Unit 3 - Chemistry

Fuel

Substance that stores chemical energy which can be released, through combustion or nuclear reaction to produce heat, light or power

Exothermic reaction

(draw diagram)
- temp increases
- releases energy in form of heat
- total chemical energy of products is less than reactants
- energy is released when bonds in products form is greater than energy required to break bonds in reactants

Endothermic reactions

(draw diagram)
- temp decreases
- absorbs energy in form of heat
- total chemical energy of products is greater than reactants
- energy released when bonds in products form is less then energy required to break bonds in reactants

Enthalpy

chemical energy of a substance

enthalpy change (△H)

measures exchange of heat energy between system and surrounding

enthalpy change is dependent on:

• mole ratio of reactants and products

• states of reactants and products

• reaction is complete or incomplete combustion

Activation Energy

energy required to break bonds of reactants so reaction can proceed

Complete vs Incomplete Combustion Response (ethanol)

Complete combustion releases more energy as ethanol is fully oxidised to carbon dioxide and water, which are stable products. This means more energy released when stronger bonds are formed in CO2 + H2O.
In incomplete combustion, ethanol is partially oxidised, forming carbon monoxide or carbon. These products still contain chemical energy as partially oxidised, so less energy released overall.

Energy system needs to be:

reliable, affordable, low in carbon emissions (CH4 + CO2)

Renewable Energy/Fuels

Ability of resource to be replaced by natural processes within relatively short period of time

Sustainability

Considers how useful materials for society may be produced with least possible adverse effects on human health and environment

Fossil Fuels

• non-renewable (coal, oil, natural gas)

• not sustainable as we use them at a faster rate than they are produced

• formation occurs overs millions of years

Coal

• peat, brown coal, black coal

• combustion of coal:
  C(s) + O2(g) → CO2(g)

Natural Gas

methane (CH4), H2O, S2, CO2, N2
found trapped above crude oil

Crude Oil

mixture of hydrocarbons (alkanes) separated into range of fractions by heating them to boiling points using fractional distillation

Renewable Fuels

Biofuels, bioethanol, biodiesel
- plant materials used for this are produced by photosynthesis:
6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g)

Bioethanol

produced by fermentation (catalysed by enzymes) at 35C with no O2 gas or air:
C6H12O6(aq) + 2CH3CH2OH(aq) + 2CO2(g)
renewable, not carbon neutral

main feedstocks: sugar cane, wheat, forest waste

Biogas

Anaerobic bacteria break down large molecules (carbs and protein) in organic waste to smaller compounds CO2(g) and CH4(g)

Biodiesel

Mixture of esters, produced by reaction between vegetable oils or animal fats and alcohol (usually methanol CH3OH)

Carbon Neutral Biofuels

In theory, biofuel could be carbon neutral (absorbing same amount of CO2 in formation as released in combustion).
However, energy is required to farm, fertilise and transport biofuels, so they are not likely to be fully carbon neutral.

Renewable vs Sustainable

Renewable Energy Sources	Sustainable Energy Sources
Replaced naturally in a short time	Naturally replenished
Low environmental impact	Technology improves energy efficiency
Widely available	Long-term availability

Environmental Impacts

• CO2 emissions

• greenhouse effect

• carbon neutral

• renewability

Glucose

most important biomolecule, glucose and its polymer, starch, are main source of energy in most diets, produced by photosynthesis

Cellular respiration

• Aerobic respiration
  C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6H2O(l)

Energy from Carbs

Carbs (CH2O)n are used as storage energy molecule, during digestion of starch enzymes in saliva and small intestine break down starch molecules into glucose which undergoes cellular respiration producing energy

Fats and Oils

Triglycerides or esters, contains 3 long hydrocarbon chains attached to glycerol molecule. Digestion breaks down fat and further oxidation releases large quantities of energy, CO2 and H2O.

Proteins

if carbs and fats depleted, proteins are alternative

Distillation of Bioethanol

1. Water (b.p. 100C) condenses and falls to bottom

2. Ethanol (b.p. 79C) rises and collects at top of column

3. trace amounts of water removed from ethanol with filters, dehydrating agents

E10 vs Petrol

E10	Petrol
lower energy density, 46 kJ g⁻¹	higher energy density, 48 kJ g⁻¹
cheaper	more widely distributed
cleaner combustion	less CO₂ produced per km when driving
can be a renewable fuel
absorbs CO₂ during formation to negate the impact of CO₂ emissions during combustion - lower net CO₂ emissions
generally safer production

Factors for choice of Fuel:

1. renewability

2. production factors

3. enivronmental impacts

4. energy value or content

5. viscosity

Heat of Combustion

Enthalpy change that occurs when specified amount of the fuel is burned completely in oxygen (water product is liquid)

Law of Conservation of Mass

In a chemical reaction, the total mass of all products is equal to the total mass of all reactants

number of unknown:

Molar Volume of a gas

One mole of any ideal gas occupies the same volume given the same temperature and pressure

Ideal Gas

gas composed of particles that do not interact expect during elastic collisions

Elastic collisions

Total kinetic energy before the collision equals total kinetic energy after the collision in a system

Specific Heat Capacity

Energy required to raise the temperature of 1 gram of substance by 1 degree Celsius

Calculating heat energy

Experimental determination of combustion

known amount of fuel undergoes combustion, energy released used to heat up specific volume of water, change in temp of water is used to estimate energy released by fuel

Calorimetry

measures the heat released or absorbed by a reaction (energy changes), reaction’s heat goes into water and temp rise tells you how much energy was transferred

Heat loss to surroundings, reduced by:

• putting on a lid

• insulating beaker of water

• placing insulation material around combusting fuel

Simple/Solution Calorimetry

measures energy changes in aqueous solutions

Features of Solution Calorimetry

• contain accurately known volume of water

• thermally insulated and sealed to limit the transfer of heat to or from surroundings

• include stirrers to ensure heat energy is distributed evenly throughout the water

• include thermometers to measure changes in temp of water before and after reaction

  • may include:

  • electrical heaters for electrical calibration

  • glass bulbs to initiate reaction within

During Calorimetry, the degree to which the temp changes depends on:

• amount of substance present

• nature of substance

• nature of immediate surroundings

Calibration Factor

amount of energy required to change temp within calorimeter by 1 degree Celsius

Energy available to body

energy available to body after food is digested may be less that energy content of food because:

• incomplete absorption of nutrients

• incomplete oxidation of nutrients

• heat loss

Oxidant

causes oxidation and is itself reduced

Reductant

causes reduction and is itself oxidised

Oxidation Number Rules

KOHES (acidic conditions)

• Key elements (balance) (any elements that are not oxygen or hydrogen)

• Oxygen (balance with H2O)

• Hydrogen (balance with H+)

• Electrons (use to balance charge) (always add electrons to the more positive side)

• States (add them)

KOHES (alkaline conditions)

• Apply KOHES as usual

• Add OH- to neutralise it (match no. of H+ ions)

• Cancel out extra H2O (should only appear on one side of the equation)