Thermodynamics Flashcards
What is lattice enthalpy?
The enthalpy change when 1 mole of a solid ionic lattice forms from its gaseous ions under standard conditions
More energy released, the stronger the bonds are
Define lattice enthalpy of formation
The enthalpy change when 1 mole of solid ionic lattice is formed from its gaseous ions under standard conditions
Define standard enthalpy of formation
The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions.
Define standard enthalpy of atomisation
The enthalpy change which accompanies the formation of 1 mole of gaseous atoms from the element in its standard state under standard conditions
Define the fist ionisation energy
The enthalpy change when 1 mole of gaseous atoms is converted into a mole of gaseous iona each with a single positive charge
Define second ionisation energy
The enthalpy change when a mole of electrons I lost from 1 mole of a singly positively charged ion to form 1 mole of 2+ ions
Define the first electron affinity
The enthalpy change when 1 mole of gaseous atoms is converted to 1 mole of gaseous ions each with a single negative charge
Define the second electron affinity
The enthalpy change when 1 mole of electrons is added to 1 mole of gaseous ions each with a single negative change to form ions each with 2 negative charges
Define lattice enthalpy of dissociation
The enthalpy change when 1 mole of solid ionic compound is dissociated into its gaseous ions
Define enthalpy of hydration
The standard enthalpy change when 1 mole of gaseous ions is converted into 1 mole of aqueous ions
Define enthalpy of solution
The enthalpy change when one mole of dilute dissolved completely in sufficient solvent to form a solution where the ions are far enough apart to not interact with one another.
Define the mean bond enthalpy
The enthalpy change when 1 mole of gaseous molecules each breaks a covalent bond to form 2 free radicals averaged over a range of compounds
What are he stages in a born haber cycle?
- Atomisation of elements
- Ionisation if elements
- Electron affinity of elements
- Lattice enthalpy
What is the formula for charge density?
Charge density = charge/size
What happens when we dissolve a solid?
First the ionic solid is broken down.
Then free ions become part of the solution (hydration)
This has an enthalpy change (enthalpy change of solution)
This can be an exo or endothermic reaction.
How is the ionic solid broken down when dissolving a solid compound?
Break the solid into gaseous ions
NaCl ——> Na+ + Cl-
The reverse of this is lattice enthalpy of formation
Therefore break down = - lattice enthalpy of formation
Breaking down a lattice is always endothermic
What factors influence the size of lattice enthalpy?
Small ions - smaller ions can get close to the molecules and therefore form stronger bonds
Greater charge density- increased attractive force so the bonds are stronger
Therefore lattices made from smaller more highly charged ions will have the strongest bonds and give out he most energy on formation (polarisation)
How are ions hydrated when dissolving a solid compound?
Ionic solids must be dissolved in polar solvents (water)
This separates the ions and stops the solid reforming
Standard enthalpy of hydration
The magnitude of this is affected by the same factors as ionic bond strength
What is entropy?
A measure of disorder
More disordered something is, the higher the entropy
Explain why a gas has a higher entropy than a solid?
gas has a higher entropy than a solid as there are many more arrangements the particles can be configured/distributed compared to a solid so the entropy is higher
What is a spontaneous reaction?
A reaction without external help
Entropy must increase in a spontaneous reaction
What are the units for enthalpy?
Kjmol^-1
What are the units for entropy?
JK^1mol^-1
J/K/mol
How do you work out entropy change for the system?
Entropy change of system = sum of products - sum of reactants
How do you calculate entropy of the surroundings?
Entropy of the surroundings = - enthalpy change / temperature
How do you calculate the total entropy change?
Entropy change = entropy of system + entropy of surroundings
Only reactions with a positive entropy change are feasible
How can we predict entropy changes?
Can predict if the entropy of system will be positive or negative without calculating it.
Change of state:
Solid/liquid/gas
Entropy———> increase
Change in the number of moles:
More moles= more configurations = more disorder = more entropy
Dissolving ionic solids in water:
Breakdown of lattice = more ways to arrange free ions (increase in entropy)
Hydration of ions = ordered around water (decrease in entropy)
What is the equation for Gibbs free energy?
Gibbs energy = change in enthalpy - T(change in entropy)
Gibbs energy must be negative for a reaction to be feasible
How is entropy change and Gibbs energy related?
Change in Gibbs free energy has the opposite charge to the total entropy
Change in entropy = - Gibbs energy / temperature
How do you work out at what temperature a reaction becomes feasible?
When Gibbs energy is 0
T = enthalpy change / entropy change
Explain why the first electron affinity of Halogens has a large negative value?
Halogens have a net attractive force, between the positive chlorine nucleus and the extra negative electron.
Explain why the second electron affinities of elements are usually large positive values
Elements already have a negative charge after the gain of the first electron.
electrons repel each other so their electron affinity is more positive.
What is the perfect ionic model?
Ions are perfect spherical shapes
100% ionic bonding
No distortion in the electron clouds
why may experimental values of lattice enthalpy be higher than the theoretical values?
Due to the compounds not being purely ionic and therefore not having spherical shapes. This means that the electron clouds may overlap meaning there is some covalent character so the bonds/forces holding the lattice together are stronger and hence there is a higher lattice enthalpy.
