Chemical Changes

Page 1

Acids and Bases

pH scale is a measure of how acidic or alkaline a solution is

Lower the pH of a solution, the more acidic it is

Higher the pH of solution, the more alkaline it is

A neutral substance has pH 7

An indicator is a dye that changes colour depending on whether it's above or below a certain pH

Some indicators contain a mixture of dyes

Means they gradually change colour over a broad range of pHs

Useful for estimating pH of a solution

pH probe attached to a pH meter can also be used to measure pH electronically

Probe is placed in solution you're measuring and the pH is given as a numerical value

Acid - substance that forms aqueous solution with a pH less than 7

Acids form H+ ions in water

Base - substance with pH greater than 7

Alkali - base that dissolves in water to form a solution with pH greater than 7

Alkalis form OH- ions in water

Reaction between acids and bases is called neutralisation

Acid + base -> salt + water

Can be in terms of H+ and OH- ions

H+ (aq) + OH- (aq) -> H2O

When an acid neutralises a base, the products are neutral

An indicator can be used to show reaction is over

Page 2

Strong Acids and Weak Acids

Acids ionise in aqueous solution - produce hydrogen ions (H+)

Strong acids ionise completely in water

All acid particles dissociate to release H+ ions

Weak acids don't fully ionise solution

Only a small proportion of acid particles dissociate to release H+ ions

Ionisation of a weak acid is a reversible reaction - sets up equilibrium between undissociated and dissociated acid

Reactions of acid involve the H+ ions reacting with other substances

If concentration of H+ ions is higher, rate of reaction will be faster, strong acids will be more reactive than weak acids of the same concentration

Acid strength tells you what proportion of acid molecules ionise in water

Concentration of an acid is different

Means how much acid there is in a certain volume of water

Basically how watered down an acid is

Larger the amount of acid, the more concentrated the acid is

You have a dilute but strong acid, or a concentrated but weak acid

pH will decrease with increasing acid concentration

Page 3

Reactions of Acids

Some metal oxides and metal hydroxides dissolve in water

Soluble compounds are alkalis

Alkalis react with acids in neutralisation reactions

Bases that won't dissolve in water will still take part in neutralisation reactions

All metal oxides and metal hydroxides react with acids to form a salt and water

Acid + metal oxide -> salt + water

Acid + metal hydroxide -> salt + water

Metal carbonates are also bases

React with acids to produce a salt, water and CO2

Acid + metal carbonate -> salt + water + carbon dioxide

Soluble salts using an insoluble base

Gently use the dilute acid using a Bunsen burner, then turn it off

Add insoluble base to the acid bit by bit, until no more reacts

Excess solid will sink to the bottom of the flask

Then filter out the excess solid to get the salt solution

To get pure, solid crystals of the salt

Gently heat solution using a water bath or an electric heater to evaporate some of the water and then stop heating it and leave solution to cool

Crystals should form, which can be filtered out of the solution and the dried out - crystallisation

Page 4

The Reactivity Series

List metals in order of their reactivity towards other substances

For metals, their reactivity is determined by how easily they lose electrons - forming positive ions

Higher up the reactivity series a metal is, the more easily they form positive ions

When metals react with water or acid, they lose electrons and form positive ions

The higher a metal is on the series, the more easily it reacts with water or acid

Some metals react with acids to produce a salt and hydrogen gas

Speed of reaction is indicated by the rate at which bubbles of hydrogen are given off

More reactive the metal, the faster the reaction will go

Very reactive metals react explosively, but less reactive metals reacts less violently

In general copper won't react with cold, dilute acids

Can also investigate reactivity by measuring temp change

If you use the same mass and surface area, the more reactive the metal, the greater the temp change should be

Page 5

Separating Metals from Metal Oxides

Some common metals react with oxygen to form oxides

Process is an example of oxidation

Oxides often have ores that metals need to be extracted from

A reaction that separates a metal from its oxide - reduction reaction

Oxidation = gain of oxygen

Reduction = loss of oxygen

Some metals can be extracted from their ores chemically

Reduction using carbon

Ore is reduced as oxygen is removed from it, and carbon gains oxygen so is oxidised

Position of metal in reactivity series determines whether it can be extracted by reduction

Metals higher than carbon must be extracted using electrolysis

Page 6

Redox Reactions

Oxdiation can mean addition of oxygen

Reduction can be removal of oxygen

Loss of electrons - oxidation

Gain of electrons - reduction

Reduction and oxidation happen at the same time

Iron atoms are oxidised when they react with dilute acid

Iron atoms lose electrons - oxidised by hydrogen ions

Hydrogen ions gain electrons - reduced by iron atoms

Displacement involves one metal kicking another out of a compound

More reactive metal will displace a less reactive metal

In displacement reactions, it's always the metal ion that gains electrons and is reduced

Metal atom always loses electrons and is oxidised

Page 7

Electrolysis

Electrolysis - an electric current is passed through an electrolyte

Ions move towards the electrodes, where they react, and the compound decomposes

Positive ions move towards the cathode and gain electrons (reduced)

Negative ions move towards anode and lose electrons (oxidised)

As ions gain or lose electrons, they form the uncharged element and are discharged from the electrolyte

An ionic solid can't be electrolysed

Ions are in fixed positions and can't move

Molten ionic compounds can be electrolysed

Ions can move freely and conduct electricity

Molten ionic liquids are always broken up into their elements

Positive metal ions - reduced to the element at the cathode

Negative non-metal ions - oxidised to the element at the anode

If a metal is too reactive to be reduced with carbon or reacts with carbon

Electrolysis can be used to extract it

Aluminium is extracted from the ore bauxite by electrolysis

Bauxite contains aluminium oxide

Aluminium oxide has a very high melting point so it's mixed with cryolite to lower melting point

Molten mixture contains free ions

It'll conduct electricity

Positive Al3+ ions are attached to neagtive electrode where they each pick up three electrons and turn into neutral aluminium atoms

These sink to the bottom of the tank

Negative O2- ions are attached to positive electrode where they each lose two electrons

Neutral oxygen atoms will combine to form O2 molecules

Page 8

Electrolysis of Aqueous Solutions

In aqueous solutions, there will be hydrogen ions and hydroxide ions from the water

At the cathode, if H+ ions and metal ions are present, hydrogen gas will be produced

If metal ions form an elemental metal that is more react than hydrogen

If metal ions form an elemental metal that is less reactive than hydrogen, a solid layer of pure metal is produced

At the anode, if OH- and halide ions are present

Molecules of chlorine, bromine or iodine will form

If no halide ions are present, the OH- ions are discharged and oxygen will be formed

Once experiment is finished you can test any gaseous products to work out what was formed:

Chlorine bleaches damp litmus paper, turning it white

Hydrogen makes a "squeaky pop" with a lighted splint

Oxygen will relight a glowing splint

Half equations show reactions at the electrodes

Important thing to remember is that number of electrons needs to be the same for each half equation

Negative electrode - (2H+) + (2e-) -> H2

Positive electrode - (2Cl-) -> Cl2 + (2e-) or (2Cl-) - (2e-) -> Cl2

Combine to form ionic equation - (2H+) + (2Cl-) -> H2 + Cl2

When a halide isn't present in the aqueous solution, half equation for the anode is:

(4OH-) -> O2 + 2H2O + 4e-

or (4OH-) - (4e-) -> O2 + 2H2O