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Chemistry IG > Topic 7 > Flashcards

Topic 7 Flashcards

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1
Q

What is the oxidation number of elements?

A

Zero

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2
Q

What is the oxidation number of simple ions?

A

Charge

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3
Q

What is oxidation?

A

Increase in oxidation number, loss of electrons (electrons on RHS)

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4
Q

What is reduction?

A

Decrease in oxidation number, gain of electrons (electrons on LHS)

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5
Q

Redox meaning

A

Stands for reduction and oxidation. In a redox reaction there are two species, one is oxidised and one is reduced

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6
Q

Oxidation in terms of oxygen and hydrogen

A

Gain of oxygen, loss of hydrogen

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7
Q

reduction in terms of oxygen and hydrogen

A

Loss of oxygen, gain of hydrogen

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8
Q

What is an oxidant (oxidising agent)

A
  • Oxidises the other species
  • Gets reduced
  • Gains electrons, oxidation number decreases
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9
Q

What is a reductant (reducing agent)

A
  • Reduces the other species
  • Gets oxidised
  • Loses electrons, oxidation number increases
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10
Q

What are reactants and species?

A

Reactants are the full substance not ionic species eg ZnSO4 is the substance that acts as an oxidant but Zn2+ is the species that is reduced

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11
Q

What colour is iodide ion and chloride ion?

A

Colourless, so is bromide

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12
Q

What is electrolysis?

A

The breakdown of an ionic compound in molten or aqueous solution by the passage of the electric current

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13
Q

What compounds can confuct electricity?

A
  • Covalent compounds cannot conduct electricity hence they do not undergo electrolysis
  • Ionic compounds in the solid state cannot conduct electricity either since they haveno free ionsthat can move and carry the charge
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14
Q

What is the current for in electrolysis?

A

Provides voltage that forces electrons to be gained and lost when it wouldn’t otherwise be possible

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15
Q

Why does electrolysis occur?

A

The charged ions in the electrolyte allows electrons to flow.
- The electrons move through the external circuit towards the cathode
- These electrons are gained by the cations and transferred from the cathode. Reduction occurs at the cathode
- The electrons are lost from the anion and transferred from the anode. Oxidation occurs at the anode

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16
Q

What is an electrode?

A

Electrodes are the rods that carry the electric current to and from the electrolyte. These rods are usually inert, as in they do not react- usually graphite or platinum. Anode is positive electrode and cathode is negative electrode

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17
Q

What is electrolyte?

A

the compound that conducts electricity when molten and breaks down during electrolysis

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18
Q

Why do electrodes need to be inert?

A

need to be inert such as graphite or platinum so that they don’t participate in a side reaction with the electrolyte

19
Q

What happens during electrolysis?

A
  • During electrolysis, current needs to flow around the circuit. In order for this to occur, charge must be transferred around the circuit (current is a measure of the rate of flow of charge) by charge carriers.
  • The power supply provides thecathodewith a supply ofelectrons, causing it to become negatively charged.
  • Positive ions(cations) in the electrolyte move towards the cathode where theygain electrons to become atoms (in a positive ion, there are more protons than electrons, so in order to become an atom, they need to gain those extra electrons to become neutral). Soreduction occurs at the cathode.
  • Negative ions(anions) in the electrolyte move towards the anode where theylose electrons to become atoms (in a negative ion, there are more electrons than protons, so in order to become an atom, they need to lose those extra electrons to become neutral). Sooxidation occurs at the anode.
  • Theelectronsmove from the anode back towards the power supply which then goes back to the cathode creating a circuit.
20
Q

What are the three types of electrolyte solutions?

A
  1. Molten (liquid, no water)
  2. Dilute aqueous solution (lots of water)
  3. Concentrated aqueous solution (less water)
21
Q

What happens in the electrolysis of a molten electrolyte?

A
  • A molten ionic compound consists of only the two elements joined by ionic bonding
  • During electrolysis, they always produce the corresponding element. Thepositiveion will migrate towards thecathodeand thenegativeion will migrate towards theanode, therefore, thecathodeproduct will always be themetal,and the product formed at theanodewill always be thenon-metal
22
Q

Electrolysis of molten lead(II) bromide

A

Cathode (reduction) reaction: Pb2+(aq) + 2e-→ Pb(s)
Anode (oxidation) reaction: 2Br-(aq) → Br2(aq)+ 2e-
Observations: There is bubbling at the anode as brownbromine gas is given off. At the cathode agrey lead metalis deposited on the surface of the electrode.

23
Q

What happens in the electrolysis of a dilute aqueous solution?

A
  • In dilute aqueous electrolytes there are positive and negative ions from the salt and a large number of H+ and OH- ions
  • At the anode (oxidation) negative non-metal ions and OH- ions from water are attracted. Only O2 forms (This is because OH- is the least reactive of the negative ions)
  • At the cathode (reduction) positive metal ions and H+ ions from water are attracted. Either metal metal or H2 is formed depending on if the metal is more reactive than hydrogen (if metal is more reactive hydrogen gas forms, if metal is less reactive metal forms (ex: copper, silver))
24
Q

Most to least electronegative

A

F-
SO42-
NO3-
Cl-
Br-
I-
OH-

25
Q

Most to least electropositive

A

K+
Na+
Ca2+
Mg2+
Al3+
Zn2+
Fe2+
Sn2+
Pb2+
H+
Cu2+
Ag+

26
Q

What happens in the electrolysis of a concentrated aqueous solution?

A
  • In a concentrated aqueous solution, there are large amounts of positive and negative ions from the salt as well as H+ and OH- from water
  • At anode (oxidation), negative non-metal ions and OH- are attracted. If halide ions are present halogen gas will form, otherwise O2 will form
  • At cathode (reduction), positive metal ions and H+ are attracted. Either metal or H2 will form depending on which one is less reactive
27
Q

Electrolysis of concentrated aqueous sodium chloride

A
  • At the negative cathode: There is H+ and Na+. H+ is discharged because it is less reactive than sodium and it gains electrons to form hydrogen gas (a colourless gas that causes bubbling at the cathode). 2H+ + 2e- → H2
  • At the positive anode: There is Cl- and OH-. Cl- is discharged because it is a halide. It loses electrons to form chlorine gas (Cl2). 2Cl- → Cl2 + 2e- (pale green-yelloe gas, bubbling)
  • Left over solution: The sodium ions (Na+) and hydroxide ions (OH-) are left and form the sodium hydroxide solution (NaOH)
28
Q

Electrolysis of dilute sulfuric acid

A
  • At the cathode: There are two H+ ions meaning there is no choice and they both form hydrogen gas. 2H+ + 2e- → H2
  • At the anode: There is OH- and (SO4)2-. OH- is discharged because it is a dilute solution and sulphate is never discharged. The hydroxide ion is attracted there and loses its electrons to form oxygen gas and water. 4(OH)- → 2H2O + O2 + 4e-
  • Bubbles of gas are seen at both electrodes
29
Q

Electrolysis of aqueous copper(II) sulfate using copper electrodes

A
  • Thecathode increasesin mass while theanodedecreases. This occurs as copper atoms areoxidisedat the anode and form ions while copper ions arereducedat the cathode, forming copper atoms. The gain in mass by the negative electrode is thesameas the loss in mass by the positive electrode. Therefore, the copper deposited on the negative electrode must be thesamecopper ions that are lost from the positive electrode
  • The copper atoms in the electrode on the anode lose electrons and become copper ions. These are sent over to the cathode where they are turned back into copper atoms. Therefore, the anode loses weight and the cathode gains weight.
30
Q

Equation for oxygen at the anode

A

4OH- -> 2H2O + O2 + 4e-

31
Q

Equation for hydrogen at the cathode

A

2H+ + 2e- ->H2

32
Q

What is electroplating?

A

Electroplating uses electolysis to coat one metal on another or to purify an impure metal (a process where the surface of one metal iscoatedwith a layer of a different metal.)

33
Q

Common metals used for electroplating

A

Copper
Chromium
Silver
Tin

34
Q

How to do electroplating through electrolysis?

A

Theanodeis made from thepuremetal you want to coat your object with. Thecathodeis theobjectto be electroplated. The electrolyte is an aqueous solution of an ionic compound of the plating metal. The atoms in the electrode on the anode lose electrons and become ions. These are sent over to the cathode where they are turned back into atoms and plate the cathode.

35
Q

Two reasons for electroplating

A
  1. To provide a protective coating to prevent corrosion
  2. To create a shiny appearance
36
Q

Halogens and their electronic configuration

A

All diatomic molecules
Fluorine (2,7)
Chlorine (2,8,7)
Bromine (2,8,18,7)
Iodine (2,8,18,18,7)
Astatine (2,8,18,32,18,7)

37
Q

Trends in Group VII

A
  • Density increases as you go down the group (states change)
  • Reactivity decreases as you go down the group
  • Colour becomes darker as you go down the group
  • Melting and boiling point increases as you go down the group
38
Q

Halogens colour and state

A

Fluorine (yellow, gas)
Chlorine (pale yellow-green, gas)
Bromine (red-brown liquid)
Iodine (Purple-black solid)
Astatine (grey-black, solid)

39
Q

What are halogen displacement reactions?

A

This occurs when a more reactive halogen replaces a less reactive halide in a compound. In a displacement reaction,the more reactive halogen atoms oxidise the less reactive halide ions, causing the halide ions to lose electrons and form halogen atoms
Ex: Fluorine will displace chlorine, bromine, iodine and astatine

40
Q

What is a fuel cell?

A

an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode. H2→ 2H++ 2e-, O2+ 4e-→ 2(O)2-

41
Q

What does a hydrogen-oxygen fuel cell do?

A

Uses hydrogen and oxygen to produce electricity (a voltage) with water as the only chemical product

42
Q

Advantages of hydrogen-oxygen fuel cells

A
  • Can convert most of the chemical energy to electrical energy (no waste heat)
  • Water is the only product, therefore no harmful emissions (more ecofriendly)
  • Can be a renewable source of energy if hydrogen comes from a renewable source
  • Releases more energy per kilogram compared to petrol or diesel
  • Quieter so less noise pollution
  • No power is lost in transmission as there is no moving parts
43
Q

Disadvantages of hydrogen-oxygen fuel cells

A
  • Hydrogen is obtained from methane and carbon is rejected so it contributes to greenhouse effect
  • Hydrogen is more difficult and expensive to store compared to petrol as it is very flammable and easily explodes when under pressure
  • Each fuel cell only makes a small voltage. A large number of fuel cells must be linked to produce a large voltage or current which is expensive
  • Fuel cells are affected by low temperatures and become less efficient
  • Only small number of hydrogen filling stations available
44
Q

Explain why the carbon anodes need to be replaced periodically

A

burns in oxygen / reacts with oxygen / oxidised by oxygen / forms carbon dioxide / forms
carbon monoxide

Chemistry IG (9 decks)

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