5. Electricity & Chemistry / 6. Chemical Energetics Flashcards
(125 cards)
1
Q
What happens to a molten ionic compound when a current is passed through it?
A
When an electric current is passed through a molten ionic compound the compound decomposes or breaks down
2
Q
In what forms can an ionic compound undergo electrolysis? Why?
A
- in a molten state
- in an aqueos state
Ionic compounds in the solid state cannot conduct electricity either since they have no free ions that can move and carry the charge
3
Q
Why can covalent compounds not undergo electrolysis?
A
Covalent compounds cannot conduct electricity hence they do not undergo electrolysis
4
Q
Electrode definition
A
Electrode is a rod of metal or graphite through which an electric current flows into or out of an electrolyte
5
Q
Electrolyte definition
A
Electrolyte is the ionic compound in molten or dissolved solution that conducts the electricity
6
Q
Anode definition
A
Anode is the positive electrode of an electrolysis cell
7
Q
Anion definition
A
Anion is a negatively charged ion which is attracted to the anode
8
Q
Cathode definition
A
Cathode is the negative electrode of an electrolysis cell
9
Q
Cation definition
A
Cation is a positively charged ion which is attracted to the cathode
10
Q
How would you set up the equipment for the electrolysis of lead (III) bromide)
A
Add lead (II) bromide into a beaker and heat so it will turn molten, allowing ions to be free to move and conduct an electric charge
Add two graphite rods as the electrodes and connect this to a power pack or battery
Turn on power pack or battery and allow electrolysis to take place
11
Q
What happens at the anode during the electrolysis of lead (III) bromide?
A
Negative bromide ions move to the positive electrode (anode) and lose two electrons to form bromine molecules. There is bubbling at the anode as brown bromine gas is given off
12
Q
What happens at the cathode during the electrolysis of lead (III) bromide?
A
Positive lead ions move to the negative electrode (cathode) and gain electrons to form a grey lead metal which deposits on the surface of the electrode
13
Q
Half equation for anode
electrolysis of lead (III) bromide
A
2Br- —> Br2 + 2e-
14
Q
Half equation for cathode
electrolysis of lead (III) bromide
A
Pb2+ + 2e- —> Pb
15
Q
When using an aqueos solution, what needs to be taken into account?
A
Aqueous solutions will always have water (H2O)
H+ and OH– ions from the water are involved as well
16
Q
To which electrode are OH- ions attracted?
A
OH– ions and non-metal ions (anions) are attracted to the positive electrode
17
Q
What will be formed at the anode? (electrolysis of an aqueos solution)
A
Either OH– or non-metal ions will lose electrons and oxygen gas or gas of non-metal in question is released, eg. chlorine, bromine, nitrogen
18
Q
What is the rule for deciding what non-metal will be formed at the anode? (electrolysis of an aqueos solution)
A
The product formed depends on which ion loses electrons more readily, with the more reactive ion remaining in solution
19
Q
What is the reactivity series for anions?
A
More reactive → SO42- → NO3- → OH- → Cl- → Br- → I- Less reactive
20
Q
What will form at the cathode? (electrolysis of an aqueos solution)
A
H+ and metal ions attracted to the negative electrode but only one will gain electrons
Either hydrogen or metal will be produced
21
Q
What is the rule for deciding what will be formed at the cathode? (electrolysis of an aqueos solution)
A
If the metal is above hydrogen in reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode
22
Q
What factor (apart from reactivity) also influences the products of the electrolysis of an aqueos solution?
A
Concentrated and dilute solutions of the same compound give different products
23
Q
What is the rule for which anion will be produced in a concentrated solution?
A
For anions, the more concentrated ion will tend to be produced over a more dilute ion
24
Q
In the electrolysis of a molten compound, what will be produced at the cathode and the anode?
A
cathode - metal
anode - non-metal
25
How can you determine that hydrogen was produced at the cathode?
If the gas produced at the cathode burns with a ‘pop’ when a sample is lit with a lighted splint then the gas is hydrogen
26
How can you determine that oxygen was produced at the anode?
If the gas produced at the anode relights a glowing splint dipped into a sample of the gas then the gas is oxygen
27
What colour is bromine gas?
red-brown
28
What colour is chlorine gas?
yellow-green
29
What colour is fluorine gas?
pale yellow
30
How would you set up the equipment for copper refining?
In the set-up, the impure metal is always the anode, in this case the impure copper
The cathode is a thin sheet of pure copper
The electrolyte used is an aqueous solution of a soluble salt of the pure metal at the anode, e.g: CuSO4
31
Why is electrolysis used to purify metals?
Electrolysis can be used to purify metals by separating them from their impurities
32
What happens to copper atoms during copper refining?
Copper atoms at the anode lose electrons, go into solution as ions and are attracted to the cathode where they gain electrons and form now purified copper atoms
33
What happens to the anode during copper refining?
The anode thus becomes thinner due to loss of atoms and the impurities fall to the bottom of the cell as sludge
34
What happens to the cathode during copper refining?
The cathode gradually becomes thicker
35
Where do electrons move during electrolysis?
During electrolysis the electrons move from the power supply towards the cathode
36
Where do positive ions move during electrolysis?
Positive ions within the electrolyte move towards the negatively charged electrode which is the cathode
37
What happens to positive ions at the cathode?
Here they accept electrons from the cathode and either a metal or hydrogen gas is produced
38
Where do negative ions move during electrolysis?
Negative ions within the electrolyte move towards the positively charged electrode which is the anode
39
What happens to negative ions at the anode, if it is inert?
If the anode is inert (such as graphite or platinum), the ions lose electrons to the anode and form a nonmetal or oxygen gas
40
What happens to negative ions at the anode, if it is reactive?
If the anode is a reactive metal, then the metal atoms of the anode lose electrons and go into solution as ions, thinning the anode
41
What is an electrochemical cell a source of?
An electrochemical cell is a source of electrical energy
42
What is the simple design of an electrochemical cell?
The simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external circuit
43
How does an electrochemical cell work, using zinc and copper as an example?
Zinc is the more reactive metal and forms ions more easily, releasing electrons as its atoms form ions
The electrons give the more reactive electrode a negative charge and they then flow around the circuit to the copper electrode
44
What causes a voltage to be produced by an electrochemical cell?
The difference in the ability of the electrodes to release electrons causes a voltage to be produced
45
What is the correlation between the reactivity of a metal and the voltage produced? (electrochemical cell)
The greater the difference in the metal’s reactivity, the greater the voltage
46
Electroplating definition
Electroplating is a process where the surface of one metal is coated with a layer of a different metal
47
What is the rule when deciding which metal is used to coat the other one? (electroplating)
The metal being used to coat is a less reactive metal than the one it is covering
48
What is the basic setup of equipment for electroplating?
The anode is made from the pure metal used to coat
The cathode is the object to be electroplated
The electrolyte is an aqueous solution of a soluble salt of the pure metal at the anode
49
What are 2 uses of electroplating?
Electroplating is done to make metals more resistant to corrosion or damage, e.g: chromium and nickel plating
It is also done to improve the appearance of metals, e.g: silver plating cutlery
50
Conductor definition
Conductors of electricity allow electrical charge to pass through them easily
51
3 examples of conductors
Conductors can be:
Solids such as metals or graphite
Liquids such as molten lead bromide or molten metals
Solutions such as sodium chloride solution
52
What metal is a good conductor and is used in electrical wiring? What other quality does it have which makes it suitable?
Copper is used extensively in electrical wiring as it is an excellent conductor and is malleable and easy to work with
53
What metal is used in overhead cables?
Aluminium is used in overhead cables which are reinforced with a steel core
54
Why do aluminium cables have a steel core?
The steel core provides extra strength and prevents the cable from breaking under its own weight
55
Why is aluminium used in overhead cables rather than copper?
Although not as good a conductor as copper, it is less dense and cheaper than copper
56
Insulator definition
Insulators resist the flow of electricity and do not conduct
57
What are 3 examples of insulators?
Most insulators are solids of plastic, rubber or ceramic
58
How are plastics used as insulators?
Plastics are used as insulators and are placed around electrical wiring and for some tool and machine handles
59
When are ceramics used as insulators?
Ceramics are used in very high voltage lines where contact between the power line and the metal of the pylon would be dangerous
60
What type of metals and metal compounds does the earth's crust contain? What is the problem?
The Earth’s Crust contains metals and metal compounds such as Gold, Iron Oxide and Aluminium Oxide, but when found in the Earth, these are often mixed with other substances
61
What needs to be done with metals found in the earth's crust to make them useful?
To be useful, the metals have to be extracted from their ore
62
What processes can be used to extract a metal from its compound?
electrolysis, using a blast furnace or by reacting with more reactive material
63
How are metals above carbon extracted?
Extracted by electrolysis of molten chloride or molten oxide.
64
How are metals below carbon extracted?
Extracted by heating with a reducing agent such as carbon or carbon monoxide in a blast furnace.
65
What is the difference in costs between electrolysis and reduction?
E - large amounts of electricity required, so an expensive process
R - cheap process as carbon is cheap and can be source of heat as well
66
1. What is bauxite purified to?
| Extraction of aluminium
The Bauxite is first purified to produce Aluminium Oxide Al2O3
67
2. What is aluminium oxide dissolved in and why?(Extraction of aluminium)
Aluminium Oxide has a very high melting point so it is first dissolved in molten cryolite producing an electrolyte with a lower melting point, as well as a better conductor of electricity than molten aluminium oxide. This also reduces expense considerably
68
3. What is the electrolyte?
| Extraction of aluminium
The electrolyte is a solution of aluminium oxide in molten cryolite at a temperature of about 1000 °C.
69
4. What keeps the electrolyte molten?
| Extraction of aluminium
The cell operates at 5-6 volts and with a current of 100,000 amps. The heat generated by the huge current keeps the electrolyte molten
70
5. How is aluminium extracted from the cell?
| Extraction of aluminium
The molten aluminium is siphoned off from time to time and fresh aluminium oxide is added to the cell.
71
What is the half-equation at the cathode?
| Extraction of aluminium
The Aluminium melts and collects at the bottom of the cell and is then tapped off:
Al3+ + 3e- → Al
72
What is the half-equation at the anode?
| Extraction of aluminium
2O2- – 4e– → O2
73
What happens at the anode which leads to it being reduced in size?
(Extraction of aluminium)
Some of the Oxygen Produced at the positive electrode then reacts with the Graphite (Carbon) electrode to produce Carbon Dioxide Gas:
C (s) + O2 (g) → CO2 (g)
*This causes the carbon anodes to burn away, so they must be replaced regularly.
74
What is brine?
Brine is a concentrated solution of aqueous sodium chloride
75
What are the products when brine is electrolysed?
When electrolysed it produces chlorine, hydrogen and sodium hydroxide
76
What ions does brine contain?
The electrolyte is concentrated sodium chloride which contains the following ions: H+, Cl– and OH–
77
What happens at the cathode during the electrolysis of brine?
The H+ ions are discharged at the cathode as hydrogen gas
78
What happens at the anode during the electrolysis of brine?
The Cl– ions are discharged at the anode as chlorine gas
79
What remains behind after the electrolysis of brine?
The Na+ and OH– ions remain behind and form the NaOH solution
80
Exothermic definition
A reaction in which energy is given out to surroundings (temperature of environment increases)
81
What are 3 examples of exothermic reactions?
Combustion of fuels
Reaction of acids and metals
Neutralisation reactions
82
Endothermic definition
A reaction in which energy is taken in from surroundings (temperature of environment decreases)
The energy change is positive
83
What are 3 examples of endothermic reactions?
Thermal decomposition of carbonates
Electrolysis
First stages of photosynthesis
84
What determines whether a reaction is endo/exothermic?
| energy
Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when the new bonds are formed
85
How can you determine if a reaction is endothermic by the energy?
If more energy is absorbed than is released, this reaction is endothermic
More energy is required to break the bonds than that gained from making the new bonds
86
Is the change in energy positive or negative if the reaction is endothermic? Why?
The change in energy is positive since the products have more energy than the reactants
- positive ΔH value
87
What symbol is used to show the change in heat energy?
The symbol ΔH (delta H)
88
What is H the symbol for, and what does this mean?
H is the symbol for enthalpy, which is a measure of the total heat of reaction of a chemical reaction
89
How can you determine if a reaction is exothermic by the energy?
If more energy is released than is absorbed, then the reaction is exothermic
More energy is released when new bonds are formed than energy required to break the bonds in the reactants
90
Is the change in energy positive or negative if the reaction is exothermic? Why?
The change in energy is negative since the products have less energy than the reactants
Therefore an exothermic reaction has a negative ΔH value
91
What are energy level diagrams?
These are graphical representations of the heat changes in chemical reactions
92
What is shown on the y-axis?
| energy level diagram
The enthalpy of the reactants and products is displayed on the y-axis
93
What is shown on the x-axis?
| energy level diagram
The reaction pathway is shown on the x-axis
94
What do the arrows show?
| energy level diagram
Arrows on the diagrams indicate whether the reaction is exothermic (downwards pointing) or endothermic (upwards pointing)
95
In an exothermic reaction, do the reactants or products have more energy? Why?
During an exothermic reaction, energy is given out
This means that the energy of the products will be lower than the energy of the reactants, so the change in enthalpy (ΔH) is negative
96
How is an exothermic reaction represented on an energy level diagram?
This is represented on the energy-level diagram with a downwards arrow (from the reactants to the products) as the energy of the products is lower than the reactants
97
In an endothermic reaction, do the reactants or products have more energy? Why?
During an endothermic reaction, energy is absorbed
This means that the energy of the products will be higher than the energy of the reactants, so the change in enthalpy (ΔH) is positive
98
How is an endothermic reaction represented on an energy level diagram?
This is represented on the energy-level diagram with an upwards arrow (from the reactants to the products) as the energy of the products is higher than the reactants
99
What does each chemical bond have which is specific to it?
Each chemical bond has a specific bond energy associated with it
100
What is bond energy?
This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed
101
What can bond energy be used to calculate?
This energy can be used to calculate how much heat would be released or absorbed in a reaction
102
What is necessary to calculate the heat released or absorbed during a reaction?
To do this it is necessary to know the bonds present in both the reactants and products
103
How can you calculate the heat released or absorbed during a reaction?
Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’
Add together the bond energies for all the bonds in the products – this is the ‘energy out’
Calculate the energy change: Energy change = energy in – energy out
104
Fuel definition
A fuel is a substance which releases energy when burned
105
What is produced upon combustion when a fuel is a hydrocarbon?
When the fuel is a hydrocarbon then water and carbon dioxide are produced in combustion reactions
106
What shows the efficiency of a fuel?
The efficiency of a fuel refers to how much energy is released per unit amount
107
How can we measure the efficiency of fuels?
We can measure the efficiency of fuels by calorimetry
108
How can we calculate the heat of combustion?
A known mass of the fuel is combusted and used to heat up a known mass of water to calculate its heat of combustion
109
What is the method for an experiment with a calorimeter?
Using a measuring cylinder, place 100 cm3 of water into a copper can
Measure and record the initial temperature of the water
Fill the spirit burner with test substance and measure and record its mass
Place the burner under the copper can and light the wick
Stir the water constantly with the thermometer and continue heating until the spirit burner burns out
Measure and record the highest temperature of the water
110
What calculations are needed? (experiment with a calorimeter)
Temperature change of water = final temperature – initial temperature
Number of moles burned = change in mass ÷ molecular mass
Amount of energy = change in temperature x mass of water x specific heat capacity
Amount of energy per mole (J mol-1) = total amount of energy ÷ moles burned
111
How is hydrogen used as a fuel?
Hydrogen is used in rocket engines and in fuel cells to power some cars
112
What are advantages of hydrogen as a fuel?
It releases more energy per kilogram than any other fuel (except for nuclear fuels)
It does not pollute as it only produces water on combustion, no other product is formed
113
What are disadvantages of hydrogen as a fuel?
Expensive to produce and requires energy for the production process
Difficult and dangerous to store and move around (usually stored as liquid hydrogen in highly pressurised containers)
114
How is Uranium-235 used in power stations?
Uranium-235 undergoes decay and gives off heat energy which nuclear power stations harness
The heat it produces is used to heat water to steam, which in turn is used to power turbines to generate electricity
115
What are advantages of nuclear fuel?
Nuclear fuel energy is clean as it does not produce pollutants such as CO2 or oxides of nitrogen or sulfur
116
What are disadvantages of nuclear fuel?
But nuclear power plants are expensive to build and maintain as well as being potentially dangerous in the event of an accident as radioactive materials may be released
117
What is a fuel cell?
A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode
118
In which industry are fuel cells becoming more common?
These cells are becoming more common in the automotive industry to replace petrol or diesel engines
119
What is pumped through the electrodes in a fuel cell?
H2 and O2 are pumped through two porous electrodes where the half-reactions occur
120
What is the reaction at the anode? (fuel cell)
2H2 → 4H+ + 4e-
121
What is the reaction at the cathode? (fuel cell)
4H+ + O2 + 4e- → 2H2O
122
What is the overall reaction in a fuel cell?
2H2 + O2 → 2H2O
123
How does a fuel cell work?
The electrons move around the external circuit from the cathode to the anode
This movement of electrons is used to drive an electric motor
124
What are advantages of fuel cell?
They do not produce any pollution
They produce more energy per kilogram than either petrol or diesel
No power is lost on transmission as there are far fewer moving parts than in an internal combustion engine
125
What are disadvantages of fuel cell?
Materials used in producing fuel cells are expensive
High-pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts
Fuel cells are affected by low temperatures, becoming less efficient
Hydrogen is expensive to produce and store
