Metal Oxides & The Reactivity Series | Chemistry for Grade 10 PDF Download

Metal Oxides

• Metals react with oxygen in the air to produce metal oxides
• Oxidation and reduction involve the addition or removal of oxygen from a substance and are called redox reactions
• Oxidation is a reaction in which:
  • Oxygen is added to an element or a compound
• Reduction is a reaction in which:
  • Oxygen is removed from an element or a compound
• A common example is the reaction with red-brown copper metal to produce black copper oxide:
  2Cu + O₂ ⟶ 2CuO
• In this reaction copper metal has been oxidised since oxygen has been added to it
• Another example is the reaction of zinc oxide with carbon:
  ZnO + C ⟶ Zn + CO
• In this reaction the zinc oxide has been reduced since it has lost oxygen. The carbon atom has been oxidised since it has gained oxygen. 

Exam Tip

REDOX are simultaneous reactions as they occur at the same time in the same reaction.

The Reactivity Series of Metals

• Based on how they react with other substances, a reactivity series can be produced in which the metals are placed in order of their reactivity
• Metal atoms form positive ions by loss of electrons when they react with other substances
• The tendency of a metal to lose electrons is a measure of how reactive the metal is
• A metal that is high up on the series loses electrons easily and is thus more reactive than one which is lower down on the series
• Note that although carbon and hydrogen are non-metals, they are included in the series as they are useful in extracting metals from their oxides by reduction processes
  
  Diagram of the reactivity series of metals
• There are several reactivity series mnemonics to help you remember the order of the metals
• One that we like goes as follows: “Please send lions, cats, monkeys and cute zebras into hot countries signed Gordon”

Reactivity Series Mnemonic Table

You can learn the reactivity series with the help of a silly phrase

Chemical properties of metals

• The chemistry of metals is studied by analysing their reactions with water and dilute acid.

Metals Reacting with Water

• Some metals react with water
• Metals above hydrogen in the reactivity series will react with water
  • For some metals such as iron, the reaction may be very slow
  • For other metals such as the alkali metals, the reaction may be quick and potentially hazardous because of their reactivity
• Metals that react with cold water form a metal hydroxide and hydrogen gas:
  metal + water → metal hydroxide + hydrogen
• For example, calcium:
  Ca       +    2H₂O     → Ca(OH)₂      +      H₂
  calcium + water → calcium hydroxide + hydrogen
• Magnesium reacts very slowly with cold water when finely divided
• Magnesium reacts with gaseous water to form a metal oxide and hydrogen gas:
  Mg       +    H₂O     → MgO      +      H₂ 
  magnesium + water → magnesium oxide + hydrogen
• Titanium is another example of a metal that reacts with water to form the metal oxide and hydrogen

Metals Reacting with Acids

• Most metals react with dilute acids such as HCl
• Only the ones below hydrogen in the reactivity series will not react with acids
• When acids and metals react, the hydrogen atom in the acid is replaced by the metal atom to produce a salt and hydrogen gas:
  metal + acid → metal salt + hydrogen
• For example iron:
  Fe    +     2HCI    →    FeCl₂    +    H₂
  iron + hydrochloric acid   →  iron(II)chloride + hydrogen
• In both these types of reactions (water and acids) the metals are becoming positive ions
• The reactivity of the metals is related to their tendency to become an ion
• The more reactive the metal the more easily it becomes an ion (by losing electrons)

The Reactions of Metals with Cold Water and Dilute Acids Summary Table

Exam Tip

Sometimes metals can fool us with their reactions. Aluminium is high in the reactivity series, but it does not react with water and the reaction with dilute acids can be quite slow. This is because it has a protective oxide layer that prevents reaction with these reagents. It reminds us that these reactions are trends or patterns rather than rules about chemical behaviour.

Non-Metals in the Reactivity Series

• Why do non-metals appear in the reactivity series of metals?
• A reactivity series will usually contain the elements carbon and hydrogen
• This is because these elements play different roles in our understanding of the reactions of metals and our ability to predict how metals can be extracted from their ores
• From the reactions with water and acids we have seen that whether a reaction takes place depends on the position of the metal in the reactivity series relative to hydrogen
  • A reaction takes place if the metal is able to displace hydrogen from water or acids
• Carbon is a cheap reducing agent which can be used to remove oxygen from metal oxide ores
  • Placing carbon in the reactivity series allows us to see whether a metal oxide can be reduced or not by carbon
• Metals below carbon can be extracted by heating the oxide with carbon
• Metals higher than carbon have to be extracted by other methods, such as electrolysis

Displacement Reactions

• The reactivity of metals decreases going down the reactivity series.
• This means that a more reactive metal will displace a less reactive metal from its compounds
• Two examples are:
  • Reacting a metal with a metal oxide (by heating)
  • Reacting a metal with an aqueous solution of a metal compound
• For example, it is possible to reduce copper(II) oxide by heating it with zinc.
• The reducing agent in the reaction is zinc:
  Zn    +     CuO    →    ZnO    +    Cu
  zinc + copper oxide → zinc oxide + copper

Metal Oxide Displacement Table

Displacement reactions between metals & aqueous solutions of metal salts

• The reactivity between two metals can be compared using displacement reactions in salt solutions of one of the metals
• This is easily seen as the more reactive metal slowly disappears from the solution, displacing the less reactive metal
• For example, magnesium is a reactive metal and can displace copper from a copper sulfate solution:
  Mg + CuSO4  →  MgSO₄ + Cu
• The blue colour of the CuSO₄ solution fades as colourless magnesium sulfate solution is formed.
• Copper coats the surface of the magnesium and also forms solid metal which falls to the bottom of the beaker
  
  Diagram showing the colour change when magnesium displaces copper from copper sulfate

Other displacement reactions

Exam Tip

Displacement reactions occur when the solid metal is more reactive than the metal that is in the compound.