Reactivity 1.3 Flashcards
Combustion reaction (endo/exo)
Exothermic
Standard enthalpy of combustion
Enthalpy change when one mole of substance in its standard state combusts completely in oxygen under standard conditions
Specific energy of a fuel formula
Energy released from the fuel / mass of fuel consumed
Specific energy of a fuel units
kJ kg-1
Fossil fuel formation
Formed from buried organisms that have undergone reduction under pressure
Main fossil fuels
Coal, crude oil, natural gas
Crude oil
Mixture of hydrocarbons (mainly alkanes)
Natural gas pros/cons
- Clean, easily transportable, higher specific energy density
- Global warming, limited lifespan, uneven distribution (fracking)
Oil pros/cons
- Easily transported, convenient, feedstock
- Global warming/acid rain, limited lifespan, risk of pollution
Coal pros/cons
- Present in large quantities, can be converted to synthetic liquid/gases, energy dense, longer lifespan
- Global warming/acid rain, not easily transported, dangerous mining, dirty
Greenhouse effect
Incoming radiation from the sun is UV/visible, most is absorbed, some reflected as IR. Greenhouses gases absorb some radiated infrared heat and reradiate it to the Earth.
Why do greenhouse gases absorb infrared radiation?
Molecules increase their vibrational state when they absorb infrared radiation. Three of their vibrational modes are IR active and the dipole changes as they vibrate.
Which vibrational mode of carbon dioxide does not produce a change in dipole and is IR inactive?
Symmetrical stretch as the molecule remains symmetrical.
Which vibrational modes of carbon dioxide produce a change in dipole and are IR active?
Asymmetrical stretch and two symmetrical bends
How can you find the CO2 equivalent of a greenhouse gas?
By multiplying its 100 year global warming potential (GWP)
Carbon footprint
Total amount of greenhouse gases produced during human activities
Largest contributing factor of carbon footprint
Supply and distribution of goods
Photosynthesis process
Chlorophyll in plants absorb the energy of sunlight (0.6% of total that reaches earth) and convert water/co2 into glucose (carbohydrates)/ (light to chemical energy)
Photosynthesis endo/exo
It is endothermic and enthalpy change for every mole of glucose formed is +2816 kJmol
How can energy from biomass be obtained?
Burned directly
How can liquid fuel be obtained from biomass?
Fermentation
Gasohol
Mixture of 10% ethanol and 90% gasoline.
Biomass fuel cons/pros
- Renewable, low carbon footprint, safe production
- Uses up land, may pollute water, remove nutrients from soil, small scale production
Hydrogen fuel cell
Energy produced during combustion of hydrogen in oxygen to form water is given out as electricity. It can give out a continuous supply of electricity.
Hydrogen fuel cell unique to rechargable batteries
Do not need an external source of electricity and are non polluting as the only product is water
Overall reaction in a hydrogen fuel cell (alkaline and acidic)
2H2 (g) + 02 (g) -> 2H20 (l)
Anode equation alkaline hydrogen fuel cell
H2 + 2OH- -> 2H2O + 2e-
Cathode equation alkaline hydrogen fuel cell
O2 + 2H2O +4e- -> 4OH-
Types of electrochemical cells
Voltaic -> redox occur spontaneously (chemical - electrical)
Electrolytic cell -> Electrical energy is used to cause reaction (electrical - chemical)
Methanol fuel cell
When fuels other than hydrogen can be used in fuel cells to convert chemical energy directly to electrical energy
Anode equation acidic hydrogen fuel cell
H2 -> 2H+ + 2e-
Cathode equation acidic hydrogen fuel cell
O2 + 4H+ + 4e- -> 2H2O
Anode equation methanol fuel cell
CH3OH + H2O -> CO2 + 6H+ + 6e-
Cathode equation methanol fuel cell
3O2 + 12H+ +12e- -> 6H2O
