Chapter 3 - Converting chemical energy to electrical

Question 1

When using oxidation numbers oxidation is an

Answer 1

increase in number

Question 2

When using oxidation numbers reduction is a

Answer 2

decrease in number

Question 3

what are the main oxidation number rules?

Answer 3

1. oxygen is always 2-
2. hydrogen in mostly 1+
3. in elemental form O.N is 0
4. O.N on a simple ion is its charge
5. in a covalent compound without O or H the more electronegative element has the lower O.N

Question 4

Constructing half equations what do we do?

Answer 4

We look for conjugate pairs. If a reactant or product has more than just one element we need to take another molecule from the overall reaction and add it in too.

Question 5

How to tell if it is an acidic or alkaline environment?

Answer 5

We first read the question carefully. We also look at the electrolyte.

H+ = acidic 
OH- = alkaline

Question 6

How do we balance in an alkaline environment>

Answer 6

We use KOHES but then just add OH- at the end to cancel out the H+. We add it to both sides and must cancel out the waters created.

Question 7

What is a galvanic cell?

Answer 7

A cell that concerts chemical energy into electrical energy.

Question 8

Why do substances in galvanic cells need to be separated?

Answer 8

As a galvanic cell uses spontaneous reactions. if the reactants were placed in the same container they would just react in the container and therefore the energy could not be harnessed and turned into electrical energy.

Question 9

how to we calculate Standard electrode potential?

Answer 9

E˚cell = E˚cathode - E˚anode

Question 10

What is standard electrode potential?

Answer 10

A galvanic cell’s ability to produce a current. the values are based on how well a substance attracts electrons when compared to a hydrogen half cell.

Question 11

positive EMF

Answer 11

more likely to attract electrons

Question 12

negative EMF

Answer 12

less likely to attract electrons

Question 13

Define a battery

Answer 13

A battery is a series of many cells connected to each other producing a large current

Question 14

Define a cell

Answer 14

A cell is a single unit producing a current

Question 15

What is a wet cell

Answer 15

A cell with a liquid electrolyte

Question 16

What is a dry cell

Answer 16

A cell with a paste electrolyte

Question 17

What is a primary cell

Answer 17

NOT RECHARGEABLE

Question 18

What is a secondary cell

Answer 18

RECHARGEABLE

Question 19

Factors affecting selection of cells

Answer 19

PHYSICAL

• size
• shape
• weight
• temperature of surroundings

EFFICIENCY

• voltage
• current
• memory effect
• shelf life
• rechargeable?

WORLDLY

• manufacturing costs
• operating costs
• ease of disposal

Question 20

What reactant is always at the cathode of a fuel cell

Answer 20

Oxygen gas

Question 21

What does the electrolyte do in a fuel cell?

Answer 21

• Controls ion movement

- acts as a salt bridge/ maintains charge

Question 22

Describe the electrodes of a fuel cell

Answer 22

• porous (allow molecules to pass into electrolyte)
• coated with catalyst
• allow for redox reactions to occur
• break up molecules (e.g: into electrons & protons)

Question 23

Advantages of a fuel cell

Answer 23

• Have the potential to produce electricity forever as long as they are continuously supplied with fuel/reactants.
• high efficiency
• less pollution released

Question 24

Disadvantages of a fuel cell

Answer 24

• parts are expensive (electrodes coated with a catalyst)

- expensive to store fuels (like hydrogen gas)

Question 25

What is “poisoning the catalyst”?

Answer 25

The less pure a substance is, the more likely it will poison the catalyst and render it useless. reaction would still occur but very slowly.

Question 26

when asked to the polarities (e.g top and bottom) of a physical battery we think….

Answer 26

which substance is acting as the anode and cathode? then which ever is closer to the top has the same polarity. e.g: if zinc acts as the anode and zinc powder is closest to the top of a primary button cell, then the top is negative too

Question 27

sometimes fuel cells are more efficient at very high temps and pressures. so why don’t we use these conditions?

Answer 27

too expensive to maintain these conditions. satisfactory results can be achieved with less extreme conditions.

Question 28

how do we observe if a galvanic cell is working?

Answer 28

the anode will decrease in size and the cathode will increase in size.