chemistry c3 part one

Chemistry C3 Part One

Revision PowerPoint - Big ideas

The Periodic Tableorganising elements into

groups

Newlands and Mendeleev

• Newlands listed the elements in order of atomic weights. He spotted repeating patterns (octaves) in behaviour• Mendeleev arranged elements into

groups and periods to fit repeating patterns. He left gaps for undiscovered elements.

Modern Periodic Tables

• Modern Periodic Tables arrange elements by increasing atomic number.• The number of electrons in the

highest energy level (outermost shell) indicates the group number of the element.

Group 1 - The Alkali Metals• Elements in Group 1 (The Alkali

Metals) are soft, low density metals.

• Alkali metals react rapidly with water, forming alkalis & hydrogen.

• Alkali metals form ionic compounds with non-metals.

• Group 1 ions have the charge +1

Reactivity down the group• Group 1 elements become more

reactive further down the group.• Large atoms lose electrons from

their outer shells more easily because they are further from the positive nuclear charge force and shielded by more inner shells of electrons.

Group 7 – The Halogens• Elements in Group 7 (The

Halogens) are coloured non-metals.

• More reactive halogens displace less reactive halogens from solutions containing halide ions, e.g. Cl2 + 2Br– → Br2 + 2Cl–

• Group 7 ions have a charge of –1

Chemistry C3 Part Two


Hard water

Causes of hard water• Hard water is caused by calcium and

magnesium ions dissolving when acidic rainwater flows through rocks.• Temporary hard water is caused by

calcium hydrogencarbonate, can be removed by boiling the water.• Permanent hardness caused by calcium

sulphate isn’t removed by boiling.

Softening and desalinating

•Washing soda and ion-exchange resins can soften hard water.• Seawater can be desalinated by distillation which needs a lot of energy

Tap water treatment• Water is filtered to remove

solids and sterilised with chlorine to kill microbes. • Dissolved substances are

removed by specialised filters or by ion exchange.

Chemistry C3 Part Three


Chemical energyand calorimetry experiments

Calorimetry• In calorimetry the energy released from a chemical reaction is transferred to water. • The energy transferred, E = m x c x ∆T

Calorimetry• Energy level diagrams show the change in chemical energy as reactants change into products.• Energy is released when new chemical bonds form. • Energy is required to break chemical bonds.

Chemical energy diagrams• In an

exothermic reaction chemical energy in the reactants in transformed into thermal energy• The

surroundings become warmer

Chemical energy diagrams• In an

endothermic reaction thermal energy is taken in from the surroundings and is stored as chemical energy in the products

• The surroundings cool down

Activation energy• Catalysts

reduce the minimum amount of energy needed to start a reaction • This is called

the activation energy

Hydrogen fuel• Hydrogen releases energy when it

reacts with oxygen in combustion or in fuel cells.

2H2 + O2 → 2H2O• Hydrogen burns exothermically

Chemistry C3 Part Four


Chemical Analysis

Flame tests 1• Colours used in flame

tests are used to identify Group 1 ions.

• Each salt produces a characteristic flame colour:

• Lithium (Li+) – magenta• Sodium (Na+) – golden

yellow• Potassium (K+) – lilac

Flame tests 2Other metal ions also produce coloured flames. • Calcium (Ca2+) – brick

red• Barium (Ba2+) – apple

green• Copper (Cu2+) - green

with blue flashes

Precipitation reactionsPrecipitation reactions are used to identify other metal ions. Test the solutions of the salts with sodium hydroxide solution.A coloured precipitate of a metal hydroxide forms.

light blue jelly ppt. copper (II) ions, Cu2+

dirty green jelly ppt. iron (II) ions, Fe2+

rusty red- brown jelly ppt.

iron (III) ions, Fe3+

White hydroxide precipitates?When you test solutions of the salts with sodium hydroxide solution, sometimes the precipitate is white.This is how to distinguish between magnesium, calcium and aluminium ions …

Add extra (excess) sodium hydroxide solution to the white precipitate.

The precipitate dissolvesin excess sodium hydroxide= aluminium ions

The precipitate does not dissolve in excess sodium hydroxide= calcium or magnesium ions

Sulphate test• Energy level diagrams show the change in chemical energy as reactants change into products.• Energy is released when new chemical bonds form. • Energy is required to break chemical bonds.

Carbonate test• Energy level diagrams show the change in chemical energy as reactants change into products.• Energy is released when new chemical bonds form. • Energy is required to break chemical bonds.

Chloride bromide iodide test

• Test the solution of the salt with silver nitrate solution, acidified by nitric acid

silver iodide ppt. is yellow

silver bromide ppt. is cream

silver chloride ppt. is white

Chemistry C3 Part Five


Acid-alkali titrationsvolumetric analysis of acids &

alkalis

Bases are the

chemical opposites

of acids. Alkalis are

bases that are

soluble in water.

A = AcidB = Base (alkali)

Apparatus• Titrations involve

reactions between solutions of acids and solutions of alkalis

• Alkalis are delivered using pipettes

• Acids are delivered using burettes

Solution concentrationThe concentration of a solution depends on the mass of the solute dissolved in a certain volume of water.Mass is measure in grams, volumes in dm3

Concentration = Mass of solute, in g in g/dm3 Volume of solution, in dm3

Molar concentrationThe concentration of a solution depends on the number of moles of the solute dissolved in a one decimetre cubed of solution.Concentration, c = m ÷ Mr in mol/dm3 V m = mass of solute in gramsMr = molar mass of the solute in g/molV = volume of solution in dm3

How many moles are delivered?

The number of moles, nA, of acid delivered during the titration depends upon:• the volume of acid, VA added

to the flask• the molar concentration

cA of the solution added to the flasknA = cA x VA

Titration question

HCl + NaOH → NaCl + H2O1 mole 1mole

25cm3

0.2mol/dm3

20cm3

The concentration of the acid is unknown

Calculation to find acid concentrationACID, A ALKALI (Base), BnA = cA x VA nB = cB x VB

1 = cA x 20 1 = 0.25 x 25

20 x cA = 0.25 x 25

cA = 0.25 x 25 20 cA = 0.31 mol/dm3

Indicators and colour changes

Indicators switch their colours at certain pH’sMatch the type of titration with the most suitable indicator using the equivalence point pH.

Equivalence points and pHThis titration graph shows what happens to the pH when a strong alkali is slowly added to a strong acid. Use methyl orange.

This titration graph shows what happens to the pH when a strong alkali is slowly added to a weak acid. Best indicator = phenolphthalein

Chemistry C3 Part Six


The Haber Processmaking ammonia from its

elements

Ammonia NH3

• Ammonia is manufactured when nitrogen gas reacts with hydrogen gas – the Haber Process• Nitrogen - N2 is obtained from air• Hydrogen is obtained from natural

gas (methane) reacting with steam CH4 + H2O → CO + 3H2

Faster reaction• Nitrogen and hydrogen gases are mixed

and passed over a catalyst of iron filings• The catalyst speeds up this slow reaction• Iron filings have a larger surface area

(better than a block of iron)• A reversible exothermic reaction

takes placeN2 + 3H2 ↔ 2NH3 + heat ∆H = –92 kJ/mole

Conditions for the reaction• Ammonia is being

produced by the forward reaction

• Ammonia is broken down by the backward reaction into nitrogen and hydrogen

• Conditions are chosen to produce a reasonable yield of ammonia as quickly as possible

Temperature considerations 1N2 + 3H2 ↔ 2NH3 + heat ∆H = –92 kJ/mole• Raise the temperature of the reaction and

molecules of gas will have more kinetic energy, move faster and collide more violently.

• Violent collisions lead to old chemical bonds breaking – but be careful, ammonia molecules may decompose faster too!!

• 450℃ is a good compromise for this exothermic reaction – you get a reasonable yield of ammonia at a reasonable rate.

Temperature considerations 2N2 + 3H2 ↔ 2NH3 + heat ∆H = –92 kJ/mole

Temperature in ℃

Yie

ld o

f am

mon

ia

Rate of th

e amm

onia reaction

High yield

Low yield

Fast rate

Slow rate

450 is the ℃ optimum temperature for a reasonable rate and a reasonable yield.

You can’t have a fast rate and a high yield.Use the Goldilocks principle … not too hot … not too cold … but just right! 450 ℃

450℃

550350 450

Pressure considerations 11N2 + 3H2 ↔ 2NH3 + heat ∆H = –92 kJ/mole

• Notice that the number of molecules on the left hand side of the equation is 3+1 = 4 and there are 2 molecules of ammonia on the right.

• For all reversible reactions involving gases an increase in pressure effects the equilibrium position favouring the reaction that produces fewer molecules

Pressure considerations 21N2 + 3H2 ↔ 2NH3 + heat ∆H = –92 kJ/mole

• So, an increase in the pressure shifts the equilibrium to the right and increases the yield of ammonia

• A high pressure of 100 - 200 atmospheres is chosen for the Haber process

• Using higher pressures adds to building costs (walls for reaction chambers will be thicker) and running costs (faster electric pumps)

Chemistry C3 Part Seven


Reversible reactions

Reversible reactions 1These are chemical reactions which can proceed in two directions• the forward reaction changes the

reactants into producte.g. NH3 (g) + HCl (g) → NH4Cl (s)

• the product can break down to re-create the original reactantse.g. NH4Cl (s) → NH3 (g) + HCl (g)

Reversible reactions 2The equations for reversible reactions contains the two-way arrow reaction signe.g. NH3 (g) + HCl (g) ↔ NH4Cl (s) By changing the reaction conditions we can favour either the forward reaction or the reverse reaction.Other reversible chemical reactions include the decomposition of limestone:CaCO3 (s) ↔ CaO (s) + CO2 (g)

Dynamic chemical equilibrium 1Ammonia and hydrogen chloride gases can react in a closed system such as the beaker in the diagram. Nothing can get in & nothing can escape.Over time the reaction reaches dynamic chemical equilibrium. The forward and backward reactions are both happening.

Dynamic chemical equilibrium 2At this point of dynamic chemical equilibrium, the rate of the forward reaction and the rate of the backward reaction are exactly equal.The amount of each substance remains constant and the forward and backward reactions continue to proceed.

X

Y

At X the heated ammonium chloride solid decomposes. NH4Cl (s) → NH3 (g) + HCl (g)

At Y the cooled gases combine together to form solid ammonium chloride.NH3 (g) + HCl (g) → NH4Cl (s)

Chemistry C3 Part Eight


Organic Chemistryalcohols, carboxylic acids and

esters

Alcohols CnH2n+1OH• Alcohols, such as ethanol, are a

family (homologous series) of compounds containing the –OH functional group• They make excellent fuels (e.g.

bioethanol is a petrol substitute)• They are used as solvents and fuels• They mix with water easily (whisky)

Carboxylic acids (e.g. vinegar)• Ethanol is oxidised to ethanoic acid.

This happens when we leave wine open to the air.• Carboxylic acids have a –COOH

functional group• Carboxylic acids are weak acids.

Acid molecules are partially ionise. They release small quantities of H+ ions into the water, pH = 4

Carboxylic acids are weak acids

• The vinegar = pH3 to 4Vinegar is a weak, partially ionised acid• Hydrochloric acid = pH0 to 1

HCl is 100% dissociated into ions and it is a strong acid

Carboxylic acid reactions 1

• Carboxylic acids react with carbonates like marble chips to form carbon dioxide. CaCO3 + 2H+ → CO2 + H2O + Ca2+

• The reaction is slow because they are weak acids.

Carboxylic acid reactions 2• Carboxylic acids react with alcohols in

the presence of an acid catalyst to form esters• Esters are sweet-smelling liquids found

in perfumes and fruits like oranges and strawberries• Esters also give some fruits a special

flavour

Esters – nice niffs & flavours

• The ester ethyl ethanoate has the smell of pears• It is made when ethanol reacts with

ethanoic acid in the presence of an acid catalyst• Esters contain the –COO- functional

group.

chemistry c3 part one

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