Thermodynamics: Lattice Enthalpy Flashcards
What is the symbol for enthalpy change
🔼H
Define enthalpy change
Heat energy transferred in a reaction at constant pressure
Give the symbol for enthalpy change measured under standard conditions
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Why do exothermic reactions have a negative value
Heat energy is given out
Heat energy is absorbed
Endothermic reactions
Define enthalpy change of formation and give its symbol
ENTHALPY CHANGE when 1 MOLE of a compound is formed from its elements in their standard states under standard conditions e.g
• 2C(s) + 3H2(g) + 1/2O2(g) -> C2H5OH(l)
• Symbol- check physical flashcards for answers
Define bond association enthalpy and give its symbol
ENTHALPY CHANGE when all the BONDS OF THE SAME TYPE in 1 MOLE of GASEOUS MOLECULES are broken. e.g Cl2(g) -> 2Cl(g)
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Define enthalpy change of atomisation of an element and give its symbol
Enthalpy change when one mole of gaseous atoms is formed from an element in its standard state e.g
• 1/2Cl2(g) -> Cl(g)
• Symbol- check physical flashcards for answers
Define enthalpy change of atomisation of a compound and give its symbol
Enthalpy change when 1 mole of a compound in its standard state is converted to gaseous atoms e.g
• NaCl(s) -> Na(g) + Cl(g)
• Symbols- check physical flashcards for answers
Define first ionisation enthalpy change and give its symbol
The enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms e.g
• Mg(g) -> Mg+ (g) + e-
• Symbol- check physical flashcards for answers
Define enthalpy change of second ionisation and give its symbol
Enthalpy change when 1 mole of gaseous 2+ ions is formed from 1 mole of gaseous 1+ ions e.g
• Mg+(g) -> Mg2+(g) + e-
• Symbols- check physical flashcards for answers
Define enthalpy change of first electron affinity and give its symbol
Enthalpy change when 1 mole of gaseous 1- ions is formed from one mole of gaseous atoms e.g
• O(g) + e- -> O-(g)
• Symbol- check physical flashcards for answers
Define enthalpy change for second electron affinity
Enthalpy change when 1 mole of gaseous 2- ions is formed from 1 mole of gaseous 1- ions e.g
• O-(g) + e- -> O2-(g)
• Symbol- check physical flashcards for answers
Note
• Ionisation is right
• Affinity is left
Define enthalpy change of hydration and give its symbol
Enthalpy change when 1 mole of aqueous ions is formed from 1 mole of gaseous ions e.g
• Na+(g) -> Na+ (aq)
• Symbol- check physical flashcards for answers
Define enthalpy change of solution and give its symbol
Enthalpy change when 1 mole of solute is dissolved in enough solvent that no further enthalpy change occurs on further dilution. e.g
• NaCl(s) -> NaCl(aq)
• Symbol- Check physical flashcards for answers
What is lattice enthalpy
A measure of ionic bond strength
Define enthalpy lattice of formation and give its symbol
The enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions
symbol- check physical flashcards for answers
What type of process is lattice formation and lattice dissociation?
Lattice formation - exothermic
Lattice dissociation- endothermic
Define enthalpy lattice of dissociation
The enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions under standard conditions
How is lattice enthalpy calculated ?
Using Born-Haber Cycles
Define Hess’s law
The total enthalpy change of a reaction is always the same no matter which route is taken under constant pressure
Why is Born-Haber Cycle used to calculate lattice enthalpy
• Lattice enthalpy can’t be calculated directly
• Born-Haber cycle is used to figure out what the enthalpy would be if you took another, less direct, route
State the stages in the Born-Haber cycle
- Start with the enthalpy of formation
- Then put enthalpy of atomisation and ionisation energy above this
- The electron affinity goes up on the right side
- lattice enthalpy goes down the right side
- Arrows go downwards, upwards and downwards
Note
Reverse the sign of what you’ve got when you go against the arrow
Draw the diagram of a born-Haber cycle of NaCl
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Why are group 2 elements different when drawings Born-Haber Cycle
If the formation of a compound involves ions with charges of more than 1, you just have to add the extra ionisation energies and possibly electron affinities
What is theoretical lattice enthalpy
Calculations based purely on the ionic model of a lattice. The purely ionic model of a lattice assumes that all the ions are spherical and have their charge evenly distributed around them
Why are theoretical lattice enthalpies often different from experimental values
• Most ionic compounds have some covalent character
• Positive ions polarise neighbouring negative ions to different extents
• The more polarisation there is, the more covalent the bonding will be
How can comparing lattice enthalpies tell you ‘how ionic’ an ionic lattice is. Use examples between magnesium and sodium
- The differences between experimental and theoretical lattice enthalpies are much bigger for magnesium halides than sodium halides
- The bonding in magnesium halides is stronger than the ionic model predicts, so the bonds are strongly polarised and have quite a lot of covalent character
- Bonding in sodium halides is similar to the predictions of the ionic model so the compounds are close to being purely ionic
What does dissolving involve?
Enthalpy Changes
What happens when a solid ionic lattice dissolved in water?
• The bonds between the ions break to give free ions-endothermic
• Bonds between the ions and the water are made- exothermic
• The enthalpy change of solution is the overall effect on the enthalpy of these two things
Why do water molecules bind to the ions and how?
• Because oxygen is more electronegative than hydrogen so it draws electrons towards itself creating a dipole
• The dipole means the positively charged hydrogen atoms form bonds with negative ions
• And negatively charged oxygen atoms form bonds with positive ions
Note
• Substances generally only dissolve if the energy released is roughly the same, or greater than the energy taken in
• So soluble substances tend to have exothermic enthalpies of solution
How is enthalpy change of solution calculated?
Using the Born-Haber Cycle
How is the Born-haber cycle used to find the enthalpy change of solution
• Break the lattice into separate gaseous ions and then dissolve the gaseous ions in water
• This forms the lattice dissociation enthalpy and the enthalpies of hydration
• Then construct the Born-Haber Cycle
• In reality, you don’t turn the ions into gases but the net effect is the same so the energy change is the same too
Describe in words how a Born-haber cycle is drawnfor the enthalpy change of solution
- Put the ionic lattice and the dissolved ions on the top, connect them by the enthalpy change of solution. this is the direct route
- Connect the ionic lattice to the gaseous ions by the lattice dissociation enthalpy. The breakdown of the lattice has the opposite enthalpy change to the formation of the lattice
- Connect the gaseous ions to the dissolved ions by the hydration enthalpies of each ion. This completes the indirect route
Work out the enthalpy change of solution for sodium chloride
• Lattice dissociation enthalpy= +787kjmol-1
• Enthalpy of hydration of Na+(g)= -406kjmol-1
• Enthalpy of hydration of Cl-(g)= -364kjmol-1
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Use the Born-Haber cycle to calculate the enthalpy change of solution for SrF2
• Lattice dissociation enthalpy= +2492KJmol-1
• Enthalpy of hydration of Sr2+(g)= -1480KJmol-1
• Enthalpy of hydration of F-(g)= -506kjmol-1
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Define the term electron affinity for chlorine
The enthalpy change/ heat change for the formation of one mole of chloride ions from chlorine ions in the gaseous state
Explain why there is a difference between the hydration enthalpies of the magnesium and sodium ions
• Magnesium ions have a higher charge density than sodium ions
• Mg2+ attracts water more strongly
How is enthalpy change calculated when hydration and lattice of formation values are given
Enthalpy change = 🔼latticefH + (sum of 🔼hydH ions)
Describe the steps you would take to determine an accurate minimum temperature that is not influenced by heat from the surroundings
• Start a clock when the solute is added to water
• Record the temperature every subsequent minute for about 5 minutes
• Plot a graph of temperature vs time
• Extrapolate back to time of mixing = 0 and determine the temperature
Explain why the lattice enthalpy of dissociation magnesium chloride is more different from its theoretical value than calcium
• Magnesium is a smaller ion than calcium
• So it forms stronger ionic bonds
Give the equation formula for the first electron affinity for chlorine
Cl(g) + e- -> Cl-(g)
Explain why the lattice enthalpy of MgCl2 is more exothermic than that of NaBr
• Mg2+ has a higher charge density than Na+ because it has double the charge
• Cl- has a higher charge density than Br- because it has a smaller ionic radius. Strongest electrostatic attraction between ions with the highest charge density
Explain why the second ionisation energy of magnesium is more endothermic than the first ionisation energy
- There is less repulsion between the outer electrons in Mg+
- The outer electron is closer to the nucleus in Mg+
- The outer electron is more strongly attracted to the nucleus in Mg+
Suggest why the second electron affinity of oxygen is endothermic (2)
- O- (g) + e- -> O2-
- There is a repulsion between the O- ion and the electron so energy is required to overcomes this repulsion
Describe how and explain why the enthalpy change of hydration of potassium ions differs from that of lithium ions (2)
- Enthalpy of hydration of potassium ions is less exothermic
- Potassium ions have a larger ionic radius and a lower charge density so weaker electrostatic forces of attraction to water molecules
The likelihood of molecules forming is dependent on what?
Their stability
A stable compound will have what type of reaction?
Exothermic
Describe the difference between a perfect ionic model and not perfect ionic model
Perfect ionic model
- Ions are perfect spheres
- Experimental and theoretical value is similar
- 100% ionic
- purely electrostatic attractions
Not so perfect ionic model
- Big difference between experimental and theoretical values
- Model has some covalent interaction/character
- Polarisation causes covalent character
Write an equation for the process that has an enthalpy change equal to the electron affinity of chlorine
Cl + e- -> Cl-(g)
In terms of electrostatic forces, suggest why the electron affinity for fluorine has a negative value
• There is an attraction between the nucleus/protons and the electrons
• Energy is released when the electron is gained
Explain why the theoretical enthalpy of lattice dissociation for silver fluoride is different from the experimental value that can be calculated using a born haber cycle
• Silver fluoride has some covalent character
• Theoretical lattice enthalpy value assumes a Bly ionic energy interaction
The theoretical enthalpy of lattice dissociation for silver chloride is +770 kjmol-1
Explain why this value is less than the value for silver fluoride
• Attraction between Ag+ and Cl- is weaker than Ag+ and F-
• Chloride ion is larger than fluoride ion
Suggest why the second electron affinity is usually an endothermic process
• The negative ion replies the electron added
Explain why the enthalpy for ionisation energy is sometimes larger than atomisation enthalpies (3)
- Electrons nearer to the nucleus
- Electrons removed from a positive species/ more strongly attracted
- Attraction of the nucleus and electron gives out heat
What is the value of 🔼S when a reaction occurs at room temperature even though it is endothermic
🔼S is positive and large if a gas has evolved
Explain why a reaction may not occur at high temperatures even though the reaction is exothermic
• 🔼S is negative
• More moles of the reactant than the product
• T🔼S is larger than 🔼H and 🔼G is positive
What must be the value of 🔼G for a reaction to occur
🔼G must be less than zero
Note
Change subtraction/addition values in born haber cycle as you go backwards
Give one reason why the bond enthalpy that you calculated in part (c) is different from the mean bond enthalpy quoted in the data book
Data book value is derived from a number of different compounds
Suggest and explain another change to the experiment that would decrease the percentage uncertainty in the use of the same thermometer
• Increase magnitude of temperature change
• By increasing the concentration of the acid/alkali
The enthalpy of lattice formation for caesium iodide in Table 1 is a value obtained by experiment.
The value obtained by calculation using the perfect ionic model is −582 kJ mol−1
Deduce what these values indicate about the bonding in caesium iodide.
Almost/ Mostly purely/ perfectly ionic
Note 2
Answers in 3s.f
Write an equation for the process that has an enthalpy change equal to the electron
affinity of chlorine.
Cl(g) + e- → Cl-(g)
In terms of electrostatic forces, suggest why the electron affinity of fluorine has a
negative value.
- There is an attraction between the nucleus / protons and (the added)
electron(s) - Energy is released (when the electron is gained)
Give the formula for enthalpy change
enthalpy of formation of products - enthalpy of formation of reactants
The temperature used for this manufacture of hydrogen is usually about 1300 K.
Suggest one reason, other than changing the position of equilibrium, why this
temperature is used rather than the value that you calculated in part (b).
To speed up the rate of reaction
Hydrogen can also be obtained by reaction of carbon monoxide with steam.
CO(g) + H2O(g) CO2(g) + H2(g) ΔH = −41 kJ mol−1
, ΔS = −42 J K−1 mol−1
(i) Explain, using a calculation, why this reaction should not occur at 1300 K. (3)
- For reaction to be feasible ΔG must be negative or zero
1 - T when ΔG = 0 = ΔH / ΔS = 976K
1 - ΔS is −ve so ΔG must be +ve at temperatures above 976K / at 1300 K
