4.4 Chemical Changes

How can you measure the pH of a solution?

universal indicator - contain mixture of dyes that means they gradually change colour over range of pH with red/yellow for acidic and blue/purple for alkaline and green for 7 or neutral
a pH probe - pH given on digital display

What is the neutralisation reaction using a base?

acid + base = salt + water
Acid form H+ ions in water
Alkali is a base which dissolves to make OH- ions
H+ + OH- = H20

What is the titration method?

1) add set volume of alkali using a pipette to flask with indicator

2) Using funnel, fill burette with acid of unknown conc. and record vol
3) add acid to alkali gradually and swirl alkali

4) alkali changes colour when neutralised. stop burette and record volume

5) calculate volume of acid used

What colour does phenolphtlein change?

Pink in alkalis and colourless in acids

What is oxidation?

Loss of electrons, gain of oxygen

Acronym for reactivity series

Please send lions, cats, monkeys, and cute zebras into ten hot countries signed Gordon.

What is reactivity determined by?

How easily they lose electrons to form positive ions
When metals react with water or acid, they lose electrons and form positive ions.

The more easily a metal loses electrons, the more reactive it is, as it forms positive ions (cations) more readily

What is a displacement reaction?

A reaction in which a more reactive element takes the place of a less reactive element in a compound

How are metals extracted

-Metals less reactive than carbon can be reducted, but metals more reactive than carbon have to be extracted through electrolysis

What type of reaction is a displacement reaction?

redox reaction
Fe-> Fe2+ + 2e- Cu2+ + 2e- -> Cu

Oxidised. Reduction

What is the neutralisation reaction using a carbonate?

Metal carbonate + acid = salt + water + carbon dioxide

What is the neutralisation reaction for metal oxide & hydroxide?

Acid + metal oxide = salt + water
Acid + metal hydroxide = salt + water
Soluble compounds are alkalis and a base + alkali is a neutralisation reaction

How to make a soluble salt?

React acid with excess of solid metal

Required practical: forming a soluble salt from a insoluble base?

Measure 50cm3 acid in a beaker
Warm acid(to speed up reaction) using Bunsen burner
Add spatulas of solid base and stir
Keep adding till no more reaction or base in excess (solid sinks to the bottom)
Filter off excess
Pour solution into evaporating basin
Heat in water bath
Leave to crystallize
Dry crystals on filter paper

What is the reaction of acid and a metal?

acid + metal -> salt + hydrogen
Speed of reaction determined by rate at which bubbles of hydrogen are given off
More reactive the metal, faster reaction will go

Reaction of metal and water

metal + water -> metal hydroxide + hydrogen
More reactive metals like sodium will react with water
Less reactive metals like zinc won't react with water

How to separate metal from its oxide?

reduction reaction

Copper oxide reduced to copper

How to reduce with carbon?

Ore is reduced as oxygen removed and carbon gains oxygen so is oxidised

How does position of reactivity series determine whether it can be reduced by carbon or not?

Metals higher than carbon in reactivity series must be extracted by electrolysis which is expensive
Metals below carbon can be extracted as carbon can only take the oxygen away from metals which are less reactive than itself

What happens when a strong acid is placed in water?

It completely ionises causing the solution to become acidic

What happens when a weak acid is placed in water?

It does not completely dissociate in water. Partial ionization.
It is reversible which sets up equilibrium between undissociated and dissociated acid

What happens for every decrease of 1 on pH scale?

Conc of H+ goes up by factor of 10

What is the difference between a strong and concentrated acid?

Strong means proportion of the acid molecules which will ionise in water
Concentrated means the amount of acid there is in a certain volume of water

What is electrolysis?

the breakdown of a substance containing ions by electricity

What happens at the cathode?

Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced.

REDUCTION

What happens at the anode?

Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised.

OXIDATION

Why can't an ionic solid be electrolysed?

Because the ions are in fixed positions and can't move

Reaction for cathode

2H+ + 2e- = H2

Reaction for anode

40H- = 02 + 2H20 + 4e-

How to extract aluminium by electrolysis

Extracted from bauxite
Aluminum oxide has high melting point so mixed with cryolite
Anode and cathode made of carbon
Aluminum ions attracted to cathode and gain 3 electrons to become liquid aluminum
Sinks to bottom

What is the overall reaction of extraction of aluminum oxide by electrolysis?

Aluminum oxide -> aluminum + oxygen
2Al2O3 -> 4Al + 3O2

What happens at the cathode in aqueous solutions?

Hydrogen is produced if metal is more reactive than hydrogen but if it is less reactive, a solid layer of the pure metal is produced

What happens at the anode in aqueous solutions?

Oxygen is produced unless solutions contains halide ions

Required practical: electrolysis

Graphite electrodes and measuring cylinder gathering oxygen at anode
Aqueous copper sulphate so copper forms at cathode and forms
Blue colour fades as copper ions removed

How electroplating works

Metal you want as anode
Cathode as thing you want plated
Electrolyte must contain ions of plating metal

Reasons for electroplating

aesthetic reasons, so make the object look better

prevent metal object from corroding

increasing hardness of surface

reduce costs by not using pure metal