Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Chemistry U3/4 > U3 AOS 1: Energy > Flashcards

U3 AOS 1: Energy Flashcards

Fuels, galvanic cells, fuel cells

You may prefer our related Brainscape-certified flashcards:
AP® Chemistry flashcards Periodic Table flashcards Chemistry 101 flashcards Organic Chemistry 101 flashcards
1
Q

Fuel

A
  • Substance that can release stored energy relatively easily
  • Unit for energy is joule (J)

NOTE: All chemicals contain stored energy but not all can be used as a fuel.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Fossil fuels

E.g. coal, natural gas and petrol

A
  • Naturally formed from the decomposition of buried dead organisms (plants, animals, microorganisms)
  • In the absence of oxygen, heat and pressure alters the chemical structure of the organic matter
  • Retains chemical energy accumulated via photosynthesis
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Coal

A
  • Made from wood and plant material that progressively becomes peat, brown coal then black coal
  • Formed under intense pressure & high temperatures
  • Becomes richer in energy over time
    • Carbon content increases
    • Amount of hydrogen and oxygen decreases
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Presence of water in coal

A
  • ↑ water = ↓ potential energy
  • When coal is burnt, energy released causes water to vaporise, reducing the net amount of heat released
  • Black coal has very small amounts of water and, thus, a high amount of potential energy
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Natural gas

A
  • Found in deposits in the earth’s crust
    • In gas resorvoirs trapped between layers of rocks
    • As a component of petroleum deposits
    • Bonded to the surface of coal deposits (CSG)
    • Trapped in shale rock (shale gas)
  • Mainly composed of methane (CH4), together w/ ethane, propane, water, sulphur, nitrogen and carbon dioxide
  • Exctracted via fracking/drilling (as is with crude oil)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Petrol (crude oil)

A
  • Mixture of hydrocarbon molecules (mostly alkanes)
  • Fractional distillation used to extract its components into more useful, pure substances that serve as fuels
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Electricity from natural gas

A
  • Combustion of natural gas
    • CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
  • Gases produced causes air to expand in a combustion turbine which generates electrical energy
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Biofuel

E.g. bioethanol, biogas and biodiesel

A
  • Derived from renewable plant materials
    • E.g. grains, sugar cane, veg waste & veg oils
  • Can be carbon neutral as plants photosynthesise (removes CO₂ from atmosphere, offseting release of CO₂ from combustion)
  • NOT produced by natural processes
  • Energy is still required for farming, fertilising and transport
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Why are biofuels are more environmentally friendly than fossil fuels?

A
  • Removes CO₂ from environment as plant materials used to create biofuels are produced by photosynthesis
  • Net impact is less as the absorption of CO₂ offsets the release of CO₂ during combustion
  • Recycles waste from industries such as farming, whereas fossil fuels are derived from raw materials
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Why are biofuels not widely used?

A
  • They currently provide less energy than fossil fuels do
  • Currently, wind and solar energy is steadily replacing fossil fuels
  • To sustain biofuel production, crops specific for this purpose would be required (poses issues e.g. land degradation, clearing of forests, competition for food supplies)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Bioethanol

A
  • Produced by fermentation of starches and sugars
  • Process is acelerated using enzymes
  • Enzymes catalyse the breakdown of these components into sugars which are then fermented into ethanol in the absence of O₂

NOTE: While better than fossil fuels, bioethanol is not as ideal as other biofuels because CO₂ is produced in both its production and combustion.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Fermentation equation

Formation of bioethanol

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Distillation

A
  • Using diff boiling points to separate a mixture’s components
  • Solution is heated in a column (temp ↓ as height ↑)
  • Water falls to the bottom and ethanol is collected at the top
  • A purer substance is obtained (water in bioethanol reduces energy content and prevents its combustion)

NOTE: Distillation requires a significant amount of energy. This means that bioethanol production is not a carbon-neutral process.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Advantages and disadvantages of bioethanol

A
  • Advantages
    • Renewable
    • Can be made from waste
    • CO₂ absorbed during photosynthesis
    • Burns smoothly
  • Disadvantages
    • Requires farmland that can be used to grow food
    • Intensive crop farming can degrede / erode land
    • Lower energy content than petrol
    • CO₂ produced in production and combustion
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Biogas

A
  • Formed from the anaerobic breakdown of organic waste
  • Anaerobic bacteria decompose complex molecules (e.g. carbs and proteins) into smaller compounds (e.g. CO₂ and CH₄)
  • Can be obtained from rotting rubbish, decomposing plant material, sewage works, chicken farms and piggeries
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Advantages and disadvantages of biogas

A
  • Advantages
    • Renewable
    • Can be made from waste (reduces waste disposal)
    • CO₂ absorbed during photosynthesis
    • Low running costs
  • Disadvantages
    • Limited supply of raw waste materials
    • Low energy content due to low amount of methane
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Biodiesel

A
  • A mixture of organic compounds called esters
  • Formed via transesterification where veg oils / animal fats react w/ an alcohol (methanol) in the presence of a catalyst (KOH)
  • Can be made from canola / palm oil and restaurant grease

NOTE: Biodiesel molecules are polar and larger than petrodiesel/petrol molecules. Thus, they have the stronger intermolecular forces and higher viscosity.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Renewability vs sustainability

A
  • Renewability – replaced by natural processes within a relatively short period of time
    • Biofuels are renewable as they are made from organic matter that can be grown in a short period of time
  • Sustainability – minimal impact on the environment

TIP: Re(newability) = re(placed).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Photosynthesis

A
  • Endothermic reaction (requires energy from the sun)
  • Chlorophyll in leaves assists w/ the collection of solar energy
  • Solar energy is converted into chemical energy (glucose)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Photosynthesis equation

A

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

TIP: Reversed cellular respiration reaction.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Cellular respiration

A
  • Exothermic reaction (releases energy)
  • Stored chemical energy in glucose is released for the body’s use
    • Thermal energy for warmth
    • Electrical energy in our nerves
    • Chemical energy to produce other moelcules
    • Mechanical energy in our muscles

NOTE: Glucose is oxidised as the primary carbohydrate energy source.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Cellular respiration equation

Same equation if glucose were to undergo complete combustion

A

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

TIP: Reversed photosynthesis reaction.

23
Q

Exothermic vs endothermic

A
  • Exothermic – system releases energy to environment
    • Energy of reactants > energy of products
    • Products are thus more stable (less energy)
    • Negative △H and non-spontaneous
    • E.g. respiration and combustion
  • Endothermic – system absorbs energy from environment
    • Energy of reactants < energy of products
    • Products are thus less stable (more energy)
    • Positive △H
    • E.g. photosynthesis
24
Q

Enthalpy (H)

A
  • The chemical energy / heat content of a substance
  • Enthalpy change (△H) is a measure of the amount of energy absorbed or released during chemical reactions
    • △H = Hp - Hr

NOTE: △H is usually measured in kJ/mol.

25
Q

Comple vs incomplete combustion

A
  • Complete – occurs when oxygen is plentiful
    • The only products are CO₂ and H₂O
  • Incomplete – occurs when oxygen is limited
    • Not all the carbon can be converted into CO₂
    • CO and / or C are produced instead
26
Q

Thermochemical equations

A
  • A balanced chemical equation including △H
  • Doubled coefficients = doubled △H value
  • Halved coefficients = halved △H value
  • Reversed equation = △H sign changes (value stays the same)
27
Q

Energy from carbohydrates, proteins and lipids (fats and oils)

A
  • Carbohydrates
    • Broken down into glucose by enzymes during digestion
    • Glucose transported to cells where respiration can occur
  • Proteins
    • Rarely used by body for energy unless intensive exercise depletes glycogen and fat stores
  • Fats and oils
    • Made up of triglycerides (consist of glycerol and FAs)
    • Broken down into components that are oxidised, releasing CO₂, H₂O and energy

NOTE: Humans cannot digest most fibre (cellulose), so the energy it contains is not available. Thus, energy obtained by humans after digestion is less than the energy released when food is burned.

28
Q

Why do fats and oils have a higher energy value than carbohydrates and proteins?

A
  • Fats and oils have greater oxidation potential (release more energy during combustion due to higher carbon content)
  • Carbon molecules have a higher degree of oxidation
  • Carbohydrates are already partially oxidised as they contain a higher proportion of oxygen
29
Q

Solution calorimetry

A
  • Estimates the △H of a reaction that occurs in solution
  • Stirrer maintains uniform temperature for accuracy
  • H₂O will ↑ in temp for exo reactions and ↓ in temp for endo

NOTE: Cannot be used to measure the △H of combustion reactions.

30
Q

Reducing heat loss in calorimetry

A
  • Put a lid on the container holding the water
  • Insulate the beaker of water (with flameproof material)
  • Insulate the burning fuel whilst ensuring that sufficient oxygen is available to the fuel for complete combustion to occur

NOTE: Heat loss is a systematic error. It makes it difficult to accurately calculate heat of combustion from experimental data.

31
Q

Calibration factor when a calorimeter is not insulated

A
  • Results in a higher calibration factor
  • Lack of insulation will cause a smaller △T
  • More energy required to raise the temp due to heat loss
32
Q

Reasons for calculated energy content being lower than the stated value in the food’s nutrition information

A
  • Other molecules (apart from fats, proteins and carbohydrates) may produce energy
  • Data book figures are rounded to whole numbers, resulting in higher / lower calculations
33
Q

Given that a calorimeter is acurately calibrated, what factors may contribute to a difference in the theoretical and experimental (calculated) energy content?

Popcorn example

A
  • Nutrition labels are based on average quantities (popcorn is not homogenous and the sample in the calorimeter may have been less energy dense)
  • The popcorn may contain water that reduces the amount of combustible material
  • Only one sample of popcorn was analysed (average of multiple samples may have been closer to the theoretical energy content)
34
Q

Redox reactions

A
  • The transfer of electrons from one species to another
  • Oxidation and reduction occur simultaneously
  • Oxidation – loss of electrons
  • Reduction – gain of electrons (e⁻ on reactants side)

TIP: ReRe (reactants, reduction).

35
Q

Oxidising agent (oxidant) vs reducing agent (reductant)

A
  • Oxidants cause oxidation
    • Substance itself is reduced and accepts electrons
    • Strong oxidants accept electrons more readily
  • Reductants cause reduction
    • Substance itself is oxidised and loses electrons
    • Strong reductants donate electrons more readily
36
Q

Conjugate redox pairs

A
  • Always two pairs in a redox reaction
  • Electron donor and electron acceptor
37
Q

Spontaneous vs non-spontaneous reactions

A
  • Spontaneous
    • Occur naturally
    • Do not need to be driven by an external source of energy
    • Occur in primary/secondary galvanic and fuel cells
  • Non-spontaneous
    • Require an external source of energy
38
Q

Galvanic cells

Aka voltaic cells

A
  • Type of electrochemical cell where chemical energy is converted into electrical energy
  • Involve spontaneous, exothermic reactions
  • AN OIL RIG CAT
    • Anode (-) is where oxidation occcurs
    • Cathode (+) is where reduction occurs

NOTE: Electrochemical cells are devices that convert chemical energy into electrical energy (or vice versa).

39
Q

Battery

A
  • Combination of several cells in series to obtain a higher potential difference or voltage
40
Q

Salt bridge

A
  • Enables ions to move between the two half-cells, balancing the charge and completing the electrical circuit
  • Anode loses electrons – lost neg. charge is replenished as anions (e.g. NO₃-) from salt bridge move toward the anode
  • Cathode gains electrons – increase in neg. charge is offset as cations (e.g K⁺) move toward the cathode

NOTE: They are often made from filter paper soaked in a salt solution (e.g. KNO₃) that must be soluble in water, unreactive and not form a precipitate (ions must be spectator ions).

41
Q

What occurs when there is no salt bridge?

A
  • Reaction would not proceed due to buildup of charge
  • One half cell would accumulate a negative charge and the other would accumulate a positive charge
42
Q

External vs internal circuit

A
  • External circuit (wires) allows electrons to flow
  • Internal circuit (salt bridge) allows ions to flow

TIP: EE and II.

43
Q

Electrode materials

A
  • Each half-cell has an electrode in contact with an electrolyte
  • If the half-cell has a solid metal
    • The given metal is used
    • E.g. Ag⁺(aq)/Ag(s): silver electrode
  • If the half-cell only has liquid/gaseous reactants (no metal)
    • An inert electrode (e.g. platinum/graphite) is used
    • E.g. Fe³⁺(aq)/Fe²⁺(aq): platinum/graphite electrode

NOTE: The inert electrode must be unreactive (does not react with any species in its half-cell), solid and electrically conductive.

44
Q

Using the electrochemical series to predict galvanic half-cell reactions

A
  • Strongest oxidant (F2(g)) is at the top left
  • Strongest reductant (Li(s)) is at the bottom right
  • Negative gradient
45
Q

Why might an expected redox reaction not occur?

A
  • Slow reaction rate
  • Not under standard conditions
  • Greasy electrodes (less electrically conductive)
46
Q

Uses and limitations of the electrochemical series in predicting galvanic half-cell reactions

A
  • Use
    • Tell us which chemical reactions can feasibly occur
  • Limitations
    • E⁰ values are only under standard conditions (values and order on the series can vary under diff conditions)
    • No information about the rate of reaction

NOTE: Standard conditions = 25 °C, 100kPa (pressure) and 1M solutions (concentration).

47
Q

Standard electrode potential (E⁰)

Aka electromotive force (emf) or voltage

A
  • Voltage measured when a half-cell is connected to a standard hydrogen half-cell at standard conditions

Cell potential difference
= higher half-cell E⁰ – lower half-cell E⁰

48
Q

Fuel cells

Type of galvanic cell

A
  • Generate electricity from redox reactions
  • Chemical energy is directly converted into electrical energy
  • Reactants are not stored and must be continuously supplied
  • Always involve combustion (oxidant is always oxygen)
  • Either acidic or alkaline
49
Q

Fuel cell design

A
  • O₂ is always reduced at the cathode (+)
  • Gaseous fuel is always oxidised at the anode (–)
  • Electrodes
    • Must be conducting and porous
    • Allows H₂ and O₂ to contact ions in the electrolyte
    • Size determines the size of the current drawn from cell
  • Electrolyte (e.g. KOH)
    • Completes the circuit
    • Allows ions from anode & cathode to come into contact
    • Does not allow transfer of electrons
    • Can be acidic (H+) or alkaline (OH-)
  • Catalysts
    • Often coat electrodes
    • Incease reaction rate and current produced by the cell
50
Q

Fuel cells with aqueous vs non-aqueous electrolytes

A
  • Aqueous electrolytes
    • Generally operate at ~80 °C
    • Can evaporate at higher temperatures or freeze at lower temperatures (stops the cell from operating)
  • Non-aqueous electrolytes e.g. polymer membranes
    • Operate at over 100 °C to maximise reaction rate
    • Prevents leakage
    • Expensive to produce (from non-renewable crude oil)
51
Q

Advantages and disadvantages of fuel cells

A
  • Advantages
    • 40-60% efficient due to direct energy conversion
    • Reduced greenhouse gas emissions
    • Can use a variety of fuels
    • Generate electricity as long as fuel is supplied (whereas conventional batteries need to be recharged/replaced)
  • Disadvantages
    • Require a constant fuel supply
    • Expensive to manufacture (requires porous electrodes, catalysts, high temperatures, etc.)
    • Storage and safety concerns (explosive fuels)
52
Q

How are fuel cells designed to maximise
energy efficiency?

A
  • Catalysts speed up reactions, decreasing the amount of time
    for energy to escape the system and be lost to the atmosphere
  • Porous electrodes allow the efficient diffusion of gaseous reactants (more reactants come into contact with catalysts)
53
Q

How can fuel cells use renewable feedstocks to produce energy?

A
  • Can use CO₂ as the oxidant and H₂ as the fuel
  • CO₂ is renewable as it is produced by the combustion of biomass and by humans as they exhale

Chemistry U3/4 (6 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions