Redox Processes: Electron Transfer & Changes in Oxidation Number (Oxidation State) (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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Define oxidation number.
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Define oxidation number.
An oxidation number (or oxidation state) is a number assigned to each atom in a compound to track electron transfer. A positive value indicates loss of electrons and a negative value indicates gain of electrons.
What is the oxidation number of sulfur in SO42-?
Oxygen contributes 4 x (-2) = -8. The overall charge is -2. So S = -2 - (-8) = +6.
True or False?
The oxidation number of oxygen is always -2 in all compounds.
False.
Oxygen is usually -2, but it is -1 in peroxides (e.g. H2O2) and +2 in F2O, where fluorine is more electronegative.
The sum of oxidation numbers in a neutral compound is .......... . The sum in an ion equals .......... . The oxidation number of any uncombined element is .......... .
The sum of oxidation numbers in a neutral compound is zero. The sum in an ion equals the charge on the ion. The oxidation number of any uncombined element is zero.
What is the oxidation number of nitrogen in NH3?
Hydrogen contributes 3 x (+1) = +3. Overall charge = 0. So N = 0 - 3 = -3.
True or False?
Hydrogen always has an oxidation number of +1.
False.
Hydrogen is +1 in most compounds, but -1 in metal hydrides such as NaH, where it is the more electronegative element.
Define fixed oxidation number.
Some elements have fixed oxidation numbers in compounds. Group 1 metals are always +1, Group 2 metals are always +2, and fluorine is always -1 because it is the most electronegative element.
In ClO2-, oxygen contributes .......... and the overall charge is -1. The oxidation number of chlorine is therefore .......... .
In ClO2-, oxygen contributes -4 and the overall charge is -1. The oxidation number of chlorine is therefore +3.
What is the oxidation number of phosphorus in P2O5?
Five O atoms = 5 x (-2) = -10. Overall charge = 0. So 2 P = +10, giving each P atom an oxidation number of +5.
Define disproportionation reaction.
A disproportionation reaction is one in which the same element is simultaneously oxidised and reduced. For example, chlorine reacting with hot concentrated NaOH forms Cl- (reduced) and ClO3- (oxidised).
In the reaction Cu2+ + Mg → Mg2+ + Cu, which species is oxidised and which is reduced? Use oxidation numbers to justify your answer.
Mg is oxidised (oxidation number 0 to +2, loss of electrons). Cu2+ is reduced (oxidation number +2 to 0, gain of electrons).
True or False?
Oxidation involves the gain of electrons.
False.
Oxidation is the loss of electrons (and causes an increase in oxidation number). The acronym OIL RIG (Oxidation Is Loss, Reduction Is Gain) is a useful memory aid.
Oxidation causes an .......... in oxidation number. Reduction causes a .......... in oxidation number. In a redox reaction, .......... and .......... always occur together.
Oxidation causes an increase in oxidation number. Reduction causes a decrease in oxidation number. In a redox reaction, oxidation and reduction always occur together.
Define redox reaction.
A redox reaction is a reaction in which oxidation and reduction occur simultaneously. One species loses electrons (is oxidised) while another gains electrons (is reduced).
List the five steps for balancing a redox equation using oxidation numbers.
Write the unbalanced equation and identify atoms that change in oxidation number. 2. Deduce the oxidation number changes. 3. Balance the oxidation number changes. 4. Balance the charges. 5. Balance the atoms.
True or False?
In the reaction 2Na + Cl2 → 2NaCl, chlorine is oxidised.
False.
Chlorine is reduced (oxidation number changes from 0 to -1, a gain of electrons). Sodium is oxidised (0 to +1, a loss of electrons).
Oxidation can be defined as the gain of .......... , the loss of .......... , or the loss of .......... . Reduction is the reverse of each of these.
Oxidation can be defined as the gain of oxygen, the loss of hydrogen, or the loss of electrons. Reduction is the reverse of each of these.
In the reaction 3Cl2 + 6OH- → 5Cl- + ClO3- + 3H2O, why is this classified as a disproportionation reaction?
Chlorine (oxidation number 0 in Cl2) is both reduced to Cl- (oxidation number -1) and oxidised to ClO3- (oxidation number +5). The same element undergoes both oxidation and reduction.
Define oxidising agent.
An oxidising agent is a substance that oxidises another species by causing it to lose electrons. The oxidising agent itself gains electrons and is reduced, so its oxidation number decreases.
In the reaction Mg + Fe2+ → Mg2+ + Fe, which species is the oxidising agent and why?
Fe2+ is the oxidising agent. Its oxidation number decreases from +2 to 0 (it gains electrons and is reduced), which means it causes Mg to be oxidised.
True or False?
A reducing agent loses electrons and its oxidation number increases.
True.
A reducing agent is itself oxidised in the process of reducing another species. It donates electrons, causing its oxidation number to rise.
An oxidising agent is itself .......... in a redox reaction. A reducing agent is itself .......... . Both an oxidising and a .......... agent must be present for a redox reaction to occur.
An oxidising agent is itself reduced in a redox reaction. A reducing agent is itself oxidised. Both an oxidising and a reducing agent must be present for a redox reaction to occur.
Define reducing agent.
A reducing agent is a substance that reduces another species by donating electrons to it. The reducing agent itself is oxidised, losing electrons and increasing in oxidation number.
How are Roman numerals used in naming compounds of transition metals? Give two examples.
Roman numerals indicate the oxidation state of the transition metal. For example, FeO is iron(II) oxide (Fe2+) and Fe2O3 is iron(III) oxide (Fe3+).
True or False?
In the reaction Cl2 + 2Br- → 2Cl- + Br2, Br- acts as an oxidising agent.
False.
Br- is the reducing agent. Its oxidation number increases from -1 to 0 (it is oxidised). Cl2 is the oxidising agent, being reduced from 0 to -1.
The systematic name of FeCl2 is .......... because iron has an oxidation number of .......... . The systematic name of NO2 is .......... oxide.
The systematic name of FeCl2 is iron(II) chloride because iron has an oxidation number of +2. The systematic name of NO2 is nitrogen(IV) oxide.
Can the same substance act as both an oxidising agent and a reducing agent? Give a brief explanation.
Yes. Some substances can act as either an oxidising or a reducing agent depending on the reaction conditions and the species they react with. Their role is determined by the relative oxidising and reducing power of the other reactant.
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