Chapter 9: Designing for Energy Efficiency and Sustainability Flashcards
Over time, batteries cannot be adequately recharged and will need replacing. Why?
- Electrodes can be damaged
- Chemicals are consumed by side reactions
Primary vs secondary cells
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Primary – galvanic cells that cannot be recharged
- ‘Go flat’ when reaction reaches equilibrium
- Products move away from electrodes / are consumed by side reactions, preventing them from being recharged
- E.g. alkaline cells
-
Secondary – galvanic cells that can be recharged
- Aka rechargeable cells or accumulators
- E.g. lithium-ion cells
Limitations of Ni-Cad cells
- ‘Memory problems’ – if the cells are not fully discharged / recharged, voltage and cell capacity decreases
- Long term exposure to Cadmium and its compounds is associated with cancer
Conditions that favour high reaction rates and high equilibrium yields
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High reaction rates
- ↑ concentration / pressure
- ↑ temperature
- ↑ surface area of solids
- Use of a catalyst
-
High equilibrium yield
- Pressure depends on reactant and product particles
- ↓ temperatures for exothermic reactions (–△H)
- ↑ temperatures for endothermic reactions (+△H)
- Addition of excess reactant
- Removal of product as it forms
Effect of temperature on equilibrium yield for exothermic and endothermic reactions
- Exothermic: ↓ temperature = ↑ equilibrium yield
- Endothermic: ↑ temperature = ↑ equilibrium yield
Conflict in exothermic reactions between rate and equilibrium yield
- High temperature – fast reaction rate, low yield
- Low temperature – low reaction rate, high yield
Effect of changing conditions in the Haber process (exothermic)
- Addition of catalyst
- No effect on equilibrium yield
- ↑ reaction rate
- Increasing temperature
- ↓ equilibrium yield
- ↑ reaction rate
- Increasing pressure
- ↑ equilibrium yield
- ↑ reaction rate
Implications of working with high pressure
- Gases are dangerous if there is a leak or if equipment is faulty
- Risk is heightened if the gases are explosive/flammable
- Equipment that contain gases at high pressure are expensive
Discharge vs recharge
- Discharge – a spontaneous reaction occurs in a cell to produce electrical energy from chemical potential energy
- Recharge – an electrolytic process whereby a voltage is applied to the cell to reform the original reactants
Energy transformations in a secondary cell
- During discharge, it acts as a galvanic cell
- Chemical → electrical energy
- During recharge, it acts as an electrolytic cell
- Electrical → chemical energy
NOTE: The voltage required to recharge a cell must be slightly higher than the potential difference of the cell during discharge.
How are secondary cells recharged?
- They are connected to an electric power supply that drives a non-spontaneous reaction
- Positive terminal of charger → cell’s positive electrode
- Negative terminal of charger → cell’s negative electrode
- Power supply must have a potential difference (voltage) larger than that produced by the cell during discharge
- Electrodes must not be damaged and products must remain in contact with the electrodes
What factor increases the likelihood of a battery being rechargeable?
- If discharge products remain in contact with electrodes
- Allows recharging; products can be converted back into reactants through electrolysis
How can catalysts be used to improve the efficiency of exothermic reversible reactions
- Exothermic, reversible reactions have a high equilibrium yield that requires a low temperature
- The catalyst negates some of the impact on reaction rate of the temperature being lowered, therefore improving yield
NOTE: A catalyst does not change the yield of a reaction at a given temperature.
Hydrogen sustainability colour scheme
- Brown hydrogen – derived from fossil fuels
- Grey hydrogen – derived from industrial processes
- Blue hydrogen – derived from fossil fuels with carbon capture (stored underground; carbon not released into the atmosphere)
- Green hydrogen – derived using renewable energy
Advantages and disadvantages of hydrogen gas as a fuel
-
Advantages
- High energy density: 282 kJ mol-1
- Abundant; present in H2O and most carbon compounds
- Sole product of its combustion is water
-
Disadvantages
- Not found as an element; energy needed to produce it
- Very low boiling point; requires lots of energy to liquif
- High pressures required to efficiently store it as a gas
- Explosive; requires careful handling and storage
- Difficult to transport safely as a gas or liquid
