Y13 Enthalpy and entropy Flashcards
Lattice enthalpy definition
The enthalpy change when ONE MOLE of an ionic compound is formed from its GASEOUS ions under standard conditions
True or false: lattice enthalpies are always positive
False (energy is released when the ionic bonds are made = exothermic)
Why does LiF (-1038) have a lower enthalpy than KF (-821)?
(Charge is the same)
Li+ has a smaller ionic radius than K+
Li+ has a stronger attraction to F-
More energy is released when Li+ attracts F-
Lattice enthalpy is more negative
What are the 4 marking points for a Born-Haber cycle?
Species must be on the lines (only 1 thing changes at a time)
State symbols
Arrows must be labelled and point in the correct direction (up = endothermic, down = exothermic)
Show calculation
Enthalpy change of formation definition
Enthalpy change when one mole of a compound is formed from its constituent elements in their standard states = exothermic
Enthalpy change of atomisation
Enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state e.g. Na(s) –> Na(g) = endothermic
First electron affinity definition
Enthalpy change when 1 mole of gaseous atoms each gain 1 electron e.g. Cl(g) + e- –> Cl-(g) = exothermic
First ionisation enthalpy definition
Enthalpy change when 1 mole of gaseous 1+ ions are formed from 1 mole of gaseous atoms e.g. Na(g) –> Na+(g) + e- = endothermic
Why is second electron affinity endothermic instead of exothermic?
More energy is required to overcome the repulsion between the negatively charged e- and negative ion
Enthalpy change of solution definition
Enthalpy change when ONE MOLE of a solid dissolves in a solvent under standard conditions e.g. NaCl(s) + aq –> Na+(aq) + Cl-(aq)
When calculating △solH, how do you work out m in the q=mc△T equation?
Mass of water AND solid
Is △solH endo or exothermic?
Can be either
When is △solH exothermic?
When the magnitude of the enthalpy change of hydrations is GREATER than the magnitude of the lattice enthalpy
When is △solH endothermic?
When the magnitude of the enthalpy of hydrations is LESS than the magnitude of the lattice enthalpy
Enthalpy of hydration definition
Enthalpy change when GASEOUS ions dissolve in water to form ONE MOLE of AQUEOUS ions e.g. Na+(g) + aq –> Na+(aq) =exothermic
Compare the enthalpies of hydration of Mg2+ and Na+
Mg2+ has a higher charge than Na+
Mg2+ have a smaller ionic radius than Na+
Mg2+ will have a stronger attraction to WATER MOLECULES
△hydH of Mg2+ would be more negative
What is the symbol and units of entropy?
Symbol: S, Units: J K-1 mol-1
Why is the entropy of water 0 at 0K?
Particles have no energy so no disorder
When is △S positive and negative?
If a reaction leads to MORE order = NEGATIVE △S
If a reaction leads to LESS order = POSITIVE △S
What affects entropy?
No. moles of gas
2Na(s) + Cl2(g) –> 2NaCl(s) Will this change in entropy be positive or negative?
Entropy will decrease because the number of moles of gas decreases which increases order
What is the equation for calculating △S?
△S = (sum of S of products) - (sum of S of reactants)
What is the equation for calculating △G (feasibility)?
△G = △H - T△S
What are the unit conversions when calculating △G?
Temperature (K): °C –> K = +273
△S (kJ K-1 mol-1): J K-1 mol-1 –> kj K-1 mol-1 = /1000
What must be the case for a reaction to be feasible?
△G must be < 0 / △G must be negative
What is the equation for calculating the minimum temperature a reaction is feasible at?
T = △H / △S
When is a reaction feasible when △H is negative and △S is positive?
△G is always negative so the reaction is always feasible at all temperatures
When is a reaction feasible when △H is positive and △S is negative?
△G is always positive so the reaction is never feasible at any temperature
When is a reaction feasible when both △H and △S are negative?
Reaction is only feasible at LOW temperatures
When is a reaction feasible when both △H and △S are positive?
Reaction is only feasible at HIGH temperatures
How does an increase in temperature affect feasibility when both △H and △S are negative?
△G = △H - T△S
For the reaction to be feasible, △G must be negative
When both △H and △S are negative, the reaction is feasible at high temperatures
As T increases, T△S becomes more positive
Eventually T△S will have a greater magnitude than △H, leading to a negative △G value
Feasibility increases as T increases
What are the 2 limitations of predicting feasibility using △G?
Reactions may have a:
1. Large activation energy
2. Slow rate
