Redox Chemistry & Acid-Base Titrations (Edexcel International A Level (IAL) Chemistry): Flashcards
Exam code: YCH11
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What is an oxidation number?
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What is an oxidation number?
An oxidation number is the charge that would exist on an individual atom if the bonding in a compound were completely ionic. It represents the electronic status of an element.
What is the oxidation number of any element in its free (uncombined) state?
The oxidation number of any element in its free state is 0. For example, Fe, O2, and Cl2 all have oxidation number 0.
In metal hydrides such as NaH, hydrogen has an oxidation number of .......... rather than its usual +1.
In metal hydrides such as NaH, hydrogen has an oxidation number of –1 rather than its usual +1.
What is Stock notation?
Stock notation is the use of Roman numerals in a compound name to indicate the oxidation number of an element with a variable oxidation state, such as iron(II) oxide or iron(III) oxide.
What rule governs the sum of oxidation numbers in a neutral molecule or compound?
The sum of all oxidation numbers in a neutral molecule or compound must equal zero. For example, in CO2: the oxidation number of C is +4 and each O is –2, giving a total of 0.
Oxygen is usually assigned an oxidation number of .......... in compounds, but in peroxides such as H2O2 its oxidation number is .......... .
Oxygen is usually assigned an oxidation number of –2 in compounds, but in peroxides such as H2O2 its oxidation number is –1.
What is the oxidation number of nitrogen in the nitrite ion, NO2–?
The oxidation number of nitrogen in NO2– is +3.
Each oxygen contributes –2 (total –4). The ion has overall charge –1, so N + (–4) = –1, giving N = +3.
True or False?
Transition metals always have a fixed oxidation number in all their compounds.
False.
Transition metals are characterised by having variable oxidation numbers. Roman numerals are used in compound names to show which oxidation state is present.
What is the oxidation number of manganese in potassium manganate(VII), KMnO4?
The oxidation number of manganese in KMnO4 is +7.
K is +1 and each O is –2 (total –8). For the compound to be neutral: +1 + Mn + (–8) = 0, so Mn = +7.
What is an oxidising agent?
An oxidising agent is a substance that oxidises another atom or ion by causing it to lose electrons. The oxidising agent itself gains electrons and is reduced, so its oxidation number decreases.
What happens to the oxidation number of an atom when it is oxidised?
The oxidation number increases when an atom is oxidised. Oxidation involves the loss of electrons, which raises the oxidation number.
In a redox reaction, the oxidising agent itself gets .......... and its oxidation number .......... .
In a redox reaction, the oxidising agent itself gets reduced and its oxidation number decreases.
What does the acronym OIL RIG stand for in redox chemistry?
OIL RIG stands for: Oxidation Is Loss (of electrons) and Reduction Is Gain (of electrons). It is a memory aid for the definitions of oxidation and reduction.
True or False?
An oxidising agent loses electrons during a redox reaction.
False.
An oxidising agent gains electrons and is itself reduced. It is the reducing agent that loses electrons.
In the reaction 2NH3 + 3Br2 → N2 + 6HBr, which species is the reducing agent and why?
Nitrogen (in NH3) is the reducing agent. Its oxidation number increases from –3 to 0, meaning it loses electrons and is oxidised. It reduces the bromine by donating electrons.
What is a reducing agent?
A reducing agent is a substance that reduces another atom or ion by causing it to gain electrons. The reducing agent itself loses electrons and is oxidised, so its oxidation number increases.
Can a substance act as both an oxidising agent and a reducing agent?
Yes, some substances can act as both an oxidising agent and a reducing agent. Their role depends on what they are reacting with and the reaction conditions.
For a reaction to be classified as a redox reaction, there must be both an .......... and a .......... present.
For a reaction to be classified as a redox reaction, there must be both an oxidising agent and a reducing agent present.
What makes a disproportionation reaction different from other redox reactions?
In a disproportionation reaction, the same species acts as both the oxidising agent and the reducing agent. This means one element changes to a higher and a lower oxidation number at the same time.
What is a disproportionation reaction?
A disproportionation reaction is a reaction in which the same element is simultaneously oxidised and reduced in a single reaction.
What are the key steps to balance a disproportionation reaction?
Write the unbalanced equation and identify atoms that change oxidation number.
Deduce the oxidation number changes.
Balance the oxidation number changes using coefficients.
Balance the charges.
Balance the remaining atoms.
When Cl2 reacts with hot concentrated NaOH, what two chlorine-containing ions are produced?
The two chlorine-containing ions produced are Cl– and ClO3–. This is a disproportionation reaction: Cl2 is simultaneously reduced to Cl– and oxidised to ClO3–.
True or False?
In a disproportionation reaction, two different elements are simultaneously oxidised and reduced.
False.
In a disproportionation reaction, it is the same element that is simultaneously oxidised and reduced.
What is the oxidation number of chlorine in Cl2 and in the Cl– ion?
Chlorine in Cl2 has oxidation number 0. In the Cl– ion, its oxidation number is –1. The change from 0 to –1 shows that chlorine has been reduced.
In a disproportionation reaction, the same species is simultaneously .......... and .......... in a single reaction.
In a disproportionation reaction, the same species is simultaneously oxidised and reduced in a single reaction.
What is the oxidation number of chlorine in ClO3– and how does it compare to its oxidation number in Cl2?
Chlorine in ClO3– has oxidation number +5. In Cl2 it is 0, so the change from 0 to +5 shows that chlorine has been oxidised.
What is a half equation?
A half equation shows what happens to the electrons for one species in a redox reaction — either the gain or the loss of electrons. It represents only half of the overall electron transfer process.
What is the correct name for Cu2O using oxidation numbers?
Cu2O is named copper(I) oxide. The oxidation number of copper is +1 because each O is –2 and the two Cu atoms together must contribute +2.
Non-metals generally form .......... ions by .......... electrons, so their oxidation number decreases.
Non-metals generally form negative ions by gaining electrons, so their oxidation number decreases.
What generally happens to metals during chemical reactions in terms of oxidation number?
Metals generally lose electrons to form positive ions (cations). Their oxidation number increases, meaning they are oxidised.
What is an ionic equation?
An ionic equation shows the net change in terms of ions in a chemical reaction. It is formed by combining two half equations so that electrons cancel out.
What are the steps to balance a half equation for a species being reduced in acidic conditions?
Write the unbalanced equation and balance the atom being reduced or oxidised.
Add H2O to balance oxygen atoms.
Add H+ to balance hydrogen atoms.
Add e– to balance the charge.
True or False?
In a half equation, electrons appear on the left-hand side when oxidation occurs.
False.
When oxidation occurs, the species loses electrons, so electrons appear on the right-hand side. For example: 2Br– → Br2 + 2e–.
How do you combine two half equations to form an overall ionic equation?
Multiply one or both half equations so that the number of electrons in each is equal. Then add all reactants and products together. The electrons cancel and do not appear in the final ionic equation.
When balancing the half equation for MnO4– being reduced to Mn2+, .......... is added to balance oxygen atoms and .......... is added to balance hydrogen atoms.
When balancing the half equation for MnO4– being reduced to Mn2+, H2O is added to balance oxygen atoms and H+ is added to balance hydrogen atoms.
What is the oxidation number of chromium in K2CrO4?
The oxidation number of chromium in K2CrO4 is +6. Each K is +1 (total +2) and each O is –2 (total –8). For the compound to be neutral: +2 + Cr + (–8) = 0, so Cr = +6.
What are the colours of methyl orange in acid and in alkali?
Methyl orange is red in acid and yellow in alkali.
What is an acid-base indicator?
An acid-base indicator is a weak acid or base whose conjugate form has a different colour. It changes colour reversibly depending on the concentration of H+ ions in the solution.
What are the colours of litmus in acid and in alkali?
Litmus is red in acid and blue in alkali.
True or False?
Phenolphthalein is colourless in acid and turns pink in alkali.
True.
Phenolphthalein is colourless in acidic conditions and turns pink in alkaline conditions.
Why is litmus not suitable for use in acid-base titrations?
Litmus does not give a sharp enough colour change at the equivalence point. A good indicator for titrations must change colour sharply at the equivalence point.
When using phenolphthalein in a titration, it is better to place the .......... in the burette because it is easier to observe the .......... appearance of a pink colour.
When using phenolphthalein in a titration, it is better to place the base in the burette because it is easier to observe the sudden appearance of a pink colour.
What is the formula for the concentration of a solution in mol dm–3?
Concentration (mol dm–3) = number of moles of solute (mol) ÷ volume of solution (dm3).
What is the formula for percentage uncertainty?
Percentage uncertainty (%) = (total uncertainty ÷ measured value) × 100.
How is the total experimental percentage uncertainty calculated when multiple pieces of equipment are used?
The total experimental percentage uncertainty is found by adding together all the individual percentage uncertainties from each piece of equipment used in the experiment.
Number of moles (mol) = concentration (mol dm–3) × ..........
Number of moles (mol) = concentration (mol dm–3) × volume (dm3)
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