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CHEMISTRY U3/4 > FUELS > Flashcards

FUELS Flashcards

1
Q

fuel

A
  • is a substance with stored chemical energy than can easily be for use (heat/power)
  • high energy content→ release a large amount of energy

energy use: transport, heating, electricity generation

all chemicals contain stored energy but they can’t all be used as fuels

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2
Q

non-renewable fuels

A
  • a fuel is considered non-renewable when it cannot be replenished at the rate at which it is consumed
  • eg. fossil fuels → coal, crude oil (petroleum), natural gas
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3
Q

fossil fuels

A
  • formed from the decomposition of buried dead organisms (plants, animals, microorganisms)
  • compacted underground w/ high pressure + heat
  • retains chemical energy accummulated through photosynthesis

formation occurs over millions of years

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4
Q

fossil fuel: coal

A
  • made from wood and plant material
  • overtime, water content decreases
    • carbon content increases
    • amount of hydrogen and oxygen decreses
  • presence of water decreases its energy content
    • black coal - very small amount of water, high carbon content + high amount of potential energy
    • formed under increased temp and pressure
    • deeper undergorund
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5
Q

combustion of coal

A

C(s) + O2(g)→CO2(g)
energy release: 32kjg^-1

CHEMICAL ENERGY CONVERTED TO ELECTRICITY

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6
Q

energy from coal

A
  • coal is burnt → chemical energy converted to thermal energy
  • heat from burning → boil water
    • thermal energy transferred to steam
  • steam passes through a turbine
    • thermal energy in steam becomes mechanical energy as the turbine spins
    • least efficient step
  • electricity is produced from a generator driven by the turbine
    • mechanical energy is converted to electrical energy
  • energy is lost at each step → mostly as heat (30-40% efficiency)
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7
Q

fossil fuel: crude oil

AKA petroleum

A
  • unrefined liquid extracted from below Earth’s surface
  • a mixture of hydrocarbon molecules (mostly alkanes)
  • needs to be separated via fractional distillation to extract smaller fractions that serve as fuel
    • crude oil itself has no use as a fuel
    • petrol: includes octane + other alkanes
    • petrodiesel: includes alkanes that are longer than in petrol
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8
Q

fossil fuel: natural gas

A
  • found in deposits in the earths crust
  • composed mainly of methane, small amounts of ethane and propane, water sulphur, nitrogen and cvarbon dioxide
  • extracted via drilling
    • once a hole has been drilled into the ground, gas will naturally migrate to the surface for capture
  • found in gas reservoirs trapped between layers of rocks
  • trapped in shale rock (shale gas)
  • coal deposits (coal seam gas)
  • fracking is used to extract natural gas
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9
Q

electricity from natural gas

A

combustion of natural gas formula:
CH4(g) +2O2(g) →CO2(g) +2H2O(g)
* gases are ised to spin turbines
* this generates electrical energy
* 55.6kjg^-1 energy released

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10
Q

biofuel

A
  • derived from plant materials
    • grains, sugar cane and vegetable waste (oils)
  • have less impact on the environment than fossil fuels
  • could be carbon neutral: plant materials used are produced via photosynthesis (removes carbon dioxide from atmosphere (net zero)
  • the release of carbon dioxide from combustion is offset by the carbon dioxide absorbed by plants through photosynthesis
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11
Q

bioethanol

A
  • produced by fermentation of starches and sugars
    • accelerated using enzymes
    • enzymes catalyse the breakdown of these components into sugars (then fermented into ethanol in the absence of oxygen
    • Bioethanol formation (equation) C6H12O6(aq) → 2CH3CH2OH(aq) + 2CO2(g)

  • can be blended with petrol for use in motor vehicles
    • reduces the emission of harmful gases such as oxides of nitrogen
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12
Q

disadvantages of bioethanol

A
  • require land which could be used to grow food
  • require harsh pre-treatment to break down material
  • energy content around 38% less than petrol
  • lower energy content is a result of carbon atoms in ethanol being partly oxdised
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13
Q

production of ethanol using fermentation

A
  • yeasts containing enzymes that catalyse fermentation
    • starches → sugar
    • sugar → ethanol + carbon dioxide
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14
Q

distillation

post treatment of bioethanol

A
  • used to separate water from ethanol to obtain a purer product
  • water falls to the bottom and ethanol is collected from the top
  • distillation uses the different boiling points of liquid to separate the components in the mixture

BIOETHANOL IS PRODUCED AS AN AQUEOUS SOLUTION

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15
Q

combustion of bioethanol

A

C2H5OH(l)+3O2(g)⟶2CO2(g)+3H2O(l)

  • releases 29.6kJg^-1
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16
Q

biogas

A
  • formed from anaerobic breakdown of organic waste
  • anaerobic bacteria involved in the decomposition of organic wastes break complex molecules (carbohydrates and proteins) into smaller molecular compounds like CO2 and methane
  • small scale electricity generators rather largepower plants
  • lower efficiency -> biogas has less methane than natural gas
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17
Q

biodiesel

A
  • a mixture of esters
  • a renewable fuel produced in a reaction between a vegetable oil or an animal fat (trigylcerides) + a small alcohol molecule (methanol) in the presence of a catalyst (KOH)
  • known as transesterification
    • the proess of exchanging organic functional groups of an ester with the organic group of an alcohol
    • used as fuel for diesel engine
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18
Q

renewable fuels

A
  • a fuel is renewable if it can be replenished at the rate at which it is consumed
  • biofuels - biogas, bioehtanol and biodiesel are renewable
    • are all produced form organic matter that can be grown in a short period of time
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19
Q

purpose of food

A
  • supplies energy for our bodies
  • provides materials for growth + repair of tissues
  • provides materials for the control of body processes

humans obtain less energy from food by digestion than the energy released from the direct combustion of food

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20
Q

energy content in food

A
  • carbohydrates (sugars and starch)
  • lipids (fat and oils)
  • proteins
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21
Q

photosynthesis

A
  • endothermic reaction → energy from sun is required for reaction to proceed
  • chlorophyll in the leaf assists with the collection of solar energy
  • glucose moelcules are transported in the sap of a plant to its cells
  • combine to form polymers starch and cellulose

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

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22
Q

cellular respiration

A
  • occurs in the mitochondria in humans
  • substances from food react w oxygen to release energy required by the cell
  • exothermic reaction
  • chemical energyis stored in glucose and is released for use by the body
  • can be converted to:
    • thermal energy for warmth
    • electrical energy in our nerves
    • chemical energy to produce pther moelcules
    • mechanical energy in our muscles

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

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23
Q

energy conservation

A
  • law of conservation of energy states that energy cannot be created or destroyed
  • it can change forms
  • a chemical equation is described as being a system
  • energy released or absorbed by a system is in the form of thermal energy
    • can be converted into other types of energy → light, electricity, kinetic
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24
Q

exothermic reactions

A
  • the total chemical energy of the products is less than the total energy of the reactants
    • energy is released from the system and into the surroundings
  • ΔH will be negative
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25
Q

endothermic reactions

A
  • total chemical energy of the products is greater than the total energy of the reactants
    • energy is absorbed from the surroundings
  • ΔH will be positive
26
Q

enthalpy

A
  • the total energy stored in a substance (also known as the heat content
  • enthalpy change is a measure of the amount of energy absorbed or released during chemical reactions
  • ΔH is the symbol for change in enthalpy and usually in kJ
  • enthlapy change formula
    • ΔH = enthalpy of products - enthalpy of reactants
27
Q

activation energy

A

the minimum amount of energy required to initiate a chemical reaction

28
Q

thermochemical equation

A
  • when the ΔH of the equation is stated → thermochemical equation
  • amount of energy of ΔH corresponds to mole amounts specified by coefficients in the equation
29
Q

states of matter

in enthalpy

A

involve enthalpy changes

30
Q

excess and limiting reactants

A
  1. calculate number of moles of each reactant
  2. divide each by coefficient of the BALANCED CHEMICAL EQUATION
  3. smaller number belongs to the limiting reactant
  4. use the amount of the limiting reactant to work out the amount of profuct formed
31
Q

energy from carbohydrates

A
  • comes from starch which is a polymer made up of repeating glucose units
  • carbohydrates is broken into gluose by enzymes during digestion
  • energy stored in bonds in thew large molecules are released during digestion
32
Q

energy from fats + oils

A
  • are triglycerides
  • are large non-polar molecules with 3 long hydrocarbon chains attached to a glycerol molecule
  • digestion breaks down fats which is further oxidised in body cells into CO2 + H2O
  • PRODUCES MORE ENERGY THAN CARBOHYDRATES
  • carbohydrates are already partially oxidised as it contains a relatively high proportion of oxygen atoms

oxidation in the case of cellular respiration involves reaction with oxygen

33
Q

energy from protein

A
  • rarely used as an energy source in the body
  • used during intensive exercise when glycogen and fat are exhausted
34
Q

forms of energy

A
  • potential energy: stored energy
    • gravitational, chemical, magnetic, elastic, nuclear, electrical
  • kinetic energy: energy associated with movement in doing work
    • heat, sound, light, chemical, mechanical
35
Q

chemical energy

A
  • chemical energy is stored in bonds between atoms and molecules
  • results from
    • attraction between electrons and protons in atoms
    • attraction + repulsion between nuclei in molecules
    • repulsion between electrons
    • movement of electrons
    • vibration + rotations around bonds
36
Q

what happens during a chemical reaction

A
  • occurs when particles collide and rearrange to form new particles
  • bond in reactants must be broken + new bonds in products form
  • all chemical reactions involve energy changes
  • MOLECULES COLLIDE ALL THE TIME BUT CHEMICAL REACTIONS DON’T ALWAYS TAKE PLACE
37
Q

system and surroundings

A
  • system is the chemical reaction
  • surroundings includes everything else
38
Q

energy profile diagram

A

represents the amount of energy used to break bonds in the reactants + form new bonds in the products

39
Q

combustion

A
  • releases chemical energy as heat + light energy
  • require energy for reaction to occur
  • involves reactants combining with oxygen to form oxides
  • exothermic and often called oxidation reactions
40
Q

complete combustion

A
  • occurs when plenty of oxygen is available
    • the only products are CO2 + H2O

WATER IS A GREENHOUSE GAS OMG

41
Q

incomplete combustion

A
  • occur when not enough oxygen is abailable
    • carbon monoxide is produced and/or carbon
    • hydrocarbon burns w/ yellow smoky flames caused by glowing carbon particles

C(s) can also be a product of incomplete combustion

42
Q

heat of combustion

A
  • the heat energy released when a specified amount (eg. 1g, 1mol, 1L) of fuel burn completely in oxygen
  • measured under standard conditions
    • 298K - 25 degrees celsius (water in liquid state)
    • 100kPA
  • because of the definition (energy RELEASE not energy CHANGE) heat of combustion always has positive values
  • if the fuel is a pure substance - kJ/mol
  • if the fuel is not a pure substance (most fuels)
    • has no specific chemical formula or molar mass
    • measured as kJ/g or kJ/L
43
Q

enthalpy of combustion

A
  • negative value with the same numerical value as heat of combustion
44
Q

molar enthalpy

A
  • enthalpy of a substance given per mole
    • same as enthalpy of combustion for a fuel
45
Q

effect of ΔH when chemical reaction is reversed

A

reversing a chemical equation changes the sign of ΔH but doesn’t change the magnitude/amount

46
Q

changes of states

A
  • change of a state is physical rather than a chemical change
    • involve energy being absorbed or released
    • described as endothermic or exothermic processes
47
Q

importance of states

A
  • heat required to convert liquid water to gas (endothermic)
  • states must be written in thermochemical equations as physical changes involve enthalpy change too
  • ΔH is different depending on whether the water produced is gas or liquid
48
Q

gas volume-volume stoichiometry

A
  • mole ratio of gases will also be volume ratios only when:
    • ALL reactants and products are in gas state
    • when temperature and pressure are constant
      • bc all gases occupy equal volumes at the same temp and pressure
49
Q

what affects the amount of energy released from fuels

A
  • amount of fuel burned
  • whether the combustion is complete or incomplete
    • more energy is released when the combustion is complete for the same fuel
  • type of fuel burned (energy content of the specific fuel)
    • equal amounts of different fuels release different quanitities of energy
50
Q

combustion and greenhouse gases

A
  • are gases that can absorb infrared radiation
  • combustion of fuels increases greenhouse gases in atmosphere
  • contributes to enhanced greenhosue effect → global warming
  • CO2 CH4 H2O are the major greenhouse gases
    • CH4 has the greatest effect out of all three
51
Q

specific heat capacity of water

A
  • 4.18J g^-1 oC^-1
  • very high because of the strength of hydrogen bonding
    • more energy required for water molecules to vibrate
52
Q

heat of combustion of pure fuels

A

= q/n

  • q is energy absorbed by water
  • n is number of moles of fuel
53
Q

energy content of food/fuel that isn’t pure

A
  • energy content = energy transferred to water/change in mass of fuel during combustion
    • q/ΔM
    • ΔM = initial mass - final mass
54
Q

reducing heat loss

A
  • put a lid on container holding water
  • insulate beaker of water w/ flameproof material
  • placing insulation around the burning fuel
  • temp of water doesn’t increase as much due to heat loss to surroundings
    • lower temp change ΔT → lower q value
55
Q

calorimeters

A
  • a specifically designed device to measure energy changes during chemical reactions
  • minimises energy loss to the environment
56
Q

solution calorimetry

A
  • used to determine enthalpy changes when acids/metals react with bases and when solids dissolve in water
  • CANNOT BE USED FOR FUEL BURNING IN OXYGEN (COMBUSTION)
  • stirrer matintains uniform temperature for accuracy
  • water will increase in temp for exothermic reactions + decrease in temp for endothermic reactions
57
Q

calibrating a calorimeter

A
  • process of working out CF
  • calibration factor: how much energy is required to change the temperature of water by 1 degrees celsius
58
Q

electrical calibration

A
  • known quantity of thermal energy released from electrical heater can be calculated using
    • E = V x I x T
      • E = energy (J)
      • I = current (amps)
      • V = voltage (volts)
      • t = time (seconds)
  • temp rise measured
  • CF = E/ΔT
59
Q

chemical calibration

A
  • performing a reaction in the calorimeter that releases a knwon quantity of thermal energy
  • energy released or absorbed
    • E = n x ΔH (enthalpy of solution)
    • CF = E/ΔT
  • more accurate estimate of ΔT can be done by extrapolating line of best fit back to the time heating COMMENCED
    • after heat is turned off - temp decreases slowly due to heat losses and delay in the transfer of heat through the water
60
Q

using a calibrated calorimeter to determine enthalpy of reaction

A
  • solution calorimeter is first calibrated by the electrical or chemical method
  • enthalpy of reaction for acid/base reactions, metals w acid and dissolving solid in water can be determine experimentally
  • CF is used to determine energy responsible for temp change that occurs during the reactions NOT DURING CALIBRATION
    • E = CF x ΔT
  • enthalpy change ΔH in kJ is calculated using
    • ΔH = E/n
      • n = number of mole of limiting reactant
61
Q

energy transformation efficiency

A

% energy efficiency = useful energy/energy input x 100

CHEMISTRY U3/4 (7 decks)

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