Chapter 13 Flashcards
What is lattice energy a measure of?
Ionic bond strength
What is the definition of standard lattice energy?
Standard lattice energy is the energy change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions
What is the equation for the standard lattice energy of sodium chloride?
Na+(g) + Cl-(g) —> NaCl(s)
What are standard conditions?
298K and 100kPa
What does the more negative the lattice energy mean?
The stronger the bonding
What 2 factors affect lattice energy?
Ionic charge and size
What does the higher the charge on the ions mean about the energy releases when an ionic lattice forms?
The higher the charge on the ion, the more energy released when an ionic lattice forms, due to the stronger electrostatic forces between the ions
Are lattice energies for compounds with 2+ or 2-ions or compounds with 1+ or 1- ions more exthermic?
Compounds with 2+ or 2- are more exothermic
Why does MgCl2 have a higher lattice energy (more negative) than NaCl?
Magnesium has a greater charge than sodium
Why does MgS have a higher lattice energy than MgCl2?
Because both Mg and S have double charges
How does ionic radius affect the size of the lattice energy?
The smaller the ionic radius of the ions involved, the more exothermic (the more negative) the lattice energy is
Why do ions with smaller ionic radius have larger lattice energy values?
Smaller ions have higher charge density and their smaller ions can pack more closely together
What does Hess’s law say?
The total enthalpy change of a reaction is always the same, no matter the route taken
What is Hess’s law also known as?
The conservation of energy
As you can’t calculate lattice energy directly, what do you have to use in order to calculate it?
A born-Haber cycle
What can a Born-Haber cycle be used to calculate?
Lattice energies
What does a Born-Haber cycle start with on the LHS?
The enthalpy of formation
What is the enthalpy of formation?
The change of enthalpy during the formation of 1 mole of the substance from its constituent elements, with all substances in their standard states.
What direction does the arrow for the enthalpy of formation face in a Born-Haber cycle?
Downward
After the enthalpy of formation, what comes next in a Born-Haber cycle?
The enthalpy changes of atomisation and ionisation
In a Born-Haber cycle, what direction do the enthalpy changes of atomisation and ionisation face?
Upwards
What is the definition of the enthalpy change of atomisation?
The enthalpy change of atomisation is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state
What is the equation for the atomisation of sodium?
Na(s) —>Na(g)
What is the equation for the atomisation of chlorine?
1/2 Cl2(g) —> Cl(g)
What is the equation for the first ionisation energy of sodium?
Na(g) —> Na+(g) + e-
After the enthalpies of atomisation and ionisation, what comes next in a Born-Haber cycle?
The electron affinity
What is the equation for the first electron affinity of chlorine?
Cl(g)+e- –> Cl-(g)-
What direction does electron affinity face?
Downwards
After the electron affinity, what comes next in a Born-Haber cycle?
The lattice energy
What is the definition of electron affinity?
The amount of energy released when an electron is attached to a neutral atom or molecule in the gaseous state to form a negative ion
When dealing with a Born-Haber cycle containing a G2 element, why do you times the value of the atomisation enthalpy by 2?
Because there are 2 moles of the G7 ion in each mole of the compound
When dealing with a Born-Haber cycle containing a G2 element, why do you have to include the second ionisation energy?
Because group 2 elements form 2+ ions
When dealing with a Born-Haber cycle containing a G2 element, why do you times the electron affinity by 2?
As 2 electrons are being added to the G7 element during the electron affinity
What does a large, negative lattice energy mean in terms of bond strength?
The more negative the value, the stronger the bonding
What is the experimental way of calculating Lattice energies?
Using experimental enthalpy values in a Born-Haber cycle
What is the theoretical way of calculating Lattice energies?
Doing some calculations based on purely the ionic model of a lattice
To work out a theoretical lattice energy, what 2 things do we assume?
- Ions are spherical
- Ions have their charge evenly distributed
What can comparing theoretical and experimental lattice energies tell you?
How ionic an ionic lattice is
If the experimental and theoretical values for lattice energies are a pretty close match, what does this tell you, and what does this tell you about the structure?
It tells you that these compounds fit the ‘purely ionic’ model very well. The structure of the lattice is close to being purely ionic
If the experimental and theoretical values for lattice energies are a quite far apart, what does this tell you?
The compound isn’t that close to being ‘purely ionic’
What are close to being ‘purely ionic’ out of soidum halides and magnesium halides?
Sodium halides
As magnesium halides aren’t as close to being ‘purely ionic’ as sodium halides are, what does this tell you?
They have some covalent character, whereas sodium halides have almost no polarisation and little covalent character
What does polarisation of ionic bonds lead to?
Covalent character in ionic lattices
What is the equation of charge density?
Charge density = Charge / Volume
In a sodium halide, as the cation(Na+) only has a small charge what does this mean?
It can’t really pull electrons from the anion towards itself, so the charge is evenly distributed around the ions (almost no polarisation).
What is an anion?
A negatively charged ion
What is a cation?
A positively charged ion
Why do the theoretical calculations for the lattice energies of sodium halides fit so well with the experimental values?
Sodium ion can’t really pull electrons from the halide ion (charge of only 1+) so there is almost no polarisation as the charge is evenly distributed across the compound
In a magnesium halide, as the cation (Mg2+) has a large charge, what does this mean?
Mg2+ has a larger charge than the halide, so it is able to pull electrons from it towards itself a bit, polarising the bond
What is the relationship between the charge density of the cation and how well matched the theoretical and experimental values of the lattice energies are?
The greater the charge density of the cation, the poorer the values will match
What is polarisation?
When the positive charge on the cation attracts electrons towards it from the anion
Why ae small cations with a very high charge very polarising?
They have a high charge density, so they can pull electrons towards itself
Why are large anions with a high charge polarised more easily than smaller ones with a lower charge?
Because their electrons are further away from the nucleus and there is more repulsion between the electrons, so can be pulled away more easily to the cations
If a compound contains a cation with a high polarising ability, and an anion which is easily polarised, what can happen?
Some of the anion’s electron charge cloud will be dragged towards the positive cation
If a compound is polarised enough, what can be formed?
A partially covalent bond
What happens if an ionic bond is polarised more and more?
The more the bond is polarised, the more covalent character it gains.
What happens if an ionic bond starts to gain more covalent characters?
It results in compounds with different properties to those with only ionic bonds
Is Na+Cl- ionic or covalent?
Ionic
Is Mg2+Cl- ionic or covalent?
It is mostly ionic
Is Al3+Cl- ionic or covalent?
Mostly covalent
Is Si4+Cl- ionic or covalent?
Covalent
What is the Pauling Scale used to measure?
The electronegativity of an atom
What is electronegativity?
The ability of an atom to attract the bonding electrons in a covalent bond
For what value are bonds polar?
Bonds are polar when the difference in Pauling electronegativity values are more than about 0.4
Predict whether a C-Cl bond will be polar, given that the pauling electronegativity values of carbon and chlorine are 2.5 and 3.0:
3-2.5=0.5, so the bond will be slightly polar with the chlorine atom having a slight negative charge and the carbon atom having a slight positive charge
When an ionic lattice dissolves in water, what tow things happen?
- The bonds between the ions break
- Bonds between the ions and water are made
When an ionic lattice dissolves in water and bonds between the ions break, is this an endothermic or exothermic process?
It is exothermic
When an ionic lattice dissolves in water and bonds are formed between the ions and water molecules, is this an exothermic or endothermic process?
It is an exothermic process
When hydrated ions ae formed, why do the water molecules surround the positive charge?
The oxygen is slightly negative, so it is attracted to the cation
When hydrated ions ae formed, why do the water molecules surround the neagtive charge?
The hydrogen atoms are slightly positive, so are attracted to the anion
What is the enthalpy change of hydration?
The enthalpy change when 1 mole of gaseous ions dissolve in water
What is the enthalpy change of solution?
The enthalpy change when 1 mole of a solute dissolves in water
What is the equation for the enthalpy change of solution for AgCl(s)?
AgCl(s) —> Ag+(aq) + Cl-(aq)
What is the equation for the enthalpy change of hydration for Ag+(g) + Cl-(g)?
Ag+(g) + Cl-(g) –> Ag+(aq+ + Cl-(aq)
When do substances generally dissolve?
When the energy released is roughly the same, or greater than the energy taken in
Do soluble substance tend to have endo or exo enthalpies of solution?
Exothermic
Describe how the enthalpy change of solution can be calculated:
- Put ionic lattice and dissolved ions at the top, connected by enthalpy change of solution
- Connect ionic lattice to the gaseous ions by lattice energy
- Connect gaseous ions to dissolved ions by hydration enthalpies
What 2 factors affect the enthalpy of hydration?
Ionic charge and ionic radius
Why do ions with a higher charge have a greater enthalpy of hydration?
Ions with a higher charge are better at atttacting water molecules than those with lower charges , so the electrostatic attraction is greater, so more energy is required to break the bonds, giving them a more exothermic enthalpy of hydration
Why do smaller ions have a greater enthalpy of hydration?
They have higher charge densities so they attract water molecules more easily and have a more exothermic enthalpy of hydration
How many more water molecules surround a cation with a 2+ charge compared to a cation with a 1+ charge?
Double the amount
What will have a more exothermic enthalpy of hydration, magnesium or sodium?
Magnesium as it is smaller and has a higher charge
What is entropy a measure of?
The disorder of a system- it tells you the number of ways particle can be arranged and the number of ways that energy can be shared between the particles
What happens as the entropy increases?
The more disordered the particles are
What does a large, positive value of entropy show?
High levels of disorder
What are the three main things that affect entropy?
- Physical state
- Dissolving
- Number of particles
Why do solids have the lowest entropy?
Solid particles wobble about a fixed point, so there is hardly any randomness, so they have the lowest entropy
Why do gas particles have the highest entropy?
They move around wherever they like, so they have the most random arrangements of particles, so they have the most entropy
If magnesium (s) reacts with Oxygen(g) to form solid magnesium oxide, what happens to the levels of entropy?
The levels of entropy lower
Why does dissolving a solid increase its entropy?
Dissolved particles can move more freely as they are no longer in place
Fit solids, liquids and gases into random, some disorder and ordered:
Solids are ordered
Liquid have some disorder
Gases are random
Why do more particles mean more entropy?
The more particles you have, the more ways they and their energy can be arranged
In the reaction N2O4(g) –> 2NO2(g), why does the entropy increase?
Because the number of moles has increased
When are substances more energetically stable?
When there’s more disorder
What will particles move to try and increase?
Their entropy, so that they are more energetically stable
Why are some reaction feasible when their energy change is exothermic?
An increase in entropy overcomes the change in enthalpy
What are the units of entropy?
JK-1mol-1
What does delta S system represent?
Entropy change
What does delta S sytstem equal?
S products - S reactants
What is S products?
The entropy of the products
What is S reactants?
The entropy of reactants
Why does the entropy of the surroundings change aswell?
Because energy is transferred to or out from the system
What is the total entropy change a sum of?
The entropy changes of the system and surroundings
What does delta S total equal?
Delta S system + Delta S surrounding
What does delta S system equal?
- deltaH / T
What is delta H?
Enthalpy change
What is delta G?
The free energy change
For feasible reactions, what must delta G be?
Negative or zero
What does delta G equal?
delta H -T(delta S system)
As when delta G is zero or negative the reaction is feasible, what equations allows us to calculate at which temperature the reaction is feasible?
T = delta H / delta S system
What is the equilibrium constant a measure of?
The ratio of the concentration of products to reactants at equilibria for a reversible reaction
What must the value for the equilibrium constant be for reaction with delta G <0?
Greater than 1
What must the value for the equilibrium constant be for reaction with delta G > 0?
Smaller than 1
What equation links free energy change and the equilibrium constant?
delta G = -RT lnk
Why doesn’t negative delta G always guarantee a reaction?
The value of the free energy change doesn’t tell you anything about the rate, and the activation energy may be extremely high
