Chapter 2: Carbon-based fuels

Question 1

Fuels

Answer 1

• substances with stored chemical energy that can be readily released to produce heat or power
• high energy content → release a large amount of energy
• all chemicals contain stored energy but they can’t all be used as fuels

Question 2

Fossil fuels

Answer 2

• natural fuels formed from the decomposition of buried dead organisms (plants, animals, microorganisms) over millions of years
• have a high carbon content, ideal for burning

Question 3

Natural Gas

(Coal seam gas)

Answer 3

• a fossil fuel consisting of small hydrocarbon molecules
  
  • methane, small amounts of ethane and propane, water sulphur, nitrogen and cvarbon dioxide
• found in Earth’s crust deposits and accessed by drilling
  
  • extraction involves fracking
  • injecting high-pressure liquid into rocks to force open cracks and extract the gas

Question 4

Coal

Answer 4

• formed from the pressure and heat that compressed the remains of wood and plant material
• mixture of carbon, hydrogen, sulfur and nitrogen
• overtime water content decreases
  
  • carbon content increases
  • amount of hydrogen and oxygen decreases
• presence of water decreases energy content
  
  • black coal: small amount of water + high carbon content → high amount of potential energy
  • formed under increased temp and pressure
  • is deeper undergorund

Question 5

Crude Oil

(Petroleum)

Answer 5

• a mixture of hydrocarbon molecules (mostly alkanes)
• crude oil itself has no use as a fuel
  
  • needs to be separated via fractional distillation to extract smaller fractions that serve as fuel
  • petrol: includes octane + other alkanes
  • petrodiesel: includes alkanes that are longer than in petrol

Question 6

Name and describe the process by which crude oil is turned into petrol

Answer 6

Fractional distillation
* uses boiling points of different alkanes to separate crude oil to produce desired products
* crude oil itself has no use as a fuel

Question 7

Combustion reaction

Answer 7

Combustion reactions involve a substance (usually a fuel) reacting with oxygen.
* always exothermic because there is a net release of energy during these reactions
* they are a type of oxidation reaction.

Question 8

Non-renewable vs renewable resource

Answer 8

Non-renewable resources cannot be replaced by natural processes within a relatively short period
* eg. fossil fuels

Renewable resources are capable of being replaced by natural processes within a relatively short time.
* eg. biofuels

Question 9

Sustainability

Answer 9

the ability to produce at a rate equal to or greater than consumption

Question 10

Fossil fuels and biofuels are both derived from organic material. Justify which one is the renewable and non-renewable resource.

Answer 10

Fossil fuels are non-renewable resources
* formed from the decomposition of plant and/or animal remains over millions of years.

Biofuels are renewable resources
* produced from organic matter in a relatively short time

Question 11

How does extraction and use of fossil fuels harm the environment

Answer 11

• Emission of sulfur dioxide from burning coal and contaminated petrol contributes to acid rain formation
• Fossil fuel-fired electric power stations emit carbon dioxide, contributing to the enhanced greenhouse effect and global warming
• Drilling and mining for fossil fuels disrupt natural landscapes and can contaminate water tables if spills occur

Question 12

Biofuels

Answer 12

a fuel sourced directly from organic matter, such as sugarcane, soya beans, wheat, or vegetable oils.
* Eg. biogas, biodiesel, bioethanol

is carbon neutral (according to VCAA)
* the release of carbon dioxide from combustion is offset by the carbon dioxide absorbed by plants through photosynthesis
* have less impact on the environment than fossil fuels

Question 13

Production of biogas

Answer 13

• organic matter is broken down by bacteria into gas
  
  • this process is anaerobic
• the gas produced is a mixture of different gases: predominantly methane and a large proportion of carbon dioxide

Question 14

Production of biodiesel

Answer 14

• produced by a transesterification reaction
  
  • the proess of exchanging organic functional groups of an ester with the organic group of an alcohol
• involves animal fats and plant oils like sunflower and canola
• these fats and oils are triglycerides
  
  • triglycerides → fat or oil composed of three fatty acid tails and a glycerol backbone
• an alcohol, typically methanol, is added to the triglyceride
• a catalyst such as KOH or NaOH is used in the reaction
• the result is biodiesel, a mix of fatty acid methyl esters

Question 15

Production of bioethanol

Answer 15

Produced by the anaerobic fermentation of glucose in biomass
* accelarated using enzymes
* the resulting solution undergoes distillation to remove water and create pure bioethanol
* ethanol is only considered bioethanol when produced via the fermentation and distillation process

Can be blended with petrol
* eg. E10 petrol which has 10% bioethanol
* reduces the emission of harmful gases such as oxides of nitrogen

Question 16

Explain why proteins, carbohydrates, and fats all have different energy content

Answer 16

Due to the different types of bonds broken and formed during metabolism

Question 17

Energy content

Answer 17

the amount of heat energy (in joules or kilojoules) produced upon burning 1 gram of a pure substance

Question 18

Photosynthesis

Answer 18

The process in which sunlight is converted into chemical energy
* involves the absorption of sunlight by chlorophyll in plant cells
* absorbed light splits water molecules into oxygen and hydrogen ions
* hydrogen ions and electrons are used to create carbohydrates from carbon dioxide
* produces oxygen as a by-product
* endothermic reaction → energy from sun is required for reaction to proceed

The chemical equation is:
6CO2(g) + 6H2O(l) + solar energy → C6H12O6(aq) + 6O2(g)

Question 19

Describe cellular respiration and its process

Answer 19

Cellular respiration is the process by which organisms convert glucose into energy
* glucose is absorbed into the bloodstream transported to cells and converted into energy via cellular respiration
* polymers in food are broken down into monomers such as glucose by digestion
* glucose is then converted into energy by an oxidation process
* exothermic reaction - chemical energyis stored in glucose and is released for use by the body

The chemical equation is:
C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6H2O(l) + Chemical energy

Question 20

Enthalpy change

Answer 20

Enthalpy change (ΔH) represents the total change in chemical energy during a reaction
* measured in units of kJ
* molar enthalpy of a pure sample of a chemical compound is measured in kJ per mole
* enthalpy of a mixture of chemical compounds is measured in kJ per gram
* the enthalpy change of the reverse reaction is the same magnitude as the forward reaction but with the opposite sign
* activation energies of the forward and reverse reactions will be different

Question 21

Activation energy

Answer 21

Activation energy (Ea) is the minimum initial amount of chemical energy required to break the chemical bonds in the reactants to initiate the reaction

Question 22

Exothermic reactions

Answer 22

Energy released as chemical bonds in the products are formed is greater than the energy absorbed to break bonds in the reactants
* ‘step down’ on energy profile diagram
* energy is released from the system and into the surroundings
* ΔH will be negative

Question 23

Endothermic reactions

Answer 23

Energy absorbed to break the chemical bonds in the reactants is greater than the energy released as bonds in the products are formed
* ‘step up’ on energy profile diagram
* energy is absorbed from the surroundings
* the activation energy will always be larger in the endothermic reaction
* ΔH will be positive

Question 24

Energy profile diagram

Answer 24

represents the amount of energy used to break bonds in the reactants (Ea) and form new bonds in the products

Question 25

Complete combustion

Answer 25

Complete combustion when oxygen is in excess
* produces only carbon dioxide and water
* excess oxygen can create explosive conditions

Question 26

Incomplete combustion

Answer 26

Incomplete combustion when oxygen supply is limited
* produces carbon-based products in addition to water, which includes: carbon monoxide gas (CO), carbon dioxide gas (CO2), and soot (C)
* harmful particles can be released into the atmosphere

Incomplete combustion forms fewer chemical bonds, producing less energy than complete combustion reactions of the same fuel

Question 27

Thermochemical equations

Answer 27

a balanced chemical equation with states and ΔH (in kJ or kJ per mol) on the right
* the ΔH value in kJ represents the total enthalpy change in a chemical reaction
* the amount of energy in kJ signified by the ΔH value corresponds to the mole amounts specified by the coefficients in the equation
* if the mole ratio is changed, ΔH will change correspondingly
* if the chemical equation is reversed, ΔH is reversed but its magnitude remains the same

Question 28

Disadvantage of bioethanol

Answer 28

• require land which could be used to grow food
• require harsh pre-treatment to break down material
• energy content less than petrol (around 38% less)
• lower energy content is a result of carbon atoms in ethanol being partly oxdised

Question 29

E10 vs Petrol

Answer 29

E10
* 46 kJ/gram for E10
* cheaper
* cleaner combustion
* absorbs CO2 during formation

Petrol
* 48 kJ/gram for petrol
* more widely distributed
* less CO2 produced per km

Question 30

Energy content in food

Answer 30

• carbohydrates (sugars and starch)
• lipids (fat and oils)
• proteins

Question 31

Carbohydrates

Answer 31

• made up of small units called monomers
  
  • specifically called monosaccharides
  • are polymers made of glucose such as starch
• composed of carbon, hydrogen, and oxygen
• carbohydrates is broken into gluose by enzymes during digestion
• energy stored in bonds in the large molecules are released during digestion

Question 32

Fats and oils

Answer 32

• are triglycerides
• are large non-polar molecules with 3 long hydrocarbon chains attached to a glycerol molecule
• digestion breaks down fats
  
  • components of this breakdown can can be oxidsed to CO2 and H2O, releasing large quantities of energy
• produces more energy than carbohydrates
  
  • fats and oils have greater potential for oxidation
  • carbohydrates are already partially oxidised as it contains a relatively high proportion of oxygen atoms

Question 33

Protein

Answer 33

• rarely used as an energy source in the body
• used during intensive exercise when glycogen and fat are exhausted
• made up of smaller units
  
  • amino group
  • carboxyl group
  • R group (variable side chain)

Question 34

States in enthapy

Answer 34

• change of a state is physical rather than a chemical change
• physical changes involve enthalpy
  
  • energy being absorbed or released
  • described as endothermic or exothermic processes
• ΔH is different depending on whether the water produced is gas or liquid
  
  • heat required to convert liquid water to gas (endothermic)

Question 35

Reverse reaction

Answer 35

• read the curve from right to left on the energy profile diagram
• magnitude of ΔH remains constant, but opposite sign is used
• activation energies of the forward and reverse reactions will be different
• the activation energy will always be larger in the endothermic reaction

Question 36

Distillation

Post treatment of bioethanol

Answer 36

• water needs to be seperated from ethanol to obtain a purer product and be useful
• uses the different boiling points of liquid to separate the components in the mixture
• water falls to the bottom and ethanol is collected from the top

Question 37

Hydrolysis

Answer 37

Water is used to breakdown the chemical bonds in large molecules

Question 38

Condensation

Answer 38

When molecules combine to form larger ones and water is released