OXIDATION AND REDUCTION REACTIONS Flashcards
1
Q
- What happens in this reaction where Sodium Magnesium Metal is placed in a solution of Hydrochloric Acid?
A
- the magnesium dissolves in the acid
- there are bubbles of the colourless hydrogen gas that
are released
2
Q
- Why is this an example of a Redox Reaction?
A
- Magnesium loses electrons
- it is oxidised
- Mg (s) → Mg2+ (aq) + 2e-
- the Hydrogen ions from the hydrochloric acid gain
electrons - they become hydrogen gas
- they gain electrons
- this is known as reduction
- 2H+ (aq) + 2e- →H2 (g)
3
Q
- What are the rules of Assigning Oxidation Numbers?
A
- An atom in its elemental form has an oxidation
number of 0 - An atom in a monatomic ion has an oxidation number
equal to its charge - An atom in a polyatomic ion or in a molecular
compound has the same oxidation number as it
would have if it were a monatomic ion - The sum of oxidation numbers is 0 for neutral
compounds
. it is also equal to the total charge for a polyatomic ion
4
Q
- What are the exceptions for the Oxidation Numbers
Rules?
A
- Hydrogen can either be +1 or -1
- When Hydrogen is bonded to a metal:
- it will have a -1 charge
- When Hydrogen is bonded to a nonmetal:
- it will have a +1 charge - Oxygen usually has a charge of -2
- except in peroxides
- which contain the O₂2- anion
- this is where oxygen has a charge of -1
- Halogens (Group 7 elements) are usually -1
- except when bonded to Oxygen or Fluorine
- these are highly electronegative elements
- halogens have a positive oxidation number when
bonded to these elements
5
Q
- What are the Oxidation Numbers for CdS?
A
- S is in Group 6
- it has a 2- charge
- Cd has a charge of 2+
6
Q
- What are the Oxidation Numbers for AlH3?
A
- H has a charge of 1-
- 3 H’s have a charge of 3-
- Al has a charge of 3+
7
Q
- What are the Oxidation Numbers for (S2 O3)2-?
A
- O has a charge of 2-
- 3 O’s have a charge of 6-
- the total charge of the polyatomic ion of 2-
- the two S’s must add up to 4+
- this means that one S has a charge of 2+
8
Q
- What are the Oxidation Number for Na2 Cr2 O7?
A
- Na has a charge of 1+
- 2 Na’s have a charge of 2+
- O has a charge of 2-
- 7 O’s have a charge of 14-
- Na2 Cr2 O7
- should have an overall charge of 0
- the two Cr’s must add up to 12+
- each Cr has a charge of 6+
9
Q
- What are the Oxidation Number for FeSO4?
A
- SO4 has a charge of 2-
- Fe needs to have a charge of 2+
- O has a charge of 2-
- 4 O’s have a charge of 8-
- S must have a charge of 6+
10
Q
- Learn these.
A
- deal
11
Q
- Which elements are oxidised and which are reduced
in the following equation:
H2 (g) + ½ O2 (g) → H2O (l)
A
- the oxidation number for Hydrogen goes from 0
- to 1+
- it loses an electron for each hydrogen atom
- two electrons are last in total
- the hydrogen is oxidised
- the Oxygen goes from 0 to 2-
- it is reduced
12
Q
- What is a substance that causes reduction called?
A
- a reducing agent
- the reducing agent is oxidised
13
Q
- What is a substance that causes oxidation called?
A
- an oxidising agent
- the oxidising agent is reduced
14
Q
- In this equation, which is the oxidising and which is the reducing agent?
Mg + Cl2 → Mg2+ + 2Cl-
A
- Mg loses 2 electrons
- it is oxidised
- Cl gains one electron
- it is reduced
- Cl2 is the oxidising agent
- Mg is the reducing agent
15
Q
- In this equation, which is the oxidising and which is the reducing agent?
Ca (s) + 2 H+ (aq) → Ca2+(aq) + H2 (g)
A
- Ca loses 2 electrons
- it is oxidised
- H+ gains one electron
- it is reduced
- H+ is the oxidising agent
- Ca is the reducing agent
16
Q
- In this equation, which is the oxidising and which is the reducing agent?
2FE²⁺ (aq) + Cl2 (g) → 2 Fe3+(aq) + 2Cl-(aq)
A
- Fe2+ loses 2 electrons
- it is oxidised
- Cl2 gains one electron
- it is reduced
- Cl2 is the oxidising agent
- Fe2+ is the reducing agent
17
Q
- What is the result of a reaction with an aqueous cation with a free element?
AQUEOUS CATION = metal ion
A
- it gives a different cation
- it gives a different free element
(this is an example of simple redox reaction)
18
Q
- What happens when an iron nail reacts with a copper sulphate solution?
Fe(s) + Cu2+(aq) → Fe2+ (aq) + Cu (s)
A
- this is a spontaneous reaction
- it can happen naturally
- there is no added energy needed to complete the
reaction - the Copper Sulphate solution will go from blue to
green - this indicates an aqueous solution of Fe2+
- the iron nail will form a layer of elemental copper on it
- it is brown/orange in colour
19
Q
- Would this reaction be able to work in reverse:
Fe(s) + Cu2+(aq) → Fe2+ (aq) + Cu (s)
A
- no
- unless there is an energy input
- this reaction is not spontaneous
20
Q
- What does this image show?
A
- the Activity Series
- the most reactive elements are at the top of the
table - they are the most easily oxidised
- elements at the bottom of the table do not oxidise easily
- they do not rust easily
21
Q
21.1. Does this reaction occur spontaneously?
Mg(s) + 2 H+(aq) → Mg2+(aq) + H2(g)
21.2. Would it be spontaneous if we replaced the
Magnesium with Gold?
A
21.1. yes
- the magnesium will dissolve in the acid
- this forms a solution
- this is because Mg is higher on the table than
hydrogen (H2)
21.2. no
- no reaction will take place
- this is because Gold is a very non reactive
metal
- it is lower than H2 on the Activity Series
- this means that it will not react with an acid
22
Q
- Will this reaction take place spontaneously as written?
Hg2+(aq) + Zn(s) → Hg(l) + Zn2+(aq)
A
- yes
- Hg2+ is being reduced
- Zn is being oxidised
- Zn is higher than Hg on the Activity Series
- this means that it prefers to be in ionic form
- this means that Hg prefers to be in Metallic form
- this matches the written reaction
23
Q
- Will this reaction take place spontaneously as written?
Cu(s) + 2 H+(aq) → Cu2+(aq) + H2(g)
A
- no
- Cu is oxidised
- H+ is reduced
- Cu is below the H2 in the Activity Series
- it prefers to stay in its metallic form
- H2 prefers to be in ionic form
- this does not match the written reacton
23
Q
- Will this reaction take place spontaneously as written?
Cu(s) + 2 H+(aq) → Cu2+(aq) + H2(g)
A
- no
- Cu is oxidised
- H+ is reduced
- Cu is below the H2 in the Activity Series
- it prefers to stay in its metallic form
- H2 prefers to be in ionic form
- this does not match the written reacton
24
Q
- Read through this summary.
Does everything make sense?
A
- yes
