22.2 The oxides in elements of period 3
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Describe the melting points of the metal oxides eg sodium, magnesium and aluminium
. High melting points They form giant ionic lattices where the bonding expands through the compound
Why is the melting point of Al2O3 lower than MgO
Because aluminium forms a very small ion with a large positive charge. This means it has some covalent character as well as ionic as the Al3+ ion can approach easily to the O2- and distort its electron cloud. Ionic bonding is stronger than covalent due to the larger difference in electronegativity of the elements.
Describe the overall trend in melting point of period 3 oxides
Melting point decreases overall, however MgO has the highest melting point as it is ionic so has strong electrostatic forces between Mg2+ and O2-. It is a smaller molecule also so has a higher charge density than Na2O
How can you predict the ionic character of a bond
.Look at the differences in electronegativity- the bigger the difference the more ionic it is
Describe the structure of silicon oxide Why does it have a high melting point
it is a giant covalent (macromolecular) structure, and this bonding extends throughout the structure These covalent bonds are very strong and require lots of energy to overcome these forces and melt it
Describe the structure of the non metal oxides: Phosphorus oxide (P4O10) and sulfur Why do they have low melting points
They are molecular They both exist as separate covalently bonded molecules. The phosphorus oxides are solids The intermolecular forces are weak van der walls and dipole-dipole forces, so don’t require a lot of energy to overcome them
Name and show the chemical formula of the oxides for the period 3 elements Sodium oxide Magnesium oxide Aluminium oxide
Na2O MgO Al2O3
Name and show the chemical formula of the oxides for the period 3 elements Silicon Phosphorus and the two sulfur
SiO2 P4O10 SO2 and SO3
What is the structure of phosphoric acid
H3PO4 is the formula O || H- - -O- - - P ---- O-----H | O | H
Describe the reaction of the basic oxides with water sodium oxide
Sodium oxide reacts with water to give sodium hydroxide solution which is strongly alkaline Na2O + H2O(l) –> 2NaOH And the PH is 14 NaOH is 2Na+(aq) + 2OH-(aq) and it is the OH- that makes it alkaline
How does the basic oxide magnesium oxide react with water
It reacts with water to give a magnesium hydroxide solution which is sparingly soluble and produces a somewhat alkaline solution MgO(s) + H2O(l) –> Mg(OH)2(s) -> <- (reversible) Mg2+(aq) + 2OH- PH of solution is 9
What are the insoluble oxides
Aluminium oxide and silicon dioxide
Describe the acidic oxides
The non metal elements on the right side of the periodic table make acidic solutions usually Phosphorus and sulfur They are covalent molecules and react with water to form acidic solutionas they release H+ ions
How does phosphorus pentoxide react with water Write the equation What happens when the phosphuric acid ionises
It reacts violently to produce an acidic solution of phosphoric (v) acid which ionises so the solution is acidic P4O10(s) + 6H20(l) –> 4H3PO4(aq) H3PO4 –>(reversible) H+(aq) + H2PO4-
How does sulfur dioxide react with water
Sulfur dioxide is fairly soluble in water and reacts to give an acidic solution of sulfuric(IV) acid (sufurous acid) This partially dissociates to produce H+ ions which make the solution acidic SO2(g) + H2O(l) –> H2SO3(aq) H2SO3(aq) –> H+(aq) + HSO3-(aq) <–
How does sulfur trioxide react with water
. It reacts violently with water to produce sulfuric acid SO3(g) + H2O(l) –> H2SO4(aq) H2SO4(aq) –> H+(aq) + HSO4-(aq) <–
Why do the sodium and magnesium oxides make alkaline solution with water Why is magnesiums solution with water less alkaline than sodiums
. They are both composed of ions so contain the O2- ion which is a very strong base as it attracts protons . So it readily reacts with water to make hydroxide ions Because it is less soluble so less OH- ions dissolve in solution
Why are aluminium oxide and silicon dioxide insoluble in water
Aluminium oxide is ionic but the bonding in the ionic lattice is too strong to separate the ions. Partly because of the covalent bonds it has Silicon dioxide is a giant macromolecule and water will not affect this type of structure
Whats the general trend of the oxides PH with water across the period
They go from alkaline to acidic