Chapter 22 - Enthalpy and entropy Flashcards
What is lattice enthalpy?
Enthalpy change that accompanies the formation of 1 mole of an ionic compound from its gaseous ions under standard conditions.
Write the equation for lattice enthalpy for KCl
K+ (g) + Cl- (g) -> KCl(s)
Is lattice enthalpy endothermic or exothermic? Why?
- Exothermic as bonds are being made
What does Hess law state?
Total enthalpy change of a reaction is always the same, no matter which route is taken
What energy cycle is used to calculate lattice enthalpy?
Born - haber cycle
Define the standard enthalpy of formation (delta fH standard conditions)?
Enthalpy change that takes place when one mole of a compound is formed from it’s elements under standard conditions, with all reactants and products in their standard states
Write the equation for standard enthalpy change of formation for NaCl
Na(s) + 1/2 Cl2 (g) -> NaCl(s)
- reactants in standard states
- 1 mole of NaCl formed
Define standard enthalpy change of atomisation? (delta at H standard conditions)
enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions
Write equation for standard enthalpy change of atomisation for Na(s).
Na(s) -> Na(g)
Is standard enthalpy change of atomisation exothermic or endothermic? Why?
Endothermic as bonds are broken to form gaseous atoms
What is standard enthalpy change of atomisation for the element when it is a gas in it’s standard state?
Related to the bond enthalpy of the bond being broken
Define enthalpy change of first ionisation energy (delta IE H standard conditions)?
enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions
Are ionisation energies endothermic or exothermic? Why?
Endothermic as energy is required to overcome attraction between a negative electron and the positive nucleus.
What is the enthalpy change of first electron affinity? (delta EA H standard conditions)
enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
Are first electron affinities exothermic or endothermic?
Exothermic because the electron being added is attracted in towards the nucleus
Draw a born-haber cycle
Are second electron affinities endothermic or exothermic? Why?
Endothermic as energy must be put into force the negatively-charged electron onto the negative ion
Draw the section of a born-haber cycle of the 2 electron affinities of oxygen
What happens when slat dissolves in water?
water molecules break up the giant ionic lattice structure and overcome the strong electrostatic attractions
Define the standard enthalpy change of solution? (delta sol H standard conditions)
enthalpy change that takes place when one mole of a solute dissolves in a solvent
Write an equation for the enthalpy change of solution of sodium chloride in water
Na+Cl- (s) + aq -> Na+(aq) +Cl-(aq)
What is the enthalpy change of hydration? (delta hyd H)
enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions
Write enthalpy changes of hydration equations for dissolving of NaCl(s)
Na+ +aq -> Na+(aq)
Cl-(g) + aq -> Cl- (aq)
gaseous ions -> aqueous ions
Is the enthalpy change of solution endothermic or exothermic?
Can be either depending on relative sizes of the lattice enthalpy and the enthalpy changes of hydration
Draw enthalpy cycle of dissolving of an ionic compound in water
Look at paper flashcards
How do you calculate enthalpy change of solution for dissolving of an ionic compound in water?
Enthalpy change of hydration
What are the factors affecting lattice enthalpy?
- Ionic charge
- Ionic radius
What is the effect of ionic charge on lattice enthalpy?
Higher the charge, the stronger the electrostatic attractions between the ions (greater nuclear attraction), so more energy released when a ionic lattice forms - more negative/exothermic
What is the effect of ionic radius on lattice enthalpy?
Smaller the ionic radius, the higher the charge density of the ion. This means the electrostatic attraction between the ions is greater (greater nuclear attraction) so the lattice enthalpy is more negative/ exothermic.
What are the factors affecting enthalpy of hydration?
- Ionic charge
- Ionic radius
What is the effect of ionic charge on enthalpy of hydration?
- Ions with greater charge have greater enthalpy of hydration.
- Ions with higher charge are better at attracting water molecules. More energy released when bonds are made, giving them more exothermic enthalpy of hydration.
What is the effect of ionic radius on enthalpy of hydration?
Smaller ions have a greater enthalpy of hydration. Smaller ions have a higher charge density than bigger ions. They attract water molecules better and have a more exothermic enthalpy of hydration.
What is entropy?
Measure of the dispersal of energy in a system. Increases as disorder increases
Why do particles move to try to increase entropy?
Substances are more energetically stable when there’s more disorder
What 3 things affect entropy?
- Physical state
- Dissolution
- Number of gaseous particles
Which physical state has the highest entropy?
Gases as they have the most disordered arrangements. More ways of arranging energy
What does dissolving a solid do to it’s entropy?
Increases it’s entropy as dissolved particles can move freely as they’re no longer held in place
What does more (moles of) gaseous particles mean for it’s entropy?
Increases entropy. More (moles of) gaseous particles you’ve got, the more ways they and their energy can be arranged
Less order and more randomness
What is standard entropy, S (standard conditions), of a substance?
entropy of one mole of a substance, under standard conditions
How do you calculate entropy change of a system, delta s?
delta s = S(products) - S(reactants)
units: JK^-1mol^-1
- Most reactions don’t happen unless delta s is positive
What is free energy change, delta G?
A measure used to predict whether a reaction is feasible (will carry on to completion, without any energy being supplied to it.
What’s the Gibbs equation fro free energy change, delta G?
delta G = delta H - T x delta S
unit:(Jmol^-1)
How can you tell if a reaction is feasible from delta G?
- If delta G is negative then the reaction is likely feasible.
- But not if: - really high activation energy or so slow you don’t notice if happening at all.
If a reaction is exothermic (negative delta H) and has a positive entropy change (delta S) then is the reaction feasible?
Yes, as delta G is negative
If a reaction is endothermic and delta S is positive, then is the reaction feasible?
Yes, above a certain temperature
What’s the equation for the temperature at which a reaction becomes feasible (in K)?
T = delta H / delta S
Why is the entropy of water 0 at 0K?
Particles at maximum state of order
Why can a reaction take place spontaneously?
Delta H is less than T x delta S
