1.08 Thermodynamics Flashcards
What does Hess’ law state ?
That the enthalpy change of a reaction is independent of the route taken
Define standard enthalpy of formation
The enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions, with all products and reactants in their standard states.
What is the standard enthalpy of an element ?
0
Define standard enthalpy of combustion
The enthalpy change when one mole of a substance is completely burnt in an excess of O2
Define standard enthalpy of atomisation
The enthalpy change when one mole of gaseous atoms is formed from a compound in its standard states and in standard conditions.
Define the first ionisation energy
Enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous +1 ions.
Define second ionisation energy
The enthalpy change when one mole of electrons is removed from one mole of 1+ ions to form one mole of 2+ ions.
Define first electron affinity
The enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- ions.
Define second electron affinity
Enthalpy change when one mole of gaseous 1- ions gains another mole of electrons to form one mole of gaseous 2- ions.
Define lattice enthalpy of formation
Enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions.
Define lattice enthalpy of dissociation
Enthalpy change when one mole of solid ionic lattice is dissociated (broken into) it’s gaseous ions
Define enthalpy of hydration
Enthalpy change when one mole of gaseous ions become hydrated (dissolved in water where water molecules completely surround the ion).
Define standard enthalpy of solution
The enthalpy change when one mole of solute dissolves completely in a solvent to infinite dilution
Define mean bond dissociation enthalpy
The enthalpy change when one mole of a certain type of covalent bond (eg Br-H) is broken, with all species in the gaseous state.
What is a Born-Haber cycle ?
•A cycle showing all the enthalpy changes involved in the formation of an ionic compound
•Start with elements in their standard states (enthalpy of 0)
What factors affect the lattice enthalpy of an ionic compound ?
Size of the ions and their charge
How can you increase the lattice enthalpy of a compound? Why does this increase it ?
•By having smaller ions, as the charge centres will be closer together
•By having an increased charge as there will be a greater electrostatic force
How can we use Born-Haber cycles to see if compounds theoretically exist ?
•Use known data to predict certain values of theoretical compounds
•You can then see if these compounds are thermodynamically stable
What happens when a solid is dissolved (in terms of interactions of the ions of water molecules)
•The water molecules break the lattice -> the now gaseous ions are then surrounded by the water molecules
What is the perfect ionic model ?
•Assumes that ions are perfectly spherical and that there is an equal distribution of charge (100% polar bonds).
•In reality, compounds have covalent character. This increases bond strength.
Why is the perfect ionic model often not accurate ?
Ions are not perfectly spherical. Polarisation often occurs when small positive ions or large negative ions are involved, making the ions gain covalent character.
Which kinds of bonds are the most ionic? Why?
Between large positive ions and small negative ions eg CsF (atomic radius’ effect on e- loss)
Define the terms spontaneous / feasible
If a reaction is spontaneous / feasible, it can take place of its own accord; does not take account of the rate of reaction
Is a reaction with a positive or negative enthalpy change more likely to be feasible ?
Negative.
Define entropy
Randomness / disorder of a system.
Higher entropy value = more disorder
What units is entropy measured in ?
JK-1mol-1
What is the second law of thermodynamics ?
Entropy (of an isolated system) always increases, as it is overwhelmingly more likely for molecules to be disordered than ordered.
Is a reaction with positive or negative entropy change more likely to be feasible ?
Positive - reactions always favour increasing disorder
Compare the general entropy of the different phases
Solid < liquid < gas
How would you calculate the entropy change for a reaction
Entropy change = sum of products’ entropy - sum of reactants’ entropy
Define Gibbs energy using an equation
•ΔG = ΔH - TΔS
•G = Gibbs free energy
•H = Enthalpy
•S = Entropy
•T = Temperature.
What does the value for Gibbs free energy show for a reaction
•If G ≤ 0 then the reaction is feasible
•If G > 0 then the reaction is not feasible.
What is the significance of the temperature where G = 0 ?
This is the temperature (in kelvin) at which the reaction becomes feasible.
How would you calculate the temperature at which a reaction becomes feasible ?
Rearrange for T = (ΔH)(ΔS) as G= 0
What are the limitations of using G as an indicator whether the reaction will occur ?
Gibbs free energy only indicates if a reaction is feasible, it does not account for rate of reaction.
A reaction may theoretically be feasible at a certain temperature but the rate is so slow that it’s basically not happening.
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 reaction is always feasible - product favoured
If a reaction is endothermic and entropy decreases, what is the value of G and what does this mean ?
G is always positive, so reaction is never feasible.
If a reaction is exothermic and entropy decreases, what is the value of G and what does this mean ?
The feasibility is temperature dependant
If a reaction is endothermic and entropy increases, what is the value of G and what does this mean ?
The feasibility of the reaction is temperature dependant
Why is entropy 0 at 0k ?
There is no disorder, molecules are not moving or vibrating and so cannot be arranged in a different way - it is the maximum state of order.
What are the two key things to look out for to decide if entropy increases/decreases or stays constant ?
•Increase in number of moles increases entropy
•Going from solid to gas increases entropy
How is it possible for the temperature of a substance undergoing an endothermic reaction to stay constant
The heat that is given out escapes to the surroundings.
