Chemistry Unit 3: Chemical Processes

Area of Study 1: Supplying Energy

Carbon-Based Fuels

Fuel Definition: A substance releasing energy as heat through reactions.
- Releases energy via combustion reactions.
- SI unit of energy: Joules (J).
- $1 \text{ kJ} = 1000 \text{ J} = 10^3 \text{ J}$
- $1 \text{ MJ} = 1,000,000 \text{ J} = 10^6 \text{ J}$
- $1 \text{ GJ} = 10^9 \text{ J}$
Fossil Fuels: Formed over millions of years from remains of living organisms; the main energy source.
- Examples: Coal, petrodiesel, natural gas, coal seam gas, uranium.
Biofuels: Sourced directly from organic matter; an alternative to fossil fuels.
- Examples: Biogas, bioethanol, biodiesel.
Renewability: A renewable fuel can be replaced in a relatively short period. Non-renewable fuels are consumed faster than they can be produced.

Fossil Fuels Details

Formation: Long-term decomposition of organic matter (thousands to millions of years).
Types and Characteristics:
- Coal:
  - Composition: Mainly carbon, with some hydrogen, sulfur, oxygen, and nitrogen.
  - Origin: Plant matter, 50-225 million years old.
  - Main use: Electricity generation.
  - Features: Brown coal is younger and has higher moisture content than black coal.
- Crude Oil (Petroleum):
  - Composition: Hydrocarbons, with some sulfur.
  - Origin: Marine algae, 60-180 million years old.
  - Main use: Transport and heating.
  - Features: Must be separated into fractions of similar boiling points before use.
- Natural Gas:
  - Composition: Methane (CH4) mainly, with some ethane (C2H6), carbon dioxide (CO2), sulfur, and nitrogen.
  - Origin: Decomposed matter trapped under rock beds, millions of years old.
  - Main use: Heating and electricity.
  - Features: Often trapped above crude oil and mined at the same site.
- Coal Seam Gas:
  - Composition: Methane (CH4) mainly, with some ethane (C2H6), carbon dioxide (CO2), sulfur, and nitrogen.
  - Origin: Natural gas associated with coal beds.
  - Main use: Heating and electricity.
  - Features: Extracted using fracking (water used to crack rocks to release gas).
- LPG (Liquid Petroleum Gas):
  - Composition: Propane (C3H8) & Butane (C4H10) mainly
  - Origin: See Natural Gas
  - Main use: Heating/transport
- Combustion Products: All these produce CO2 when combusted.

Biofuels Details

Methane Example: Methane from natural gas vs. methane from vegetable scraps.
- Natural gas methane: Fossil fuel due to long-term fossilization.
- Vegetable scrap methane: Biofuel, directly from organic matter.
Petrodiesel: Mixture of hydrocarbons (8-21 carbon atoms).
Biodiesel: Mixture of fatty acid methyl esters (10-15 carbons).
- Made from triglycerides (fats and oils) reacting with methanol to form methyl esters.
Biodiesel Production - Transesterification:
- Lipids (fats and oils) are triglycerides (three long-chain fatty acids bonded to glycerol).
- Triglyceride hydrolysis separates fatty acids from glycerol.
- Methyl esters form by reacting fatty acids with methanol in transesterification.
- Overall Reactions:
  - Triglyceride + 3 Water Molecules → Glycerol + 3 Fatty Acids
  - 3 Fatty Acids + 3 Methanol → 3 Methyl Esters

Costs and Benefits of Fuels

Factors: Energy efficiency, economic costs, environmental considerations, sustainability.
Energy Output: Combustion of organic fuels is fairly inefficient.
- Example: Coal-fired power stations (e.g., LaTrobe Valley) are about 35% efficient due to heat loss during energy transformations.
Efficiency of Energy Production:
- Coal-fired power station: Only 35% efficient due to energy losses as heat during transformations.
- Gas-fired power stations (e.g., Newport) are more efficient because some electrical energy is produced directly from a gas turbine.
Sustainability:
- An energy source is sustainable if it meets current needs without compromising future generations.
- Generally requires the energy source to be renewable.
Renewable:
- A fuel or energy source is renewable if it can be replaced faster than it is used.
- Fuels consumed faster than replacement are non-renewable.

Environmental Considerations

Particulates and Pollutants: Impure fuels release pollutants (NO2, SO2, CO, solid/liquid particulates) when burned.
Carbon Emissions:
- Organic fuels release carbon as CO2, CO, or C upon combustion.
- CO2 is a significant greenhouse gas.
- Fossil fuels add to atmospheric greenhouse gases.
- Biofuels have almost zero net carbon emissions.
  - Carbon in biofuels is taken from the atmosphere during plant growth and released back upon combustion.
Other Considerations: Land use, water requirements, and competition with food production.

Thermochemical Equations

Key Concepts: Bond making/breaking, endothermic/exothermic reactions, limiting reactants, energy profile diagrams.
Chemical Enthalpy (H): Heat energy released from fuel combustion.
- Enthalpy is affected by attractions/repulsions within a substance and kinetic energy of electrons/atoms.
Understanding Bond Energy:
- Bonds in molecules are broken and formed during reactions.
- Products have different enthalpy from reactants.
- Change in enthalpy ( $\Delta H$ ) causes temperature change in surroundings.
  - If energy is released, temperature increases.
  - If energy is absorbed, temperature decreases.
- $\Delta H = \text{Enthalpy of Products} - \text{Enthalpy of Reactants}$
Types of Enthalpy Changes:
- Heat of solution: Enthalpy change when one mole of a substance dissolves in water.
- Heat of neutralization: Enthalpy change when an acid reacts with a base to form one mole of water.
- Heat of vaporization: Enthalpy change when one mole of liquid converts to gas.
- Heat of combustion: Enthalpy change when a substance burns in air; always exothermic.

Energy Profile Diagrams

*Shows energy of reactants and products in a chemical reaction.
*Activation energy: energy required to break bonds.
*Energy evolved: when new bonds are formed.
* $\Delta H$ : energy released during the reaction.
* $\Delta H$ = -ve

*Change in enthalpy (heat), $\Delta H$ = Hproducts - Hreactants
*Breaking bonds is endothermic while forming bonds is exothermic.

Exothermic and Endothermic Reactions

Combustion of Fuels

Exothermic Reactions:
- $\Delta H$ is negative.
- Heat is emitted.
Endothermic Reactions:
- $\Delta H$ is positive.
- Heat is absorbed.
Enthalpy (H) is also called "heat content" or "chemical energy".
$\Delta H$ is measured in kJ mol-1 for pure substances and kJ g-1 or MJ/tonne for mixtures.

Factors Affecting $\Delta H$

The value of $\Delta H$ depends on the number of moles of the substances that react.
Halving the coefficients results in halving the $\Delta H$ value.
Reversing the reaction results in changing the sign of the $\Delta H$ value.
*Combustion of Fuels
*Complete Combustion occurs when there is an excess of oxygen gas. Produces carbon dioxide and water.
*Incomplete Combustion occurs when there is limited oxygen. Carbon in the fuel is only partially oxidised so less energy is released. The reaction produces carbon monoxide gas, CO or carbon, C, in the form of soot.

Fuel Sources for Plants and Animals

Food provides energy for the many millions of chemical reactions in the body. The source of all this energy is the Sun.
Plants convert this energy via photosynthesis into simple carbohydrates.
Other food molecules are proteins and lipids (fats and oils).
Carbohydrate, protein and lipid molecules.
Photosynthesis is an endothermic reaction that converts light energy, carbon dioxide and water into glucose and oxygen

Glucose - the Primary Energy Source

Simple carbohydrates, such as glucose, are produced by plants during photosynthesis.
$6CO2(g) + 6H2O(l) \rightarrow C6H{12}O6(aq) + 6O2(g)$
$\Delta H \cong + 2.8 \times 10^3 \text{kJmol}^{-1}$
The efficiency of this endothermic reaction depends on the type of plant, how much chlorophyll is present, and the presence of other nutrients.

Cellular respiration occurs in all cells in the body

Energy is released when bonds in glucose molecules are broken.

Aerobic process (uses oxygen).

Thermochemical equation:

Anaerobic respiration occurs when no oxygen is present
duces less energy than respiration.

Takes place in tissues where there is a high demand for energy such as working muscles.

Produces lactic acid which can cause muscle soreness.

Equation: C6H12O6 (aq) → 2CH3CH(OH)COOH (aq)

Fermentation occurs in plants and yeast cells, producing ethanol and carbon dioxide.

C6H12O6 (aq) → 2C2H5OH (aq) + 2CO2 (g)