Chapter 22: Enthalpy Flashcards
What is lattice enthalpy, and its symbol?
- The enthalpy change when 1 mole of an ionic lattice is formed from its gaseous ions
- ∆LEH
Write the equation for lattice enthalpy of potassium chloride, and is this enthalpy change endothermic or exothermic, and why?
- K+ (g) + Cl- (g) -> KCl (s)
- Exothermic as bonds are being formed
What do lattice enthalpy values show?
- The more exothermic the lattice enthalpy, the stronger the ionic bonding in a giant ionic lattice
How is lattice enthalpy found?
- It cannot be measured directly (heating would give gaseous ion pairs), so it is calculated indirectly using a type of Hess cycles called Born-Haber cycles, which break lattice enthalpy up into different steps
Generally describe the steps lattice enthalpy can be broken into.
- Splitting the ionic compound into its elements in their standard states
- Turning the elements in their standard states to gaseous atoms
- Turning the metal atom into a cation
- Turning the non-metal atom into an anion
What is the first step known as, and what is its definition and symbol?
- The standard enthalpy of formation
- The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
- ΔfH⦵
Write the equation for the enthalpy of formation of potassium chloride, and is it endothermic or exothermic, and why?
- K (s) + 1/2 Cl2 (g) -> KCl (s)
- It is exothermic for salts
What is the second step known as, and what is its definition and symbol?
- The standard enthalpy of atomisation
- The enthalpy change that takes place when 1 mole of gaseous atoms is formed from the element in its standard state
- ΔatH⦵
Write the equations for the enthalpy of atomisation of potassium chloride, and are values for this enthalpy change endothermic or exothermic, and why?
- K (s) -> K (g)
- 1/2 Cl2 (g) -> Cl (g)
- Endothermic because bonds are broken to form gaseous atoms
What is the third step known as, and what is its definition and symbol?
- Ionisation energy
- First ionisation energy: the energy required for the removal of 1 mole of electrons from 1 mole of gaseous atoms of an element
- ΔIE1H⦵
Write the equation for the ionisation energy in potassium chloride, and are values for this enthalpy change endothermic or exothermic, and why?
- K (g) -> K+ (g) + e-
- Endothermic because energy is required to overcome the attraction between the positive nucleus and the negative electron
What is the fourth step known as, and what is its definition and symbol?
- Electron affinity
- First electron affinity: the enthalpy change when 1 electron is added to each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1- ions
- ΔEA1H⦵
Write the equation for the electron affinity in potassium chloride.
- Cl (g) + e- -> Cl- (g)
Are electron affinities endothermic or exothermic, and why?
- The first electron affinity is exothermic as the first electron is attracted by the nucleus more strongly than it is repelled by the electrons
- After this, the negative ion repels electrons, so the second and successive electron affinities are endothermic
How are these other enthalpy changes used to calculate lattice enthalpy? Give 8 details.
- You draw a Born-Haber cycle with:
- the cations, electrons and non-metal atoms at the top
- the ionic solid at the bottom
- on the right is the lattice enthalpy
- on the left are all the other enthalpies with the exception of electron affinities
- arrows point down for exothermic energy changes
- arrows point up for endothermic energy changes
- because of this, electron affinities have to go on the right above lattice enthalpy
- Lattice enthalpy = the total of all of the other enthalpy changes (follow the arrows)
What are 4 things to be careful of when using Born-Haber cycles?
- If you have multiples of the atoms/ ions, do not forget to multiply the enthalpy changes
- Remember to show all of the ionisation energies where there are more than one
- If given values of bond dissociation enthalpy instead of enthalpy change of atomisation, remember to adapt for 1 mole of product
- Second electron affinities
What limitation is there on the use of average bond enthalpy to find the enthalpy change of atomisation?
- It only works if the standard state of the element is gaseous and the element has covalent bonds
- This is what the definition of average bond enthalpy specifies
How are Born-Haber cycles drawn when there are 2 electron affinities?
- The ions are the highest level on the right
- The first electron affinity is shown below this
- Between the 2 levels on the left is the cation, electrons and non-metal atom
- This way, there is an arrow down for the first electron affinity (exothermic) and an arrow up for the second (endothermic)
What is another way of calculating lattice enthalpy?
- Using the enthalpy changes of solution and hydration
What is the definition of the standard enthalpy change of solution and its symbol?
- The enthalpy change that takes place when 1 mole of a solute dissolves in a solvent
- ΔsolH⦵
What is the definition of the standard enthalpy change of hydration and its symbol?
- The enthalpy change that takes place when 1 mole of gaseous ions forms hydrated ions
- ΔhydH⦵
What happens when ionic compounds dissolve in water?
- The lattice breaks up
- The ions are separated and react with polar water molecules to form hydrated ions
- These hydrated ions are held together by ion-dipole forces
Write the equation for the enthalpy of solution of sodium chloride, and is it endothermic or exothermic, and why?
- NaCl (s) -> Na+ (aq) + Cl- (aq)
- It can be either endothermic or exothermic as the lattice breaks up apart (endothermic) and ion-dipole forces form (exothermic), and the sum of these gives the enthalpy of solution
Write the equations for the enthalpy of hydration of sodium chloride, and is it endothermic or exothermic, and why?
- Na+ (g) -> Na+ (aq)
- Cl- (g) -> Cl- (aq)
- It is exothermic since ion-dipole forces are being formed
How are the enthalpies of solution and hydration used to find lattice enthalpy?
- In a Hess cycle
- Using an energy level diagram
How would you use a Hess cycle to find lattice enthalpy if given the values for the enthalpies of hydration and solution?
- Lattice enthalpy would go at the top
- The aqueous ions would go at the bottom at the bottom
- As a result, the enthalpy of hydration would be on the left, and the enthalpy of solution on the right
- Both equations of hydration would be combined into one step
What decides which way energy level diagrams to find lattice enthalpy from the enthalpies of solution and hydration should be drawn?
- Whether the enthalpy of solution is endothermic or exothermic
What do all energy level diagrams have in common?
- The gaseous ions go at the top
- Lattice enthalpy goes on the left and is exothermic (arrow goes down)
- The enthalpies of hydration go on the right (order doesn’t matter) and are also exothermic (arrows go down)
How are energy level diagrams drawn when the enthalpy of solution is endothermic?
- The ionic solid is at the bottom
- The enthalpy of solution goes on the right and is the final step
- It is endothermic, and therefore points up
How are energy level diagrams drawn when the enthalpy of solution is exothermic?
- The ionic solid is on the left
- The aqueous ions are at the bottom
- The enthalpy of solution is on the left so that it can point down without clashing
What factors affect lattice enthalpy?
- Ionic radius
- Ionic charge
How does ionic radius affect lattice enthalpy?
- As the radius increases:
- the attraction between the ions decreases
- therefore the lattice enthalpy becomes less exothermic
How does ionic charge affect lattice enthalpy?
- As the charge of ions increases:
- the attraction between the ions increases
- the lattice enthalpy becomes more exothermic
What does it mean if a compound has a more exothermic lattice enthalpy?
- The more exothermic the lattice enthalpy of an ionic compound, the higher its melting point is
What factors affect enthalpy of hydration?
- Ionic radius
- Ionic charge
How does ionic radius affect enthalpy of hydration?
- As the radius increases:
- the attraction between the ion and the (polar) water molecules decreases
- the enthalpy of hydration becomes less exothermic
How does ionic charge affect enthalpy of hydration?
- As the ionic charge increases:
- the attraction with the water molecules increases
- the enthalpy of hydration becomes more exothermic
How does the enthalpy of hydration of a compound help determine its solubility?
- To dissolve ionic compounds, the attraction between the ions has to be overcome
- This therefore requires a quantity of energy equal to the lattice enthalpy
- If the sum of the enthalpies of hydration is greater than or equal to the lattice enthalpy, this indicates that the compound is soluble
