Elements Of The Sea Flashcards
Electrolysis is?
Breaking a substance down using electricity.
If you pass an electric current though an IONIC substance that is molten or in a solution, it breaks down into the elements that it’s made of. This is electrolysis.
It requires a liquid to conduct the electricity, called the electrolyte. The electrolyte contains free ions, which is usually molten or dissolved ionic substance.
In either case it’s the free ions which conduct the electricity.
For the circuit to be complete, there just be a flow of electrons:
Negative ions (anions) go to the positive electrode (the anode), and lose electrons.
Positive ions (cations) go to the negative electrode (the cathode), and gain electrons.
The electrodes must be able to conduct electricity, so they much have free electrons - graphite and platinum are used. They must also be inert so they don’t react.
Anode and cathode formation?
Negative ions are formed at the anode.
E.g. 2Br- —> Br2 + 2e-
Positive ions are formed at the cathode.
E.g. Pb2+ + 2e- —> Pb
Cathode - reduction occurs. Ions are accepted.
Anode - oxidation occurs. Ions are donated.
Procedure for carrying out the electrolysis of an aqueous solution?
- Use wires and clips to connect each electrode to the power supply. The electrode connected to the positive pole will be the anode, and the electrode connected to the negative pole will be the cathode.
- Use inert electrodes (such as platinum or carbon electrodes) so that they don’t start reacting and interesting with the electrolysis.
- Place the electrodes into a beaker containing the electrolyte, making sure the electrodes are not touching.
- Turn the power supply on.
- Depending on what electrolyte your using, the products will form as metals (thin layer on surface of the cathode, called ‘plating’) or as gases (bubbles at the cathode or anode).
Look at notes for diagram.
The half equations for anodes and cathodes will show?
Half equations for anodes will show negative ions losing electrons to form atoms.
Half equations for cathodes will show positive ions gaining electrons for form atoms.
E.g. molten zinc chloride (ZnCl2):
Anode: 2Cl-(l) —> Cl2 (g) + 2e-
Cathode: Zn2+ (l) + 2e- —> Zn(s)
Atoms and charges must both be balanced.
Electrolysis of a molten salt?
A molten salt is just a molten ionic compound.
The product will be elements, because the only ions available are the ones that make up the salt.
For example, PbBr2 is an ionic compound.
Pb is lead - positive. So it is a cation, that will go to the cathode, and gain electrons.
Pb2+ (l) + 2e- —> Pb (s)
Br - negative. So it is an anion, that does to the anode, and loses electrons.
2Br- —> 2e- + Br2 (s)
Electrolysis of an aqueous solution?
This is slightly more complicated than ionic compounds.
In aqueous solutions, you have H+ and OH- ions from the water as well as the ions from the ionic compounds.
The products formed at each electrode depend on the reactivity of the ions, as well as the concentration of the salt solution.
There are a few rules to help you work out what will happen at each electrode. They’re on next flashcard.
How to know what is formed in an aqueous electrolysis reaction?
The products formed at each electrode depends on the reactivity of ions and the concentration of the salt solution.
Look at reactivity series.
The cathode:
- if the metal is less reactive than hydrogen, a metal will form.
- if a metal is less reactive than hydrogen, hydrogen gas is formed (from H+ of water in aqueous solution).
The anode:
- if the solution doesn’t contain a halide ion (F-, Cl-, etc), then oxygen will form (from OH- in water).
- if the solution is concentrated, and has a halide ion, then the halogen is formed (Cl2),
- if it is dilute, oxygen is formed.
Reactivity series?
(Most reactive)
Potassium Sodium Calcium Magnesium Aluminium - Carbon Zinc Iron Tin Lead - Hydrogen Copper Silver Gold Platinum
(Least reactive)
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Purification of copper?
The purification of copper is an electrolysis reaction, takes place in an aqueous solution.
Diagram on notes.
At the anode, electrons are lost. This means the copper that makes up the electrode looses electrons. This forms Cu2+ ions.
The ions are attracted to the cathode. The cathode gains electrons, and so a plate of Cu2+ ions forms on the copper cathode.
The anode breaks down and loses mass whilst the cathode builds and gains mass.
Cathode: Cu2+ +2e- —> Cu
Anode: Cu —> Cu2+ + 2e-
Rules for electrolysis of an aqueous solution: at a cathode?
Cathode:
1. If the metal is less reactive than hydrogen (e.g. silver or copper), than the metal will be formed.
- If the metal is more reactive than hydrogen (e.g. all group 1 and group 2 metals, and aluminium), hydrogen gas will be formed (from the hydrogen ions in the water).
Rules for the electrolysis of an aqueous solution: at an anode?
Anode:
1. If the solution doesn’t contain a halide, oxygen will be formed (from hydrogen ions in water).
- If the solution is concentrated and contains a halide, then the halogen will be formed.
- If the solution contains a halide but is dilute, oxygen will be formed again (from hydrogen atoms in water).
Half equation for the production of oxygen at an anode in an aqueous solution?
If the solution doesn’t contain a halide, oxygen will be formed at the anode in an aqueous solution.
Half:
4OH- (aq) —> O2 (g) + 2H2O (l) + 4e-
Group 7?
The halogens.
They exist as diatomic molecules (covalent). Usually Cl2, F2, Br2, etc.
In compounds with metals, they exist as 1- ions.
We call these halides: Cl- is chloride, F- is fluoride, Br- is bromide, I- is iodide, As- is astatide.
The halides are ionic.
Melting and boiling points increase down the group.
Physical properties of fluorine?
F2
At room temperature: pale yellow gas
Melting point: 53K
Boiling point: 85K
Solubility at 298K/grams per 100g of water: reacts with water.
Physical properties of chlorine?
Cl2
At room temperature: green gas
Melting point: 172K
Boiling point: 239K
Solubility at 298K/grams per 100g of water: 0.6g
Physical properties of bromine?
Br2
At room temperature: dark red volatile liquid
Melting point: 266K
Boiling point: 332K
Solubility at 298K/grams per 100g of water: 3.5g
Physical properties of iodine?
I2
At room temperature: shiny gray/black solid - sublimes to give a purple vapour on warming
Melting point: 387K
Boiling point: 457K
Solubility at 298K/grams per 100g of water: 0.03g
Dissolution of group 7 elements?
Halogens are more soluble in non-polar solvents (like cyclohexane) then they are in polar solvents (like water).
- Chlorine is pale green when dissolved in water.
- Bromine is orange/yellow when dissolved in water.
- Iodine is brown when dissolved in water.
- Chlorine is pale green when dissolved in cyclohexane.
- Bromine is orange/brown/red when dissolved in cyclohexane.
- Iodine is violet when dissolved in cyclohexane.
Redox half equations of halogens?
These more clearly show the redox processes occurring:
F2 (aq) + 2 I- (aq) → 2 F- (aq) + I2 (aq)
Half equation for oxidation:
2 I-(aq) → I2 (aq) + 2e (aq)
Half equation for reduction:
F2 (aq) + 2e → 2F- (aq)
Oxidation has the e on the right side of the arrow, reduction has the e on the left side of the arrow.
Boiling point trends of halogens?
Boiling points increases as you go down the group.
This is because there are more instantaneous dipole-induced dipole bonds.
There are more of these bonds because the relative mass of the atoms increases down the group.
Electronegativity of halogens?
Electronegativity decreases down the group.
Electronegativity is the ability of an atom to attract electrons towards itself in a covalent bond.
This is because the atoms get larger and so there is an increased distance between the nucleus and the outer electrons as you go down the group.
This means there is more shielding (more shells between nucleus and outer electrons).
