Thermodynamics ( A-level ) Flashcards
What does Hess’s law state?
The enthalpy change for a reaction is independent of the route taken by the reactants
What is the standard enthalpy of an element?
Zero
Define standard enthalpy of atomisation
the enthalpy change when of gaseous compound is formed from a compound in its standard state
Define first electron affinity
Enthalpy change when one mole of gaseous atom gains one mole of electron to form one mole of a gaseous 1- ions
Define second affinity
Enthalpy change when one mole of gaseous 1- ion gains one mole of electron to form one mole of gaseous 2- ion
Define lattice enthalpy of formation
Enthalpy change when one mole of a solid ionic compound is formed from its consistuents ions in their gaseous state
Define lattice enthalpy of dissociation
Enthalpy change when mole of a solid ionic compound dissociates into its consistuent ions in their gaseous state
Define enthalpy of hydration
Enthalpy change when one mole of gaseous ions dissolves completely in water to form a infinitvely dilute solution
Define enthalpy of solution
Enthalpy change when one mole of a solid ionic compound dissolves completely in water to form a infinitvely dilute solution
Define mean bond dissociation enthalpy
Enthalpy change when one mole of covalent is broken, with all species in their gaseous state
What is the Born-Haber cycle?
a cycle that shows all the enthalpy changes involved in the formation of an ionic compound. starting with all elements in their standard state
what factors affect the lattice enthalpy of an ionic compound?
Size of the ions, charge on the ions
what happens when a solid is dissolved in terms of interactions of the ions with water?
- The lattice is broken forming gaseous ions
- gaseous ions are dissolved to form aqueous ions
- The aqueous ions are surrounded by the water molecules due to the polarity of the O-H bond
What is a perfect Ionic model?
- model that suggests that ions are perfectly spherical and there is even charge distribution
- acts as point charges
why is the perfect ionic model not accurate?
- ions are not perfectly spherical
- polarisation occurs between the interaction of small positive ions or large negative ions leading to covalent character
what kind of bonds will be the most ionic?
bonds between large positive ions and small negative ions
define entropy
-measure of randomness/disorder of a system
- High value for entropy = more disorder
what unit is entropy measured in?
JK-1mol-1
what is the second law of thermodynamics?
entropy always increases, as its overwhelmingly more likely for molecules to be disordered than ordered
compare the general entropy values for solids,liquids and gases
solids- low entropy
liquid- medium entropy
gases- high entropy
define Gibbs free energy using an equation
ΔG = ΔH - TΔS
what does the value for Gibbs free energy in a reaction show?
- if G < 0, reaction is feasible
- if G = 0, reaction is just feasible
- if G > 0, reaction is not feasible
how would you calculate the temperature at which a reaction becomes feasible?
T = ΔH / ΔS
if the reaction is exothermic and the entropy increases, what is the value of G and what does it mean?
G is always negative so the reaction is always feasible
if the reaction is endothermic and entropy decreases, what is the value of G and what does it mean?
G is always positive so the reaction is never feasible
why is entropy 0 at 0K?
molecules/atoms are not moving or vibrating and cannot be arranged in any other way
what are two key things to look out for to decide if entropy increases/decreases/stays relatively constant?
- Number of moles, if more moles are made there is an increase in entropy
- Going from solid to liquid/gas or liquid to gas
