thermodynamics Flashcards
define enthalpy change of formation
the enthalpy change when one mole of a compound is formed from its constituent elements (in their standard state) under standard conditions (100KPa, 298K, 1mol of reactants)
define enthalpy change of combustion
the enthalpy change when 1 mole of
a substance is completely burned in O2 under standard conditions with all
reactants and products in their standard states.
define bond dissociation enthalpy
the enthalpy change when 1 mole of
covalent bonds are broken under standard conditions in the gaseous state.
define enthalpy change of atomisation
the enthalpy change for the formation of 1 mole of gaseous atoms from the element/compound in its standard state.
define first ionisation energy
enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous atoms to form one mole of gaseous ions with a +1 charge.
define second ionisation energy
enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous ions with a charge of +1 to form one mole of gaseous ions with a charge of +2
define first electron affinity
enthalpy change when 1 mole of gaseous
atoms form 1 mole of gaseous ions w/ a charge of -1.
values for first electron affinity are typically negative, as the process is exothermic
values of the second electron affinity are…
+ve as the process is
endothermic.
This is due to the repulsion from the negative ion toward the
negative electron being added.
define enthalpy of lattice dissociation
the enthalpy change when 1 mole of an ionic compound is separated into its constituent gaseous ions.
define enthalpy of lattice formation
the enthalpy change when 1 mole of
an ionic compound is formed from its constituent ions in the gaseous state
define enthalpy of hydration
the enthalpy change when 1 mole of gaseous
ions is converted into 1 mole of aqueous ions.
define enthalpy of solution
the enthalpy change when 1 mole of an ionic
substance dissolves in enough solvent to form an infinitely dilute
solution
how do we work out enthalpy of solution?
enthalpy of lattice dissociation + enthalpy of hydration of positive ions + enthalpy of hydration of negative ions
why can ionic substances dissolve in water?
because water is a polar molecule - it has a positive dipole (hydrogens) and negative dipole (oxygen)
what is hess’ law?
The enthalpy change for a chemical reaction is independent of the route taken
what do arrows pointing up and arrows pointing down represent on a born-haber cycle?
arrow pointing up = endothermic
arrow pointing down = exothermic
the greater the value of the lattice enthalpy…
the stronger the ionic bonding
what factors cause stronger ionic bonding?
- smaller ions; more closely packed in the lattice, so are more
attracted to each other, resulting in high lattice enthalpy. - higher positive charge; stronger electrostatic forces of attraction
between the ions which results in high lattice enthalpy
what two assumptions is the perfect ionic model based upon
- the bonding is completely ionic
- the ions are regarded as perfect spheres and are not distorted (charge is perfectly distributed)
what causes covalent character in an ionic compound?
a small and highly charged cation can distort a large anion so
that there is covalent character to the ‘ionic compound’. This means that ions are distorted and not spherical (That IS assumed by the perfect ionic model)
what does it mean if the compound has covalent character?
the forces of attraction in the compound are found to be greater than predicted by the perfect ionic model.
*This leads to a difference between the values of the experimental lattice enthalpy and the theoretical lattice enthalpy. The greater the difference in the values, the greater the covalent character in the
compound.
what is a feasible reaction?
a reaction which can occur at a particular temperature and, once started, will carry on to completion without any energy being supplied
in every feasible reaction, what happens to the entropy of the system?
entropy of the system increases
what is entropy?
a numerical value of the disorder within a system
what is entropy measured in?
j/kelvin/mol
how does physical state affect entropy?
gases are more disordered than liquids and solutions.
Liquids and solutions are more disordered than solids
how does dissolution affect entropy?
dissolving a solid increases its entropy as dissolved particles can move freely because they are no longer held in fixed positions.
how does number of particles affect entropy?
more moles of particles increased entropy. The
more particles there are, the more ways they, and their energy can be
arranged.
what is the formula for entropy change in a reaction?
∆S = [sum of ∆SƟ products] - [sum of ∆SƟ reactants]
at what temperature is a crystalline form of a structure a perfect crystal?
absolute zero (0k)
what is the formula for gibbs free energy?
∆G = ∆H – T∆S
what is gibbs free energy?
a measure used to predict whether a reaction will be feasible at a particular temperature
for a reaction to be feasible, what should gibbs be?
equal to or less than zero
