Noadswood Science, 2012. To understand and be able to represent ionic bonding Sunday, May 10, 2015.

Noadswood Science, 2012

To understand and be able to represent ionic bonding

Tuesday, April 18, 2023

Appearance

The appearance of a substances (colour, state etc…) gives you the first clues about what it could be

However this is not that reliable – a big suspicious bag of white powder could be anything (it could be cocaine, it could be talcum powder, it could be rat poison)

Behaviour

The behaviour of a compound gives the next clues as to what it is – this is determined by the structure of the bonding within the substance…

Salt under a microscope

Ionic Bonding

When metals react with non-metals, electrons are transferred from the metal atoms to the non-metal atoms, forming ions – the resulting compound is an ionic compound

Consider reactions between metals and non-metals, e.g.

sodium + chlorine → sodium chloride

magnesium + oxygen → magnesium oxide

calcium + chlorine → calcium chloride

In each of these reactions, the metal atoms give electrons to the non-metal atoms – the metal atoms become positive ions and the non-metal atoms become negative ions

Ionic Bonding – Demo

There is a strong electrostatic force (generated by differences in electric charge) of attraction between these oppositely charged ions, called an ionic bond

Many ionic bonds in an ionic compound are arranged in giant lattice structures (a regular grid-like arrangement of atoms)

Ionic Compounds

Ionic bonds form when a metal reacts with a non-metal – metals form positive ions; non-metals form negative ions

Ionic bonds are the electrostatic forces of attraction between oppositely charged ions (+ve and –ve)

The oppositely charged ions are arranged in a regular way to form giant ionic lattices (ionic compounds often form crystals as a result)

Lattice

In an ionic compound, millions and millions of ions are packed together in a regular cubic arrangement, joined by ionic bonds forming a giant 3D structure called a (giant) ionic lattice

Ionic Properties

Ionic compounds have the following properties: -

High melting and boiling points - ionic bonds are very strong (a lot of energy is needed to break them) causing ionic compounds have high melting and boiling points

Conductive when liquid - ions are charged particles, but ionic compounds can only conduct electricity if their ions are free to move so ionic compounds only conduct electricity when dissolved in water or melted

Ionic Properties

Ionic compounds do not conduct electricity when they are solid because the ions are packed together and cannot move

When molten, however, the lattice breaks up and the ions are free to move (as there are charged particles they can carry an electric current)

Ionic compounds are usually soluble in water because water molecules have a slight electrical charge and can attract the ions away from the lattice – when dissolved, the ions are free to move and can carry an electric current

Ionic Properties

Ionic compounds are brittle – they shatter when they are hit

When the lattice is hit, a layer of ions is shifted so that ions with the same charges are lined up together

These like charges repel each other, thereby splitting the lattice

Ionic Properties

Ionic Compound

Properties

Sodium Chloride

NaCl

High melting point: 800ºC

Non-conductive in its solid state, but when dissolved in water or molten NaCl will conduct electricity

Magnesium Oxide

MgO

High melting point: 2,800ºC

Mg2+ and O2- ions have a greater number of charges, so they form stronger ionic bonds than the Na+ and

Cl- ions in sodium chloride

As magnesium oxide stays solid at such high temperatures it remains non-conductive (high-

temperature electrical insulation)

Salt Water Conductivity

Your task is to design an experiment to test if water / sugar water / salt water is a good conductor of electricity…

Explain using a diagram how you would test this

Taking a beaker add either the water, sugar water or salt water to it – stir the mixture and connect a circuit up – if the bulb lights the solution is able to conduct…


Water and sugar water do not conduct, however salt water does – explain why this is the case…

Consider what is needed for the flow of electrical current…


Salt dissociates into ions when it dissolves in water allowing conduction of an electrical current (the large salt crystals separates into smaller ions)

This process produces ions which can freely move, transferring electrical charges (and the flow of electrical current)

A solution can only conduct an electric current it it contains freely moving ions – water and sugar are both lacking in these

Soluble (Higher)

Some ionic compounds dissolve, whilst others do not (useful for forensice scientists when trying to identify ionic compounds)…

Sodium (Na); Potassium (K); and Ammonium (NH4) salts are soluble in water

Nitrates (NO3) are soluble in water

Most chlorides (Cl) are soluble in water, except silver and lead which are insoluble in water

Most sulfates (SO4) are soluble in water, except barium and lead which are insoluble in water (calcium is slightly soluble in water)

Most oxides (O), hydroxides (OH) and carbonates (CO3) are insoluble in water, except sodium and potassium which are soluble in water

Charges (Higher)

Different ions have different charges – the ionic compound must have a neutral charge…

*Carbon and silicon (group IV) usually form covalent bonds (sharing electrons)

**Elements within group VIII do not react with other elements to form ions

I II III IV V VI VII VIII

Element

example

Na Mg Al C N O Cl He

Charge 1+ 2+ 3+ * 3- 2- 1- **

Ion symbol Na+ Mg2+ Al3+ * N3- O2- Cl- **

Charges (Higher)

Different ions have different charges – the ionic compound must have a neutral charge…

Na +Cl NaCl (sodium chloride)

Mg + O MgO (magnesium oxide)

Al + Cl AlCl3 (aluminium chloride)

Na +O NaO2 (sodium oxide)

Al + O Al2O3 (aluminium oxide)

Na + N NaN (sodium cyanide)

Properties Quiz

Summary

Ionic compounds: - Form giant lattices Are made of charged particles (ions) Metal ions are +ve Non-metal ions are –ve Forces of attraction between the ions are very strong –

ionic compounds have a high melting point and are solids at room temperatures

Some ionic compounds dissolve in water

Noadswood Science, 2012. To understand and be able to represent ionic bonding Sunday, May 10, 2015.

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Transcript of Noadswood Science, 2012. To understand and be able to represent ionic bonding Sunday, May 10, 2015.