SAC 1 - Content Crucial for revision Flashcards
Define Fuel
- releases heat energy in combustion reaction and stores chemical energy
What are fossil fuels?
- derived from fossils remains of living organisms
- found in earth’s crust formed through decompositions under anaerobic intense heat and pressure that alter chemical structure over millions of years
- non-renewable
What are the type of fossil fuels?
Coal, Natural Gas and Petrol
Define Coal
- most abundant fossil fuel mined in ground and formed by gradual chemical changes to decomposing wood and plant material where carbon content increases
Black coal vs brown coal
Higher energy content because higher carbon content
Define Natural Gas
Methane (90%) ethane, propane, CO2 and nitrogen
Define Coal Seam Gas
- found in earth’s crust deposits
- extracted by drilling and fracking (inject liquid at high pressure into coal to force open fissures)
Crude Oil
- mixture of hydrocarbons
- separated into different hydrocarbons through fractional distillation
- often contaminated with sulfur
Environmental constraints of fossil fuels
- drilling/mining disrupts natural landscape and natural water table if spills occur
- produce CO2 contributing to enhanced greenhouse effect
- contamination with sulfur compounds can cause acid rain through SO2 emissions
What are some organic sources of glucose?
Sugarcane, soya beans and corn
Environmental and sustainability considerations of biofuels
- land and water supplies are limited in some countries and biofuel production should not reduce amount of food available
- to reduce greenhouse gas emissions, immense land for growing required
- non-renewable fuels used in transport
Glucose
Simple carbohydrate and primary source of energy
Photosynthesis
plants, algae, bacteria convert light energy from sun into chemical energy in form of glucose.
Activation energy
Minimum energy required to break bonds in reactants (larger activation energy indicates stronger bonds)
Exothermic reactions
releases energy to the environment, energy required to break bonds is less than energy released when new bonds form
Endothermic reactions
- absorbs energy
- energy required to break bonds is greater than the energy released when new bonds form
Reactants
Consumed during a reaction
Reagents
Not neccersarily consumed e.g. a catalyst
Combustion of methane
- 10 times more potent than CO2
- combustion of methane to produce carbon dioxide in some instances better
NET amount formula
Amount produced - amount consumed
Specific heat capacity
amount of heat energy required to raise the temperature of 1g of a substance by 1 degree.
Limitations of water method
- low accuracy due to large amount of heat loss to surroundings
- mass of water being heated will change due to evaporation
- not all fuels are available in form to be safely combusted
Primary Cells
- cannot be recharged
- fixed quantity of stored reactants
- operate in closed environments
Consideration of Galvanic Cells
- environment species that may interfere with the cell function
- products/reactants/malfunctions dangerous
- environmental impacts e.g. heavy metal
- ability to source material
Purpose of an inert electrode
- used to conduct current
- non-reactive
- solid
- conduct electricity
Features of the electrochemical series
- only valid under SLC
- 25 degrees
- 100kPa
- 1M concentration
Limitations of the Electrochemical series
- doesn’t indicate reaction rate
- only valid under SLC
What is a Galvanic Cell?
cell that converts chemical energy to electrical energy
- must have two half cells so that electrons are forced to move through external circuit and produce electrical energy rather than heat energy
Observation at anode
- decrease mass, increase colour
Salt bridge
- completes circuit
- provides free moving ions to compensate for those lost/gained
- must be soluble, ionic, not interfere, not form a precipitate
Fuel Cells
Cell that continuously converts chemical energy into electrical energy via redox reaction where reactants continuously supplied.
Porous Electrode
Material with many holes used in a fuel cell to maximise the ability of gaseous reactants to come into contact with the electrolyte
Fuel Cell efficiency
- more efficient than power stations at converting chemical energy to electrical energy (40-60% compared to 30-36%)
- reduces greenhouse emissions, remove reliance on fossil fuels
Similarities between fuel and galvanic cells
- convert chemical to electrical energy
- cells separated
- contains electrolyte
- produce voltage
- connected to a load (something that consumes electrical energy)
- can be stacked
Differences between fuel and galvanic cells
- reactants continuously supplied from an external source
- open systems often involving gaseous reactants
- membranes often used as electrolytes, polymer layer that conducts H+ ions known as proton electrolyte membrane
- electrodes are porous to improve contact between gas and electrolyte. (size allows certain molecules, increase surface area, catalysts, increase rate of reaction)
Acidic conditions
- greater concentration of H+ ions
Advantages of fuel cells
- lower emissions of harmful sulfur-nitrogen containing compounds compared to direct combustion
- low maintenance and running costs
- quiet
- no direct CO2 emissions from hydrogen fuel cells.
- greater efficiency as less waste heat/energy conversions
Disadvantages of fuel cells
- storage and safety issues associated with highly explosive hydrogen fuel
- expensive to manufacture
- require new infrastructure for hydrogen fuel
- not as convenient as batteries.
Faraday’s First Law
The amount of electrical charge carried by a galvanic cell is directly proportional to the mass of anode lost/fuel used or mass gained by the cathode.
Feedstock
Raw material used for producing another product
- renewable feedstocks can increase sustainability
Sustainable
Produced at a rate greater or equal to the consumption rate without compromising future generations
Factors contributing to the energy efficiency of fuel cells
- catalysts
- electrode porosity and nanomaterials
- combined heat and power
- hybrid systems
- polymer membrane electrolytes
- operating conditions
- durability
Catalysts
- increase rate of reaction
- reduce amount of time for energy to escape system increasing efficient
- effective catalysts are nonrenewable and expensive
Electrode porosity and nanomaterials
- porous allows diffusion of gaseous reactants
- smaller + more numerous pores provide greater surface area
- increases contact between reactants and catalysts
Combined heat and power
Heat, a by product of fuel cells is captured and used to heat other things.
Hybrid systems
- combining fuel with another energy system
- allows excess energy to be stored minimising energy loss
Polymer membrane electrolytes
Maximize speed of ion movement
more resistant to temperature changes
- expensive and derived from crude oil
Operating conditions
- higher temperatures and pressures increase rate of reaction
- maintaining high temperatures requires large amounts of energy
Durability
Increasing lifespan of fuel cell will increase long term efficiency
First generation feedstock source
- edible crops e.g wheat/sugarcane
- intensive farming required to meet demand, potentially damaging soil
- competes with valuable food sources
Second generation feedstock source
non-food crops e.g grass
- waste readily available
- difficult to convert biomass to usable fuels.
Function of Seperator
- separates reactants
- allows for the flow of ions
Disadvantage of hydrogen as a fuel
- lack of refueling stations
- storage of gas under high pressure.
When would a galvanic cell stop?
- one of ion in salt bridge depleted
- electrodes no longer immersed in solution
- wires are disconnected
Top of energy diagram is
Transition energy
