Comprehensive Notes on Patterns and Properties of Metals

C8 Patterns and Properties of Metals

C8.01 The Alkali Metals

• Alkali metals are Group I metals known for their reactivity, especially their reaction with cold water.
• Due to their high reactivity and quick tarnishing, alkali metals have limited uses and are typically stored under oil.
• Sodium is used in sodium vapour lamps, which produce the yellow light seen in street and motorway lighting.
• The melting points of alkali metals decrease gradually down the group.
• The hardness of alkali metals also decreases down the group; they are all soft, low-density metals.
• Lithium is the hardest alkali metal but can still be cut with a knife; the metals become easier to cut as you descend the group.
• The density of alkali metals tends to increase down the group, with potassium being an exception as it is slightly less dense than sodium.
• Common Properties of Alkali Metals:
  • They are all reactive metals and must be stored under oil.
  • They are soft and can be cut with a knife.
  • They form positive ions with a single positive charge (e.g., $Li^+$, $Na^+$, $K^+$.
  • They form compounds with similar formulae (e.g., lithium carbonate $Li<em>2CO</em>3$, sodium carbonate $Na<em>2CO</em>3$, and potassium carbonate $K<em>2CO</em>3$).
  • They react strongly and directly with non-metals to form white, crystalline, ionic solids that dissolve in water.

Reaction with Water

• Alkali metals react spontaneously with water to produce hydrogen gas and the metal hydroxide. This reaction is exothermic.
  • $metal + water \rightarrow metal hydroxide + hydrogen$
  • Example: $2Na(s) + 2H<em>2O(l) \rightarrow 2NaOH(aq) + H</em>2(g)$
• The reactivity of alkali metals with water increases down the group.
  • Lithium reacts steadily without melting, and the hydrogen doesn't ignite.
  • Sodium reacts more strongly, melting, and the hydrogen may ignite if movement is restricted.
  • Potassium reacts vigorously, and the hydrogen gas ignites spontaneously, sometimes causing an explosion. The flame is lilac.
  • Rubidium and caesium explode upon contact with water.
• The metal hydroxide produced makes the water alkaline.

Reactions with Air

• Lithium tarnishes slowly in air, forming an oxide layer.
• Sodium tarnishes quickly in air, forming an oxide layer.
• Potassium tarnishes very quickly in air, forming an oxide layer.

Flame Tests

• Alkali metal compounds produce characteristic colors in a Bunsen flame, which can be used for identification.
  • Lithium (Li) produces a red flame.
  • Sodium (Na) produces a yellow flame.
  • Potassium (K) produces a lilac flame.

Group II Metals (Alkaline Earth Metals)

• Group II metals, known as alkaline earth metals, exhibit similar trends in reactivity to Group I metals but are generally less reactive.
• Magnesium burns intensely with a brilliant white light and is used in distress flares, flashbulbs, and fireworks.
  • $2Mg(s) + O_2(g) \rightarrow 2MgO(s)$

Trends in Reactivity

• Reactivity increases down the group. Beryllium is the least reactive, and barium is the most reactive.
• Magnesium reacts very slowly with cold water but more vigorously with steam, producing hydrogen and magnesium oxide.
  • $Mg(s) + H<em>2O(g) \rightarrow MgO(s) + H</em>2(g)$
• Calcium reacts strongly with cold water, producing hydrogen rapidly and forming calcium hydroxide (limewater).
  • $Ca(s) + 2H<em>2O(l) \rightarrow Ca(OH)</em>2(aq) + H_2(g)$
• Calcium hydroxide is more soluble than magnesium hydroxide, resulting in an alkaline solution and a white suspension.

C8.03 The Transition Elements

• Transition elements possess general metallic properties such as hardness, strength, high melting points, high densities, and good conductivity of heat and electricity.
• They are less reactive than Group I and II metals and often exhibit excellent corrosion resistance.
• Tungsten's high melting point ($3410 °C$) makes it suitable for light bulb filaments.

Distinctive Properties

• Many transition metal compounds are colored.
• Transition metals often exhibit multiple valencies, forming more than one type of ion.
• Transition metals and their compounds are frequently used as catalysts.
• Iron, cobalt, and nickel are strongly magnetic.

Coloured Compounds

• Salts of transition elements are often colored and produce colored solutions when dissolved in water.
• Examples include vanadium compounds, which can be yellow, blue, green, or purple.
• The colors of gemstones like sapphire (due to titanium and iron ions) and ruby (due to chromium ions) are produced by trace amounts of transition metals.
• Transition elements contribute to the colors in stained glass windows.
• Hydroxide precipitates formed by transition elements have characteristic colors, aiding in chemical analysis (e.g., iron(II) hydroxide is grey-green, while iron(III) hydroxide is red-brown).

Catalytic Properties

• Catalysts speed up chemical reactions without being consumed. Many industrial catalysts are transition elements or their compounds (e.g., iron in the Haber process).

Reactions of Certain Transition Elements

Iron

• Iron is moderately reactive and reacts with steam or acids to displace hydrogen gas.
  • $Fe(s) + 2HCl(aq) \rightarrow FeCl<em>2(aq) + H</em>2(g)$

Copper

• Copper is relatively unreactive and does not react with dilute acids to produce hydrogen.
• When heated in air, copper forms a black layer of copper(II) oxide.
  • $2Cu(s) + O_2(g) \rightarrow 2CuO(s)$
• Copper statues and roofs develop a green layer of basic copper(II) carbonate when exposed to the atmosphere.
• Copper(II) carbonate decomposes upon heating to release carbon dioxide.
  • $CuCO<em>3(s) \rightarrow CuO(s) + CO</em>2(g)$

Zinc

• Zinc is moderately reactive and displaces hydrogen from steam or dilute acids.
  • $Zn(s) + H<em>2O(g) \rightarrow ZnO(s) + H</em>2(g)$
  • $Zn(s) + 2HCl(aq) \rightarrow ZnCl<em>2(aq) + H</em>2(g)$
• Zinc carbonate decomposes on heating to produce zinc oxide and carbon dioxide.
  • $ZnCO<em>3(s) \rightarrow ZnO(s) + CO</em>2(g)$
• Zinc oxide is yellow when hot but turns white upon cooling.