Chapter 22 Flashcards
What is lattice enthalpy (ΔlattHꝋ)?
The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.
A measure of the strength of the ionic bonds in a giant ionic lattice.
Is lattice enthalpy endothermic or exothermic?
exothermic, as when ions are combined to form an ionic solid lattice there is an extremely large release of energy. The value will always be negative.
What do we use to measure lattice enthalpy?
The Born- Haber cycle.
What is enthalpy change (ΔH)?
The amount of heat energy transferred during a chemical reaction, at a constant pressure
What is the enthalpy of formation (ΔfHꝋ)?
The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions
Is the enthalpy of formation (ΔfHꝋ) endothermic or exothermic?
The ΔHfꝋ can be endothermic or exothermic as the energy change is the sum of the bonds broken and formed, so the enthalpy change can have positive or negative values.
What is ionisation enthalpy (ΔieHꝋ)?
The amount of energy required to remove an electron from a gaseous atom of an element to form a gaseous ion under standard conditions
Is ionisation enthalpy (ΔieHꝋ) exothermic or endothermic?
Endothermic, energy is needed to overcome the attraction between an electron and the nucleus
What is the first ionisation energy (ΔieHꝋ)?
Energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions.
What is the standard enthalpy change of atomisation (ΔatHꝋ)?
The enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions.
Is the standard enthalpy change of atomisation (ΔatHꝋ) endothermic or exothermic?
Endothermic, energy is always required to break any bonds between the atoms in the element, to break the element into its gaseous atoms
What is the first electron affinity (ΔEAHꝋ)?
The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form 1 mole of gaseous 1- ions.
Do endothermic reactions go up or down in a born haber cycle?
Up, energy of substances increases.
so exothermic reactions go down..
What states do atoms go through in the Born-Haber cycle (in order)?
The cycle shows all the steps needed to turn atoms into gaseous ions and from gaseous ions into the ionic lattice.
What states do atoms go through in the Born-Haber cycle (in order)?
The cycle shows all the steps needed to turn atoms into gaseous ions and from gaseous ions into the ionic lattice.
What is a successive electron affinity and are they endothermic/ exothermic?
When an atom has a charge like Mg2+ it has to go from Mg1+ to that 2+. Successive electron affinities are endothermic, a second electron is being gained by a negative ion (which repels it) so energy must be put in to force it on.
1st electron affinity goes down (exothermic) second goes up (endothermic).
What’s the order of the enthalpy changes in a Born-haber cycle?
1- Enthalpy change of formation
2- Enthalpy change of atomisation (1st element)
3- Enthalpy change of atomisation (2nd element)
4- First ionisation energy
5- First electron affinity
6- lattice enthalpy
How do you calculate lattice energy?
ΔHfꝋ = ΔHatꝋ + ΔHatꝋ + IE + EA + ΔHlattꝋ
rearrange..
ΔHfꝋ = ΔH1ꝋ + ΔHlattꝋ
ΔH1ꝋ = the sum of all of the various enthalpy changes necessary to convert the elements in their standard states to gaseous ions.
so its- ΔHlattꝋ = ΔHfꝋ - ΔH1ꝋ
What is the standard enthalpy change of solution (ΔHsol)?
The enthalpy change when 1 mole of a solute dissolves in a solvent.
If the solvent is water- the ions from the ionic lattice finish surrounded with water molecules.
What is the standard enthalpy change of hydration (ΔHhyd)?
The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions.
What are the two processes that occur when a solid ionic compound dissolves in water?
1- the ionic lattice breaks up and forms separate gaseous ions. (Opposite of lattice energy)
2- The separate gaseous ions interact with polar water molecules to form hydrated aqueous ions.
What are the two key factors that affect lattice enthalpy?
The charge of the ions that make up the crystalline lattice.
The radius of the ions that make up the crystalline lattice.
How does the ionic radius increasing affect lattice energy and why?
The lattice energy becomes less exothermic.
This is because the charge on the ions is more spread out over the ion when the ions are larger.
The ions are also further apart from each other in the lattice. Therefore, the electrostatic forces of attraction between the oppositely charged ions in the lattice are weaker.
How does the ionic charge increasing affect lattice energy and why?
The lattice energy gets more exothermic.
The greater the ionic charge, the higher the charge density.
This results in stronger electrostatic attraction between the oppositely charged ions in the lattice.
What are the two key factors that affect lattice enthalpy of hydration?
It is affected by the amount that the ions are attracted to the molecules and this is controlled by the ionic charge and radius of the atoms.
How does ionic radius decreasing affect the enthalpy change of hydration?
ΔHhydꝋ becomes more exothermic.
Smaller ions have a greater charge density resulting in stronger ion-dipole attractions between the water molecules and the ions in the solution.
Therefore, more energy is released when they become hydrated.
How does ionic charge increasing affect the enthalpy change of hydration?
ΔHhydꝋ is more exothermic.
Ions with large ionic charges have a greater charge density resulting in stronger ion-dipole attractions between the water molecules and the ions in the solution.
So more energy is released when they become hydrated.
What is entropy (S)?
The dispersal of energy within the chemicals making up the chemical system.
The greater the entropy the greater the dispersal of energy and the greater the dispersal.
greater entropy- means that the molecules are more disordered.
What units are used for entropy?
JK-1mol-1
Does entropy increase or decrease when ice melts (solid-> liquid)?
Entropy increases in changed in state that give a more random arrangement of particles.
What is the standard entropy of a substance?
The entropy of one mole of a substance under standard conditions.
How do you calculate entropy?
ΔSꝋ = ΣSproductsꝋ - ΣSreactantsꝋ
Sum of the standard entropy of products - the sum of the standard entropy of reactants.
What does it mean for a reaction to be feasible?
reactions can take place of their own accord, in other words, they are energetically favourable.
What does feasibility take no account of? What is an issue with this?
feasibility takes no account of the rate of reaction and states only what is possible, not what actually happens. A feasible reaction might be incredibly slow, such as the rusting of iron - which may not be helpful.
What equation is used to calculate if a reaction is feasible or not?
The Gibbs equation.
What is the Gibbs equation?
ΔGꝋ = ΔHreactionꝋ - TΔSsystemꝋ
free energy change = enthalpy change with surroundings - temperature in kelvin x entropy change of system.
What is free energy change?
The overall change in energy during a chemical reaction.
What 2 energies is the free energy change made up of?
1- the enthalpy change. (heat transfer between the chemical system and the surroundings)
2- the entropy change at the temperature of the reaction. (the dispersal of energy within the chemical system itself)
For a reaction to be feasible should the free energy change be above or below 0?
There must be a decrease in free energy.
ΔG < 0
What units is ΔH and ΔS shown in?
ΔH- shown in KJmol-1
ΔS- shown in JKmol-1
so the units of ΔS need to be divided by 1000 to match KJ
When ΔGꝋ is negative is the reaction feasible?
Yes
When ΔGꝋ is positive is the reaction feasible?
No
What is a limitation of using Gibs equation
doesn’t account for rate of reaction- the actual rate could be very slow.
