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
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
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
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
5
Q
combustion of coal
A
C(s) + O2(g)→CO2(g)
energy release: 32kjg^-1
CHEMICAL ENERGY CONVERTED TO ELECTRICITY
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)
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
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
9
Q
electricity from natural gas
A
combustion of natural gas formula:
CH4(g) +2O2(g) →CO2(g) +2H2O(g)
* gases are used to spin turbines
* this generates electrical energy
* 55.6kjg^-1 energy released
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
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
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
13
Q
production of ethanol using fermentation
A
-
yeasts containing enzymes that catalyse fermentation
- starches → sugar
- sugar → ethanol + carbon dioxide
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
- the presence of water in bioethanol will reduce its energy content/prevent it from combusting
BIOETHANOL IS PRODUCED AS AN AQUEOUS SOLUTION
15
Q
combustion of bioethanol
A
C2H5OH(l)+3O2(g)⟶2CO2(g)+3H2O(l)
- releases 29.6kJg^-1
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
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
18
Q
renewable fuels
A
- a fuel is renewable if it can be replaced by natural processes within a relatively short period of time)
- biofuels - biogas, bioehtanol and biodiesel are renewable
- are all produced form organic matter that can be grown in a short period of time
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
20
Q
energy content in food
A
- carbohydrates (sugars and starch)
- lipids (fat and oils)
- proteins
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)
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)
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
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
- More energy is required to break the bonds in the products than in the reactants.
- The products are more chemically stable compared to the reactants.
goal of atoms is to be stable
IF THE BONDS IN A MOLECULE ARE WEAK, THEN THE CHEMICAL POTENTIAL ENERGY IS HIGH AND STABILITY IS LOW
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
- Less energy is required to break the bonds in the products compared to the reactants.
- The products are less stable as they have more chemical energy than the reactants.
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
- calculate number of moles of each reactant
- divide each by coefficient of the BALANCED CHEMICAL EQUATION
- smaller number belongs to the limiting reactant
- 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 greenhouse 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 in calorimetry
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 maintains 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)
- E = V x I x T
- 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
extrapolating may not always be done - try it with if not a mcq ans try without extrapolating
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
- ΔH = E/n
Make sure that you keep ∆T (calibration) distinct from ∆T(reaction) in your mind and calculations
61
Q
energy transformation efficiency
A
% energy efficiency = useful energy/energy input x 100
62
Q
determining enthalpy change from bond enthalpies
A
- sum of enthalpies of bonds broken - sum of enthalpies of bonds formed
- have to take into account the mole ratio
- be careful is double or triple or single bond
63
Q
carbon neutral
A
- carbon released as greenhouse gas is approx the same amount absorbed during the process of fuel creation (photosynthesis)
- process of making biofuel isn’t necessarily carbon neutral
- transport releases co2
64
Q
definition of sustainable fuels
A
- able to meet the neeeds of the current and future generations
