Chemical Analysis - Chapters 4.83 Spec ONLY

Testing Ionic Compounds

• Ionic compounds are made up of

• Positive Ions (cations)

• Negative Ions (Anions)

• Tests are carried out to find specific ions

Flame Tests - Positive Metals Ions

• Lithium ions Li⁺ gives a crimson flame

• Sodium Ions Na⁺ gives a yellow flame

• Potassium ions K⁺ gives a lilac flame

• Calcium ions Ca²⁺give an orange-red flame

• Copper ions Cu²⁺ give a green flame

Steps for flame test

1. Clean a nichrome wire loop by dipping it in some HCL and the holding it in a blue flame from a bunsen burner until it burns without any colour

2. Dip the loop into sample and put back into the flame record the colour of the flame

3. Colours can be used to detect and identify different ions only works for samples that contain a single metal ion

4. If sample texted contains a mixture of metal ions multiple flame colours will be present hinding each other

Precipitation reactions - Positive Metal Ions

•  Calcium Ca²⁺ → White

• Copper(II) Cu²⁺ →  Blue

• Iron (II) Fe²⁺ → Green

• Iron (III) Fe³⁺ → Brown

• Aluminium Al³⁺ → White first,then redissolves in excess NaOH to form a colourless solution

• Magneisum Mg²⁺ → White

Positive Metal Ions - IONIC EQUATIONS

• Ca²⁺ 2OH^- →Ca(OH)₂

• format is (aq)+(aq) —> (s)

• Just put the metal and charge + (number of charge)OH^-

• plus element(OH)₂

Testing for carbonate ions - negative ions

• carbonate reacts with dilute acid you get salt water and carbon dioxide

• you can use co2 test for carbonate ions if you react a substance with an acid and it gives off a gas that turns limewater cloudy then the gas is c02 and the substance must contain carbonate ions

• acid and carbonate ions will produce co2 then it will go through the pipe and go into limewater which will make it cloudy

Testing for halide ions - negative ions (aq,aq = s)

• Chloride Cl^- - White - Ag⁺ +Cl^- → AgCl

• Bromide Br^- - Cream - Ag⁺ +Br^- → AgBr

• Iodide I^- - Yellow - Ag⁺ +l^- → AgI

• Important to use dilute nitric acid rather than HCL as chloride ions would be present meaning that a white percipitate will always be formed

Flame Emission Spectroscopy

• Sample placed in flame

• Ions heat up and their electrons become excited

• When electrons drop back to their original energy level they transfer energy as light

• light passes through a spectroscope which can detect different wavelengths of light to produce a line spectrum

• combination of wavelengths emitted by an ions depends on its charge and its electron arragment

• since no two ions have the same charge and same electron arrangement different ions emit different wavelengths of light

• so each ion has a different line spectrum

Flame Emission spectroscopy of mixtures

• it can be used to identify different ions in mixtures

• more useful than flame tests which only work for substances that contain a single metal ion

• it can also give quantitative results in numerical form

• intensity of spectrum indicates concentration of ion

Precipitate

a solid that forms and separates from a liquid solution due to a chemical reaction

Instrumental Methods

Methods that are accurate,sensitive rapid and can be automated 

Advtages of flame emission spectroscopy vs flame tests

• easier to identify ions

• quantitative

• very sensitive

• very fast 

• very accurate

Disadvantages of flame emission spectroscopy vs flame tests

• expensive

• measurement errors

• destructive

• only identifies the presence of elements/ions doesnt identify compounds

• large machinery required