Physical 8: Thermodynamics Flashcards
What is enthalpy change?
Heat change in a reaction at constant pressure
What are standard conditions?
100kPA
298K
Define standard molar enthalpy of formation.
The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions with all reactants and products in their standard states
Define standard molar enthalpy of combustion.
The enthalpy change when one mole of a compound is burned completely in oxygen
Define standard enthalpy of atomisation.
The enthalpy change which accompanies the formation of one mole of gaseous atoms from the element in its standard state under standard conditions
e.g. Mg(s) –> Mg(g)
Define first ionisation energy.
The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of positively charged ions
Define second ionisation energy.
The energy required to remove one mole of electrons from one mole of 1+ ions to form one mole of 2+ ions
Define first electron affinity.
The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions with a single negative charge
E.g. O(g) + e- –> O-(g)
Is first electron affinity exothermic or endothermic, and why?
First electron affinity is exothermic because energy is given out when electrons are attracted to the positiely charged nucleus
Define second electron affinity.
The standard enthalpy change when one mole of electrons is added to a mole of gaseous 1- ions to form one mole of 2- ions
Are second electron affinities exothermic or endothermic?
Endothermic because it is more difficult for a negative ion to gain an electron due to charge repulsion, so energy is needed to overcome this repulsion
Define lattice enthalpy of formation.
The standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions
Define lattice enthalpy of dissociation.
The standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions
How does lattice enthalpy vary with size of ion?
Larger ions = smaller lattice enthalpy
This is because the opposite charges do not approach each other as closely when the ions are larger
Define enthalpy of hydration.
The standard enthalpy change when one mole of gaseous ions become aqueous ions
E.g. Na+(g) + aq –> Na+(aq)
Enthalpy of hydration is exothermic because bonds are made between the ions and water molecules
Define enthalpy of solution.
The standard enthalpy change when one mole of solute dissolves completely in sufficient solvent to form a solution in which the molecules or ions are far enough apart to not interact
E.g. NaCl(s) –> Na+(aq) + Cl(aq)
Define mean bond enthalpy.
The standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms
What is a Born-Haber Cycle?
A thermochemical cycle that includes all the enthalpy changes involved in the formation of an ionic compound
They are constructed by starting with the elements in their standard states, the enthalpy of which will be zero
Draw a Born-Haber Cycle for sodium chloride.
Describe how hydration occurs.
Solvent molecules (i.e. water) cluster around the charged ions:
The positive dipoles face towards negative ions
The negative dipoles face towards positive ions
Draw a general thermochemical cycle to calculate enthalpy of solution.
Draw a thermochemical cycle to calculate the enthalpy of solution of NaCl.
What is entropy and what are its units?
A mathematical expression of the ‘randomness’ of a system
Given the symbol S
And the units JK-1mol-1
What happens to entropy as temperature increases?
As temperature increases, entropy also increases, because the particles have more kinetic energy so have more freedom of movement, so randomness increases
What are the two factors that govern the feasibility of a chemical reaction?
Enthalpy change
Entropy change
What is Gibbs free energy equation?
ΔG = ΔH - TΔS
When would a reaction not be feasible?
When ΔG is positive
When is a reaction feasible?
When ΔG is less than or equal to zero
