Thermodynamics Flashcards
Define enthalpy change
Heat energy change at constant pressure
Define enthalpy of lattice formation
Enthalpy change when 1 mol of solid ionic compound is formed from its gaseous ions
How would you calculate Entropy change for a reaction (delta S)
Sum of products - reactants
How do you calculate Gibbs free energy?
AG = AH - TAS
H= enthalpy change
S= entropy change
T= temp
When is G (gibbs) feasible and not feasible
G is 0 or less than 0 (negative) = feasible
G is more than 0 = not feasible
How would you calculate what temperature the reaction becomes feasible
T = AH/ (AS/1000)
Define enthalpy of hydration
Enthalpy change when one mole of gaseous ions becomes aqueous ions
Define enthalpy of atomisation
Enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions.
Define electron affinity
Enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- ions
Define mean bond dissociation enthalpy
Enthalpy change when one mole of covalent bonds is broken with all species in the gaseous state
Define enthalpy change of combustion
Enthalpy change when 1 mol of a compound is burned completely in O2 with all reactants and products in their standard states under standard conditions
What is Hess’ law
The enthalpy change for a chemical reaction is independent of the route taken
What’s the perfect ionic model
- assumes bonding is 100% ionic (has no covalent character)
- all ions are perfect spheres
According to the perfect ionic model, what would be the difference between LDE values
There would be no difference between the experimental and theoretical
Why does LDE decrease
The size of the negative ion increases
What does a bigger difference between LDEexp and LDEtheo mean
The bigger the difference between the values the more covalent character
What effect does the covalent character have on bonds
It causes the strength of the bonds to increase
What does the difference between the experimental LDE and theoretical/perfect ionic LDE mean
- because there is a difference it shows there is some degree of covalent character
How do you work out activation energy (Ea) in the Arrhenius equation
Ea = RT(lnA-lnk)
If a reaction is carried out at a higher temperature how does it effect (AG) delta G
TAS gets bigger
How does disorder effect entropy (delta s)
An increase in disorder and entropy will lead to a positive entropy change
When will an increase in entropy occur
- state change from solid/liquid to gas
- increase in number of molecules between products and reactants
How do AS and AH effect AG/feasibility
- increased entropy (+ve AS) and exothermic (-ve AH) will make AG -ve so will be feasible
When temp increases what happens to the feasibility if entropy (AS) decreases/is negative
AG is more likely positive so reaction is not feasible so won’t occur
When temp increases what happens to the feasibility if entropy (AS) increases/is positive
AG is negative so reaction is feasible
When temp increases what happens to the feasibility if entropy (AS) is 0
AG will not change much
How does larger ion size effect enthalpy of formation
- Bigger ion means smaller charge density
- so enthalpy of formation will be less negative/exothermic
- because there is a weaker attraction between ion and halide
Define enthalpy of solution
Enthalpy change when 1 mol of an ionic solid is dissolved in water to infinite dilution so that the ions no longer interact under standard conditions
Define enthalpy of lattice dissociation
Enthalpy change when 1 mole of an ionic crystal lattice form is separated into its constituent ions in their gaseous states
Define enthalpy change of formation
The energy transferred when 1 mol of a compound is formed from its elements under standard conditions, and all reactants and products in their standard states
Define mean bond enthalpy
Enthalpy change needed to break 1 mol bonds averaged over a range of molecules
Why is electron affinity endothermic
The ion and the electron both have a negative charge so energy is required to overcome repulsion
How do you work out AH from a graph
Y intercept
How do you work out AS from a graph
gradient x 1000
How do you work out what temperature a reaction will become feasible from a graph
The temperature at the X intercept and above will be feasible
How do you work out bond enthalpy
Reactants - products
What’s the difference between enthalpy of solution and hydration
Solution - (s) —> (aq)
Hydration - (g) —> (aq)
