Enthalpy, Entropy and Free Energy Flashcards
What is enthalpy change?
The heat energy transferred in a reaction at constant pressure
What are the units of enthalpy change, ΔH?
kJ mol^-1
What are standard conditions?
1 atm (1000 kPa) of pressure and 298K (25°C)
Is -ve ΔH endothermic or exothermic?
Exothermic
Is +ve ΔH endothermic or exothermic?
Endothermic
Why is the ΔH value a negative number for exothermic?
Because heat energy is given out (the chemicals lose energy)
Why is the ΔH value a positive number for endothermic?
Because heat energy is absorbed (the chemicals gain energy)
What is enthalpy change of formation?
The enthalpy change when 1 mole of a compound is formed from its elements
What is enthalpy change of atomisation of an element?
The enthalpy change when 1 mole of gaseous atoms is formed from an element
Give an example of a reaction that has enthalpy change of formation
Cl2 (g) + Ca (s) —> CaCl2 (s)
Give an example of a reaction that has enthalpy change of atomisation of an element
1/2Cl2 (g) —> Cl (g)
What is enthalpy change of atomisation of a compound?
The enthalpy change when 1 mole of a compound is converted to gaseous atoms
Give an example of a reaction that has enthalpy change of atomisation of a compound
NaCl (s) —> Na (g) + Cl (g)
What is the first ionisation energy?
The energy needed to change 1 mole of gaseous atoms into 1 mole of gaseous 1+ ions
Write an equation representing the first ionisation energy of Mg
Mg (g) —> Mg+ (g) + e-
What is the second ionisation energy?
The energy needed to change 1 mole of gaseous 1+ ions into 1 mole of gaseous 2+ ions
Write an equation representing the second ionisation energy of Mg
Mg+ (g) —> Mg2+ (g) + e-
What is the first electron affinity?
The energy needed to change 1 mole of gaseous atoms into 1 mole of gaseous 1- ions
Write an equation for the first electron affinity of O
O (g) + e- —> O-
What is the second electron affinity?
The energy needed to change 1 mole of gaseous 1- ions into 1 mole of gaseous 2- ions
Write an equation for the second electron affinity of O
O- + e- —> O 2-
What is lattice enthalpy?
The enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions
Write the reaction of Na+ and Cl- that has lattice enthalpy
Na+ (g) + Cl- (g) —> NaCl (s)
What is enthalpy change of hydration?
The enthalpy change when 1 mole of gaseous ions is dissolved in water
Write the reaction of Na+ that has enthalpy change of hydration
Na+ (g) —> Na+ (aq)
What is enthalpy change of solution?
The enthalpy change when 1 mole of solute is dissolved in a solvent (e.g. water)
Write the reaction of NaCl that has enthalpy change of solution
NaCl (s) —> NaCl (aq)
What do ionic compounds form?
Regular structures called giant ionic lattices
What is a giant ionic lattice?
Where oppositely charged ions are held together by electrostatic attraction
What is lattice enthalpy a measure of?
Ionic bond strength
What happens when gaseous ions combine to make a solid ionic lattice?
Energy is given out - called the lattice enthalpy
The more negative the lattice enthalpy, the…?
…stronger the ionic bond
What 2 factors affect the lattice enthalpy?
Ionic charge
Ionic radius
How does ionic charge affect lattice enthalpy?
The higher the charges on the ions, the stronger the electrostatic attraction between ions, so the more energy is released when an ionic lattice forms, so the lattice enthalpy will be more negative
How does ionic radius affect the lattice enthalpy?
The smaller the ionic radii of the ions involved, the higher the charge density of the ion. This means the electrostatic force of attraction between the ions is greater so the lattice enthalpy is more negative
What is the trend in ionic radius down a group?
Ionic radius increases down the group
Why does the ionic radius increase down a group?
Ionic radii increase down a group as more shells are added (per period)
What is the direct route to form an ionic lattice from elements in their standard states?
Converting the elements in their standard states directly into an ionic lattice
What is the indirect route to form an ionic lattice from elements in their standard states?
Forming gaseous atoms and converting them into gaseous ions, them form the ionic lattice
What are the 4 stages to form an ionic lattice from atoms in their standard states by the indirect route, and state what the energy change is called for each one?
- Gaseous atoms form (enthalpy change of atomisation)
- Atoms of one element lose electrons to form a positive ion (first ionisation energy)
- Atoms of one element gain electrons to form a negative ion (first electron affinity)
- Positive and negative ions form a solid ionic lattice (lattice enthalpy)
What is a Born-Haber cycle?
An enthalpy cycle that allows you to calculate the lattice enthalpy for a system
What is Hess’s Law?
The total enthalpy change of a reaction is always the same, no matter which route is taken
How do you find the lattice enthalpy?
It can’t be measured directly, so if you know all the other enthalpy changes, use Hess’s Law and the Born-Haber cycle to work out the lattice enthalpy
What are the 3 differences on a Born-Haber cycle for a Group 2 element compared to other compounds?
- There is an extra ionisation energy (2nd ionisation energy) to form a 2+ ion
- You need to multiply the atomisation enthalpy of the non-Group 2 element by the number of moles of the element there are in 1 mole of ionic lattice
- You need to multiply the first electron affinity of negative ion by the number of moles of ions formed
What 2 things happen when a solid ionic lattice dissolves in water?
- The bonds between the ions break to give gaseous ions
- Bonds between the ions and water are formed
When a solid ionic lattice dissolves in water, the bonds between the ions break to give gaseous ions. Is this an exothermic or endothermic process?
Endothermic
What is the enthalpy change equal to when the bonds between the ions in a solid ionic lattice break to give gaseous ions?
Negative lattice enthalpy
When a solid ionic lattice is dissolved in water, bonds form between the ions and the water. Is this process endothermic or exothermic?
Exothermic
When a solid ionic lattice is dissolved in water, bonds form between the ions and the water. What is this type of enthalpy change called?
Enthalpy change of hydration
What is enthalpy change of hydration?
The enthalpy change when 1 mole of gaseous ions is dissolved in water
Give an example of an equation for the enthalpy change of hydration?
Na+ (g) —> Na+ (aq)
Why does water form bonds with ions?
Water is a polar molecule, where the oxygen is more electronegative than the hydrogen, so it has a dipole. The positive ions then form weak bonds with the partial negative charge on the oxygen and the negative ions form weak bonds with the partial positive charge on the Hydrogen
What is the enthalpy change of solution?
The enthalpy change when 1 mole of solute is dissolved in a solvent
Substances generally only dissolve if the energy…?
released is the same, or greater than, the energy taken in
A reaction or state change is more likely when there is a… entropy change?
positive
What effect on entropy does dissolving have?
Normally causes an increase in entropy
Why does dissolving small, highly charged ions normally causes a decrease in entropy?
When water molecules surround the ions, there become more organised and orderly
What are the 3 steps for drawing an enthalpy cycle to work out the enthalpy change of solution?
- Put the ionic lattice and dissolved ions at the top and connect them with a line for enthalpy change of solution (direct route)
- Connect the ionic lattice to the gaseous ions with a line for negative lattice enthalpy
- Connect the gaseous ions to the dissolved ions with a line for hydration enthalpies of each ion
What extra do you have to do when drawing an enthalpy cycle for 2+ or 2- ions?
You need to multiply the enthalpies of hydration by the number of moles of the ions that are reacting
What 2 factors affect the enthalpy of hydration?
Ionic radius
Ionic charge
Why do ions with a greater charge have a greater enthalpy of hydration?
Ions with a higher charge are better at attracting water molecules than ions with lower charge, so more energy is released when the bonds are made, so they have a more exothermic enthalpy of hydration
Why do smaller ions (smaller ionic radius) have a greater enthalpy of hydration?
Smaller ions have a greater charge density than bigger ions. They attract water molecules better, so have a more exothermic enthalpy of hydration
What is entropy?
A measure of dispersal of energy in a system (number of ways particles can be arranged)
Which has a higher entropy, a gas or a solid?
Gas has a higher entropy because it’s more disordered
What 3 factors affect entropy?
Physical state
Dissolution
Number of gaseous particles
How does physical state affect the entropy?
Particles in a solid are fixed in a regular pattern, so they are very ordered, so have a low entropy. Gas particles are not fixed at all, so they can go wherever they want, do they’re very disordered so has a high entropy
Why does dissolving a solid increase entropy?
Dissolved particles can move freely as they’re no longer fixed in one place
How does the number of gaseous particles affect the entropy?
The greater the number of gas particles, the higher the entropy
Why does a greater number of gaseous particles mean a greater entropy?
The more particles there are, the more combinations of ways that their energies can be arranged so a higher entropy
Why is there an increase in entropy when water evaporates?
Because there is a state change from liquid to gas
What is the equation to calculate entropy change, ΔS?
ΔS = S products - S reactants
What are the units of entropy change?
J K^-1 mol^-1
What is free energy change, ΔG?
A measure used to predict whether a reaction is feasible
What is a feasible reaction?
A reaction that, once started, will carry on to completion, without any energy being supplied to it
Is a reaction feasible if the ΔG (free energy change) value is negative or zero?
The reaction is theoretically feasible
What is the equation for free energy, ΔG?
ΔG = ΔH - TΔS
ΔG: Free energy change
ΔH: Enthalpy change
T: Temperature
ΔS: Entropy change
Even if the value of ΔG is negative or zero, why might a reaction not happen?
It might have a very high activation energy or very slow rate so that it’s not noticeable
Why is an exothermic (-ve ΔH) reaction with a positive entropy change always feasible?
Because ΔG is always negative because ΔG = ΔH-TΔS
Why is an endothermic (+ve ΔH) reaction with a negative entropy change not feasible?
Because ΔG would be positive because ΔG = ΔH-TΔS
If a reaction is exothermic (-ve ΔH) and has a negative entropy change, will the reaction be feasible?
It’ll be feasible below a certain temperature where ΔG is zero or negative
