Enthalpy Flashcards
What is the definition of lattice enthalpy?
The enthalpy change when one mole of an ionic solid is formed from its gaseous ions in standard conditions
Give an example of a lattice enthalpy reaction
Na+ (g) + Cl- (g) -> NaCl (s)
What factors result in a greater magnitude of lattice enthalpy?
- Higher charges on the ions
- Greater number of cation-anion interactions
- Smaller distance between the centres of the two ions (inter-ionic distance)
What must all lattice enthalpy values be, and why?
All values are negative, as it is a formation reaction meaning energy is released
What is the definition of standard enthalpy change of atomisation?
The enthalpy change when one mole of GASEOUS ATOMS is formed from an element in its standard state
Give two examples of an atomisation reaction
C (s) -> C (g)
1/2H2 (g) -> H (g)
What is important to remember in atomisation?
The product must be gaseous
It is one mole of atoms, not one mole of molecules (e.g. not one mole of H2 but one mole of H)
What is the first electron affinity of an element?
The enthalpy change when each atom in one mole of gaseous atoms gains an electron to form a 1- ion
Give an example of an electron affinity reaction
Cl (g) + e- -> Cl- (g)
Is first electron affinity exothermic or endothermic?
Exothermic (negative value)
What is the exception to first electron affinity being exothermic?
For noble gases, first electron affinity is endothermic
Is second electron affinity exothermic or endothermic?
Endothermic (positive value)
Why is second electron affinity endothermic?
As energy has to be put in to overcome the forces of repulsion from the already negative ion
What is the definition of standard enthalpy of neutralisation?
The enthalpy change when an acid and an alkali react to form 1 mole of water, under standard conditions
What is the definition of standard enthalpy of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions
What is the definition of standard enthalpy change of formation?
The enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions
Give an example of standard enthalpy of formation
Na (s) + 1/2Cl2 (g) -> NaCl (s)
What assumptions are made when find theoretical lattice enthalpy?
- The ions are in contact with one another
- The ions are perfectly spherical
- The charge on each ion is evenly distributed around
What causes differences between theoretical and experimental lattice energy?
Covalent character in the ionic bonding
What causes covalent character?
The polarisation of the anion by the cation
What is the effect of covalent character in ionic bonding on the value of lattice enthalpy?
Greater covalent character = more negative experimental value compared to the theoretical value
Why does the cation polarise the anion?
The positive cation will attract electrons from the anion, resulting in a distortion of the electron density of the anion. Some electrons will be drawn closer to the cation (covalent character)
What two cation factors increase the level of polarisation?
The cation will have a greater polarising effect when it is smaller, and has a higher charge (i.e. has a higher charge density
What two anion factors increase the level of polarisation?
The anion will be more easily polarised when it is larger, and has a higher charge
What is the definition of enthalpy change of solution?
The enthalpy change when one mole of an ionic solid dissolves in water to form an infinitely dilute solution
Give an example of an enthalpy change of solution reaction
NaCl(s) -> Na+ (aq) + Cl- (aq)
What is an infinitely dilute solution?
A solution in which further dilution has no measurable effect on enthalpy of solution
What is enthalpy change of hydration?
The enthalpy change when one mole of an ion in gaseous state is completely hydrated
Give an example of an enthalpy change of hydration reaction
Cl- (g) -> Cl- (aq)
What factors affect the enthalpy of hydration?
- The charge on the ions
- The size of the ion
How and why does the charge on the ions affect the enthalpy of hydration?
The greater the charge on the ions, the stronger the force of attraction between the ion and water, the more negative the value for enthalpy of hydration
How and why does the size of the ions affect the enthalpy of hydration?
The smaller the size, the stronger the force of attraction between the ion and water, the more negative the value for enthalpy of hydration
What is the relationship between enthalpy of solution, lattice enthalpy and enthalpy of hydration?
Enthalpy of solution = Enthalpy of hydration - Lattice enthalpy
What is the relationship between entropy and disorder?
Increasing entropy = increasing disorder
What is a spontaneous process?
A process that takes place without continuous intervention from us
What is the natural direction of change in terms of entropy?
Toward increasing entropy
What is entropy?
A property of matter that is associated with the degree of disorder/randomness of the particles
What is total entropy change equal to?
Total entropy change = Entropy of system + Entropy of surroundings
For a reaction to be spontaneous, what must total entropy change be?
Total entropy change must be positive for a reaction to be spontaneous
What is the equation to work out entropy of system?
Entropy of system = Entropy of products - Entropy of reactants
What is the equation to work out entropy of surroundings?
Entropy of surroundings = -Enthalpy change/Temperature
In exothermic reactions, entropy of surroundings will always be…
Positive, as energy is released to the surroundings causing increased disorder
In endothermic reactions, entropy of surrounding will always be…
Negative, as energy is taken in from the surrounding causing decreased disorder
How does an increase in temperature impact the entropy of a cold object?
Supply heat energy to a cold object will increase the movement of molecules and the entropy significantly
How does an increase in temperature impact the entropy of an already hot object?
Supply heat energy to an already hot object will still increase the movement of molecules and the entropy to some extent, but not as much as with a cold object. This is because relatively the increased degree of movement is less for hot objects
What does entropy change with changes in state?
Entropy increases in the following order:
Solid
How does a change in the number of moles from reactants to products affect entropy of system?
More moles = more particles = more ways in which the particles can be arranged = increased entropy
