Thermodynamics Flashcards
give a definition for enthalpy change
The difference in energy taken in when bonds are broken, and given out when bonds are formed, during a reaction, at constant pressure.
give a definition for enthalpy of formation
the enthalpy change when 1 mol of a substance is formed from its constituent elements, with all substances in their standard states.
give a definition for the enthalpy of combustion
the enthalpy chnage when 1mol of a substance is completely burnt in oxygen, with all substances in their standard states
give a definition for first ionisation energy
enthalpy change when each atom in 1mol of a gaseous substance loses on electron, to form 1mol of gaseous ions with a 1+ charge
give a definition for seconed ionisation energy
enthalpy change when each ion in 1mol of gaseous ions with a 1+ charge lose 1 electron, forming 1mol of gaseous ions with a 2+ charge
give the definition for first electron affinity
enthalpy change when each atom in 1mol of a gaseous substance, gain 1 electron, forming 1 mol of gaseous with a 1- charge
the sign is -ve
give the definition for the seconed electron affinity
enthalpy change when each ion in 1mol of gaseous ions with a 1- charge, gain one electron to form 1mol of gaseous ions with a 2- charge
the sign is +ve (there is a repulsion between the -ve ion and the electron)
give the definition for bond dissociation
enthalpy change when 1mol of covalent bonds are broken, in the gaseous state e.g. I2(g) –> 2I(g)
give the definition for enthalpy of atomisation of an element
enthalpy change when 1mol of gaseous atoms are formed from an element in its standard state
e.g. 1/2 I2 (g) –> I (g)
give the definition for enthalpy of atmoisation of a compound
enthalpy change when 1mol of gaseous atoms are formed from a compound in its standard state
give a definition for lattice enthalpy of formation
enthalpy change when one mol of a solid ionic compound is formed from its constituent ions in the gas phase
give a definition for lattice enthalpy of dissociation
enthalpy change when 1mol of a ionic compound is broken up into its constituent ions in the gas phase
give a definition for hydration enthalpy
enthalpy change when 1 mol of an ionic solid is hydrated
give a definition for the enthalpy of solution
enthalpy change when 1mol of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with eachother
what is hess’s law
states that enthalpy change for any reaction is independent on the route taken
what does the size of the arrow in a born habor cycle represent
the size of the enthalpy change
Explain why the lattice dissociation enthalpy of NaBr is a bit less than that of NaCl
Lattice dissociation enthalpy is a measure of the forces of attractions between the positive and
negative ions. Bromide ions are bigger than chloride ions, and so the distance between the centres
of the positive and negative ions in the lattice is greater in NaBr. Increasing distance weakens the
forces of attraction between them, and hence the lattice enthalpy
Explain why the lattice dissociation enthalpy of MgO is about 5 times greater than that of NaCl.
Magnesium ions are smaller than sodium ions, and oxide ions are smaller than chloride ions. That
means that the distance between the positive and negative ions is quite a lot less in MgO than in
NaCl, and so the forces of attraction will be greater in MgO.
But the main factor is the charge on the ions. In MgO, both positive and negative ions carry two
charges. In NaCl, they only carry one. The strength of the attractions is much greater in MgO than
in NaCl.
The standard atomisation enthalpy of bromine is +112 kJ mol-1, and for sodium is +107 kJ mol-1
.
Write equations for the reactions that these figures relate to
1/2 Br2 (g) –> Br(g)
Na(s)–> Na(g)
write an equation for the lattice enthalpy of sodium chloride
Na+ (g) + Cl- (g) –> NaCl (s)
why are the theoretical values usually lower than the experimental values
because of polarisation
why do most ionic compounds dissolve in water
water is a polar molecule because of the difference in electronegativity of the oxygen and hydrogen in the molecule. the hydrogen has a +ve partial charge, and the oxygen has a -ve partial charge
give three stages that dissolving an ionic compound can be broken down into
- breaking the solid into gaseous ions
- hydrating the +ve ions
- hydrating the -ve ions
give an equation for calculating the enthalpy of solution
delta sol = delta LE + delta hydration
what does gibbs free energy determine
whether or not a reaction will occur
give an equation for gibbs free energy change
deltaG = deltaH - TdeltaS
what value must gibbs free energy be for the reaction to be feasible
less than or equal to 0
rearrange the equation to calculate the temperature
T= (deltaH-deltaG)-deltaS
