Thermodynamics Flashcards
What is enthalpy change?
It is the heat energy transferred in a reaction at constant pressure.
What is enthalpy of formation?
It is the enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions.
What is the enthalpy change of atomisation of an element?
It is the enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state.
What is the first ionisation energy?
It is the enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms.
What is the first electron affinity?
It is the enthalpy change when 1 mole of gaseous 1- ions is formed from 1 mole of gaseous atoms.
What is the enthalpy change of hydration?
It is the enthalpy change when 1 mole of aqueous ions is formed from 1 mole of gaseous ions.
What is the bond dissociation enthalpy?
It is the enthalpy change when all the bonds of the same type in 1 mole of gaseous molecules are broken.
What is the enthalpy change of atomisation of a compound?
It is the enthalpy change when 1 mole of a compound in its standard state is converted into gaseous atoms.
What is the second ionisation energy?
It is the enthalpy change when 1 mole of gaseous 2+ ions is formed from 1 mole of gaseous 1+ ions.
What is the second electron affinity?
It is the enthalpy change when 1 mole of gaseous 2- ions is formed from 1 mole of gaseous 1- ions.
What is the enthalpy of solution?
It is the enthalpy change when 1 mole of solute is dissolved in enough solvent that no further enthalpy change occurs on further dilution.
What is lattice enthalpy?
It is a measure of ionic bond strength
What is lattice enthalpy of formation?
It is the enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions.
What is the lattice enthalpy of dissociation?
It is the enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions under standard conditions.
What is the enthalpy of combustion?
It is the enthalpy change when 1 mole of a substance is completely burnt in oxygen with all the reactants and products in their standard states under standard conditions.
What is the enthalpy of neutralisation?
It is the enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions.
What is the enthalpy of vaporisation?
It is the enthalpy change when 1 mole of a liquid is turned into a gas.
What is the enthalpy of fusion?
It is the enthalpy change when 1 mole of a solid is turned into a liquid.
What is Hess’ Law?
The enthalpy change for a reaction is independent of the route taken
Which factors affect lattice enthalpies?
The size of the ions
The charges on the ions
How does size affect the lattice enthalpy?
The larger the ions, the less negative the enthalpies of lattice formation. The charges are further apart, therefore, there is a weaker force of attraction between them.
How do charges on the ions affect the lattice enthalpy?
The larger the charge on the ion, the greater the attraction between the ions, therefore, the stronger the lattice enthalpy.
What are the assumptions of theoretical lattice enthalpies?
The assumptions are of a perfect ionic model
- the ions are 100% ionic
- the ions are spherical
- the attractions are purely electrostatic
Why are the theoretical and experimental values different?
This is evidence that the ionic compounds have some covalent character. The positive ions polarise the neighbouring negative ions.
What type of reaction is the lattice enthalpy of dissociation and why?
The bonds between the ions break to give free ions
What type of reaction is the enthalpy change of hydration and why?
Exothermic
Bonds between the ions and the water are made
Why can water molecules bond to the ions?
Oxygen is more electronegative than hydrogen, and therefore draws electrons towards itself, creating a dipole. The dipole means that the positively charged hydrogen atoms can form bonds with negative ions and the negatively charged oxygen can form bonds with positive ions.
When do substances generally dissolve?
If the energy released is roughly the same or greater than the energy taken in.
What is entropy?
It is a measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles.
What is the effect of change in physical state on the entropy?
Solids have the lowest entropy because the particles vibrate about fixed points.
Gases have the highest entropy because the particles move around rapidly and randomly.
What is the effect of having more particles on the entropy?
The more particles there are, the more ways there are for them and their energy to be arranged
How does temperature affect entropy?
The higher the temperature, the faster the particles vibrate/move and so the greater the entropy.
Why is the entropy increase from liquid to gas greater than the increase from solids to liquids?
There is a large amount of disorder in gases compared to solids and liquids.
How do you calculate entropy(S) change?
change in S = S(products) - S(reactants)
What is free energy change(G)?
It is a measure used to predict whether a reaction is feasible/spontaneous.
What is the condition for a reaction to be feasible?
change in G must be negative or zero
Why might a reaction not happen or be so slow even if it is feasible?
The reaction might have a high activation energy
How can change in G be calculated?
change in G(J/mol) = enthalpy change(J/mol) - temperature(K)*entropy change(J/K/mol)
What is G when the reaction is exothermic and has a positive entropy change?
change in G is always negative
The reactions are feasible at any temperature
What is G when the reaction is endothermic and has a negative entropy change?
change in G is always positive
The reactions are not feasible at any temperature
How do you calculate the temperature at which the reaction is just feasible?
T= enthalpy change/entropy change
What is the standard entropy of a substance?
It is the entropy of 1 mole of that substance under standard conditions.
