Unit 5 Thermodynamics Flashcards
Why are first electron affinities always exothermic?
There is an electrostatic attraction between the negative external electron and positive nucleus of the atom.
What must the overall enthalpy change be for each reaction in the Born-Haber cycle?
0 kJ/mol
What is the Gibbs free energy equation?
ΔG = ΔH - TΔS
Why is the melting of water endothermic?
Energy must be supplied to break hydrogen bonds between water molecules.
In the Born-Haber cycle, what is the direction of the arrow for the 1st electron affinity?
Down - exothermic
In the Born-Haber cycle, what is the direction of the arrow for the 2nd electron affinity?
Up - exothermic
In the Born-Haber cycle, what is the length of each arrow proportional to?
The size of the enthalpy change
What is meant by the Hydration enthalpy?
The enthalpy change when one mole of gaseous ions become hydrated (dissolved in water).
What is meant by the Enthalpy of solution?
The enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other.
Gibbs free energy is ……….. for a feasible reaction
Negative
What are the units of entropy
JK-1mol-1
Joules per kelvin per mole
What happens when Gibbs free energy is zero?
The temperature at which a reaction becomes feasible
What is the formula to calculate enthalpy of solution using lattice dissociation?
Enthalpy of solution = lattice enthalpy of dissociation + sum of hydration enthalpies
What is meant by lattice formation enthalpy?
The standard enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions
What is meant by first electron affinity?
The standard enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous ions, each with a single negative charge
What is meant by first ionisation energy?
The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single positive charge
What is meant by mean bond energy?
The enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form two free radicals, averaged over a range of compounds
How do you calculate enthalpy change using mean bond enthalpies?
Sigma bonds broken - sigma bonds formed
If the equation for Gibbs free energy is applied to the equation of a straight line y = mx + c, what is the gradient of the graph equal to?
-delta S
What are the units of entropy?
J K^-1 mol^-1
Joules per Kelvin per mole
What does covalent character do to lattice enthalpy values?
Increases it.
What does covalent character do to lattice enthalpy values?
Increases them.
What has 0 entropy?
Perfect ionic crystal, at 0K
What is the state of a reaction, if ΔG=0?
At equilibrium.
What is meant by enthalpy of atomisation?
The enthalpy change when one mole of gaseous atoms, are formed from their elements in their standard states, under standard conditions.
What is meant by enthalpy change?
The change in heat, at constant pressure (100kPa)
Why is chelation “thermodynamically favourable”?
Increase in entropy, due to more molecule on RHS of equation.
State Hess’s Law.
The enthalpy change of a reaction is independent of the route taken.
In terms of Gibbs free energy, when is a reaction feasible?
ΔG < 0
