Chemical Cells

Question 1

Chemical Cell (definition)

Answer 1

Devices for converting chemical energy (from chemical reaction) into electrical energy (electricity)

Question 2

Components (definition)

Answer 2

1. Electrodes (metals or inert electrodes Pt / Ni)
2. Electrolyte (ionice compounds / acids / alkaline in aqueous state)
3. External circuit

Question 3

Negative electrode (definition)

Answer 3

The place from which electron flow into an external circuit

Question 4

Positive electrode (definition)

Answer 4

The place into conversion of a cell intro electrical energy

Question 5

Discharge (definition)

Answer 5

The conversion of chemical energy to electrical energy

Question 6

Recharge (definition)

Answer 6

The conversion of electrical energy into chemical energy

Question 7

How is electricity produced in a chemical cell?

Answer 7

By electrons flow in the external circuit and movement of mobile ions in the electrolyte.

Question 8

All chemical cells produce electricity through ___________ ________

Answer 8

Redox reactions

Question 9

The two electrodes in a chemical cell

Answer 9

1. Anode (negative electrode)
   AN OX
   - the electrode at which oxidation takes place
2. Cathode (positive electrode)
   RED CAT
   - the electrode at which reduction takes place

Question 10

Chemical cells (3 types)

Answer 10

1. Dry cell (electrolyte in the form of paste)
   - e.g. zinc-carbon cell
2. Wet cell (electrolyte in liquid form)
   - simple chemical cell, car battery
3. Fuel cell (example: hydrogen-fuel cell)

Question 11

primary cell

Answer 11

–> not rechargeable
–> convert chemical energy to electrical energy
eg. silver oxide cell, alkaline manganese cell and zinc-carbon cell (all examples are dry cells)

Question 12

secondary cell

Answer 12

–> V rechargeable
–> interconverts between chemical and electrical energy by charging and discharging
1. lithium-ion battery
2. nickel-cadmium rechargeable battery
3. nickel metal hydride (NiMH) cell
4. lead-acid accumulator (wet cell)
–>others are dry cells

Question 13

factors when choosing chemical cells

Answer 13

cost, size, weight, electromotive force / vpoltage, charge capacity, current generated, discharge curve, rechargeability, shelf life, service life

important: cost, service life, charge capacity, voltage

Question 14

Primary cell: Zinc-carbon cell

voltage over discharge, steady current?, current, service life, shape, prixe, other characteristics, conditions for use, uses

Answer 14

1. falls rapidly
2. no steady current
3. small current
4. short service life
5. cylindrical
6. cheap
7. :-< leakage of electrolyte occurs un unsealed tyoes
8. suitable for low-drained or intermittenetly-ised electrical appliances
9. uses: torches, portable radios, clocks, calculators, remote control

Question 15

Primary cell: Alkaline Manganese Cell

voltage over discharge, steady current?, current, service life, shape, prixe, other characteristics, conditions for use, uses

Answer 15

1. falls slowly
2. has a steady current
3. large current
4. medium (3 times as expensive as zinc-carbon cell)
5. :-> small internal resistance, leak proof
6. suitable for high-drained, continuously-used electrical appliances (requiring a steady current)
7. used in motorised toys, shavers, photo flashguns, MP3 players

Question 16

Primary cell: Silver oxide Cell

voltage over discharge, steady current?, current, service life, shape, prixe, other characteristics, conditions for use, uses

Answer 16

1. constant
2. has a steady current
3. small current
4. button-shaped
5. expensive
6. :-> light weight, small size and flat discharge curve
7. suitable for small-sized continuously-used electrical appliances requiring a small steady current
8. quartz watches, calculators, hearing aids, small electronic instruments

Question 17

Secondary cell: lithium-ion rechargeable cell

voltage over dischaarge, weight, average service life, approximate shelf life, shape, price, conditions for use, uses

Answer 17

1. constant
2. light
3. long service life
4. short shelf life
5. cylindrical and prismatic
6. expensive
7. suitable for high-drained electrical appliances
8. used for mobile phones, digital cameras and laptop computers

Question 18

Secondary cell: nickel-metal hydride (NiMH) cell

voltage over dischaarge, weight, average service life, approximate shelf life, shape, price, conditions for use, uses

Answer 18

1. constant
2. moderate
3. long service life
4. shelf life is much shorter than the others
5. cylindrical
6. very expensive
7. suitable for high-drained electrical appliances
8. used for indoor wireless phones, digital cameras

Question 19

Secondary cell: (wet cell) lead-acid accumulator

voltage over dischaarge, weight, average service life, approximate shelf life, shape, price, conditions for use, uses

Answer 19

1. falls slowly
2. very heavy
3. long service life
4. short shelf life
5. box-like
6. very expensive
7. suitable as an uninterruptible power supply, suitable for high-drained electircal appliances
8. used for emergency lighting, wheelchairs, telecom back-up power supplies, engine starters

Question 20

Advantages of lithium-ion batteries over other batteries

[important]

Answer 20

1. it provides the greatest amount of energy for a given size or weight of all commercial rechargeable cells –> high energy density
2. as compared with other batteries of same dimension, lithium-ion batteries are lighter –> commonly used in mobile phones and other portable electronic devices
3. causes less harmful effect to the environment than other batteries on disposal as it does no toxic metals

Question 21

When the question gives you some information of a chemical reaction of discharge / recharge …

Answer 21

you should be able to the reverse reaction
1. simply write down the reverse reaction (product –> reactant, reactant –> product)
2. calculate the oxidation number of the elements (to see which element undergoes reduction and oxidation)
3. AN OX, RED CAT (for anode and cathode half equations)

Question 22

nickel-carmium cell (the process of discharge)

what can be concluded in the chemical equation???

Answer 22

1. all the reactants or products are solid or liquid state
2. their concentrations remain unchanged during reaction, hence the battery maintains a steady voltage during discharge

Question 23

lead-acid accumulator (the process of discharge)

Answer 23

1. anodic and cathodic reactions are reversible and there is no loss of materials and no side products formed –> lead-acid cell is rechargeable
2. when the car is in motion, it drives a generator which charges the battery
3. the density changes upon discharging and charging
   * during discharging process, H2SO4 is consumed –> density of battery acid decreases
   * during recharging process, H2SO4 is produced –> density of battery acid increases
4. The energy density of lead-acid accumulators is low among three rechargeable cells –> provide smallest amount of energy for a given size or weight

Question 24

Comparison between primary cell (non-rechargeable cell) and secondary cell (rechargeable cell)

Answer 24

primary cells are not environmentally friendly as disposal of dry cells causes land pollution. It will produce more solid wastes than rechargeable cells. secondary cells are more environmentally-friendly in the sense that they can be used for a prolonged period of teim. –> possess great hazard (because of heavy metals like Pb, Ni and Cd.

Question 25

for environmental protection, ## Footnote for primary cells (aspect)

Answer 25

- use mains electricity whenever possible - choose to use rechargeable cells instead of disposable cells - use zinc-carbon cells and alkaline manganese cell with little or no energy - recycle rechargeable cells

Question 26

Working principle of a lemon ccel (simple chemical cell)

Answer 26

- two different metal strips are inserted into a lemon - a digital ultimeter or a high resistance voltmeter --> connected to the metal strips Working principle: the lemon juice containing citric acid acts as an electrolyte (allows electricity to flow through it to complete the circuitb **electrons flows through a more reactive metal to a less reactive metal** because **the more reactive metal has a higher tendency to lose electrons than the less reactive metal** electrons flow from anode to cathode (i.e. from negative pole to positive pole) if multimeter is positive --> meaning the current flow is from Mg to Cu (electron flow and current flow are in opposite direction)

Question 27

Dipping two metal strips into the electrolyte ## Footnote structure of the observations

Answer 27

the anode dissolves gradually as it loses electrons to the cathode.

Question 28

relationship in the ECS

Answer 28

The further apart the metals are in the electrochemical series, the greater the voltage of the cell. The closer the metals, the smaller the voltage.

Question 29

Oxidising power ## Footnote definition

Answer 29

the tendency to undergo reduction (to gain electrons)

Question 30

Reducing power ## Footnote defintion

Answer 30

tendency to undergo oxidation (to lose electrons)

Question 31

FOR THE ECS, if cannot remember all, still need to remember the following orders

Answer 31

1. reducing power of metals (similar to metal reactivity series) 2. oxidising power of metail ions (Ag+ > Cu2+ > H+ > Zn2+ > Mg2+ > Na+ 3. oxidising power of halogens (F2 > Cl2 > Br2 > I2) 4. reducing power of halides (I- > Br- > Cl- > I-)

Question 32

Simple chemical cells with metallic electrodes

Answer 32

1. Compare the position of two metallic electrodes 2. More reactive metals (stronger reducing power) will lose electrons. 3. Compare the position of cation in the electrolyte with H+ (aq) ion (from water) in ECS. The cations with strong oxidising power will gain electrons. ## Footnote Mark on each diagram: 1. electron flow 2. +ve / -ve pole 3. anode / cathode

Question 33

Deduce the direction of electron flow through the external circuit in the chemical cell. ## Footnote how to answer this question type?

Answer 33

Since the reading of the voltmeter is positive, it means the electrons flow from Mg to Cu through the external circuit as Mg has a stronger reducing power than Cu. Since the reading of the voltmeter is negative, it means the electron flows from Zn to Cu through the external circuit. Zn has a higher tendency to lose electrons than Cr, thus Zn is more reactive than Cr.

Question 34

You are provided the following items: Lemon, multimeter, connecting wires, Zn strips, Cu strip and Ag strip With the aid of a labelled diagram, suggest how you can perform an experiement to confirm (with explanation) the order of reducing power of metals as Zn > Cu > Ag ## Footnote Essay-type question (how to answer) DSE 2021 1B Q8

Answer 34

Model answer: 1. Construct a chemical cell by connecting Zn and the Cu to the negative terminal of a multimeter respectively, a positive voltage can be recorded. 2. The result of this experiment indicates that Zn has a higher tendency to release electrons than Cu thus Zn has a higher reducing power than that of Cu. 3. Repeat the experiment by replacing the Zn with Ag, a negative voltage can be recorded. 4. The result of this experiment indicates that Cu has a higher reducing power than that of Ag. Therefore, the order of reducing power of metals can be confirmed as Zn > Cu > Ag | Similar question CE 05 Q7 ## Footnote A chemical cell can be made from 2 metal strips and a lemon. Given the following materials and equipment, outline how you can set up a chemical cell with the maximum output voltage. (Your answer should include variables that need to be controlled.)

Question 35

Limitation of simple chemical cell with two metal electrodes in an electrolyte

Answer 35

H2 is not a good conductor. When H2 (g) is evolved at the copper electrode, this causes an increase in the resistance of the cell. The current in the external circuit drops quickly upon the formation of the H2 (g) at the Cu electrode. In addition, Zn electrode also reacts directly with dilute H2SO4 --> direct reaction direct reaction betwene Zn and H2SO4 convert chemical energy into heat energy. ## Footnote can add a salt bridge in between or porous pot / partition

Question 36

Use of a salt bridge

Answer 36

1. To complete the circuit by allowing the ions to move from one half-cell into the other. The ions in electrolytes of beaker can move up to the salt bridge and the ions in salt bridge can m ove down to the beaker. 2. To provide ions to balance the charge in the solutionsof the two half-cells / to maintain charge balance. ## Footnote can be prepared by soaking a filter paper in an electrolyte (usually an aqueous solution of ionic compound) that provides mobile ions (eg. saturated KNO3)

Question 37

Use of a porous pot / porous partition ## Footnote which prevents direct mixing of the two electrolytes

Answer 37

1. It completes the circuit by allowing the ions to move between the two solutions through its small holes. 2. The charge balance in the half-cells is maintained. --> the small holes on the porous pot allow the ions to pass through the wall slowly. the ions passed through the porous thingy can neutralise the excess positive / negative charge.

Question 38

Simple chemical cells with inert electrodes

Answer 38

Pt, graphite

Question 39

calculation involving redox reactions

Answer 39

write the overall equation --> find the ratio of increase or decrease