Ions in solids and solutions.

Ions in ionic solids. - Cl- Na+ + Sodium chloride crystals are made up of ions in an ionic lattice. Electrostatic attractions hold the ions together. Each Na+ ion is closely surrounded by 6 Cl- ions and each Cl- ion is closely surrounded by 6 Na+ ions. The nearest neighbours for each ion have an opposite charge and so attractions are stronger than repulsions in an ionic lattice.The lattice has a cubic shape and this accounts for the cubic shapes of salt crystals. Try Q6 in CI 5.2. + -

Ions in solution. - O Hydrated ions Oδ- + Most electronegative δ+ δ- δ+ δ- Hydrated ions - δ+ δ- δ+ δ- δ+ δ- δ+ δ- + δ+ δ- δ+ δ- Most electronegative Polar molecules δ+ δ- Oxygen is more electronegative than hydrogen, the bonds are polar. Because of the bent shape of the water molecule, the water molecules themselves are polar. The polar water molecules are attracted to the now separated ions. The ions are said to be hydrated. Oδ- O Hδ+ H Hδ+ H

Dissolving.

- - - - - Some bonds made Some bonds broken + + + + δ+ δ- δ+ δ- δ+ δ- Water molecules are attracted to the ions in the lattice. Bonds between water molecules are broken but bonds between water molecules and the ions are made. δ+ δ- - - δ+ δ- δ+ δ- + δ+ δ- δ+ δ- δ+ δ- δ+ δ-

- - - - - Some bonds broken Some bonds made + + + + Ionic bonds between ions in the lattice break and more water molecules are attracted to the separated ions. - - +

- - - - - Some bonds made Some bonds broken + + + + Eventually, all the ionic lattice will have been broken apart. The energy required for bond breaking is provide by exothermic bond making. - - +

Dissolving. Some bonds made Some bonds broken Exothermic Endothermic What can you say about the energy produced on bond making and the energy required for bond breaking?

For Mg(NO3)2, the ions present in solution are Mg2+(aq) + 2NO3-(aq) Try Q1 from CI 5.1. Page 40 will help you.

A precipitate is a solid made when two solutions react. Precipitation. A precipitate is a solid made when two solutions react. What is a precipitate?

Ag+ Ag+ Cl- Cl- Cl- NO3- NO3- NO3- Na+ Na+ Na+ Silver nitrate solution is added to sodium chloride solution. All the ions are hydrated. Na+ Cl-

Spectator ions Ag+ Cl- Ag+ Cl- NO3- NO3- Na+ Na+ A precipitate of silver chloride is produced. The precipitate is an ionic lattice. Some of the ions don’t change at all, they are hydrated at the start and the end of the reaction. These are called spectator ions. Cl- Ag+ Cl- Spectator ions

Ag+ Cl- Ag+ Cl- Some bonds made Some bonds broken Bonds are broken and made in this reaction. Cl- Ag+ Cl-

Precipitation. Some bonds made Some bonds broken Exothermic Endothermic What can you say about the energy produced on bond making and the energy required for bond breaking?

NaCl(aq) + AgNO3(aq)  AgCl(s) + NaNO3(aq) The stoichiometric equation for the reaction.

Spectator ions Ag+ Cl- Ag+ Cl- NO3- NO3- Na+ Na+ Remember that there were spectator ions in this precipitation reaction. Cl- Ag+ Cl- Spectator ions

NaCl(aq) + AgNO3(aq)  AgCl(s) + NaNO3(aq) Spectator ions NaCl(aq) + AgNO3(aq)  AgCl(s) + NaNO3(aq) Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq)  AgCl(s) + Na+(aq) + NO3-(aq) What ions were present in the solutions? Which were spectator ions? How did you decide? For precipitation reactions, it’s easiest to put the solid/precipitate only on the right then fill in the aqueous ions on the left to balance the ionic equation. Ag+(aq) + Cl-(aq)  AgCl(s) Ionic equation

More precipitations: Try Q3 from CI 5.1

Neutralisation.

HCl(aq) + NaOH(aq)  H2O(l) + NaCl(aq) Spectator ions HCl(aq) + NaOH(aq)  H2O(l) + NaCl(aq) H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq)  H2O(l) + Na+(aq) + Cl-(aq) Water is a covalent compound, the hydrogen and hydroxide ions no longer exist. H+(aq) + OH-(aq)  H2O(l) Ionic equation

More reactions of acids: Try Q5 from CI 5.1 A spectator ion is an aqueous ion on both sides of the equation!

Finally…. 26 5 15 0.098nm 0.057nm 0.078nm Ion Ionic radius Water molecules attached Na+ 2,8,8 0.098nm 5 Mg2+ 15 Al3+ 0.057nm + Why is the aluminium ion smaller than the sodium ion – consider core charge? Why does a magnesium ion have more water molecules surrounding it when hydrated? Although the sodium ion was larger before hydration, it isn’t after hydration. How many water molecules might surround an aluminium ion? 0.078nm 2+ 26

Water of crystallisation… Cu2+(aq) + SO42-(aq) + 5H2O(l)  CuSO4.5H2O Sometimes when crystals are formed, they are hydrated too e.g.CuSO4.5H2O Water molecules are incorporated in the ionic lattice. Try Q4 from CI 5.1

ICSD Chem Zone Lesson Dissolve & Dissociate_files The hyperlink below is a good flash animation of dissolving: ICSD Chem Zone Lesson  Dissolve & Dissociate_files http://www1.icsd.k12.ny.us/chemzone/lessons/03bonding/dissociate.htm