elements of the sea Flashcards
What are common inaccuracies in titrations?
Contaminated apparatus, incorrect burette readings.
How do you ensure accurate titration results?
Repeat until you get 3 concordant results; wash flask between repeats.
What is an electrolyte?
An ionic compound whose aqueous solution conducts an electric current.
What is the positive electrode called and what happens there?
Anode; anions move to it and lose electrons.
What is the negative electrode called and what happens there?
Cathode; cations move to it and gain electrons.
How do molten ionic compounds differ from aqueous solutions in electrolysis?
Molten has only one source of ions; aqueous contains H⁺ and OH⁻ ions too.
What forms at the cathode in aqueous electrolysis?
The less reactive between the metal ion and hydrogen.
What forms at the anode in aqueous electrolysis?
Halide ions form halogens; otherwise, oxygen is formed via 4OH⁻ → O₂ + 2H₂O + 4e⁻.
What happens when metal electrodes are used in electrolysis?
They can produce metal ions; in copper, impure anode loses electrons, and copper plates the cathode.
What is brine?
A concentrated solution of sodium chloride.
What are the products of brine electrolysis?
Chlorine gas at anode, hydrogen at cathode; Na⁺ and OH⁻ left to form NaOH.
What condition is needed for electrolysis of brine to work?
The sodium chloride solution must be concentrated.
How is bromine made from brine?
Bubble chlorine gas through brine to displace bromine, then condense and purify.
How is iodine made from brine?
Bubble chlorine through brine to displace iodine, then purify and condense to grey solid.
What are the rules for assigning oxidation states?
Uncombined elements = 0; oxygen = -2 (except peroxides/fluorides); hydrogen = +1 (except metal hydrides).
What is oxidation and reduction?
Oxidation = loss of electrons, Reduction = gain of electrons.
How do you find the concentration of an oxidising agent using iodine and thiosulfate?
Add oxidising agent to excess reducing agent to form iodine, then titrate with thiosulfate.
What is the equation for iodine-thiosulfate reaction?
I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻
Why use starch in iodine-thiosulfate titration?
Added when iodine goes pale yellow; blue to colourless end point is easier to see.
How does volatility change down group 7?
Decreases due to increased molecular size and stronger id-id forces.
How do halogens behave in water vs hexane?
Water: Cl₂ - colourless, Br₂ - yellow, I₂ - brown
Hexane: Cl₂ - colourless, Br₂ - orange, I₂ - violet
Why are halogens more soluble in organic solvents than in water?
Because they are non-polar and dissolve better in non-polar solvents like hexane.
How does reactivity of halogens change down the group?
Decreases due to increased shielding and atomic radius, making it harder to gain electrons.
How are hydrogen halides prepared?
By reacting a solid ionic halide with concentrated acid like phosphoric acid.
How does sulfuric acid react with ionic halides?
Cl⁻ forms HCl
Br⁻ forms Br₂ and SO₂
I⁻ forms I₂ and H₂S
How does bond strength of hydrogen halides change down the group?
It decreases due to larger halogen atoms and increased shielding.
Are hydrogen halides acidic?
Yes, they dissociate in water; HF is a weak acid as it doesn’t fully dissociate.
What happens when hydrogen halides react with ammonia?
They form ammonium halides.
Which hydrogen halides reduce sulfuric acid?
HBr reduces it to SO₂, HI to H₂S; HCl and HF are not strong enough reducing agents.
How is chlorine stored and transported?
As a liquid under pressure in metal cylinders.
What are common uses of chlorine?
Water treatment (kills bacteria), bleach production.
What is the formula for atom economy?
(Molecular mass of desired products / Molecular mass of all products) × 100
What is dynamic equilibrium?
A state where forward and reverse reactions occur at the same rate in a closed system.
How does changing concentration affect equilibrium?
It doesn’t change Kc; equilibrium shifts to maintain Kc.
What does Kc tell you about the position of equilibrium?
Kc > 1 = lies to the right; Kc < 1 = lies to the left.
How does pressure affect equilibrium?
Increasing pressure shifts it to the side with fewer gas molecules.
How does temperature affect equilibrium?
Increasing temp shifts it to the endothermic side; decreasing shifts to exothermic.
