Notes

Additional Notes

• Combustion that occurs in such a way that fuel is not completely oxidized ("burned up").
• The incomplete combustion of carbon-containing fuels (such as coal and oil) always results in the formation of some carbon monoxide. 
  
• The relative amounts of carbon monoxide or carbon dioxide that form during combustion depend on two factors
• The amount of oxygen present
  
• the combustion temperature

• When a large supply of oxygen is present and when the combustion temperature is high, carbon dioxide is more likely to be formed.
• With limited supplies of oxygen and at lower temperatures, carbon monoxide is produced.
  

Carbon monoxide

• Used in industry primarily as a source of energy and as a reducing agent.
• As a reducing agent, carbon monoxide is used to convert the naturally occurring oxide of a metal to the pure metal. 
• When carbon monoxide is passed over hot iron oxides, for example, the oxides are converted to metallic iron.

Carbon

Essentially carbon acts as a reducing agent as well as the carbon monoxide that is inevitably formed by its heating in air.

DEFINITIONS OF OXIDATION AND REDUCTION (REDOX)

Oxidation and reduction in terms of :
A. Oxygen transfer
Definition	·       Oxidation is gain of oxygen. ·       Reduction is loss of oxygen.
For example, in the extraction of iron from its ore:
Oxidising agent	-        An oxidising agent is substance which oxidises something else. -        Oxidising agents give oxygen to another substance. -        In the above example, the iron(III) oxide is the oxidising agent.
Reducing agent	-        A reducing agent reduces something else. -        Reducing agents remove oxygen from another substance. -        In the equation, the carbon monoxide is the reducing agent.
B. Hydrogen transfer
Definition	·       Oxidation is loss of hydrogen. ·       Reduction is gain of hydrogen.
For example, ethanol can be oxidised to ethanal:
Oxidizing agent	-        You would need to use an oxidising agent to remove the hydrogen from the ethanol. -        A commonly used oxidising agent is potassium dichromate(VI) solution acidified with dilute sulphuric acid.
Reducing agent	-        Ethanal can also be reduced back to ethanol again by adding hydrogen to it. -        A possible reducing agent is sodium tetrahydridoborate, NaBH4.
C. Electron transfer
Definition	·       Oxidation is loss of electrons. ·       Reduction is gain of electrons.
How to remember…
A simple example	The equation shows a simple redox reaction which can obviously be described in terms of oxygen transfer.
	Copper(II) oxide and magnesium oxide are both ionic. The metals obviously aren't. If you rewrite this as an ionic equation, it turns out that the oxide ions are spectator ions and you are left with:
Oxidizing and Reducing agents	-        If you look at the equation above, the magnesium is reducing the copper(II) ions by giving them electrons to neutralise the charge. -        Magnesium is a reducing agent. -        Looking at it the other way round, the copper(II) ions are removing electrons from the magnesium to create the magnesium ions. -        The copper(II) ions are acting as an oxidising agent.
How to visualize…	·       An oxidising agent oxidises something else. ·       Oxidation is loss of electrons (OIL RIG). ·       That means that an oxidising agent takes electrons from that other substance. ·       So an oxidising agent must gain electrons.
Or you could think it out like this:	·       An oxidising agent oxidises something else. ·       That means that the oxidising agent must be being reduced. ·       Reduction is gain of electrons (OIL RIG). ·       So an oxidising agent must gain electrons.

Oxidation State in Reduction-Oxidation reaction

• Free elements have oxidation state zero, e.g. Cu, Fe, N₂
• Oxidation of an ion is the charge of the ion
• e.g. Na⁺ = +1, Cu²⁺ = +2, O^2- = -2
• The oxidation state of some elements in their compounds is fixed, e.g.
• Group I Elements = +1
• Group II Elements = +2
• Hydrogen in most compounds = +1
• Iron or copper can have either +1, +2, +3, so it’s not fixed
• Oxidation states of the elements in a compound adds up to zero, e.g.
• NaCl: (+1) + (-1) = 0
• K₂O: (+1) x 2 + (-2) = 0
• Al₂O₃: (+3) x 2 + (+2) x 3 = 0
• Sum of oxidation states of elements in an ion is equal to charge on the ion,
  
• e.g. OH^-: (-2) + (+1) = -1

Examples of elements with variable oxidation states

• Iron(II) chloride has formula FeCl2 and iron oxidation state +2
• Potassium(VI) dichromate has formula K₂Cr₂O₇ and potassium oxidation state +6
• Manganese(IV) oxide has formula MnO2 and manganese oxidation state +4
  

• Oxidation is the increase of oxidation state by a substance
• Losing electrons means gain in oxidation state
• Reduction is the decrease of oxidation state by a substance
• Gaining electrons means loss in oxidation state

• Cu is oxidized as it gains oxidation state from 0 to +2. Cu is reducing agent
• H⁺ ions in HCl reduced as it loses oxidation state from +1 to 0. H⁺ ions are oxidising agent
  

• I^- ions in KI oxidized as it gains oxidation state from -1 to 0. I-ions is reducing agent
• Cl₂ is reduced as it loses oxidation state from 0 to -1. Cl₂ is the oxidizing agent

Test for Oxidising/Reducing Agents

Non-Redox Reactions

• Decomposition of carbonates by heat: CaCO₃ (s) → CO₂ (g) + CaO (s)
• The oxidation state of each element don’t change.
  
• Neutralization: NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l)
• The oxidation state of each element don’t change.
  
• Precipitation reactions: Ag⁺ (aq) + Cl^- (aq) → AgCl (s)
• The oxidation numbers of silver and chloride ions unchanged. 

Reversible Reactions

• Reversible reactions are denoted by the sign “⇌” where the arrow --> denotes forward reaction, where reactants react to form products, and the arrow <-- denotes backward reaction where products decompose to reform reactants.
  
• The reactions occur at the same time.
  
• E.g. N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Effect of Temperature on Reversible Reactions

• With higher temperature, the condition is now favored to break the bonds of the product formed (The bonding of products requires low temperatures).
  
• Thus, the products decompose to its constituents, leading to backward reaction.

Effect of Pressure on Reversible Reactions

• Increase in pressure encourages forward reaction because the higher pressure the more reactants collide to react.

Dynamic equilibrium

• Dynamic Equilibrium is the state when the rate of forward reaction is the same as the rate of backward reaction.
  
• Both reactants are reacted and products decompose at the same rate.
  
• Hence, there is no overall change in the amounts of reactants and products.
• When we remove the products, it will also encourage forward reaction as the reaction would try to achieve equilibrium.
  
• Similar thing happens when we remove the reactants, that the decomposition of products is encouraged to reach the point
  

MCQ Questions

1. Which one of the following is usually described as a reducing agent?
a. carbon monoxide

b. concentrated sulphuric acid

c. copper(II) oxide

d. Sodium oxide

2. Cu⁺ ions may be formed when
a. Cu²⁺ ions are oxidised by sulphur dioxide
b. Cu²⁺ ions are reduced by copper
c. Cu is oxidised by nitric acid
d. Zinc is placed in copper sulphate solution

3. An oxidisng agent was observed to turn from orange to green. It is most likely to be:
a. acidified potassium manganate (VII)
b. acidified potassium dichromate (VI)
c. maganese (IV) oxide
d. iron (III) chloride

4. When iron (II) oxide is heated strongly with carbon
a. iron ions gain 2 electrons
b. carbon is reduced
c. carbon is acting as the oxidising agent
d. only carbon monoxide is formed

5. When iron (III) sulphate is reduced to iron (II) sulphate, the colour of the solution turns from
a. brown to yellow
b. green to yellow
c. red to colourless
d. brown to green

 
6. In which of the following reactions is the underlined substance reduced?

a. C_l2 + 2I^- –> 2Cl^- + I₂
b. Zn + _H2SO₄ –> ZnSO₄ + H₂
c. Fe²⁺ + H₂O₂ –> Fe³⁺ + H₂O
d. CuO + H₂ –> Cu + H₂O

7. Which conversion involves the smallest change in oxidation number of the underlined element?
a. C(s) –> CO₂
b. NO₃^-(aq) –> NO₂(g)
c. SO₃^2-(aq) –> SO₄^2-(aq)
d. MnO₄^-(aq) –> Mn²⁺(aq)

8. The oxidation states of chlorine in ClO4- and Cl2O are respectively
a. +1 and +3
b. +3 and +7
c. +7 and +1
d. +7 and +2

9. Which statement is true about oxidizing agents?
a. Their oxidation state is zero
b. They are easily oxidized
c. They never contain hydrogen
d. They readily accept electrons

10. What does an oxidizing agent do?
a. It turns acidified potassium dichromate (VI) green
b. It turns potassium manganate (VII) colorless
c. It turns Universal Indicator red
d. It turns aqueous potassium iodide brown

11. When zinc reacts with dilute sulphuric acid a gas is released. What happens to the zinc and what is the gas released?