2.2 Energy Flashcards
What does Hess’ law state?
If a reaction can take place by more than 1 route and the initial and final conditions are the same, the total enthalpy change is the same for each route
Define lattice enthalpy
The enthalpy change that accompanies the formation of 1 mole of an ionic compound from its gaseous ions under standard conditions
Why is the melting point of an ionic compound so high?
- Ionic bonds are electrostatic forces of attraction between oppositely charged ions
- Ionic bonds are strong
Describe some features of lattice enthalpy
- It is an exothermic change
- It indicates the strength of the ionic lattice and is a measure of the ionic bond strength
- Cannot be measured directly because it is impossible to form 1 mole of an ionic lattice from gaseous ions
What are the key enthalpy changes used for working out lattice enthalpy?
- The standard enthalpy change of formation
- The standard enthalpy change of atomisation
- The first ionisation energy
- The second ionisation energy (for 2+ ions)
- The first electron affinity
- The second electron affinity (for 2- ions)
Define ‘the standard enthalpy change of atomisation’
The enthalpy change that takes place when 1 mole of gaseous atoms forms from the element in its standard state
Define the first ionisation energy
The enthalpy change accompanying the removal of 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions
Define the first electron affinity
The enthalpy change accompanying the addition of 1 electron to each atom in 1 mole of gaseous ions to form 1 mole of gaseous 1- ions
What is the datum line on a Born-Haber cycle?
- Zero energy
- The elements in their standard states
In the Born-Haber cycle, how can you tell which values are exothermic and which are endothermic?
- ΔH values pointing upwards are endothermic
- ΔH values pointing downwards are exothermic
What is found in the lowest energy level of the Born-Haber cycle?
The ionic solid
Define the standard enthalpy change of formation
The enthalpy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions
Define the standard enthalpy change of solution
The enthalpy change that takes place when 1 mole of a compound is completely dissolved in water under standard conditions
What are the 2 processes that take place when an ionic solid dissolves in water?
- Breakdown of the ionic lattice into gaseous ions
- Hydration of the ions
Describe the breakdown of the ionic lattice
The process is exact opposite of that producing the lattice enthalpy
- The crystal lattice is broken down
- The ions are separated
- Overcoming the attractive forces between the oppositely charged ions requires energy
- The energy involved has the same magnitude as lattice enthalpy but the sign is opposite
Describe hydration
Gaseous ions are hydrated
- The gaseous ions bond with the water molecules
- The positively charged ions will be attracted to the slightly negative oxygen atoms in the water
- The negatively charged ions will be attracted to the slightly positive hydrogen atoms in water
Define the standard enthalpy change of hydration
The enthalpy change that takes place when 1 mole of isolated gaseous ions is dissolved in water forming 1 mole of aqueous ions under standard conditions
What are the key enthalpy changes for working out the lattice enthalpy of an ionic solid?
- The enthalpy changes of hydration of the constituent gaseous ions
- The enthalpy change of solution of the ionic solid
What 2 factors affect the size of lattice enthalpy?
Ionic size
Ionic charge
How does ionic size affect lattice enthalpy?
As the ionic radius increases:
- The attraction between the ions decreases
- The lattice enthalpy becomes less negative (less exothermic)
How does ionic charge affect lattice enthalpy?
- The compounds with the most negative lattice enthalpies are those which have small, highly charged ions
- When 2 highly charged ions of opposite charge are present in the lattice they attract each other strongly
- A very exothermic reaction will take place when a lattice is formed from 2 highly charged ions
What factors affect the size of hydration enthalpy?
Ionic size
Ionic charge
How does ionic size affect the size of hydration enthalpy?
As the ionic radius decreases
- The value of the enthalpy change of hydration becomes more negative (more exothermic)
- Hydration depends on the ability of an ion to attract and bond with water molecules. Small ions exert more attraction on water molecules and more energy is released
How does ionic charge affect the size of hydration enthalpy?
As the charge on the ion increases
- There is a greater attraction for water molecules
- So the hydration enthalpy is more negative
What is entropy?
The quantitative measure of the degree of disorder in a system
Why is entropy, S, always a positive number?
Because all substances possess some degree of disorder because particles are always in constant motion
How does entropy increase?
It increases during the changes in state that give more randomness (e.g. Liquid to gas)
How do you calculate entropy changes?
Sum of products - sum of reactants
If a change makes a system more random, is the entropy change positive or negative?
Positive
What is the free energy change, ΔG?
The balance between enthalpy, entropy and temperature for a process:
ΔG = ΔH - TΔS
(T is measured in Kelvin)
What is a spontaneous process?
A process that can proceed on its own, once it has begun
When is a process spontaneous?
If a chemical system becomes more stable and its overall energy decreases
For a reaction to be feasible, what ΔG value must it have?
One that is less than 0
What is an oxidising agent?
The substance that is reduced (takes electrons from the substance that is oxidised)
What is a reducing agent?
A substance that is oxidised (gives electrons to the substance that is reduced)
What does a half cell comprise of?
An element in 2 oxidation states
- A metal placed in an aqueous solution of its ions
What is the electrode potential of a half cell?
Its tendency to gain or lose electrons in equilibrium
How is a simple electrochemical cell made?
2 half cells with different electrode potentials are connected together by a wire and a salt bridge
- 1 half cell releases electrons
- the other half cell gains electrons
- The difference in electrode potential is measured with a voltmeter
- The wire connects the 2 metals, allowing electrons to be transferred between the 2 half cells
- The salt bridge connects the 2 solutions, allowing ions to be transferred between the half cells. Can be made out of a strip of filter paper soaked in an aqueous solution of an ionic compound that does not react with either of the half-cell solutions (e.g. KNO3)
Which is the most reactive (so the greatest oxidising agent): the metal with the highest or lowest electrode potential?
Lowest (most negative)
Is the more negative, or more positive, metal at the negative terminal?
More negative
What is the problem with non-metal/non-metal ion half cells? (E.g. Hydrogen half cells)
There is no electrode to connect the wire to
- A platinum electrode is placed in the solution so that it is in contact with both H2 and H^+ ions
- The platinum is inert and does not react at all
- Its surface is coated with platinum black, a spongy coating in which electrons can be transferred between the non-metal and it’s ions
Define ‘standard electrode potential of a half cell’
The electromotive force of a half cell compared with a standard hydrogen half cell, measured at 298K with solution concs of 1 mol dm^-3 and a gas pressure of 1 atm
What are the standard conditions?
298K (25°C)
1 atm pressure (gas)
1 mol dm^-3
Which half cell is used as the reference for the measurement of voltages in electrochemical cells?
A hydrogen half cell (given an electrode potential of 0)
What is the equation for working out the standard cell potential?
Standard cell potential = Electrode potential of positive terminal - electrode potential of negative terminal
What is the standard cell potential?
The electromotive force between the 2 half cells making up the cell under standard conditions
- The difference between the standard electrode potentials of each half cell
What is an electromotive force?
The voltage produced by a cell when no current flows
Define ‘cell reaction’
The overall chemical reaction taking place in the cell
- The sum of the oxidation and reduction half equations taking place in each half cell
What are the 3 main types of electrochemical cells?
- Non-rechargeable cells
- Rechargeable cells
- Fuel cells
Explain how a non-rechargeable cell works
- Provide electrical energy until the chemicals have reacted to such an extent that the voltage falls
- The cell is then ‘flat’ and is discarded
Explain how a rechargeable cell works
- The chemicals in the cell react, providing electrical energy
- The cell reaction can be reversed during recharging
- The chemicals in the cell are regenerated and the cell can be used again
Explain how a fuel cell works
- The cell reaction uses external supplies and an oxidant, which are consumed and needs to be provided continuously
- The cell will continue to provide electrical energy so long as there is a supply of fuel and oxidant
What are the advantages of a methanol fuel cell compared with use of hydrogen gas?
- A liquid fuel is easier to store than hydrogen gas
- Methanol can be generated from biomass
What are the advantages of fuel cell vehicles?
Less pollution and less CO2
- Hydrogen-rich fuels produce only small amounts of CO2 and air pollutants
Greater efficiency
- A petrol engine is less than 20% efficient in converting chemical energy by the combustion of petrol, so much of the chemical energy is wasted as heat
- Hydrogen fuel cell vehicles are 40-60% efficient in converting the fuel’s energy. This means that fuel consumption drops by more than half compares with a petrol- or diesel-powered car
What different strategies are there for storing hydrogen?
- Hydrogen can be stored as a liquid under pressure (but this still requires a very low temp)
- Hydrogen can be adsorbed onto the surface of a solid material in a similar way that a catalyst is able to hold gases on its surface
- Hydrogen can be absorbed within some solid materials
What is the main problem in storing hydrogen?
It is a gas with a very low boiling point
What are the limitations of hydrogen fuel cells?
- The large-scale storage and transportation of hydrogen poses problems. A cost-effective and energy-efficient infrastructure needs to be in place to deliver large quantities of hydrogen fuel over long distance
- The feasibility of storing a pressurised liquid
- Current ‘adsorbers’ and ‘absorbers’ of hydrogen have a limited lifetime
- Current fuel cells have a limited lifetime, requiring regular replacement and disposal following high production costs
- Fuel cells use toxic chemicals in their production
What are the limitations of the ‘hydrogen economy’?
- The use of H as a fuel has to be accepted politically and by the general public
- There are logistical problems in the handling and maintenance of H systems
- H is an ‘energy carrier’ and not an ‘energy source’. It must first be manufactured, which could end up using more energy than is saved.
