Chemistry Unit 2 And 3b

Question 1

Recall the order of the members of the reactivity series in order of decreasing reactivity

Answer 1

(G1)
Potassium
Sodium
(G2)
Calcium
Magnesium
(G3)
Aluminium
(G4)
Carbon
(TM)
Zinc
Iron
Copper
Silver
Gold
Platinum

Question 2

Describe the reaction of these metals with oxygen, if they react at all

Answer 2

K - tarnishes readily at room temp
- reacts vigorously when heated
- burns with a lilac flame, forming a white solid

Na - tarnishes readily at room temp
- reacts vigorously when heated
- burns with an orange/yellow flame, forming a white solid

Ca - slowly tarnishes at room temp, forming a surface oxide
- reacts vigorously when heated
- burns with a brick-red flame, forming a white solid

Mg - slowly tarnishes at room temp, forming a surface oxide
- reacts readily when heated
- produces a bright white light, forming a white solid

Al - slowly tarnishes at room temp
- reacts readily when heated as a powder
- forms a white solid

Zn - slowly tarnishes at room temp
- reacts steadily when heated
- forms a yellow solid which cools to white

Fe - slowly tarnishes at room temp
- reacts readily when heated as filings
- orange sparks are produced and a black solid is produced

Cu - slowly tarnishes at room temp
- reacts slowly when heated
- glows red, forming a black solid

Question 3

Give the template equations for group 1, 2, and 3 metals with oxygen

Answer 3

Group 1 (and silver)
4x_(s) + O_{2 (g)} —> 2x₂O_(s)

Group 2 (and 2+ transition metals)
2x_(s) + O_{2 (g)} —> 2xO_(s)

Group 3 (Al + Fe³⁺)
4x_(s) + 3O_{2 (g)} —> 2x₂O_{3 (s)}

Question 4

Describe the reactions of these metals with water

Answer 4

K - reacts vigorously
- floats and spins on the surface
- fizzes and burns with a lilac flame
- heat is given off
- small explosion at the end as metal disappears, forming a colourless solution

Na - reacts vigorously
- floats and spins on the surface
- fizzes and melts into a silvery ball, can burn with an orange flame
- heat is given off
- crackles as the metal disappears, colourless solution is formed

Ca - reacts readily
- sinks and then floats, but does not move on the surface
- melts into a silvery
- heat is given off
- crackles as the metal disappears, forming a colourless solution

Mg - reacts slowly
- few gas bubbles produced
- heat is given off

Question 5

Give the template equations of group 1 and 2 metals with water

Answer 5

Group 1
2x_(s) + 2H₂O_(l) —> 2xOH_(aq) + H_{2 (g)}

Group 2
x_(s) + 2H₂O_(l) —> x(OH)_{2 (aq)} + H_{2 (g)}

Question 6

Describe the reactions of these metals with steam

Answer 6

(K, Na, and Ca are too dangerous to be reacted)

Mg - reacts vigorously
- bright white light and heat given off
- white solid formed

Al - reacts as powder
- white solid is formed

Fe - reacts as powder
- black solid formed

Question 7

Describe the formation of positive ions and how the reactivity of a metal can affect it’s tendency to form a positive ion

Answer 7

• Positive ions (called cations) are formed when metal atoms lose electrons to form ions
• more reactive metals, such as potassium and sodium, have a higher tendency to form cations
• less reactive metals, such as iron and copper, have a lower tendency to form cations

Question 8

Describe and explain displacement reactions between different metal ions in solution, provide an example

Answer 8

• a displacement reaction occurs when a more reactive element takes the place of a less reactive element in its compound
• this can be tested when one compound is dissolved in a solution and another metal is added
• if a reaction occurs the metal added is more reactive than the metal ion in the solution
• for example, in the reaction between Magnesium and Copper (II) sulfate solution, magnesium takes the place of copper, forming magnesium sulfate solution
  Observations for this reaction include:
  > blue solution fades to colourless
  > red-brown coating forms on magnesium
  >heat given out (observed in all reactions, but varies as the difference in reactivities between metals increases)

Question 9

Describe how you would experimentally determine where unfamiliar metals fit into a reactivity series

Answer 9

• Samples of solid metals and their salts are placed in a spotting tile
• a sample of each is placed in every salt, except its own
• the results are recorded in a table with the metals down the first column and the salts across the first row
• the space in the table where each metal meets its own salt is blocked out
• a tick is then placed in the spaces where a reaction occurred between a metal and a salt
• the metal with the most ticks is the most reactive, as it displaced the most other metals from their salts

Question 10

Define the term “ore”. Recall the names given to aluminium ore and iron ore, and state how the metals are extracted from their ores

Answer 10

Ore - a rock that contains a metal compound
- from which the metal can be extracted

Aluminium - ore is called bauxite
- extracted from its ore by electrolysis
- also used for all metals more reactive than carbon

Iron - ore is called haematite
- extracted from its ore by reduction using carbon, in a blast furnace
- also used for all metals less reactive than carbon, but more reactive than copper

Question 11

Describe and explain the process of phytomining and how it used to obtain copper

Answer 11

1. Plants are planted in soil that is rich in copper compounds, and allowed to grow
2. Once the plants are large enough, they are harvested, dried, and burned to produce an ash containing the copper compounds
3. Sulfuric acid is then added to the ash, containing insoluble copper compounds, to produce a leachate solution containing soluble copper compounds
4. Scrap iron is then added to the leachate. The iron displaces the copper from its compounds forming copper
   Ionic equation:
   Fe(s) + Cu²⁺(aq) —> Fe ²⁺(aq) + Cu

Question 12

State some of the advantages of phytomining compared to traditional methods of extracting copper

Answer 12

• Traditional methods of extraction involve digging, transportation and disposal of large amounts of rock which is bad for the environment
• Traditional methods also produce huge amounts of noise, dust, and visual pollution

Question 13

Prescribed Practical C5: Investigating the reactivity of metals

Answer 13

• Pour a known volume of Copper (II) sulfate solution into a polystyrene cup and record the temperature of the solution
• Add a known mass of a metal and stir
• Record the maximum temperature of the mixture and calculate the temperature rise
• Repeat the experiment with the same metal and then twice with a variety of metals
• Record the results in a table with the metals down the first column and the headings “Temperature increase/^OC”, and “average temperature rise/^OC” with the subheadings “Experiment 1” and “Experiment 2” below the first heading

Question 14

Define the terms “reduction” “oxidation” and “redox”

Answer 14

Reduction - the loss of oxygen;
- gain of hydrogen; or
- gain of electrons

Oxidation - the gain of oxygen;
- loss of hydrogen; or
- loss of electrons

Redox - a reaction
- where reduction and oxidation occur
- simultaneously

Question 15

Recognise redox in terms of electrons in a symbol/ionic/half equation, and state which species is reduced/oxidised

Answer 15

Symbol:
2Al + Fe₂O₃ —> Al ₂O₃ + 2Fe

• Oxidation is loss of electrons
• Aluminium atoms lose electrons in the reaction, aluminium is oxidised
• Reduction is gain of electrons
• Iron ions gain electrons in the reaction, Iron Oxide is reduced
• Redox is when reduction and oxidation occur simultaneously
• this is a redox reaction

Half:
Mg —> Mg²⁺ + 2e^-

• Oxidation is loss of electrons
• Magnesium atoms lose electrons in the reaction, Magnesium is oxidised

Cl + e^- —> Cl^-

• Reduction is gain of electrons
• Chlorine atoms gain electrons in the reaction, Chlorine is reduced

Ionic:
Cu + 2Ag⁺ —> Cu²⁺ + Ag

• Reduction is gain of electrons
• Silver ions gain electrons in the reaction, Silver is reduced
• Oxidation is the loss of electrons
• Copper atoms lose electrons in the reaction, Copper is oxidised

Question 16

State the scientific term for rust, its chemical formula, and describe how it looks

Answer 16

Rust - Hydrated iron (III) oxide
- Fe₂O₃.xH₂O
- it is a red-brown, flaky solid

Question 17

Describe how rust forms and how it causes iron objects to weaken over time

Answer 17

• rust is formed when iron reacts with oxygen and water
• rust flakes off of iron objects it forms on
• this causes the iron underneath to be exposed to the air and moisture, causing it also to rust
• this cycle continues and the iron continues to corrode and weaken

Question 18

Describe how you would experimentally investigate the conditions required for rust to form

Answer 18

• take 3 test tubes with an iron nail in each
• in the first test tube have some water and some air
• in the second fill with boiled water until the nail is fully submerged, and pour a layer of oil onto the water
• in the third put some anhydrous calcium chloride and no water
• the only test tube in which the nail should show rusting is the first one
• the boiled water, in the second test tube contains no dissolved oxygen, and the layer of oil prevents any oxygen from dissolving in the water, so the iron is not exposed to oxygen
• the anhydrous calcium chloride, in the third test tube removes moisture from the air, so the iron has no exposure to water
• the results from these tests combined proves that both air and water simultaneously are needed for iron to rust

Question 19

Describe some methods commonly used to prevent rusting, and some examples of each

Answer 19

Painting - acts as a physical barrier, preventing the iron from being exposed to oxygen and water
- used commonly to protect cars, bridges and railings

Oiling - acts as a physical barrier, preventing the iron from being exposed to oxygen and water
- used most commonly for moving parts, such as bicycle chains and hinges as it also doubles as a lubricant

Plastic coating - acts as a physical barrier, preventing the iron from being exposed to oxygen and water
- used to cover garden fences, fridge shelves and weights

Metal plating - a thin plate of a different metal is applied using electroplating
- acts as a physical barrier, preventing the iron from being exposed to oxygen and water
- used primarily in food cans (with tin), and buckets and chains are galvanised
- “galvanisation” is the term given to metal plating with zinc specifically

Sacrificial protection - a metal that is higher up in the reactivity series is used to cover the iron so that it will react first and corrode before the iron
- for example, zinc blocks are fastened to the hulls of ships to prevent the steel rusting, and magnesium is used to line pipes and oil rigs
- these are replaced periodically to prevent the iron from ever rusting

Question 20

Describe the process of removing iron from its ore in the blast furnace, include the (1) production of the reducing agent, (2) reduction of the ore, and (3) removal of acidic impurities and balanced symbol equations

Answer 20

(1) - Haematite (iron ore), limestone (calcium carbonate) and coke (pure carbon) are put into the top of the blast furnace
- blasts of hot air react with the coke to form carbon dioxide
- carbon dioxide further reacts with more coke to form carbon monoxide, which is used as a reducing agent:
C + O₂ —> CO₂
CO₂ + C —> 2CO
(2) - Carbon monoxide reacts with haematite to produce iron and carbon dioxide:
Fe₂O₃ + 3CO —> 2Fe + 3CO₂
- the molten iron sinks to the bottom to be tapped off
(3) - acidic impurities such as sand (silicon dioxide) are present in the ore and need to be removed
- this is done by thermally decomposing the limestone into calcium oxide and carbon dioxide:
CaCO₃ —> CaO + CO₂
- the calcium oxide reacts with the silicon dioxide to form slag (calcium silicate):
CaO + SiO₂ —> CaSiO₃
- the molten slag sinks to the bottom, but is tapped off above the iron due to its lower density

Question 21

State two reasons why iron is used in the building of bridges

Answer 21

• it is incredibly strong; and
• the ore is common, so it is very cheap

Question 22

State the equation to find the rate of a reaction and state the unit of rate

Answer 22

Eqn:
1
Rate = ———
time

Unit: s^-1

Question 23

State the three ways rate may be calculated

Answer 23

1. Mass of gas produced lost over a period of time
2. Volume of a gas produced over a period of time
3. Measuring the amount of time it takes for a certain amount of a precipitate to form

Question 24

Describe the “collision theory” and define the term “activation energy”

Answer 24

• Collision theory states that in order for a reaction to occur particles must collide with substantial energy
• this energy is called the activation energy
  Activation energy - the minimum energy required for a reaction to occur
• needed to break bonds so that the reactant particles may be rearranged to form the products
• successful collisions are those that result in a reaction, and unsuccessful collisions are those that don’t
• the rate of reaction is increased when the energy and frequency of collisions increase

Question 25

State, describe, and explain the four factors that affect the rate of a reaction

Answer 25

1. Temperature
   - the higher the temperature of a system, the greater the rate of reaction will be
   - this is because the particles have more kinetic energy so collide more, and collide more frequently
   - they also have greater energy, meaning more are more likely to have the activation energy
   - hence, the likelihood of successful collisions in the same space of time is higher, and the rate of reaction increases
2. Concentration of a solution
   - the more concentrated a solution is, the faster the rate of reaction will be
   - there are more particles available to react in the same volume of solution
   - successful collisions are more frequent
   - hence, the rate of reaction is faster
3. Surface are (S/A) of solid reactants
   - increasing the surface area of reactants increases the rate of a reaction
   - more particles are on the surface of the reactant, and are able to react
   - and as such, successful collisions between particles with the activation energy are more frequent
   - and the rate of reaction is faster
4. Presence of a catalyst
   - a catalyst is a substance that speeds up the rate of a reaction without being used up in the reaction itself
   - different reactions have different catalysts, i.e.;
   > Catalytic decomposition of hydrogen peroxide (catalyst = manganese (IV) oxide)
   > The Haber Process (catalyst = Iron)
   - a catalyst functions by providing an alternative reaction pathway of lower activation energy
   - so there is more chance of particles colliding with substantial energy to react, so there is more frequent successful collisions
   - rate of reaction is faster

Question 26

Prescribed practical C6(a): Investigating the effects of changing a variable (Temperature/concentration/presence of a catalyst) on the rate of a reaction by observing the formation of a precipitate

Answer 26

• Pour 25cm³ of sodium thiosulfate into a conical flask
• Place the flask on top of a piece of paper with a cross drawn on it
• Add hydrochloric acid; and
• Start the timer
• Stop the timer when the precipitate causes the solution to become cloudy, and the cross is no longer visible
• Alter the variable that’s being investigated and repeat the investigation
• Do this a number of times and record the results in a table with the “experiment number” down the first column and the headings “Concentration of sodium thiosulfate/ mol/dm³” “Time taken for cross to disappear/ s” “Rate of reaction/ s^-1”

Question 27

Prescribed practical C6(b): Investigating the effects of changing a variable on the rate of a reaction by measuring the volume of gas produced

Answer 27

• Pour some hydrochloric acid into a conical flask
• Add 1.0g of magnesium turnings to the flask
• Place a rubber stopper with a delivery tube leading to a gas syringe in the flask; and
• Start the timer
• Record the volume of gas produced at 1 minute intervals
• Stop the timer when the solution stops effervescing
• Alter the variable being investigated and repeat the investigation
• Do this a number of times and record the results in a line graph with “gas volume/cm³” on the y-axis and the “time/min” on the x-axis, and a line for each time the investigation was carried out with a different temperature/concentration/S/A of magnesium/catalyst

Question 28

Define the term “reversible reaction”, state how is this notated, and describe how the direction of a reversible reaction may be changed

Answer 28

Reversible reaction - a reaction in which the products can
- further react to reform the reactants

• This is shown in equations by a reversible arrow
• The direction of the reaction may be changed by changing the conditions of the system

Question 29

Define the terms “dynamic equilibrium”, “open system”, “closed system”, “position of equilibrium”, and “homogeneous”

Answer 29

Dynamic equilibrium - occurs in a closed system
- when the rates of the forward and reversed reactions are the same
- and the amounts of products and reactants are constant

Open system - a system that allows entry and exit of substances

Closed system - a system that does not allow entry or exit

Position of equilibrium - the side of a system at equilibrium with a greater relative amount of substance
- if there are more reactants than products the position of equilibrium is regarded as being to the left
- if there are more products than reactants the position of equilibrium is regarded as being to the right

Homogeneous - all products and reactants and products are in the same physical state

Question 30

State Le Châtelier’s Principle and state and describe the effects of changing concentration, temperature and pressure

Answer 30

Le Châtelier’s Principle - If a change is made to the conditions of a system then the position of equilibrium will move to oppose the change in conditions

Concentration:
- Increase in concentration of a reactant - position of equilibrium moves to the right to reduce the concentration of the reactant
- Increase in concentration of a product - position of equilibrium moves to the left to reduce the concentration of the product
- Decrease in concentration of a product - position of equilibrium moves to the right to increase the concentration of the product
- Decrease in concentration of a reactant - position of equilibrium moves to the left to increase the concentration of the reactant

Temperature:
Forward reaction is exothermic;
- Increase in temperature - position of equilibrium moves to the left (endothermic direction) to lower the temperature
- Decrease in temperature - position of equilibrium moves to the right (exothermic direction) to raise the temperature
Forward reaction is endothermic;
- Increase in temperature - position of equilibrium moves to the right (endothermic direction) in order to lower the temperature
- Decrease in temperature - position of equilibrium moves to the left (exothermic direction) in order to raise the temperature

Pressure:
More molecules on the left (i.e. 2A(g) <—> B(g));
- Increase in pressure - position of equilibrium moves to the right (side with fewer molecules) to decrease the pressure
- Decrease in pressure - position of equilibrium moves to the left (side with more molecules) to increase the pressure
More molecules on the right (i.e. A(g) + B(g) <—> 4C(g));
- Increase in pressure - position of equilibrium moves to the left (the side with fewer molecules) in order to decrease the pressure
- Decrease in pressure - position of equilibrium moves to the right (side with more molecules) in order to increase the pressure
Equal molecules on both sides (i.e. A(g) <—> B(g);
- no change in position of equilibrium in response to an increase or decrease in pressure as there is already equal pressure on both sides

Question 31

Describe the Haber Process, give the balanced symbol equation

Answer 31

The Haber Process - a reversible exothermic reaction between hydrogen and nitrogen to produce ammonia
N₂ + 3H₂ <—> 2NH₃

Question 32

Describe and explain the conditions used in industry to optimise the yield of ammonia

Answer 32

Temperature - ammonia is produced by an exothermic reaction. Decreasing temperature will cause the position of equilibrium to move to the right, producing more ammonia. However, at too low of a temperature the rate of reaction is hindered, thus a compromise temperature of 450°C is used

Pressure - If a high pressure is used the position of equilibrium moves to the right to decrease the pressure, so more ammonia is produced. However the infrastructure needed to accommodate this would be expensive to build and maintain, so a compromise pressure of 200atm is used

Use of a catalyst - the catalyst has no effect on the position of equilibrium, as it does not take place in the reaction. As such, it has no effect on the yield of ammonia, it just speeds up the rate at which ammonia is produced. For the Haber Process Iron is used as a catalyst

Question 33

Define the terms “exothermic” and “endothermic” and state the whether the energy change value is positive or negative

Answer 33

Exothermic - a reaction that gives out thermal energy
- has a negative energy change value

Endothermic - a reaction that takes in thermal energy
- has a positive energy change value

Question 34

Give 5 examples of an exothermic reaction and an endothermic reaction

Answer 34

Exothermic:
- Respiration
- Nuclear Fission
- Neutralisation
- Combustion
- Metal and acid reactions

Endothermic:
- Photosynthesis
- Evaporation
- Alkane Cracking
- Thermal decomposition
- Electrolysis

Question 35

Define “activation energy”

Answer 35

• The minimum amount of energy required for a reaction to occur

Question 36

Describe a reaction profile and how you draw one for an endothermic reaction and an exothermic reaction

Answer 36

Reaction profile - a graph showing how the energy of reactants and products changes as a reaction progresses
- with “energy” on the y-axis and “progress of reaction” on the x-axis
- gradient is called the reaction pathway

Endothermic - horizontal showing the energy of the reactants
- rises sharply and then plateaus to show the activation energy being reached
- sharp decrease and then a horizontal line to show the energy of the products
- the energy of the products will be greater than the energy of the products as the reaction is endothermic

Exothermic - horizontal showing the energy of the reactants
- rises sharply and then plateaus to show the activation energy being reached
- sharp decrease and then plateaus to a horizontal line to show the energy of the products
- the energy of the products will be less than the energy of the reactants as the reaction is exothermic

Question 37

Describe how a reaction profile can be used to find the activation energy/energy change value of a reaction

Answer 37

Activation energy - the peak of the reaction pathway is the activation energy

Energy change value - subtract energy of reactants from energy of products
- value will be positive for endothermic reactions
- value will be negative for exothermic reactions

Question 38

Describe how bonds are broken and formed and how it is determined whether a reaction is exothermic or endothermic

Answer 38

• all reactions have exothermic and endothermic stage; whether they are called one or the other depends on the net energy change value
• bond breaking takes in energy, which is the endothermic stage (Big BEN (BB = Bond Breaking, EN = endothermic))
• bond forming releases energy, which is the exothermic stage (EX BoyFriend (EX = exothermic, BF = Bond Forming))
• the energy change value of a reaction is found by subtracting the energy required to form the new bonds from the energy taken in by breaking bonds
• if the result is positive the reaction is endothermic, and if it negative then the reaction is exothermic

Question 39

Define the terms “electrolyte”, “inert”, “electrode”, “anode”, “cathode”, and “electrolysis”

Answer 39

Electrolyte - a molten or dissolved ionic compound
- that carries electricity, as particles are free to move and carry charge
- and is decomposed by electricity

Inert - unreactive

Electrode - a solid conductor, used to make contact with a non-metallic component in a circuit

Inert electrode - an electrode that allows electrolysis to occur, without reacting themselves
- usually made from graphite as it is inert, and it is a good conductor of electricity
- can also be made from platinum, but it is much more expensive

Anode - the positively charged electrode
- the electrode that the anions are attracted and move to
- forming the atom/molecule form of the element

Cathode - the negatively charged electrode
- the electrode that the cations are attracted and move to
- forming the atom form of the element

Electrolysis - decomposition of an electrolyte
- by passing a direct current through it

Question 40

Predict the products of electrolysis of molten salts such as lead (II) bromide and lithium chloride and the observations you would make at each electrode

Answer 40

Lead (II) bromide
Cathode:
- molten grey liquid forms
- reaction is reduction of lead (II) ions to lead atoms
Half eqn; Pb²⁺ + 2e^- —> Pb

Anode:
- red-brown pungent gas effervesces
- reaction is oxidation of bromine ions to bromine molecules
Half eqn; 2Br^- —> Br₂ + 2e ^-

Lithium chloride
Cathode:
- molten grey liquid forms
- reaction is reduction of lithium ions to lithium atoms
Half eqn; Li⁺ + e^- —> Li

Anode:
- yellow-green pungent gas effervesces
- reaction is oxidation of chlorine ions to chlorine molecules
Half eqn; 2Cl^- —> Cl₂ + 2e^-

Question 41

Describe the industrial extraction of aluminium from its ore via electrolysis

Answer 41

• aluminium ore is known as bauxite
• when bauxite is purified alumina (aluminium oxide) is formed
• in order for electrolysis to occur the aluminium ions must be free to flow and carry charge
• alumina has a melting point of 2072°C, and to melt it at this temperature would require a lot of energy, which would be expensive
• to decrease the melting point the alumina is dissolved in cryolite
• this reduces the melting point to 900°C and increases the conductivity
• the ore is place in a steel tank, called a cell, lined with a graphite lining, acting as the cathode
• graphite rods are then dipped into the electrolyte acting as anodes
• aluminium ions go to the cathode where they discharge to form aluminium metal
  Half eqn: Al³⁺ + 3e^- —> Al
• oxide ions go to the anodes where they are discharged to form oxygen molecules
  Half eqn: O^2- —> O^{2</sub> + 4e-}

Question 42

Describe how the closed system is maintained during the electrolysis process

Answer 42

• a crust of aluminium oxide forms on the surface of the electrolyte
• this acts as a lid by:
• keeping heat in;
• stops impurities entering; and
• prevents the aluminium reacting with the air

Question 43

List some of the disadvantages of extraction of aluminium by electrolysis and some of the advantages of recycling aluminium instead

Answer 43

Disadvantages of extraction by electrolysis:
- high cost of electricity
- high cost of energy to melt the metal compounds and keep them molten

Advantages of recycling:
- uses about 5% of the energy to required to create aluminium from bauxite, and saves money and energy as a result
- saves waste
- saves natural resources of bauxite

Question 44

Describe the products of electrolysis of sulfuric acid, include the half equations of the reactions happening at the electrodes

Answer 44

• when dilute sulfuric acid is used as an electrolyte, some of the water it is dissolved in decomposes into hydrogen cations and hydroxide anions
• dilute sulfuric acid contains hydrogen cations and sulfate anions
• because there is two types of anion, preferential discharge takes place at the anode
• hydroxide ions react more readily than the sulfate ions so they are more attracted to the anode than the sulfate ions
  Half eqn for cathode: 2H⁺ + 2e^- —> H₂
  Half eqn for anode: 4OH^- —> O²

Question 45

State how many covalent bonds each carbon atom can have and how this is commonly seen, and define the term “homologous series”

Answer 45

• every carbon atom can form 4 covalent bonds
• this can be with other carbon atoms, to form chains or rings
• these can be single, double or triple bonds
• can bond with other atoms such as hydrogen, oxygen, and chlorine
• there are in excess of 10 million known carbon compounds
• these are categorised into groups called homologous series

Homologous series - a family of elements with:
- the same general formula
- similar chemical properties
- a gradation in physical properties
- differ by ‘CH₂’ group

Question 46

Define the term “hydrocarbon”

Answer 46

• an organic compound
• that consists of hydrogen and carbon atoms
• only

Question 47

Recall the nomenclature for organic compounds

Answer 47

Carbon atom number (prefix):
Meth- -> 1 carbon atom
Eth- -> 2 carbon atoms
Prop- -> 3 carbon atoms
But- -> 4 carbon atoms

Homologous series (suffix):
-ane -> alkane
-ene -> alkene
-anol -> alcohol
-anoic acid -> carboxylic acid

Question 48

Define “alkane” and recall the general formula for alkanes, and the molecular formulae of the first 4 alkanes
Recall also, the state and colour of the first 4 alkanes at room temperature and pressure

Answer 48

Alkane - saturated hydrocarbon
- all carbon-carbon bonds are single
- does not have a functional group

General formula - C_nH_2n+2

Methane - CH₄
Ethane - C₂H₆
Propane - C₃H₈
Butane - C₄H₁₀

• The first 4 alkanes are all colourless gases at room temperature and pressure

Question 49

State the products formed from the complete and incomplete combustion of alkanes and write symbol equations for the complete and incomplete combustion of the first 4 alkanes

Answer 49

• When alkanes combust with a plentiful supply of oxygen they completely combust
• the products of this are carbon dioxide (CO₂) and water (H₂O)

Methane - CH₄ + 2O₂ —> CO₂ + 2H₂O
Ethane - 2C₂H₆ + 7O₂ —> 4CO₂ + 6H₂O
Propane - C₃H₈ + 3O₂ —> 3CO₂ + 4H₂O
Butane - 2C₄H₁₀ + 13O₂ —> 8CO₂ + 10H₂O

• when alkanes combust in a limited supply of oxygen they incompletely combust
• the products of this are carbon monoxide (CO) and water (H₂O)

Methane - 2CH₄ + 3O₂ —> 2CO + 4H₂O
Ethane - 2C₂H₆ + 5O₂ —> 4CO + 6H₂O
Propane - 2C₃H₈ + 7O₂ —> 6CO + 8H₂O
Butane - 2C₄H₁₀ + 9O₂ —> 8CO + 10H₂O