Chemistry Unit C3.3 and C3.4: Edexcel Triple Science

Analytical chemists check purity of water throughout its treatment. Two main types:

• Qualitative analysis: investigates chemicals present in a sample;
• Quantitative analysis: measures amount of each chemical present

Pollution that contaminates water can come from a number of sources:

• Industry: large amounts water and discharges toxic chemicals in its waste
• Fertilisers: contaminate rivers and lakes
• Detergents: washed down the drains contaminates local water systems

Ions: atoms that have lost or gained electrons
Ionic substances: formed between metals and non-metals 

• Metals form positive ions called cations by losing electrons
• Non-metals usually form negative ions called anions

Flame tests: qualitative analysis techniques

• Clean piece of wire dipped in solution compound in hot Bunsen flame
• Colour produces indicates the metal ions present
• Colours produces as electron in atoms move between the energy levels
• Different metals have different electron arrangements different energy levels
• Different metals give off different coloures of light

• Calcium Ca2+ brick red
• Sodium Na+ yellow
• Potassium K+  lilac
• Copper Cu2+ green

Add sodium hydroxide solution to a solution of the unknown sample

Metal ions are insoluble so a precipitation reaction can occur

Precipitate formed can identify the ion present 

• Ammonium NH4+ none
• Aluminium Al3+ white
• Calcium Ca2+ white
• Copper(II) Cu2+ blue
• Iron(II) Fe2+ green
• Iron(III) Fe3+ brown (rust)

Precipitate will be a solid hydroxide of the cation. 

Aluminium and Calcium distinguised by adding excess sodium hydroxide

• In excess sodium hydroxide, calcium precipitate is not changed
• The aluminium precipitate starts to dissolve 

To test for the ammonium (NH4+) ion:

• heat the unknown sample with concentrated sodium hydroxide
• if the ions are present the smelly alkaline gas called ammonia will be given off
• this is detected as it turns universal indicated paper blue

Sulphur dioxide nitrogen dioxide released by burning fossil fuels make water acidic

Acidic solutions contain excess H+ ions which is detected using indicators

• Litmus and universal indicator both turn red in acids
• All acids react with reactive metals (magnesium, zinc and iron)
• Hydrogen and a salt is made
• Test for hydrogen is a squeaky pop

Acids also react with metal carbonates

• acid+ metal carbonate ---> salt + water + carbon dioxide
• carbon dioxide is tested as it turns limewater cloudy

Formulae of common acids:

Hydrochloric acid: HCl Sulphuric acid: H2SO4 Nitric acid: HNO3 

Phosphoric acid: H3PO4

If indicator turns blue/purple then hydroxide ions (OH-) are present

If they aren't present, add hydrochloric acid:

• Bubbles of gas will be produced if carbonate ions or sulphite ions are present;
• hydrochloric acid + sodium carbonate > sodium chloride + CO2+ H2O
• hydrochloric acid + sodium sulphite > sodium chloride + SO2+ H2O

You can then identify which gas is formed and ion present using tests:

• Gas turns limewater milky it is Carbon dioxide and carbonate (CO3-2) 
• Gas is acidic and choking smell it is Sulphur dioxide sulphite (SO3-2)

SO4-2 Test for Sulphate ions uses precipitation reaction:

• Barium chloride solution added to sulphate
• White precipitate of barium sulphate forms

Carbonates and Sulphites also produce precipitate with barium chloride:

• To be sure which ion is present you add hydrochloric acid;
• No effect on Sulphates 
• But dissolves white precipitate of Carbonate and Sulphites

Halide ions (Cl-,Br-,I-) identified using silver nitrate solution acidified with nitric acid

• A different colored precipitate of the silver halide forms in each case:
• Cl-: white
• Br-: cream
• I-: yellow
• Dilute nitric acid used remove any carbonate sulphite hydroxide ions present as they also produce precipitate with silver nitrate

Test 1:

• carry out flame tests
• indigestion tablets(sodium): yellow
• fertilizer: none
• moss killer: none
• sterilizer(potassium): lilac

Test 2: 

• add sodium hydroxide to each solution and seeing precipitates or gases

Test 3: 

• test for anions by adding hydrochloric acid and testing gas formed

Test 4:

• addition of barium chloride and acidified silver nitrate

• Carbonate CO3-2
• Sulphite SO3-2
• Sulphate SO4-2

Quantitative analysis:

• 1dm^3= 1000cm^3= 1000ml= 1 litre

Hard water:

• ore dissolve solids like calcium ion compounds
• soap forms a scum in hard water
• hard water produces limescale
• the concentration of calcium carbonate in milligrams per cubic decimeter are converted into Clark's degrees to see how hard the water is:
• 0 to 99.9 (0 to 6.9 Clark's): soft
• 400 to 499.9 (28 to 34.9 Clark's): very hard

Relative atomic masses tell us approximately how much heavier one atom of the element is compared to a hydrogen atom 

Relative formula mass of an element is found by adding the relative atomic mass values of all the atoms in the formula together

A mole of a substance is equal to its relative formula mass in grams 

Mass = number of moles x Mr

m (g)= n (mol) x Mr (mass of 1 mole)

Mole is Avogadro Constant: equal to 6 x 10^23 particles per mole (mol-1)

Acids can be neutralised by a base to form a salt and water

If you don't have enough of the reactant there will be some left so the solution at the end will not be neutral so you need to calculate the correct amounts reactant

• 1. Write the balanced formula equation
• 2. Find the mole ratios of substances in the question
• 3. Change the units as required
• 4. Solve the problem

• At the same temperature, and pressure, equal volumes of gases will contains equal numbers of gas particles 
• Gases obey Avogadro's law because distances between molecules much bigger than in solids or liquids
• This means volume of gas depends on distance between molecules not the size:

Example

• 60cm^2 ammonia gas completely decomposed: what could be the total final volume of gases produced:
• 2NH3>N2+3H2
• Ratio of molecules is 2 molecules NH3> 1 N2+ 3 H2 
• Ratio of volumes is 2> 1+ 3 
• Substituting the volume of gas used in the question:
• 60cm^3 NH3 > 1/2 x 60cm^3 N2 + 3/2 x 60cm^3 H2 
• = 30 cm^3 N2 + 90cm^3 H2 
• Total final volume of gas is 120cm^3

One mole of any gas must have the same volume and this is called the molar vol

number of moles (mol) = volume of gas (dm^3)/ molar volume 

n= V/Vm 

At normal room temp and pressure the molar gas volume is about 24dm^3

Convert all measurements to cm^3 

Worked example:

• Mass of magnesium = 0.12g Volume of hydrogen = 118 cm^3
• Mg + H2SO4> MgSO4 + H2 1 mole Mg = 1 mole H2
• 1 mole Mg = 24g 
• 0.12g Mg = 0.12/24 g = 0.005 moles
• 0.005 moles of H2 = 118 cm^3 H2  1 mole of H2 = 118/ 0.005
• Molar volume= 23600cm^2 (23.6 dm^3)

g dm^-3 = mol dm^-3 x Mr 

Volumetric analysis through titration:

• Pipette used to accurately measure a set volume of alkali into a conical flask
• An indicator is added to the alkali in the conical flask
• the acid is placed in a burette during titiation 
• the acid in the burette slowly added to alkali in conical flask until the indicator changes colour
• the volume of acid solution added is then noted by reading the scale on the burette

In volumetric analysis we need to use solutions of an exact concentration: standard solutions:

• calculate mass of solute required
• weigh out exact mass of solute using balance
• dissolve solute in a small amount of water in a beaker
• transfer solution to volumetric flask
• wash beaker with distilled water 
• transfer washings to the flask
• add distilled water to volumetric flask to make up to graduation mark
• stopper flask and invert two or three times to mix 

number of moles (mol) = concentration (mol dm^-3) x volume of solution (dm^3)

n = C x V 

The steps involved in using standard sodium carbonate solution to analyse hydrochloric acid are outlined below:

• Use pipette to measure 25cm^3 of standard sodium carbonate in conical flask and add indicator 
• Fill a burette with the hydrochloric acid and note the level of the acid in the burette
• Add the acid to the solution until indicator changes colour and note the new level of the acid calculating how much was used
• Do this 3 times and work out an average of the amount of acid used
• This is a titration 

Properties of metals:

• sonorous (they make a ringing sound when hit)
• heavy to move around and have high density
• metals can be bent into complicated shapes (malleable)
• steal added to metals to make it stronger
• metals are also ductile 
• electrical cables made of copper because it is a good conductor

Transition metals are used as catalysts.

• Platinum metal catalytic converter of a car exhaust speeds up conversion reaction of exhaust gases 
• Iron is also used as a catalyst in the Haber process for the manufacture of ammonia.

Compounds transition metals are pigments and dyes:

• Pigments are compounds that are insolube
• Soluble compounds are used as dyes 
• Powdered pigment mixed with liquid to form suspension (paint) 
• Emulsion paints pigment dispersed in water with polymer (PVC acrylic resin)
• When water evaporates, pigment is left on the surface 
• Gloss paints use an oil-based liquid which is why the brushes have to be washed with white spirit after use rather than water.
• The paint will include solvent which evaporate as the paint dries and other chemicals that harden as they react with oxygen in the air (oxidise) to give the paint a hard-wearing finish. 

Dyes are chemicals which are soluble and form chemical bond to fabric

• Some dyes bond directly to fabric, others require mordant to bond
• Dyes made from natural organic sources 
• Raw material for these dyes coal or oil (non-renewable) fuels 
• Processes used to make the dyes involve corrosive chemicals and some are carcinogenic that can cause cancer.
• In the future we may have to depend more on renewable biomass

Cosmetics contain active ingredients and alcohol solvents 

• Alcohol can be distilled from natural products
• Alcohol means carbon compounds that contain a hydroxyl (OH) group
• A group of atoms like this is called a functional group 
• All alcohols have functional group of an OH group bound to carbon atom 

Alcohols can mix with natural oils and water

• Oils and water are immiscible and do not mix together
• The alcohol allows us to make products where the components are miscible 
• The longer the carbon chain, the less likely the alcohol is to mix with water 
• Compounds contain same functional group different carbon chain length are known as homologous series

Glycerol useful alcohol three carbon atoms and three hydroxyl groups: it mixes well with water and natural oils and so is used in skin cream

• Ethanol C2H5OH 
• Propanol C3H7OH

Ethanol and oxygen makes ethanoic acid:

• ethanoic acid is an organic acid (carboxylic acid) 
• organic acids are carbon compounds containing a carboxyl group (-COOH)
• organic acid react with alcohols to form an ester 

Esters

• esters occur naturally in fruits and use as flavourings and scents
• also used as solvents nail varnish
• board markers 
• ethanol and ethanoic acid dissolve well in water and are miscible
• esters do not mix with water and are immiscible 

Batteries: transform energy chemical reaction to electrical

Cells: chemical inside cell react so it pumps electrons around circuit

• A simple wet cell made placing zinc copper plate into beaker of conducting solution (dilute sulphuric acid)
• Wet cell like this not practical quite large: dry cell uses moist conduct paste

Dry cell

• case and bottom made of zinc 
• zinc atoms turn into ions release their electrons to travel around the circuit
• after electrons traveled around circuit they enter cell at metal cap
• chemical manganese oxide absorbs electrons
• between positive and negative poles there is a paste of an electrolyte (ammonium chloride) made of ions kept damp so ions can move 
• metal negative pole reacts too much the case too thin and cell will start leaking 

Alkaline cells use potassium hydroxide as the electrolyte 

Carbon electrodes do not usually react- they are inert

Electrolyte has to be in liquid form for electrolysis to take place

• metals and hydrogen will be formed at the negative electrode cathode
• non metals will be formed at the positive electrode cathode

Impure copper (anode) with copper sulphate (electrolyte) pure copper (cathode)

• Copper anode reacts dissolves into solution to form copper ions
• Equal amount copper deposited from electrolyte onto cathode
• This builds up a deposit of pure copper and impurities from anode fall to the bottom of the liquid

Electroplating: thin layer of metal coated on another 

• Object to be plated made cathode in electrolysis cell 
• Electrolyte solution contains metal to be plated
• Anode will also be made of the metal which is to make up the coating 

Copper purification: current flows positive metal ions attracted to cathode 

• Copper ion picks up two electrons from the negatively charged cathode 
• Ion turns into atom and solid copper deposited on cathode 
• Cu2+ + 2e-> Cu (solid atom)
• Cu- 2e-> Cu2+ (ion)

Burning magnesium oxidisation:

• Magnesium + oxygen> magnesium oxide
• Magnesium atoms turn into magnesium ions lose two electrons
• Iron oxide> iron = reduction reaction 
• metal ion converted into a metal atom 
• In this case: Fe3+ + 3e- > Fe

Electrolysis of potash made alkali metals: convert potassium ions into potassium atoms:

• Alkali metals are soft 
• Melting points are lower than other metals
• Alkali metal + water > metal hydroxide + hydrogen 

Glass

• Heat silicon dioxide (SiO2) sand until it melts (giant structure strong bonds)
• Add sodium carbonate (Na2CO3) lowers melting point to 1000 C 
• Soda glass is much easier to make not strong dissolve in water 
• Adding calcium oxide lime improves strength (soda-lime glass most common)
• Crystal glass: additives potassium and lead oxide (changes light refraction)
• Pyrex glass is heat resistant boron added to glass (low expansion rate preventing cracks for forming)

Transition metal compounds give colours to glass:

• Iron (II) oxide and chromium = dark green
• Copper oxide = turquoise
• Cobalt = blue
• Copper gold = deep red 

Sodium hydroxide electrolysis salt water (brine) 

• Sodium hydroxide formed at cathode. Positive sodium ions attracted to the negative electrode. Water molecules are decomposed in this reaction. 
• 2Na+ (aq) + 2H2O (l) + 2e- > 2NaOH (aq) + H2 (g)
• At the anode chlorine gas is produced. Some chlorine reacts with the water to make it acidic.
• 2Cl-(aq) - 2e-> Cl2(aq)
• Chlorine bleach takes colour out of indicator. 
• Two halves of reaction separate otherwise chlorine and sodium hydroxide would react with each other. 

Uses:

• Make soap, oven cleaners viscose fibers
• Viscose fibers (rayon) made from cellulose plant material (wood) 
• Sodium hydroxide used to react with wood break down chemicals 
• Cellulose now used to  turn into pure form to be spun into fibres 

• Fertilisers: used to make superphosphate fertiliser from rock phosphate (calcium phosphate)  and make ammonium sulphate (contains nitrogen)
• Paints: natural ore ilmenite contains titanium and iron. To extract titanium ilmenite reacted with sulphuric acid to make iron sulphate and titanyl sulphate. Titanyl sulphate heated to form titanium dioxide. 
• titanyl sulphate + steam > titanium dioxide + sulphuric acid
• Plastics
• Detergents
• Fibres

Plants take minerals through roots. Calcium phosphate insoluble so when it reacts with sulphuric acid it is converted into calcium hydrogen phosphate that the plant can absorb. 

Ammonia sprayed would be wasted by evaporation before it went to plant beter convert it into ammonium sulphate.

Contact process manufactures sulphuric acid:

Sulphur/ Sulphuric Ore: oxygen from air

• S + O2 > SO2
• Roasting: iron sulphide + oxygen > sulphur dioxide + iron oxide 

Sulphur Dioxide gas: oxygen from air

• 2SO2+ O2 > 2SO3
• Used vanadium oxide catalyst 

Sulphur Trioxide: mixed with 98% sulphuric acid (four converters)

Oleum: diluted in water to form sulphuric acid

• SO3 + H2O > H2SO4

Sulphuric acid

On the surface of a water droplet the overall force acts inwards pulling the water into a drop shape (surface tension): 

• A detergent is a chemical reduce surface tension of water break up droplets. It is a surfactant surface active agent. 
• Hydrophobic end of detergent molecule sticks to the grease
• The detergent starts to lift the grease off the fabric
• The grease mixes with the water 

The detergent molecules can get in between the water molecules and reduce the force of attraction between them reducing the surface tension as a result. 

• Detergent washed down drain carbon chains broken down by bacteria in water 
• Some detergents have enzymes added to them that help the detergent to break down stains with high protein content like blood 

Soap is a detergent made with fats or oils and an alkali sodium hydroxide 

• Fats and oils are esters; made from alcohols and fatty acids (organic acids contain long chain carbon and hydrogen atoms  and a carboxylic acid group COOH) 
• Glycerol (alcohol) + steraic acid (a fatty acid) > glyceryl tristerarate (ester)
• Fat reacts with alkali it is broken down into glycerol and sodium stearate (soap)
• sodium hydroxide + glyceryl tristerarate > sodium stearate + glycerol 
• Sodium Stearate (stearate ion hydrophillic) and long chain of carbon atoms (hydrophobic) 
• Soap is alkaline 

Hard water reacts with water to make scum

• Hard water contains minerals which react with soap:
• calcium sulphate + sodium stearate > calcium stearate + sodium sulphate
• calcium stearate insoluble and makes scum in water

Limestone area hard water as acids in rain water react with calcium carbonate in limestone to form soluble calcium hydrogen carbonate 

calcium carbonate + water + carbon dioxide > calcium hydrogen carbonate 

Soften hard water: you have to remove the calcium by washing soda crystals containing sodium carbonate that get rid of calcium by turning it into a solid which falls out of the water as a precipitate 

Dishwashers contain compound swap calcium ions in the water for sodium ions keeping it soft.