Module 5: Section 2 Flashcards
Define standard lattice enthalpy
Enthalpy change when 1 mol of an ionic lattice is formed from its gaseous ions, under standard conditions.
What does standard lattice enthalpy measure?
What does a negative standard lattice enthalpy show?
It is a measure of ionic bond length.
The more negative the standard lattice enthalpy, the stronger the bonding.
Give 2 factors affeceting lattice enthalpy.
- Ionic charge
2. Size of the ions.
Describe how charge on ions affects lattice enthalpy.
The higher the charge on the ions, the more energy is released when an ionic lattice forms. This is due to the stronger electrostatic forces between the ions.
Describe how ion size affects lattice enthalpy.
The smaller the ionic radii of the ions involved, the more exothermic (negative) the lattice enthalpy. Smaller ions have a higher charge density and ions can sit closer together in the lattice Attractions between the ions are stronger.
State Hess’s Law
Total enthalpy change of a reaction is always the same, no matter which route is taken.
Define the enthalpy change of hydration.
Enthalpy change when 1 mol of gaseous ions dissolves in water.
Define the enthalpy change of solution.
Enthalpy change when 1 mol of a solute dissolves in water.
Is bond breaking exothermic or endothermic?
Endothermic.
Energy is absorbed by the bonds, to break them.
Is bond making exothermic or endothermic?
Exothermic.
Energy is released as bonds form.
Give 2 factors affecting enthalpy of hydration.
- Size of the ions.
2. Charge on the ions.
How does ionic charge affect enthalpy of hydration?
Ions with a greater charge have a greater enthalpy of hydration.
Ions with a higher charge can attract water molecules better. Electrostatic attraction formed is stronger. More energy is released when bonds are made giving them a more exothermic enthalpy of hydration.
How does ion size affect enthalpy of hydration?
Smaller ions have a higher charge density than bigger ions. They attract the water molecules better and have a more exothermic enthalpy of hydration.
What does entropy tell you?
Entropy tells you how much disorder there is.
The more disorder there is, the greater the value of entropy.
What does thermodynamically stable mean?
When a substance reaches its maximum entropy state . This means it wont react any further without the input of energy.
How do you calculate entropy change of a reaction?
Give the units
ΔS = S products - S reactants
Units of entropy are JK-1mol-1
When is a reaction feasible?
When ΔG < 0 OR ΔG = 0
Give the formula for ΔG and the units for each component.
ΔG = ΔH - TΔS
ΔG = Jmol-1 ΔH = enthalpy change (J mol-1) T = temperature (K) ΔS = entropy change (J K-1 mol-1)
What does an oxidising agent do?
Accepts electrons and gets reduced.
What does a reducing agent do?
Donates electrons and gets oxidised.
What do electrochemical cells do?
Electrochemical cells make electricity.
Why is an electrochemical cell a redox process?
There are always 2 reactions within an electrochemical cell - ones an oxidation and ones a reduction.
Why is a platinum electrode sometimes used in half-cells?
Why is the metal platinum used, and not another one?
Half-cells involving solutions of 2 aqueous ions of the same element, such as Fe2+ (aq) and Fe3+ (aq) use a platinum electrode. The conversion from Fe2+ to Fe3+ occurs on the surface of the pt electrode.
Platinum is used because it is inert and doesn’t conduct electricity
What do electrode potentials measure?
How easily a metal is oxidised.
The more negative electrode potential = more easily oxidised.
Define standard electrode potential of a half-cell.
The SEP of a half-cell is the voltage measured, under standard conditions, when the half-cell is connected to a standard hydrogen electrode.
What are the standard conditions for electrode potentials?
- Solutions have a concentration of 1.00moldm-3
- Temperature must be 298K
- Pressure must be 100kPa
How do conditions affect the value of electrode potential.
The equilibrium position is affected by changes in temperature, pressure and concentration, which changes the cell potential.
How do fuel cells generate electricity?
A fuel cell produces electricity by reacting a fuel (hydrogen) with an oxidant, (usually O2).
Give 2 benefits of using electrochemical cells.
- They are more efficient at producing energy than combustion, as combustion loses energy as heat.
- They produce less pollution, as the only waste product is water.
Give 2 drawbacks of using electrochemical cells.
- Production of cells involves the use of toxic chemicals, which need to be disposed of.
- The chemicals used to make the cells are often very flammable.
