Periodicity 2 Flashcards
Describe the physical trends in the elements as you go across period three
- Sodium, magnesium and aluminium are metallic- they are shiny (when freshly exposed to air), conduct electricity and react with dilute acids to give hydrogen and salts.
- Silicon is a semi-metal (metalloid)- it conducts electricity to some extent, a property that is useful in making semiconductor devices.
- Phosphorus, sulfur and chlorine are typical non-metals- in particular they do not conduct electricity and have low melting and boiling points.
- Argon is a noble gas. It is chemically unreactive and exists and different atoms.
Which elements in period three react with water
- Sodium
- Magnesium
- Chlorine
Describe the reaction of sodium with water
- The reaction of sodium with water is vigorous
- The sodium floats on the surface of the water and fizzes rapidly, melting because of the heat energy released by the reaction.
- A strongly alkaline solution of sodium hydroxide is formed (pH 13-14).
- H2 gas is also formed
What is the equation for the reaction of sodium with water
2Na (s) + 2H2O (l) —> 2NaOH (aq) + H2 (g)
Describe the reaction of magnesium with water
- The reaction is very slow at room temperature.
- Only a few bubbles of hydrogen are formed after some days.
- The resulting solution is less alkaline (pH around 10) than in the case of sodium because magnesium hydroxide is only sparingly soluble.
- The reaction is much faster with heated magnesium and steam and gives magnesium oxide and hydrogen (instead of magnesium hydroxide and hydrogen).
Give the equation for the reaction of magnesium with water at room temperature
Mg (s) + 2H2O (l) —> Mg(OH)2 (aq) + H2 (g)
Give the equation for the reaction of heated magnesium and steam
Mg (s) + H2O (g) —> MgO (s) + H2 (g)
What elements in period 3 react with oxygen
All of them apart from argon
Describe the reaction of sodium with oxygen
Sodium burns brightly in air with a characteristic yellow flame to form white sodium oxide.
Give the equation for the reaction of sodium with oxygen
2Na (s) + 1/2O2 (g) —> Na2O (s)
Describe the reaction of magnesium with oxygen
- A strip of magnesium ribbon burns in air with a bright white flame.
- The white powder that is produced is magnesium oxide.
- If burning magnesium is lowered into a gas jar of oxygen the flame is even more intense.
Give the equation for the reaction of magnesium with oxygen
2Mg (s) + O2 (g) —> 2MgO (s)
Describe the reaction of aluminium with oxygen
- When aluminium powder is heated and then lowered into a gas jar of oxygen it burns brightly to give aluminium oxide which is a white powder.
- Aluminium powder also burns brightly in air.
Give the equation for the reaction of aluminium with oxygen
4Al (s) + 3O2 (g) —> 2Al2O3 (s)
Describe the reactivity of aluminium
- Aluminium is a reactive metal but it is always coated with a strongly bonded surface layer of oxide which prevents it from further reaction.
- This makes aluminium appear and reactive metal and is used for many everyday purposes.
Describe the reaction of silicon with oxygen
Silicon with form silicon oxide if it is heated strongly in oxygen
Give the equation for the reaction of silicon with oxygen
Si (s) + O2 (g) —> SiO2 (s)
Describe the reaction of phosphorus with oxygen
- Red phosphorus must be heated before it reacts with oxygen.
- White phosphorus spontaneously ignites in air and the white smoke of phosphorus pentoxide is given off.
- Red and white phosphorus are allotropes of phosphorus- the same element with atoms arranged differently.
- If the supply of oxygen is limited, phosphorus trioxide P2O3 is also formed.
Describe the reaction of sulfur with oxygen
When sulfur powder is heated and lowered into a gas jar of oxygen, it burns with a blue flame to form the colourless gas sulfur dioxide.
Give the equation for the reaction of sulfur with oxygen
S (s) + O2 (g) —> SO2 (g)
What is the trend in oxidation numbers of the period three elements in their oxides as you go across the group
The oxidation number of the period three element in the oxide increases across the period
Describe the properties of the period three metal oxides
- Sodium, magnesium and aluminium oxides form giant ionic lattices.
- This means they all have high melting points.
- The bonding in aluminium oxide is ionic but has some covalent character.
Explain why the bonding in aluminium oxide has some covalent character
- Aluminium forms a very small ion with a large positive charge and so can approach closely to the O2- and distort its electron cloud.
- This means that the bond has some covalent character.
Explain how to predict the covalent/ionic character of a bond by considering the difference in electronegativities of the two atoms
The bigger the difference in electronegativities, the greater the ionic character of the bond.
Describe the properties of silicon oxide
- Silicon oxide has a giant covalent (macromolecular) structure.
- It has a high melting point because many strong covalent bonds must be broken in order to melt it.
Describe the properties of phosphorus and sulfur oxides
- Phosphorus and sulfur oxides exist as separate covalently bonded molecules.
- The phosphorus oxides are solids.
- Sulfur dioxide and sulfur trioxide are both gases at 298K
- The intermolecular forces are weak Van Der Waals and dipole-dipole forces.
- They have relatively low melting points.
Describe how sodium oxide reacts with water
- Sodium oxide is a base
- It reacts with water to give sodium hydroxide which is a strongly alkaline solution with a pH of approximately 14
Give the equation for the reaction of sodium oxide with water
Na2O (s) + H2O (l) —> 2Na+ (aq) + 2OH- (aq)
Describe how magnesium oxide reacts with water
- Magensium oxide is a base
- It reacts with water to give magnesium hydroxide which is sparingly soluble in water.
- This produces a somewhat alkaline solution with a pH of approximately 9.
Give the equation for the reaction of magnesium oxide with water
MgO (s) + H2O (l) —> Mg(OH)2 (s) <=> Mg 2+ (aq) + 2OH- (aq)
Which group 3 oxides are insoluble in water
Aluminium oxide
Silicon dioxide
Which group 3 oxides are acidic oxides
- Phosphorus pentoxide
- Sulfur dioxide
- Sulfur trioxide
Describe the reaction of phosphorus pentoxide with water
- Phosphorus pentoxide is an acidic oxide.
- It reacts violently with water to produce an acidic solution of phosphorus (V) acid.
- This ionises so the solution is acidic.
How does phosphoric acid (V) ionise
In stages. The first stage is:
H3PO4 (aq) <=> H+ (aq) + H2PO4- (aq)
Give the equation for the reaction of phosphorus pentoxide with water
P4O10 (s) + 6H2O (l) —> 4H3PO4 (aq)
Describe the reaction of sulfur dioxide with water
- Sulfur dioxide is an acidic oxide
- It is fairly soluble and reacts with water to give an acidic solution of sulfuric (IV) oxide (sulfurous acid).
- This partially dissociates producing H+ ions which cause the acidity of the solution.
Give the equation for the reaction of sulfur dioxide with water
SO2 (g) + H2O (l) —> H2SO3 (aq)
Give the equation for the partial dissociation of sulfuric (IV) acid (sulfurous acid)
H2SO3 (aq) <=> H+ (aq) + HSO3- (aq)
Describe the reaction of sulfur trioxide with water
Sulfur trioxide reacts violently with water to produce sulfuric acid (sulfur (VI) acid)
Give the equation for the reaction of sulfur trioxide with water
SO3 (g) + H2O (l) —> H2SO4 (aq)
Describe the overall pattern in the reactions of the group 3 oxides as you go across the period
- The metal oxides on the left of the period form alkaline solutions in water.
- The non-metal oxides on the right of the period form acidic solutions in water.
- The ones in the middle do not react
Explain how the behaviour of the oxides can be understood by looking at their bonding and structure
- Sodium and magnesium oxides are composed of ions.
- Sodium oxide contains the oxide ion O2- which is a very strong base (it strongly attracts protons) and so readily reacts with water to produce hydroxide ions.
- This makes the solution strongly alkaline.
- Magnesium oxide also contains oxide ions.
- However, its reaction with water produces a less alkaline solution than sodium oxide because it is less soluble than sodium oxide.
- Aluminium oxide is ionic but the bonding is too strong for the ions to be separated, partly because of the additional covalency it has.
- Silicon dioxide is a giant macromolecule and water will not affect this type of structure.
- Phosphorus oxides and sulfur oxides are covalent molecules and react with water to form acid solutions.
What is the general trend in the solutions of group 3 oxides across the period
Solutions of the oxides of the elements go from alkaline to acidic across the period.
Do sodium oxide and magnesium oxide react with acids or bases
Sodium oxide and magnesium oxide react with acids to give salt and water only.
Does aluminium oxide react with acids or bases
Aluminium oxide reacts with both acids and alkalis which makes it an amphoteric oxide
Does silicon dioxide react with acids or bases
Silicon dioxide will react as a weak acid with strong bases
Does phosphorus pentoxide react with acids or bases and describe this reaction
- It reacts with alkalis.
- The reaction of phosphorus pentoxide with an alkali is really the reaction of phosphoric (V) acid, H3PO4 , because this is formed when phosphorus pentoxide reacts with water.
- Phosphoric (V) acid has three -OH groups and each of these has an acidic hydrogen atom.
- So it will react with, for example, sodium hydroxide in three stages.
- Each hydrogen atom in turn reacts with a hydroxide ion and is replaced by a sodium ion.
Does sulfur dioxide react with acids or bases
Bases
Describe the two reactions that occur when aqueous sulfur dioxide is added to sodium hydroxide
- First sodium hydrogensulfate(IV) is formed NaHSO3
- Then sodium sulfate (IV) is formed Na2SO3
