Oxidation, Reduction & Redox Equations (AQA A Level Chemistry): Flashcards
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What is the oxidation state of an atom in a compound?
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What is the oxidation state of an atom in a compound?
An average bond enthalpy is the mean energy required to break one mole of a particular type of covalent bond in the gas phase, averaged over a range of different molecules containing that bond.
True or False?
Oxidation is defined as the gain of electrons.
False.
Oxidation is the loss of electrons (and corresponds to an increase in oxidation state). The mnemonic OIL RIG helps: Oxidation Is Loss, Reduction Is Gain.
In the reaction Cu2+ + Mg → Mg2+ + Cu, magnesium is .......... because its oxidation state .......... from 0 to +2.
In the reaction Cu2+ + Mg → Mg2+ + Cu, magnesium is oxidised because its oxidation state increases from 0 to +2.
What is an oxidising agent and what happens to it during a reaction?
An ionic equation is an equation that shows only the species that actually change during a reaction, omitting spectator ions that remain unchanged in solution.
In the reaction Mg + Fe2+ → Mg2+ + Fe, which species is the oxidising agent?
Fe2+ is the oxidising agent.
It oxidises Mg (0 → +2) by accepting two electrons, and is itself reduced from Fe2+ to Fe (0).
A reducing agent is a substance that .......... electrons and is itself .......... during the reaction, so its oxidation state .......... .
A reducing agent is a substance that donates/loses electrons and is itself oxidised during the reaction, so its oxidation state increases.
True or False?
Roman numerals in compound names (e.g. iron(III) oxide) indicate the oxidation state of the transition metal.
True.
Roman numerals are used because transition metals can have more than one oxidation state. For example, Fe2+ gives iron(II) and Fe3+ gives iron(III).
Can the same substance act as both an oxidising agent and a reducing agent?
Yes. Some substances, such as H2O2, can act as either an oxidising or a reducing agent depending on what they are reacting with and the reaction conditions.
What rule governs the sum of oxidation states in a neutral molecule?
In any neutral molecule, the sum of the oxidation states of all the atoms must equal zero. The more electronegative element takes the negative value.
In SO42-, if each oxygen has an oxidation state of −2, the oxidation state of sulfur is .......... . Show your reasoning.
In SO42-, if each oxygen has an oxidation state of −2, the oxidation state of sulfur is +6.
Reasoning: x + 4(−2) = −2, so x = −2 + 8 = +6.
True or False?
The oxidation state of oxygen is always −2 in every compound.
False.
Oxygen is −2 in most compounds but is −1 in peroxides (e.g. H2O2) and +2 in OF2 (where fluorine, being more electronegative, takes the negative value).
What is the oxidation state of a pure element in its standard state?
The oxidation state of any element in its standard state is zero.
This applies to all pure elements, whether atomic (e.g. Na) or molecular (e.g. O2, Cl2).
In P2O5, if oxygen has an oxidation state of −2, the oxidation state of phosphorus is .......... .
In P2O5, if oxygen has an oxidation state of −2, the oxidation state of phosphorus is +5.
Reasoning: 2x + 5(−2) = 0, so 2x = 10, x = +5.
Why can an oxidation state sometimes appear to be a fraction, such as +2.5?
A fractional oxidation state is a mathematical result of sharing a total oxidation across multiple identical atoms. Individual atoms can only have integer oxidation states; the fraction reflects an average over all equivalent atoms in the formula.
True or False?
In a simple ion, the oxidation state of the atom equals the charge on the ion.
True.
For example, Na+ has an oxidation state of +1, Mg2+ has +2, Cl− has −1, and O2− has −2.
Oxidation state
A number assigned to an atom that represents the degree of oxidation of that atom in a compound or ion. It indicates the hypothetical charge the atom would carry if all bonds were fully ionic.
What is a disproportionation reaction?
A standard enthalpy change of atomisation is the enthalpy change when one mole of gaseous atoms is formed from an element in its standard state under standard conditions (298 K, 100 kPa).
True or False?
In a redox reaction, the total increase in oxidation state must equal the total decrease in oxidation state.
True.
Electrons are neither created nor destroyed. The number of electrons lost by the species being oxidised must equal the number gained by the species being reduced.
In the reaction MnO4− (aq) + Fe2+ (aq) + H+ (aq) → Mn2+ (aq) + Fe3+ (aq) + H2O (l), manganese changes oxidation state from .......... to .......... , a decrease of .......... .
In the reaction MnO4− (aq) + Fe2+ (aq) + H+ (aq) → Mn2+ (aq) + Fe3+ (aq) + H2O (l), manganese changes oxidation state from +7 to +2, a decrease of 5.
In the balanced equation MnO4− (aq) + 5Fe2+ (aq) + 8H+ (aq) → Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l), why are 5 Fe2+ ions needed for every MnO4−?
Each Mn decreases in oxidation state by 5 (from +7 to +2). Each Fe increases by 1 (from +2 to +3).
To balance the electron transfer, 5 Fe2+ ions are required to supply the 5 electrons gained by one Mn.
A redox reaction is one in which oxidation and reduction occur .......... . While one species is .......... , another species is .......... in the same reaction.
A redox reaction is one in which oxidation and reduction occur simultaneously. While one species is oxidised, another species is reduced in the same reaction.
True or False?
When balancing a redox equation, H+ ions are used to balance oxygen atoms.
False.
H+ ions are used to balance hydrogen atoms. Oxygen atoms are balanced by adding H2O molecules. The correct order is: balance the element being oxidised/reduced, then balance oxygen using H2O, then balance hydrogen using H+, then balance charge by adding electrons.
In a redox equation, what does it mean for a species to be 'reduced'?
Reduced describes a species that has had its oxidation state decreased (gained electrons).
A species is reduced when its oxidation state decreases. Reduction always occurs simultaneously with oxidation (redox reactions).
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