22 - Enthalpy And Entropy Flashcards
What does lattice enthalpy measure
Ionic bond strength
- energy given out when gaseous ions combine to make a solid lattice
Standard lattice enthalpy definition
The enthalpy change when 1 mole of an ionic lattice is formed from its gaseous ions under standard conditions
What are standard conditions
298k
25c
100kpa
The more negative the lattice enthalpy the…
Stronger the bonding
Standard enthalpy change of formation definition
The enthalpy change that takes place when one mole of a compound is formed from its elements in their standard states under standard conditions
enthalpy change of formation equation
The normal reaction equation
Elements in standard state - > solid compound
Standard enthalpy change of atomisation definition
The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions
enthalpy change of atomisation
Is it endothermic or exothermic
Endothermic - because bonds are broken to form gaseous atoms
Elements in standard state -> gaseous atoms
First ionisation energy
The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ atoms
Gaseous atom -> gaseous ion
First ionisation
Endo or exo
Endo
Energy required to overcome attraction between a negative electron and the positive nucleus
First electron affinity definition
The 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
First electron affinity
Endo or exo
Exo
- the electron added is attracted in towards the nucleus
Why is the second successive electron affinity endothermic
A second electron is gained by a negative ion, energy is need to force the electron onto the negative ion, due to repulsion
Enthalpy change of solution definition
The enthalpy change that takes place when one mole of a solute dissolves in a solvent under standard conditions
Ionic compound + aq -> aqueous ions
What is the experimental determination of the enthalpy change of solution
Procédure
1) weigh out sample of KCl
2) pour 25cm3 of distilled water into a plastic cup in a beaker
3) measure initial temp
4) tip all of KCl in cup - stir till dissolved, temp is constant
5) record temp to nearest 0.5c
What is the experimental determination of the enthalpy change of solution
Calculations
1) use q=mct C= 4.18 Mass of water - (1gcm-3 x volume used) Mass of solution - (water + solid) - energy = kJ
2)find moles of solid compound
3) calculate enthalpy change in solution
- 0.05 mol of KCl gains 278kj energy from solution - 1 mol would gain
Energy / mol
What is the dissolving process steps
1) the bonds between the ions break to give gaseous ions - endo = Lattice enthalpy
2) the separate gaseous ions interact with polar water to form hydrated aqueous ions
= enthalpy change of hydration
Enthalpy change of hydration definition
Enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions
Enthalpy change of hydration
Équation
Can be exo or endo - depending on relative size of LE
Gaseous ions + aq -> aqueous ions
List 2 factors affecting Lattice enthalpy
Ionic size
Ionic charge
Affect of ionic charge on LE
- ionic radius increases
- lower charge density
- ions further apart in lattice
- attraction between ions decrease
- lattice enthalpy less negative
- melting point decreases
Effect of ionic charge on LE
- ionic charge increases
- more energy released when ionic lattice forms
- attraction between ions increases
- LE becomes more negative
- mp increases
List factors affecting hydration
Ionic size
Ionic charge
Ionic size affect on hydration enthalpy
- ionic radius increases
- lower charge density
- attraction between ion and water molecules decreases
- hydration enthalpy is less negative
Ionic charge on hydration enthalpy
- ionic charge increases
- attraction with water molecules increases - stronger electro station attraction
- more energy released when the bonds are made
- hydration enthalpy more negative
Entropy definition
The dispersal of energy and disorder within chemicals making up the chemical system
The higher the entropy (positive) …
The more disordered a system
What is entropy the measure of
The number of ways that particles can be arranged
The number of ways that the energy can be shared out between the particles
How physical state affects entropy
Solid -> liquid -> gas
Gases:
Randomness increases, most disorder, have the greatest entropy
Why more particles means more entropy
The more particles
The more ways their energy can be arranged
Entropy increases because the number of moles increases
Energy is spread out more
Why are some reactions feasible
Substances like disorder, they’re more stable
Particles move to increase their entropy
What does thermodynamically stable mean
When a substance reaches its maximum entropy state
Won’t react further without energy input
How to calculate entropy change
Change in S =
S of products - S reactants
What does a positive entropy mean
The reaction is likely to be feasible
React without the addition of energy
Why does negative free energy not guarantee a reaction
Free energy change doesn’t tell you about rate of reaction
It might have a high activation energy or happen very slowly
What is free energy
The overall change in energy during a chemical reaction
Tells you if a reaction is feasible or not
What 3 factors determine whether a process will take place
Température - kelvin
Entropy - j K-1mol-1= convert by dividing by a thousand
Enthalpy - kjmol-1
Free energy change equation
G = H - tS
What two types of energy make up free energy change
The enthalpy change
- heat transfer between the chemical system and the surroundings
Entropy change at temperature of reaction
- dispersal of energy within chemical system
Limitations of predictions made for feasibility
- high activation energy
- slow rate of reaction
