U3AOS1 - Galvanic Cells as a source of energy

Question 1

Redox Reactions

Answer 1

Transfer of electrons, from one species to another, in two half reactions (oxidation and reduction)

Question 2

Oxidation Number

Answer 2

number of electrons lost or gained by an atom to form a chemical bond with another species

Question 3

Examples of Redox Reactions

Answer 3

Combustion, Photosynthesis

Question 4

How to determine if a redox reaction has occured

Answer 4

If the Oxidation Number of the molecules has changed

Question 5

What is the oxidation number of a free element (e.g. O₂)?

Answer 5

Zero

Question 6

What is the Oxidation of a simple ion (e.g. Na⁺)?

Answer 6

Equal to the charge of the ion

(so Na⁺ is +1)

Question 7

What is the sum of the oxidation numbers of a compound (e.g. KCl or SO₄^2-)

Answer 7

KCl -> ON of 0
SO₄^2- -> ON of -2

Question 8

What is Hydrogens Oxidation Number (in a compound - including special rule)

Answer 8

Normally +1

Except in metal hydrides, where its -1 (e.g. CaH₂)

Question 9

What is Oxygens Oxidation Number (in a compound - including special rule)

Answer 9

Normally -2

Except in peroxides where its -1 (e.g. H₂O₂)

Question 10

Conjugate Reducing & Oxidizing Agents

Answer 10

An electron donator and its corresponding electron reciever

Question 11

Spectator Ions

Answer 11

Ions that are not involved in the redox reaction - will have no change in oxidation number

Question 12

Do Noble Gases react in a redox reaction

Answer 12

No - they can have no change in electrons - as they have a full shell

Question 13

Oxidation (electron, ON, sample reaction)

Answer 13

Reactant loses an Electron
Increase of ON
Reactant = Product + e^-

Question 14

Reduction (electron, ON, sample reaction)

Answer 14

Reactant gains an electron
Decrease in ON
Reactant + e^- = Product

Question 15

Reduction Agent (reductants)

Answer 15

substance that causes the other substance present to undergo reduction

it will itself, undergo oxidation

Question 16

Oxidation Agents (oxidants)

Answer 16

substance that causes the other substance present to undergo oxidation

it will itself, undergo reduction

Question 17

How to balance a half equation

Answer 17

K - key elements
O - balance oxygen by adding H2O
H - balance hydrogens by adding H+
E - balance electrons
S - include states

Question 18

How to balance a basic (alkaline) half equation

Answer 18

use KOHES,

Then, for every H+ present, add OH- to BOTH sides (should cancel out, creating waters instead)

Question 19

What is a Galvanic Cell (+basic structural features)

Answer 19

Electrochemical Cell in which chemical energy from a - spontaneous redox reaction - is converted into - electrical energy -

Each cell contains two half cells and a salt bridge

Question 20

Common Design Feature of Galvanic Cells - Electrodes

Answer 20

material that is able to conduct electricity

Often a metal (which can react) or a pure carbon rod (inert)

Question 21

Common Design Feature of Galvanic Cells - Anode

Answer 21

Negatively Charged Electrode

Oxidation occurs at the anode (will lose electrons)

Label as ‘Anode (-ve)’

Question 22

Common Design Feature of Galvanic Cells - Cathode

Answer 22

Positively Charged Electrode

Reduction occurs at the cathode (gains electrons)

Label as ‘Cathode (+ve)’

Question 23

Common Design Feature of Galvanic Cells - Salt Bridge

Answer 23

connection between two half cells - allows the flow of ions into the two half cells

purpose is to maintain electrical neutrality, and complete the circuit (allowing electrons to flow through the connecting wire) by preventing polarity

Question 24

Common Design Feature of Galvanic Cells - Salt Bridge - how it balances the anode half cell

Answer 24

Over time, this cell becomes positively charged (as there is a loss of electrons - oxidation)

therefore, the salt bridge will provide a flow of negatively charged substances to maintain electrical neutrality

Question 25

Common Design Feature of Galvanic Cells - Salt Bridge - how it balances the cathode half cell

Answer 25

Over time, this cell becomes negatively charged (as there is a gain of electrons - reduction) therefore, the salt bridge will provide a flow of positively charged substances to maintain electrical neutrality

Question 26

Common Design Feature of Galvanic Cells - Salt Bridge - composition requirements

Answer 26

Must be: - soluble in water (doesn't form an insoluble compound - precipitate - as this will prevent a circuit from flowing as electrons cannot flow through solid ionic substances) - nonreactive

Question 27

Chemical Processes of Galvanic Cells - Electron Ion Flows

Answer 27

Electrons will flow form the anode to the cathode and charged substances will flow from the salt bridge

Question 28

Common Design Feature of Galvanic Cells - Separation of Half-Cells

Answer 28

allows for electrical current to be captured through the connecting wire

Question 29

Common Design Feature of Galvanic Cells - Potential Difference

Answer 29

Ecell = Ereduction - Eoxidation

Question 30

What is a Spontaneous Redox Reaction

Answer 30

reactions that can occur naturally (spontaneous), and do not need to be forced

Question 31

What is a direct, spontaneous redox reaction

Answer 31

Occurs when the reacting species are in the same vessel - with chemical energy being released as heat e.g. combustion

Question 32

What is an indirect, spontaneous redox reaction

Answer 32

Occurs when the reacting species are kept separately in different vessels chemical energy is released predominantly as electrical energy (although not 100% efficient, some energy will be released as heat) e.g. galvanic cells

Question 33

Types of Half Cells

Answer 33

Solid/Aqueous Cell: The electrode will participate in the reaction Double Aqueous Cell: Inert Electrode will be used Gas/Aqueous Cell: Inert Electrode will be used - which is often covered in a fine powdery - which increases the surface area of electrode - increasing reaction rate (for platinum electrodes - its called platinum black)

Question 34

Designing Galvanic Cells - how to influence voltage and current

Answer 34

Voltage - depends on how much energy is being released in the reaction - can be determined by looking at the electrochemical series Current - depends on the rate of reaction - with a higher rate of reaction yielding a higher current because the transfer of electrons is occurring at a faster rate