Chap 10

Question 1

in the zinc-copper daniell cell, whats the anode and whats the cathode

Answer 1

the zinc electrode is the anode as this is where zinc atoms undergo oxidation

the copper electrode is the cathode because this is where the copper(II) ions undergo reduction

Question 2

cell potential

Answer 2

the electrical potential difference between the two electrodes of a cell

Question 3

alkaline battery

Answer 3

a dry cell that has an alkaline (basic) electrolyte in the paste

Question 4

galvanizing

Answer 4

the process of covering iron with a protective layer of zinc
~ zinc acts as a protective layer; if this layer is broken, the iron is exposed to air and water however it is still protected as zinc is more easily oxidized than iron
~ zinc becomes the anode in the galvanic cell and iron acts as the cathode when zinc is present
~ zinc is the sacrificial anode

Question 5

what are proton exchange membrane fuel cells (PEM)? How does it work

Answer 5

the fuel cell first used by NASA on space flights for their Gemini program

• the anode of the PEM fuel cell is porous carbon coated with platinum to catalyze the oxidation half-reaction
• a thin solid polymer acts as the electrolyte, allowing the positively charged H ions, formed in the oxidation half-reactions, to pass through but blocking the passage of negatively charged electrons
• the electrons move from the anode, through an external circuit, to the cathode
• the source of oxygen for the cathode is air; as air passes beside the porous carbon cathode, oxygen combines with the electrons returning to the cathode from the external circuit and protons that have passed through the membrane
• the oxygen, hydrogen ions, and electrons combine to form water

Question 6

primary battery vs secondary battery

Answer 6

primary - disposable battery that cannot be recharged

secondary - rechargeable battery

Question 7

What is a salt bridge

Answer 7

an electrical connection between half-cells that contains an electrolyte solution, allowing a current to flow but preventing contact between the oxidizing agent and the reducing agent

Question 8

standard cell potential

Answer 8

the cell potential when the salt concs of all salt solutions are 1.0 mol/L under standard conditions of temp (25 degrees C) and pressure (101.325 kPa)

Question 9

explain rust

Answer 9

• rust is a hydrated iron (III) oxide; the iron consists many small galvanic cells in which electrochemical reactions form rust
• in each small cell, irons acts as the anode; the cathode is inert and may be an impurity that exists in the iron
• rain is needed for rusting to occur; carbon dioxide in the air reacts with rainwater to form carbonic acid; this weak acid partially dissociates into ions and thus it is an electrolyte for the corrosion process
• the circuit is completed by the iron itself, which conducts electrons from the anode to the cathode

Question 10

what are some health and safety concerns involving batteries

Answer 10

• can corrode and alkaline substances can leak out
• can explode if they are incinerated, releasing alkaline material and toxic heavy metals
• accidental swallowing of button batteries (if battery lodges in esophagus, it can cause serious death; the body fluids provide an electrolyte for the battery and a current then flows through the fluids, causing chemical rxns)
  ~ in some cases, the alkaline compounds released have burnt holes through the esophagus; extremely serious

Question 11

what is corrosion

Answer 11

a spontaneous redox reaction between materials and substances in their environments

many metals are easily oxidized by a powerful oxidizing agent in the atmosphere (oxygen) ; as metals are constantly in contact with oxygen, they are vulnerable to corrosion

Question 12

electrolytic cell

Answer 12

an electrochemical cell that uses an external source of energy to drive a non-spontaneous redox rxn (transforms electrical energy into chemical energy)

Question 13

What’s a galvanic cell

Answer 13

a device that uses redox reactions to transform chemical potential energy into electrical energy

Question 14

electrical potential difference

Answer 14

the amount of energy that a unit charge would gain by moving from one point to the other between 2 points (in galvanic cell = the 2 electrodes)

Question 15

What’s a daniel cell

Answer 15

a cell that consists of a zinc strip in zinc sulfate solution and a copper strip in a copper (II) sulfate solution; these two solutions are connected by a salt bridge

Question 16

what happens when an iron pipe is connected to a copper pipe in plumbing

Answer 16

by connecting an iron pipe to a copper pipe in a plumbing system, an inexperienced plumber could accidentally speed up the corrosion of the iron pipe
~ copper is less reactive than iron, therefore copper acts as the cathode and iron as the anode in numerous small galvanic cells at the intersection of the two pipes

Question 17

How to determine if the rxn is spontaneous

Answer 17

• a positive standard cell potential tells you that the reaction will proceed spontaneously, in the direction indicated
• a reaction will proceed if the reducing agent on the left is strong than the RA on the right

Question 18

inert electrode

Answer 18

an electrode made from a material that is neither a reactant nor a product of the redox rxn; instead, it can carry a current and provide a surface on which redox reactions can occur

Question 19

How does a salt-bridge work

Answer 19

it is a U-shaped glass tube filled with potassium chloride; the ends of the tube are plugged with a porous material such as cotton or glass wool
~ ions can diffuse through the plugs, but the plugs prevent the solutions from mixing
~ a wire is connected to the copper and zinc rods; the other ends of the wire can be connected to any electrical device (like a voltmeter)

Question 20

whats an anode

Answer 20

the electrode at which oxidation occurs

Question 21

what are some ways of corrosion prevention

Answer 21

• if an iron object is kept cool, dry, and clean, it will not corrode
• simple method of preventing is to paint an iron object (paint prevents air and water from reaching the metal surface)
• protective layers: grease, oil, plastic, or a metal that is more resistant to corrosion than iron
• forming an alloy with a different metal to protect iron against corrosion (ex. stainless steel)

Question 22

how efficient are PEM fuel cells

Answer 22

40% to 50% efficient and operate at temps between 60 degrees C and 100 degrees C

Question 23

factors that affect the electrolysis of water

Answer 23

• pure water is a poor conductor of electric current, so the electrolysis of pure water proceeds very slowly
• if you use a salt that will conduct current but will not interfere with the reactions, you can increase the rate of reaction
• the concs of the reactants and products

Question 24

What are electrolytes

Answer 24

substances that, when dissolved in water, conduct an electric current in the form of moving ions

Question 25

fuel cells (what is it and how does it work)

Answer 25

a battery in which reactants can be added and products can be removed while the battery is operating (a battery that can be refuelled; reactants flow in and products flow out) they convert the energy in the fuel directly into electrical energy; waste product is water very costly

Question 26

industrial extraction and refining of metals

Answer 26

- some metals, like iron, are smelted - reactive metals, like sodium, lithium, beryllium, etc. are extracted industrially by the electrolysis of their molten chlorides - one of the most important electrolytic processes is THE EXTRACTION OF ALUMINUM FROM AN ORE CALLED BAUXITE - the process of purifying a material is known as refining (after the extraction stage, some metals are refined in electrolytic cells) ex: copper is 99% pure after extraction, however, it is not pure enough for electrical wiring; therefore, some of the impure copper is refined electrolytically

Question 27

how does the lead-acid battery work (rechargeable battery)

Answer 27

- in use, it partially discharges; the cells in the battery operate as galvanic cells and produce electrical energy - the reaction in each cell proceeds spontaneously in one direction - to recharge the battery, an alternator, supplies electrical energy to the battery (in car) - the external voltage of the alternator reverses the reaction in the cells which makes it proceed non-spontaneously, resulting in the cells operating as electrolytic cells - materials: lead and an acid; electrodes in each cell are constructed using lead grinds and each electrode consists of powdered lead packed into one grind; the other electrode consists of powdered lead (IV) oxide packed into the other grid AND the electrolyte solution is sulfuric acid

Question 28

how is rust formed

Answer 28

- the iron(II) ions produced at the anode and the hydroxide ions produced at the cathode react to form a precipitate of iron (II) hydroxide - the iron (II) hydroxide undergoes further oxidation through reaction with the oxygen in the air to form iron (III) hydroxide - iron (III) hydroxide readily breaks down to form hydrated iron (III) oxide / rust

Question 29

electrolysis in water

Answer 29

- when water goes through electrolysis, some water molecules are oxidized at the anode and other water molecules are reduced at the cathode - oxygen gas is generated at the anode and hydrogen gas is generated at the cathode; to prevent these from mixing, the electrodes are usually positioned below or in the closed ends of glass tubes

Question 30

dry cell

Answer 30

galvanic cell in which the electrolyte has been thickened into a paste; stops producing electrical energy when the reactants are used up

Question 31

whats an alkaline fuel cell

Answer 31

developed for NASA's Apollo program & still used in space shuttles - The electrolyte is aqueous potassium hydroxide - when hydrogen reaches the anode, it combines with the hydroxide ions from the electrolyte and forms water - the electrons travel along the electrode to the external circuit - at the cathode, oxygen gas combines with the electrons that are returning from the external circuit and with water - they operate at temps around 90 degrees C to 100 degrees C and are about 40% to 50% efficient

Question 32

how does the electrolytic cell work

Answer 32

- the power supply "pulls' electrons from the copper electrode, thus oxidizing copper atoms in the electrode to copper (II) ions that go into solution - the copper electrode is therefore the anode, because oxidation half-rxns occur there - the power supply is forcing electrons onto the zinc electrode, causing zinc ions in the solution to become reduced to zinc atoms that become part of the zinc electrode; since zinc ions are becoming reduced at the zinc electrode, that electrode is the cathode ** THIS SYSTEM IS OPERATING OPPOSITE TO THE DANIEL CELL

Question 33

What factors must be considered when choosing the electrolyte in the salt bridge of a galvanic cell

Answer 33

- when choosing the electrolyte, make sure the electrolyte compound doesn't precipitate with the anode or cathode - choose an electrolyte that is soluble like KNO3 and KCl but make sure to use KNO3 in a solution if one of the electrodes is silver as when the chloride ion will react with silver, it'll cause precipitation

Question 34

explain the reaction for rechargeable batteries (lead-acid)

Answer 34

- the reaction consumes some of the lead in the anode, some of the lead (IV) oxide in the cathode, and some of the sulfuric acid - a precipitate of lead (II) sulfate forms - when the battery is recharged, the half-reactions and the overall cell reaction are reversed - in the reverse reaction, lead and lead (IV) oxide are re-deposited in their original locations, and sulfuric acid is ref-formed

Question 35

how is hydrogen used in fuel cells

Answer 35

- hydrogen for fuel cells must be extracted from other cmpds through an extraction process that requires energy (ex. electrical energy can split water into hydrogen and oxygen) - reforming can chemically remove hydrogen from hydrocarbons - research is being done to create fuel cells with internal reformers (systems that remove H from hydrocarbons) ; some CO2 is produced

Question 36

nicad battery (nickel-cadmium) also a rechargeable battery

Answer 36

- contains three cells in series to produce a suitable voltage for electronic devices - risks: after being discharged repeated, they eventually wear out & over time, discarded nicad baterries release toxic cadmium which makes it hazardous to the environment as cadmium can enter the food chain - exposure to cadmium can lead to high blood pressure and heart disease

Question 37

differences between galvanic cell and electrolytic cell

Answer 37

galvanic: spontaneous electrolytic: non-spontaneous galvanic: chemical energy to electrical energy electrolytic: electrical to chemical energy galvanic: anode (negatively charged); zinc electrode electrolytic: anode (positively charged); copper electrode galvanic: cathode (positively charged); copper electrode electrolytic: cathode (negatively charged); zinc electrode

Question 38

cathodic protection

Answer 38

the process of attaching a more reactive metal to an iron object that will act as an anode and prevent the iron object from corroding ~ unlike galvanizing, the metal does NOT completely cover the iron ~ as the sacrificial anode is slowly destroyed by oxidation, it must be replaced periodically

Question 39

What's a dry cell

Answer 39

a galvanic cell in which the electrolyte has been thickened into a paste

Question 40

Whats an electrode

Answer 40

conductors that carry electrons into and out of a cell

Question 41

whats a cathode

Answer 41

the electrode at which reduction occurs

Question 42

What's a battery

Answer 42

a set of galvanic cells connected in series; in series, the negative electrode of one cell is connected to the positive electrode of the next cell

Question 43

how to calculate voltage of series

Answer 43

the voltage of a set of cells connected in series is the sum of the voltages of the individual cells

Question 44

chlor-alkali process

Answer 44

an electrolysis procedure for making chlorine gas and sodium hydroxide from brine, which is an aqueous solution of sodium chloride ~ chlorine gas, hydrogen gas, and sodium hydroxide are all highly reactive with one another and must be separated as they are being produced ~ a net movement of solution into the cathode compartment in which H gas is produced ~ chlorine gas is produced at the anode, where it is removed and prevented from mixing with the hydroxide ions produced at the cathode ~ sodium hydroxide is removed from the cell periodically, and fresh brine is added to the cell; it is later dried by evaporation and packaged as a solid

Question 45

electrolysis of molten salts

Answer 45

- sodium cannot be purified by electrolysis in an aqueous solution - salts are solid at standard temps and will not conduct electric current - davy heated the salts until they melted then applied electrolysis to the molten solids - he isolated sodium, potassium, magnesium, calcium, strontium, and barium

Question 46

electrolysis

Answer 46

the process of using electrical energy to drive a non-spontaneous redox reaction (the process that takes place in an electrolytic cell)

Question 47

how does the fuel cell technology work

Answer 47

based on the principle of the galvanic cell the chemical reactants are gases that flow past the anode and cathode where the oxidation and reduction half-reactions occur; an electric current then flows through an external circuit and FINALLY, the gaseous products are eliminated from the cell

Question 48

button battery

Answer 48

a very small dry cell, usually having an alkaline electrolyte in the paste and either zinc and mercury OR zinc and silver electrodes

Question 49

whats a phosphoric acid fuel cell

Answer 49

a fuel cell where the oxidation and reduction half-reactions are the same as those for the PEM fuel cell; however, the electrolyte is PHOSPHORIC ACID these fuel cells run at slightly high temps than PEM but can achieve slightly higher efficiences; also, they can tolerate fuels that are not as pure as those for the PEM fuel cells

Question 50

sacrificial anode

Answer 50

a metal that oxidizes more easily than iron and is destroyed to protect an iron object