22 Enthalpy and Entropy:
22.3: Factors effective lattice enthalpy and hydration enthaply:
Ionic compounds
Ionic compounds are famous with scientists for having the
following general properties, high melting points and high boiling
points, soluble in polar solvents and conduct electricity when
molten or in an aqueous solution.
However amongst many ionic compounds there is actually a wide
range in melting points and solubilities. Some ionic compounds
can easily be melted by a Bunsen burner, while others have such a
high melting point that they are often used to coat the inside of
furnaces. There are even rarer examples of ionic compounds which
are liquids at room temperature these are called ionic liquids.
Solubility is even more variable with ionic compounds with most
being soluble and polar solvents such as water, but many are also
insoluable in conjunction with specific polar solvents.
The magnitude of lattice and hydration enthalpies helps to explain
the variety and melting point and boiling points and trends in
solubility seen in the ionic solids.
a.) Draw a simple labelled diagram of an example of an ionic lattice
b.) Draw a simple labelled diagram of dissolution of ions in water
Factors affecting lattice enthalpy
The two main factors that affect lattice enthalpies are ionic size and
ionic charge.
Effect of ionic size
The effect of increasing ionic size is lattice energy is less negative
and melting point the decreases. This is due to the ionic radius
increasing and therefore the attraction between the ions
decreasing.
c.) Draw a simple labelled diagram to represent this
Effect of ionic charge
The effect of ionic charge is latticed and energy becomes more
negative and therefore melting point increases. This is due to the
charge increase of the ions increasing the attraction between the
ions.
d.) Give example of cations of similar size showing this effect
Therefore across a period in the periodic table, cations from left to
right across the period increase in charge and therefore the
decrease in size of ion thereby increasing the overall charge
density of the ion.
Whereas anions had two opposing effects from from right to left
across the period there is an increasing charge of ion giving more
attraction counterbalanced by an increasing size of ion giving less
attraction.
e.) Draw out period 3 and label on directional arrows explaining the
trends.
Predicting melting points
All this information makes it possible within reason to predict
melting points based on the magnitude of lattice energy. That is
lattice energy is a very good indicator of the size of melting point
but it should be known that other factors such as the packing of
ions into the ionic lattice may also need to be considered, which
explain the existence of ionic liquids.
f.) Give example pairs of of ionic compound comparisons, where it
would be easy to predict melting point.
For each explain the reason why this is the case.
g.) Give example pairs of of ionic compound comparisons, where it
would be difficult to predict melting point.
For each explain the reason why this is the case.
Factors affecting hydration
Hydration enthalpies are also affected by ionic size and ionic
charge in a similar way to lattice enthalpies.
For example as ionic radius increases the attraction between the
ions and the water molecules decreases. This results in the
hydration energy being less negative.
Similarly as ionic charge increases the attraction with the water
molecules increases, therefore hydration energy becomes more
negative.
h.) Draw a simple labelled diagram to represent both the effect of
ionic and ionic charge on hydration.
Predicting solubility
In order that an ionic compound dissolves in water the attraction
between the ions and the ionic lattice must be overcome. This
requires a quantity of energy equal to the lattice enthalpy. In water
molecules the delta negative charge of the oxygen atom is
attracted towards positive ions and the delta positive of charge of
the hydrogen atoms is attracted towards the negative ions. The
water molecules surround the ions releasing energy equal to
hydration enthalpy.
If the sum of the hydration enthalpy is larger than the magnitude of
the lattice enthalpy, the overall enthalpy change (the enthalpy
change solution) will be exothermic and the compound should
dissolve.
However it should be noted that the many compounds with
endothermic and enthalpy changes of solution are soluble so this
does not provide the total picture. This is due to the fact that the
reason the solubility also depends on the temperature and another
factor called entropy. Entropy should be thought of as the natural
tendency for energy to spread out rather than remain concentrated
in one place.
Ionic liquids
It is generally accepted that ionic compounds have high melting
point and high boiling points, but ionic liquids have low melting
points that are below 100°C.
Ionic liquids conduct electricity, do not vaporise easily and can
dissolve in an amazing range of substances. Chemical structure of
ionic liquids typically contain a complex organic ion, which is highly
variable in shape and structure. Ionic liquids can be made from
biomass, and are seen as a potential green solvents in future to
replace oil-based derivatives as solvents.
i.) Carry out an Internet search to find some potential uses for ionic
liquids and their synthesis from biomass.22 Enthalpy and Entropy:
22 Enthalpy and Entropy:
22.3: Factors effective lattice enthalpy and hydration enthaply:
Ionic compounds
Ionic compounds are famous with scientists for having the
following general properties, _____ _____ ______ and ______ ______
_______, ______ in _____ ______ and ________ __________ when
______ or in an _________ solution.
However amongst many ionic compounds there is actually a _____
range in melting points and solubilities. Some ionic compounds
can easily be melted by a ________ _______ , while others have
such a ____ melting point that they are often used to coat the
inside of _________. There are even rarer examples of ionic
compounds which are _______ at room temperature these are
called ______ ________.
Solubility is even more _______ with ionic compounds with most
being soluble and ______ _______ such as ______ , but many are
also insoluable in conjunction with specific ______ _______.
The magnitude of ______ and _________ ___________ helps to
explain the variety and melting point and boiling points and trends
in _________ seen in the ionic solids.
a.) Draw a simple labelled diagram of an example of an ionic lattice
b.) Draw a simple labelled diagram of dissolution of ions in water
Factors affecting lattice enthalpy
The two main factors that affect lattice enthalpies are _____ ____
and _____ ________.
Effect of _____ _____
The effect of ___________ ionic size is lattice energy is _____
negative and melting point the ___________. This is due to the ionic
radius ___________ and therefore the attraction between the ions
___________.
c.) Draw a simple labelled diagram to represent this
Effect of _____ _________
The effect of _____ ________ is latticed and energy becomes _____
negative and therefore melting point ___________. This is due to the
charge ___________ of the ions ___________ the attraction between
the ions.
d.) Give example of cations of similar size showing this effect
Therefore across a _______ in the periodic table, cations from left to
right across the period ___________ in charge and therefore the
___________ in size of ion thereby ___________ the overall charge
density of the ion.
Whereas anions had two opposing effects from from right to left
across the period there is an ___________ charge of ion giving
_____ attraction counterbalanced by an ___________ size of ion
giving ____ attraction.
e.) Draw out period 3 and label on directional arrows explaining the
trends.
Predicting melting points
All this information makes it possible within reason to predict
melting points based on the magnitude of _______ ______ . That is
________ ______ is a very good indicator of the size of melting point
but it should be known that other factors such as the ________ of
ions into the ionic lattice may also need to be considered, which
explain the existence of ______ ________.
f.) Give example pairs of of ionic compound comparisons, where it
would be easy to predict melting point.
For each explain the reason why this is the case.
g.) Give example pairs of of ionic compound comparisons, where it
would be difficult to predict melting point.
For each explain the reason why this is the case.
Factors affecting hydration
Hydration enthalpies are also affected by _______ ______ and
_______ ______ in a similar way to _________ __________.
For example as ionic radius ____________ the attraction between
the ions and the water molecules ____________. This results in the
hydration energy being _____ negative.
Similarly as ionic charge ____________ the attraction with the water
molecules ____________, therefore hydration energy becomes
_____ negative.
h.) Draw a simple labelled diagram to represent both the effect of
ionic and ionic charge on hydration.
Predicting solubility
In order that an ionic compound ____________ in water the
attraction between the ions and the ionic _______ must be
overcome. This requires a quantity of energy equal to the ________
____________. In water molecules the delta ____________ charge of
the ____________ atom is attracted towards positive ions and the
delta positive of charge of the ____________ atoms is attracted
towards the ____________ ions. The water molecules surround the
ions releasing energy equal to ________ ____________.
If the sum of the ________ ____________ is larger than the
magnitude of the ________ ____________, the overall enthalpy
change (the ________ ________ __________) will be _____________
and the compound should dissolve.
However it should be noted that the many compounds with
_____________ and enthalpy changes of solution are soluble so this
does not provide the total picture. This is due to the fact that the
reason the solubility also depends on the _____________ and
another factor called ________ . _______ should be thought of as
the natural tendency for energy to spread out rather than remain
concentrated in one place.
_______ __________
It is generally accepted that ionic compounds have ____ melting
point and ____ boiling points, but _____ _________ have low
melting points that are below 100°C.
_____ _________ conduct electricity, do not vaporise easily and can
dissolve in an amazing range of substances. Chemical structure of
_____ _________ typically contain a complex _________ ___, which
is highly variable in ______ and _________. Ionic liquids can be
made from _________, and are seen as a potential green solvents in
future to replace _________ __________ as solvents.
i.) Carry out an Internet search to find some potential uses for ionic
liquids and their synthesis from biomass.
Missing Words:
Bunsen burner
ionic liquids
packing
solubility
ionic charge
molten
non-polar solvents
hydrogen
biomass
wide
hydration enthalpy/ies
lattice enthalpy/ies
water
lattice energy
furnaces
enthalpy change solution
ionic size
temperature
low
exothermic
endothermic
polar solvents
soluble
increasing/increases/increase
decreasing/decreases/decrease
dissolves
shape
lattice
period
solid
oil-based derivatives
liquids
Entropy
structure
insoluble
high melting points
high
negative
positive
narrow
less
more
oxygen
aqueous
low melting points
high boiling points
gaseous
low boiling points
organic ions
conduct electricity
variable
consistent