithium - Red Sodium - Orange Potassium - Lilac
Transition metals form coloured compounds when they react with other elements. Many of these are soluble in water and so form coloured solutions. Others are not soluble and so form precipitates (the insoluble products of these reactions). To test for these metal ions, sodium hydroxide solution is added to them.
Here is the equation for copper sulfate solution reacting with sodium hydroxide solution:
copper sulphate + sodium hydroxide → copper hydroxide + sodium sulphate
CuSO4 + 2NaOH → Cu(OH)2 + Na2SO4
(blue solution + colourless solution → blue precipitate + colourless solution)
Testing for non-metal ions
Testing for ammonium ions
To test for ammonium ions (NH4+), sodium hydroxide solution is added to the test solution and gently heated. If ammonium ions are present, ammonia gas will be given off. This can be shown in the following equation:
ammonium ions + hydroxide ions → ammonia + water
NH4+ (aq) + OH- (aq) → NH3 (g) + H2O (l)
Ammonia has a characteristic sharp, choking smell. It makes damp red litmus paper turn blue. It forms a white smoke of ammonium chloride when it comes into contact with hydrogen chloride gas, from concentrated hydrochloric acid.
Testing for halide ions
To test for halide ions (ions of the group 7 halogen elements), dilute nitric acid and sliver nitrate solution are added to the test solution. Different coloured precipitates (insoluble products) will be formed to show the presence of different halide ions.
Testing drinking water and blood
Water is regularly tested by the companies that supply water to us. This is done so they can be sure that it is safe to drink. The water is monitored to make sure that it does not contain any contaminants, such as chloride or bromide ions. It is also monitored to check that it is not too hard. Limescale is produced when hard water is boiled – and a build-up of limescale can affect our washing machines and kettles.
Scientists test the water supply for magnesium, calcium, sodium and carbonate ions.
In order for us to remain healthy we need to have small amounts of minerals in our blood. Blood tests can be done to show the presence or absence of these minerals. Minerals have different functions. Some of the more important minerals and the role they play in maintaining good health :
Calcium ; Strong bones and teeth , Potassium: Helathy heart and blood pressure , Copper: Growth , Sodium : Sending electrical signals along nerves , Chlorides : Fluid balance , Iodides : Healthy thyroid
Some areas of our country have dissolved calcium ions (Ca2+) or magnesium ions (Mg2+) in the tap water. The presence of these ions makes water ‘hard’. The ions usually come from the layers of rocks the water passed through when it fell as rain.
It is difficult to form a lather with soap when you are washing your hands with hard water. This is because the metal ions in the water react with the soap to make an insoluble precipitate - which forms a scum on the surface of the water. This means lots of soap is wasted before any is used to clean your hands.
Temporary or permanent hardness
When you boil hard water containing calcium hydrogencarbonate (Ca(HCO3)2) or magnesium hydrogencarbonate (Mg(HCO3)2), a limescale precipitate is left behind (we can often see limescale on the heating element of a kettle). This kind of water can be ‘softened’ by boiling, and is known as temporary hard water. When you boil hard water containing calcium sulfate (CaSO4) or magnesium sulfate (MgSO4), it does not lose it hardness, so it is called permanently hard water. Permanently hard water can only be softened by passing the water through an ion exchange resin, where calcium and magnesium ions are swapped for sodium ions, or by adding sodium carbonate (Na2CO3) which precipitates out the Ca2+ and Mg2+ ions as an insoluble carbonate (eg CaCO3).
The amount of a solute dissolved in a known volume of solution is called its concentration.
The most commonly used units of concentration are grams or milligrams per cubic decimetre. These are abbreviated to g dm-3 or mg dm-3. One dm3 is the same volume as 1,000 cubic centimetres or one litre.
To calculate concentration, use the following equation:
concentration (g dm-3) = mass of solute (g) ÷ volume of solution (dm3)
All substances are made up from atoms. We can measure the amount of a substance using the number of atoms or mass (in grams). The masses of atoms of different elements vary, so we need to use Avogadro’s number to convert relative atomic masses of elements to numbers of atoms (or vice versa). Avogadro’s number is 6.02 x 1023 atoms. So, 6.02 x 1023 atoms of sodium would have a mass of 23 g (sodium’s relative atomic mass). This is also what scientists would call one mole of sodium. The same applies for compounds but relative formula mass is used in place of relative atomic mass. The relative formula mass of water is 18, so one mole of water has a mass of 18 g (and has 6.02 x 1023 water molecules).
The following equations show you how to calculate moles:
number of moles of atoms of an element = mass of element (g) ÷ relative atomic mass (g mol-1)
number of moles of a compound = mass of compound (g) ÷ relative formula mass (g mol-1)
Concentrations of solutions
concentration = mass of solute (g) ÷ volume of solution (dm3)
Concentration can also be expressed in moles per cubic decimetre (dm3). The following equation shows you how to calculate this: concentration (mol dm-3) = number of moles of solute ÷ volume of solution (dm3)
Measuring substances - Examples
Calculate the mass of 5 moles of water.
relative atomic mass of H = 1
relative atomic mass of O = 16
relative formula mass of H2O = 18
Rearrange the formula above to give:
relative formula mass (g mol-1) = number of moles of a compound x mass of compound (g)
5 x 18
Preparation of soluble salts
A salt is any compound formed by the neutralisation of an acid by a base. The first part of the name of the salt comes from the metal and the second part from the acid. Water is also produced. The equation below shows the formation of sodium chloride (the most abundant salt).
hydrochloric acid + sodium hydroxide → sodium chloride + water
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
If the base dissolves in water (meaning it is soluble), you need to add just enough acid to make a neutral solution. You could check this with universal indicator paper or an acid-base titration. The only other product - water - can then be evaporated to leave the salt crystals.
Soluble salts made from insoluble bases (those that don’t dissolve) are commonly transition metal oxides or hydroxides. Another step is needed in the formation of these salts.
You add the base to the acid until no more will dissolve and you have some base left over (called an excess). You filter the mixture to remove the excess base and then evaporate the water in the filtrate to leave the salt behind.
An acid-base titration is a neutralisation reaction where hydrogen ions (H+) from the acid react with hydroxide ions (OH-) from the base to form water. The concentration of the hydrogen ions determines the strength of the acid. This can be shown in this equation: H+ (aq) + OH- (aq) → H2O (l)
The steps involved in a titration are as follows:
A pipette is used to measure accurately a volume of an alkali of known concentration (usually 25 cm3). A safety pipette filler is used to draw solution into the pipette. This is emptied into a conical flask.
A few drops of an indicator are added to the conical flask. This will show a change of colour when the titration is complete.
The acid is placed in a burette.
Acid from the burette is slowly added to the conical flask. As the end-point approaches, the acid is added one drop at a time, swirling to mix. Eventually, a colour change shows that the correct amount has been added to neutralise the alkali completely.
The volume of solution added from the burette is noted. The titration results can then be used to calculate the concentration of the acid.
Acid-base titrations calculations
Take another look at this equation:
If we know the volume and concentration of one of the solutions in an acid-base titration and the volume of the second solution needed to reach neutralisation, the concentration of this second solution can be calculated.
20 cm3 of 0.1 mol dm-3 hydrochloric acid was needed to react completely with 25 cm3 of sodium hydroxide solution. What concentration is the sodium hydroxide solution?
If we rearrange the above equation:
number of moles of solute
= concentration (mol dm-3) x volume of solution (dm3)
number of moles of hydrochloric acid
= concentration of hydrochloric acid x volume used
= 0.1 x 20/1000 (to convert 20 cm3 into dm3)
= 0.002 mol
Acid-base titration calculations 2
Now we need to know the balanced equation for the reaction:
hydrochloric acid + sodium hydroxide → sodium chloride + water
HCl + NaOH → NaCl + H2O
The equation shows us that hydrochloric acid and sodium hydroxide react in a ratio of 1:1. Therefore 0.002 moles of hydrochloric acid reacts with 0.002 moles of sodium hydroxide.
Lastly we use the following equation again to work out the concentration:
concentration (mol dm-3)
= number of moles of solute ÷ volume of solution (dm3)
concentration of sodium hydroxide
= moles of sodium hydroxide ÷ volume of sodium hydroxide
= 0.002 ÷ 0.025
= 0.08 mol dm-3
To understand electrolysis, you need to know what an ionic substance is.
Ionic substances form when a metal reacts with a non-metal. They contain charged particles called ions. For example, sodium chloride forms when sodium reacts with chlorine. It contains positively charged sodium ions and negatively charged chloride ions. Ionic substances can be broken down by electricity. Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.
For electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance is dissolved in water or molten (melted). The solution or molten ionic compound is called an electrolyte.
For example, if electricity is passed through copper chloride solution, the copper chloride is broken down to form copper metal and chlorine gas.
Here is what happens during electrolysis:
Oxidation and reduction
At the negative electrode (cathode)
At the negative electrode (cathode), positively charged ions gain electrons. This is reduction, and you say that the ions have been reduced. Metal ions and hydrogen ions are positively charged. Whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the reactivity series.
The metal will be produced if it is less reactive than hydrogen.
Hydrogen will be produced if the metal is more reactive than hydrogen.
So the electrolysis of copper chloride solution produces copper at the negative electrode. But the electrolysis of sodium chloride solution produces hydrogen.
At the positive electrode (anode)
At the positive electrode (anode), negatively charged ions lose electrons. This is oxidation, and you say that the ions have been oxidised. The table summarises some of the elements you should expect to get during electrolysis.
Oxidation and reduction 2
Negative ion in solution Element given off at positive electrode Chloride, Cl– : Chlorine, Cl2 Bromide, Br– : Bromine, Br2 Iodide, I– : Iodine, I2 Sulfate, SO42 : Oxygen, O2
The table below shows some common ionic compounds, and the elements released when their solutions are electrolysed.
Ionic substance in solutionElement at the negative electrodeElement at the positive electrode Copper chloride, CuCl2 Copper Chlorine Copper sulfate, CuSO4 Copper Oxygen Sodium chloride, NaCl Hydrogen Chlorine Sodium sulfate Na2SO4 Hydrogen Oxygen Lead bromide (molten) PbBr2 Lead metal Bromine
Half equations of electrolysis
A half equation shows you what happens at one of the electrodes during electrolysis. Electrons are shown as e-. A half equation is balanced by adding, or taking away, a number of electrons equal to the total number of charges on the ions in the equation. At the negative electrode (cathode) : Positive ions gain electrons at the negative electrode, so are reduced.
In aluminium extraction: Al3+ + 3e- → Al
In copper purification: Cu2+ + 2e- → Cu
Electrolysis of sodium chloride solution: 2H+ + 2e- → H2
At the positive electrode (anode) : Negative ions or neutral atoms lose electrons at the positive electrode and are oxidised. For example, chlorine is produced during the electrolysis of sodium chloride solution:
2Cl- - 2e- → Cl2
This half equation can be rewritten as 2Cl- → Cl2 + 2e-
In aluminium extraction: 2O2- → O2 + 4e- . In copper purification: Cu → Cu2+ + 2e-
Electrolysis of sodium chloride
Brine is sodium chloride solution. If an electric current is passed through it, hydrogen gas forms at the negative electrode (cathode) and chlorine gas forms at the positive electrode (anode). A solution of sodium hydroxide forms. You might have expected sodium metal to be deposited at the negative electrode. But sodium is too reactive for this to happen, so hydrogen is given off instead (see Oxidation and Reaction section of this Revision Bite).
Sodium is not produced in the electrolysis of sea water as it is too reactive. It can be produced when molten sodium chloride is electrolysed. The products formed during this electrolysis are sodium metal and chlorine gas.
Sodium is used in street lamps and as a coolant in some nuclear reactors. It is obtained by the electrolysis of molten sodium chloride. Calcium chloride is added to sodium chloride to reduce the melting point. Chloride ions are oxidised at the anode and sodium ions are reduced at the cathode.
Copper is a good conductor of electricity, and is used extensively to make electrical wiring and components. The extraction of copper from copper ore is done by reduction with carbon. However, the copper produced is not pure enough for use as a conductor, so it is purified using electrolysis.
Electrolysis of copper
In this process, the positive electrode (anode) is made of the impure copper which is to be purified. The negative electrode (cathode) is a bar of pure copper. The two electrodes are placed in a solution of copper(II) sulfate.
The animation shows what happens when electrolysis begins. Copper ions leave the anode and are attracted to the cathode, where they are deposited as copper atoms. The pure copper cathode increases greatly in size, while the anode dwindles away. The impurities left behind at the anode form a sludge beneath it.
This process deposits a thin layer of metal on the object being protected. Tin plating requires the object being protected to be the negative terminal and surrounded by a solution of the ions of the metal being deposited on to the object. The negative charge attracts positive ions.
The iron then becomes coated in tin atoms.
The equation for this reaction is written as:
Sn2+ + 2e- → Sn
Note that you only need to know this equation if you are doing the Higher tier.
Electroplating is commonly used to improve the appearance of metal objects – for example, cheaper jewellery is often electroplated with a more expensive metal. It is also used to help prevent metal objects from corroding. Tin cans are actually steel cans coated in tin.
Molar volumes of gases
The particles in gases are able to move freely, so determining their volume can be difficult. The scientist Amedeo Avogadro (1776-1856) stated that one mole of any substance has 6.02 x 1023 particles. This is known as the Avogadro constant. We use this to work out the number of moles of any substance by using this equation:
number of moles = mass (grams) ÷ relative atomic (or formula) mass
Avogadro’s Law states that equal volumes of all gases, at the same temperature and pressure, contain the same number of particles. It also states that the volume of one mole of any gas (the molar volume) is 24 dm3 at room temperature and atmospheric pressure. We can prove this experimentally. : If 100 cm3 of 1 mol dm-3 hydrochloric acid is reacted with excess magnesium, 1.2 dm3 of hydrogen gas is produced. This could be collected and the volume measured in a gas syringe attached to the top of a conical flask. The balanced equation for this reaction is: Mg(s) + 2HCl(aq) → MgCl2 (aq) + H2(g)
We need to convert the volume of acid used from cm3 to dm3: 100 ÷ 1000 = 0.1 dm3
And then multiply this by the concentration used to determine the number of moles: 0.1 x 1 = 0.1 moles
The balanced equation shows us that the hydrochloric acid used and hydrogen produced are in a ratio of 2:1, so half of the moles above would be hydrogen: 0.1 ÷ 2 = 0.05 moles
The experimental result was that 1.2dm3 of hydrogen gas was produced, so the volume of one mole of hydrogen is: 1.2 ÷ 0.05 = 24 dm3
Molar volume of gases calculations
The molar volume is the volume occupied by one mole of a gas. The units used for the molar volume are l mol-1 (litres per mole). Calculating molar volume from mass:
976 cm3 of oxygen was found to have a mass of 1.3 g. Calculate the molar volume of oxygen under these conditions.
Remember that the molar volume is the volume occupied by one mole of oxygen, O2.
Relative formula mass of oxygen, O2, is 32.0 g
Volume of 1 g = 976 ÷ 1.3
So 32 g = (976 ÷ 1.3) × 32 = 24024 cm3 (= 24 dm3)
24.0 is the volume of 32.0 g of O2. So, the molar volume of oxygen is 24.0 l mol-1 under these conditions. The above can be carried out for other gases, under the same conditions of temperature and pressure. The molar volume is the same for all gases, at the same temperature and pressure.
Some reactions are able to go in two directions - forward and reverse. They are known as reversible reactions.
The forward and reverse reactions occur at the same time, and never stop. As a result, they are called dynamic reactions.
When the rate of the forward reaction is equal to the rate of the reverse reaction, the reaction is said to have reached equilibrium.
At equilibrium, the concentrations of the reactants and products are constant, but are not necessarily equal.
Example: the reaction of iron(III) ions with thiocyanate ions
Fe3+(aq) + CNS-(aq) FeCNS2+(aq)
Pale yellow iron(III) ions react with colourless thiocyanate (CNS) to produce red iron thiocyanate.
When there are more products than reactants present, the position of equilibrium lies to the right.
Dynamic equilibrium 2
Example: in ester formation, there are more products than reactants at equilibrium.
CH3COOH(l) + CH3OH(l) CH3COOCH3(l) + H2O(l)
In this example of ester formation, ethanoic acid reacts with methanol to produce methyl ethanoate (the ester) and water. The equilibrium lies to the right.
When there are more reactants than products present, the position of equilibrium lies to the left.
Example: in water, only a small proportion of the molecules have split to form ions at equilibrium.
H2O(l) H+(aq) + OH-(aq)
In this example of water, the equilibrium lies to the left. Only a few molecules have split to form ions.
It doesn't matter whether the reaction starts with 100 per cent reactants or 100 per cent products, the reaction will always reach the same equilibrium position.
Process of equilibrium showing the forward reaction and reverse reaction of iron with thiocyanate
Changing dynamic equilibria
Making changes to the concentration, pressure or temperature of a reaction can alter the position of the equilibrium. The rule is that any change made to a reaction which is in equilibrium will result in the equilibrium position moving to minimise the change made (Le Chatelier's principle).
Changes to the concentration of a reactant or product
When chlorine gas dissolves in water, the following equilibrium is produced:
Cl2(g) + H2O(l) Cl-(aq) + ClO-(aq) + 2H+(aq)
If potassium chloride (a source of chloride ions) is added to the equilibrium mixture, the equilibrium will shift to the left, to remove the chloride ions added. If potassium hydroxide is added, the hydroxide ions will react with the hydrogen ions and remove them from the mixture. The equilibrium will now move to the right, to replace the lost hydrogen ions.
Changes to temperature or pressure
A good example of how changes to temperature and pressure alter the position of equilibrium can be seen in the Haber process (the industrial manufacture of ammonia). During the manufacture of ammonia, the following equilibrium is present (the position of equilibrium lies to the left): N2(g) + 3H2(g) 2NH3(g)
Changing dynamic equilibria 2
In the Haber process, the forward reaction is an exothermic reaction. Although it is not a substance, in exothermic reactions heat can be imagined to be a product: N2(g) + 3H2(g) 2NH3(g) + heat
If the temperature is increased, then the equilibrium will shift to the left (the endothermic direction), to remove the extra heat added. This is why only a moderately high temperature (380 - 450°C) is used in the Haber process. Increasing the temperature always favours the endothermic reaction. Decreasing the temperature always favours the exothermic reaction.
Adding a catalyst
A catalyst reduces the time taken to reach equilibrium, but does not change the position of the equilibrium. This is because the catalyst increases the rates of the forward and reverse reactions by the same amount.
In the Haber process, the catalyst used is iron.
The Haber process is a continuous process. Ammonia is constantly being separated from the reaction mixture and the unreacted nitrogen and hydrogen are recycled back into the reaction vessel. As a result, equilibrium is never reached. Instead, the equilibrium is constantly shifting to the right, to replace the ammonia which has been removed.
Changing dynamic equilibria 3
Reactions in which the equilibrium mixture is made up of only liquids and/or solids will not be affected by changes in pressure. If there is at least one gas present in the equilibrium mixture, then a change in the pressure may affect the position of the equilibrium. Using the balanced equation, it can be seen that there are 4 volumes of reactants and only 2 volumes of product: N2(g) + 3H2(g) 2NH3(g)
1 volume of N2(g) and 3 volumes of H2(g), giving 2 volumes of NH3(g).
If the pressure is increased, the equilibrium will shift towards the right, creating more product. This is because the volume of the product is smaller than the volume of the reactants, and so the pressure will reduce to minimise the change.
If the pressure is reduced, the equilibrium will shift towards the left, resulting in more reactants. This is because the volume of the reactants is greater than the volume of the product, and so the pressure will increase to minimise the change.
In the Haber process, the actual pressure used is around 250 atmospheres, which favours more product.
High pressure always favours the side with the lowest volume of gases, while low pressure always favours the side with the highest volume of gases.
Ammonia and the Haber process
Ammonia (NH3) is a compound of nitrogen and hydrogen. It is a colourless gas with a choking smell, and a weak alkali which is very soluble in water . It is used to make fertilisers, explosives, dyes, household cleaners and nylon. And it is also the most important raw material in the manufacture of nitric acid. Ammonia is manufactured by combining nitrogen and hydrogen in an important industrial process called the Haber process.
The Haber process
The raw materials for this process are hydrogen and nitrogen. Hydrogen is obtained by reacting natural gas - methane - with steam, or through the cracking of oil. Nitrogen is obtained from air. Nitrogen and hydrogen will react together under these conditions:
a high temperature - about 450ºC
a high pressure - about 200 atmospheres (200 times normal pressure)
an iron catalyst
The reaction is reversible.
nitrogen + hydrogen ⇌ ammonia
N2(g) + 3H2(g) ⇌ 2NH3(g)
Ammonia and the Haber process
Fertilisers and eutrophication
Fertilisers make crops grow faster and bigger so that crop yields are increased. They are minerals, which must first dissolve in water so that plants can absorb them through their roots.
Fertilisers provide plants with the essential chemical elements needed for growth, particularly nitrogen, phosphorus and potassium. The proportions of these elements in a fertiliser are often shown as N:P:K = 15:30:15.
The name or formula of a compound often suggests which elements are provided by a particular fertiliser.
A major problem with the use of fertilisers occurs when they're washed off the land by rainwater into rivers and lakes.
The increase of nitrate or phosphate in the water encourages algae growth, which forms a bloom over the water surface. This prevents sunlight reaching other water plants, which then die. Bacteria break down the dead plants and use up the oxygen in the water so the lake may be left completely lifeless.
Glucose from plant material is converted into ethanol and carbon dioxide by fermentation. The enzymes found in yeast (single-celled fungi) are the natural catalysts that can make this process happen. Here are the word and balanced formulae equations: sugar → ethanol + carbon dioxide
Fermentation occurs in warm, anaerobic conditions. It is a slow process and several weeks or more are usually needed to produce an acceptable alcoholic drink.
Beer - The sugars for beer-making come from boiling barley in water. Hops are added to adjust the flavour of the beer. Beer typically has an ethanol concentration of 3-6 per cent.
Wine - The sugars for wine-making come from grape juice. Different varieties of grapes are used to produce wines with different flavours. Wine contains a higher proportion of ethanol than beer because grape juice contains a higher concentration of sugars than barley in water. Wines typically have an ethanol concentration of 11-14 per cent.
Spirits - Some alcoholic drinks are distilled after fermentation. This produces a higher concentration of ethanol. Examples include tequila, vodka and whisky (all of which have an ethanol concentration of around 40 per cent). If alcoholic drinks have had no added sugar and have an ethanol concentration of above 20 per cent they are called spirits.
Ethanol and society
Ethanol is a depressant - it slows down signals in the nerves and brain. Small amounts of alcohol help people to relax, but greater amounts lead to a lack of self-control and loss of judgement. Drinkers may put themselves in dangerous situations, and may not realise how much they are drinking. They may fall ill, or become unconscious.
Ethanol is also toxic. Yeast are actually killed by the ethanol they produce. Long-term effects of drinking excessive amounts of alcoholic drinks include damage to certain parts of the body. It may also cause weight gain, and is addictive.
Anti-social behaviour - such as fighting and vandalism - can be triggered by people consuming too much alcohol. People who drive or use machinery when drunk can cause accidents, while people suffering from 'hangovers' may be unable to function normally the next day. This puts a strain on family and colleagues, and can lead to problems (such as domestic violence and family breakdown), lost work and missed business.
The ethanol in alcoholic drinks causes damage to the liver and brain, and leads to an increased risk of stroke or heart disease. This causes problems both for the drinker, and for those around them. Such diseases require the resources of the Health Service, which means it has less to spend on treating other illnesses
Distillation is a process that can be used to separate a pure liquid from a mixture of liquids. It works when the liquids have different boiling points. Distillation is commonly used to separate ethanol - the alcohol in alcoholic drinks - from water.
The thermometer shows the boiling point of the pure ethanol liquid. When all the ethanol has evaporated from the solution, the temperature rises and the water evaporates.
This is the sequence of events in distillation:
heating → evaporating → cooling → condensing
Ethanol is the only product. This is a continuous reaction - as long as ethene and steam are fed into the reacting vessel, ethanol will be produced.
Ethanol produced by fermentation of carbohydrates also only produces a low concentration of ethanol. Further processing (known as ‘distillation’) is often needed.
Dehydration of ethanol
You need to know the formula and properties of the following homologous series:
These compounds only contain hydrogen and carbon and possess no double bonds. They are saturated (by having only carbon single bonds, they have bonded with the maximum number of atoms). Their general formula is CnH2n+2
As alkanes increase in chain length (with an increase in the number of carbon atoms), their boiling points increase. Methane has a boiling point of -164oC, while butane has a boiling point of 0oC. Longer alkanes than butane are liquids at room temperature. The longest alkanes are solids.
These compounds also only contain hydrogen and carbon but do possess one or more double bonds. They are unsaturated (by possessing one or more carbon double bonds, they have not bonded with the maximum number of atoms). Their general formula is CnH2n.
As alkenes increase in chain length, their boiling points increase. Ethene has a boiling point of -104°C, while butene has a boiling point of around 0°C. Longer alkenes than butene are liquids and solids at room temperature.
These compounds contain hydrogen and carbon but also possess one hydroxyl group (-OH). Their general formula is CnH(2n+1)OH. As alcohols increase in chain length, their boiling points increase and their solubility in water decreases.
These compounds contain hydrogen and carbon but also possess one carboxyl group (-COOH). Their general formula is CnH2O2. As carboxylic acids increase in chain length, they change from a colourless liquid to a waxy solid. Their odour becomes less strong and their solubility in water decreases as well.
Ethanoic acid is a carboxylic acid with a formula of CH3COOH . This compound is formed when ethanol is oxidised. This commonly occurs when a bottle of wine is left open for a few days. The ethanol reacts with the oxygen in air in the following reaction: ethanol + oxygen → ethanoic acid + water C2H5OH (l) + O2 (g) → CH3COOH (aq) + H2O (l)
Ethanoic acid is the active ingredient in vinegar. It gives it its tangy, sharp taste and so is often used to flavour food. It can also act as a preservative, so some foods are stored in vinegar. This is called ‘pickling’ and works because bacteria cannot survive in this acidic environment.
Ethanoic acid is a ‘typical’ acid. This means it:
reacts with some metals to produce hydrogen
Ethanoic acid reacts with ethanol to form an ester (ethyl ethanoate) and water in the following reaction:
ethanoic acid + ethanol ethyl ethanoate + water
CH3COOH (l) + C2H5OH (l) CH3COOC2H5 (l) + H2O (l)
Esters and their formation
Esters and their formation
A general formula to represent the formation of esters is:
organic acid + alcohol ester + water
Properties of esters
Esters and their formation
Uses of esters
Some esters are used in perfumes due to their strong, pleasant smell.
Some are used as fruit flavourings in cooking ingredients.
Some are used in cosmetics.
They can be made into very long chains which contain thousands of esters joined together. These are called polyesters (‘poly’ means ‘many’). These chains are made into fibres and woven into fabrics. If plastic bottles have been made from polyesters they can be recycled and turned into lots of different things including fleece clothing.
Oils, fats and soaps
Fats and oils are esters. They are made from long carbon chain carboxylic acids. The only difference between fats and oils is their melting point - fats are solid at room temperature, whilst oils are liquids. Fats can be saturated (contain no carbon double bonds), monounsaturated (contain only one carbon double bond) or polyunsaturated (contain more than one carbon double bond). The more double bonds a fat possesses, the lower the melting point it has. So, oils tend to be unsaturated while fats tend to be saturated.
Oils, fats and soaps
Catalytic hydrogenation is the reaction of an unsaturated hydrocarbon with hydrogen to make a saturated hydrocarbon. This removes any double carbon bonds. This reaction can be used to make a hydrogenated oil - the main ingredient in margarine.
Margarine is made by bubbling hydrogen gas through vegetable oils in the presence of a nickel catalyst.
Making soap : Fats and oils can be broken down by boiling with concentrated alkali solution to produce soaps. These are sodium or potassium salts of long carbon chain carboxylic acids.
Therefore, the hydrophobic ends of the soap molecules bury themselves into the oily dirt on clothes or dishes. This leaves only the hydrophilic ends of the molecules in the water. This means the droplets of oily dirt can be more easily washed away.