Thermodynamics Flashcards
Define enthalpy change.
The heat energy change at constant pressure.
What are standard conditions?
298K, 100kPa
Define enthalpy of lattice dissociation.
The enthalpy change when one mole of an ionic compound is separated into its component gaseous ions.
Define enthalpy of lattice formation.
Enthalpy change when one mole of an ionic compound is formed from its constituent ions in the gaseous state.
Define enthalpy of formation.
Enthalpy change when one mole of a compound is formed from its elements when all reactants and products are in their standard states under standard conditions.
Define enthalpy of atomisation.
The enthalpy change for the formation of one mole of gaseous atoms from the element in its standard state.
Define bond dissociation enthalpy.
Enthalpy change when one mole of a covalent bond is broken under standard conditions in the gaseous state.
Define first ionisation enthalpy.
Enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous ions with a single positive charge.
Define second ionisation enthalpy.
Enthalpy change when one mole of electrons is removed from one mole of gaseous ions with a single positive charge to form one mole of gaseous ions with a 2+ charge.
Define first electron affinity.
Enthalpy change when one mole of gaseous atoms forms one mole of negative ions with a single negative charge.
Define second electron affinity.
Enthalpy change when one mole of gaseous ions with a single negative charge forms one mole of gaseous ions with a double negative charge.
Define enthalpy of solution.
Enthalpy change when one mole of a solute dissolved in water.
Define enthalpy of hydration.
Enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions.
Why do smaller ions have more negative values of enthalpy of hydration?
They have higher charge densities so the negative charge on the ion attracts the &+ H of the water more strongly.
Why is the first electron affinity of oxygen exothermic?
There is attraction between the nucleus of the oxygen atom and the external electron.
Why is the second electron affinity of oxygen endothermic?
The electron and negative oxygen ion repel each other, so energy is required.
What is entropy?
A measure of disorder in a system.
What are the units for entropy?
J/K/mol
What is the entropy value if there is lots of disorder?
High.
What are standard entropy values?
Calculations for 1 mole of a substance based on a scale where the substance has an entropy of 0 at 0K.
Why do substances tend to disorder?
They become more energetically stable.
Why is entropy change bigger when changing to a gas from a liquid, rather than to a liquid from a solid?
Gases are a lot more disordered.
What is the formula for entropy change?
ΔS = S(products) - S(reactants)
Why do macromolecular substances have very low entropy values?
They have very regular macromolecular structures.
Why do metallic substances have low entropy values?
They have very ordered metallic lattices.
Why do ionic compounds have low entropy values?
They have ordered ionic lattices.
Why do molecular crystals have low entropy values?
They are regular ordered crystalline structures.
Generally, what happens to the entropy value as Mr of substance, in the same state, increases?
Increases.
What happens the entropy value as number of moles increases?
Increases.
What is a feasible reaction?
A reaction that can occur at a particular temperature.
Why might a feasible reaction not happen?
*the activation energy hasn’t been met
*the reaction is too slow to observe
What are units of Gibbs free energy?
KJ/mol
What is the equation for Gibbs free?
ΔG = ΔH - TΔS
What are the units of enthalpy change?
KJ/mol
What are the units of entropy change?
J/K/mol
What must you do to the value of entropy change before using it in a calculation?
Divide by 1000 to get into KJ/K/mol.
When calculating the temperature at which a reaction becomes feasible, what is ΔG assumed to be?
0
What must the value of ΔG be for the reaction to be feasible?
Less than or equal to 0.
What does it mean if ΔG = 0?
The system is in equilibrium.
What is the feasibility of an exothermic reaction with positive entropy change?
-ΔG, so feasible.
What is the feasibility of an endothermic reaction with positive entropy change?
Feasible when T is large (ΔG negative).
What is the feasibility of an endothermic reaction with negative entropy change?
+ΔG, so never feasible.
What is the feasibility of an exothermic reaction with negative entropy change?
Feasible when T is small (ΔG negative).
What is temperature in Kelvin never less than?
0
What is the value of ΔG when changing state?
0
Is ΔG positive or negative for temperatures below melting point?
Positive.
Why is ΔG positive for temperatures below melting point?
The reaction is not feasible (cannot react with a solid).
Is ΔG positive or negative for temperatures above melting point and below boiling point.
Negative.
If ΔG is plotted on a graph, what is the gradient?
-ΔS in KJ/K/mol
If ΔG is plotted on a graph, what is the y intercept?
ΔH
If ΔG is plotted on a graph, what is on the x axis?
Temperature.
If ΔG is plotted on a graph, what is on the y axis?
ΔG
On a positive ΔG graph, what is the x intercept?
Maximum temperature for a reaction to be feasible (ΔG=0)
On a negative ΔG graph, what is the x intercept?
Minimum temperature for a reaction to be feasible (ΔG=0).
How do you work out ΔS from the gradient?
Multiply the negative gradient by 1000.
What are Born-Haber cycles used for?
To calculate lattice enthalpies as they can’t be calculated from experiments.
What is the perfect ionic model?
- ions are prefectly spherical
- charge is evenly distibuted in spheres (point charges)
How can theoretical lattice enthalpies be calculated?
From data assuming a perfectly ionic model.
What does it mean if the experimental lattice enthalpy is higher than the theoretical?
- compound doesn’t follow perfect ionic model
- has some covalent characteristics
How can an ionic compund have some covalent characteristics?
- positive ion distors charge distribution in negative ion
- positive ion polarieses negative ion
- some electron pulled to positive ion
What does more polarisation mean for covalent character?
More covalent character.
What do lattice enthalpy values tell us?
How much a substance is purely ionic.
Why is there a greater difference between theoretic and experimental lattice enthalpy values between +1 and +2 ion?
- larger distortion by positive ion
- more covalent character
What must happen for a substance do dissolve?
- substance’s bonds must break
- new bonds must form between solvent + substance
- new bonds must be same/greater strength than bonds broken
Is bonds breaking endothermic or exothermic?
Endothermic
Is bonds forming endothermic or exothermic?
Exothermic
What happens when an ionic compound dissolves in a polar solvent?
- delta + H atoms attracted to negative ions
- delta - O atoms attracted to positive ions
How can enthalpy of solution be calculated?
Using:
* lattice dissocation enthalpy
* enthalpy of hydration
What is the symbol for enthalpy of hydration?
ΔhydH
What is the symbol for enthalpy of solution?
ΔsolH
What is the symbol for enthalpy change?
ΔH
What is the symbol for enthalpy of formation?
ΔfH
What is the formula for lattice enthalpy of formation?
ΔlattH
What is the formula for lattice enthalpy of dissocation?
ΔlattH
What is the formula for bond dissocation enthalpy?
ΔdisH
or
ΔBDEH
What is the symbol for first ionisation enthalpy?
ΔIE1H
What is the symbol for second ionisation enthalpy?
ΔIE2H
What is the symbol for enthalpy of atomisation?
ΔatH
What is the symbol for first electron affinity?
ΔEA1H
What is the symbol for second electron affinity?
ΔEA2H
