5.5 Enthalpy and Entropy Flashcards
what is enthalpy change of atomisation
the enthalpy change when 1 mole of gaseous atoms are formed from an element in its standard state
Na(s) -> Na(g)
what is first ionisation energy
energy required to remove the 1st electron from each atom in 1 mole of atoms in its gaseous state
Na(g) -> Na+(g) + e-
what is electron affinity
enthalpy change when every atom in 1 mol of gaseous atom acquires an electron to form 1 mol of gaseous ions
what does first electron affinity involve?
the electron being gained experiencing an attraction to the nucleus
is an exothermic reaction
what does second electron affinity involve?
the electron being gained experiencing repulsion by the negatively charged ions
what is enthalpy of formation
the enthalpy change when 1 mole of a compound is made from its elements in their standard states
H2(g) + 1/2O2(g) -> H2O(l)
what does Hess’ law state
the enthalpy change for a chemical reaction depends only on the initial and final states and is independent on the path followed.
or
whatever route in which the reaction proceeds the overall enthalpy change for the reaction will be the same.
Born-Haber cycles
used primarily as a means of calculating lattice enthalpies which cannot otherwise be measured directly
made of many separate sub-processes each with their own enthalpies
what is the process of the Born-Haber cycle
- atomisation
- 1st ionisation
- atomisation
- electron affinity
- lattice enthalpy
- formation
what are exothermic reactions
release energy (get hot) makes bonds
what are endothermic reactions
absorb energy (get cold) breaks bond
why do solid ionic lattices form
not only as a result of the transfer of electrons, but also the huge release in energy that occurs when the 2 ions of opposite charge combine to form a solid
what is lattice enthalpy
enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions
values are used as a measure of the strength of ionic bonding in giant ionic lattices
how do you calculate standard enthalpy change of solution
- calculate energy change (q) in kJ using q=mcT
- calculate amount in moles of the thing that’s being dissolved
- calculate standard enthalpy change of solution using △H=q/n
what is enthalpy change of solution
the enthalpy change for 1 mole of solute dissolving
the process of dissolving can be endo or exothermic
what happens when a solid dissolves- breakdown
- breakdown of the ionic lattice into monatomic gas ion
opposite to lattice enthalpy
K+ + Cl- -> KCl(s)
KCl(s) -> K+ + Cl-
what happens when a solid dissolves- hydration
hydration of gaseous ions by water molecules
ions bond to water
is exothermic
slight + ion are attracted to slight - oxygen
slight - ion are attracted to slight + hydrogen
what is enthalpy change of hydration
enthalpy change for 1 mole of gaseous ions dissolving in water
energy released when the ions form bonds with water molecules
different ions have different enthalpy changes
what happens when a solid dissolves
breaking the lattice is endothermic
water interacting with the gaseous ions is exothermic
what happens if the lattice is….
if lattice enthalpy > enthalpy of hydration, then the enthalpy of solution will be endothermic
if lattice enthalpy < enthalpy of hydration, then the enthalpy of solution will be exothermic
what is the relation between ionic size of atoms and lattice enthalpy
as the radius gets smaller, there is an increased attraction, as the ions closer together.
this means that lattice enthalpy increases
what is the relation between charge and lattice enthalpy
as charge increases, there is more lattice enthalpy. this is because it can form stronger bonds and so a larger attraction
ion charge is more important than ionic size
enthalpy change of hydration
as ionic radius decreases, the enthalpy change of hydration becomes more negative
hydration depends on the ability of an ion to attract and bond with water. smaller molecules exert more attraction
what is the relation of lattice enthalpy, enthalpy change of solution and enthalpy change of hydration
enthalpy change of solution is more affected by lattice enthalpy than enthalpy change of hydration
if lattice enthalpy is smaller than hydration, the enthalpy of solution is exothermic
if lattice enthalpy is greater than hydration, the enthalpy of solution is endothermic
entropy
measure of the dispersal of energy in a system, which is greater the more disordered the system
has an irregular arrangement of particles and so has a high entropy.
has a regular arrangement of particles and so has a low entropy
symbol is S
units is JK-1mol-1
what is an idea of entropy
nature tends to move from an ordered state to one of disorder
a system becomes energetically more stable when it becomes less ordered
what are factors that affect entropy
- when a solid becomes a gas
- when a solid lattice dissolves
- when the number of gaseous molecules increases
explanation- when a solid becomes a gas
solids wobble around a fixed point
gas has high randomness
explanation- solid lattice dissolves
dissolving increases entropy
moves freely
explanation- more gas
more particles means there’s more ways they and their energy can be arranged
what is the equations for entropy
entropy= sum of products- sum of reactants
spontaneous changes
occurs in 1 particular direction and takes place without the need for work
exothermic reactions are usually spontaneous
what is free energy
a reaction is only spontaneous if it can do work and must generate a free potential
- free energy (△G) indicates a reaction capable of proceeding of its own accord
free energy and spontaneity
if free energy is < 0, it is negative and a spontaneous reaction
if free energy is = 0, the system is in equilibrium
if free energy is > 0, it is positive and a non-spontaneous reaction
what is the 2nd law of thermodynamics
based on entropy
entropy tends to a maximum
infers that all chemical and physical changes involve an overall increase in entropy
how to increase entropy
solids melt liquids boil ionic substances dissolve in water number of gas molecules increases temperature increases
what is the free energy equation
△G= △H-T△S
free energy = enthalpy change - (entropy X temperature)
if free energy in a reversible reaction at equilibrium is 0,
△S= △H/T or △H=T△S
