8. Thermodynamics Flashcards
What is enthalpy change?
The heat energy transferred in a reaction at constant pressure
Symbol for enthalpy change?
ΔH
What is standard enthalpy change?
The heat energy transferred in a reaction at 100kPa and 298K
What does the sign on an enthalpy change value depend on?
Whether the reaction is exothermic or endothermic
What does a negative enthalpy change value indicate?
The reaction is exothermic
What does a positive enthalpy change value indicate?
The reaction is endothermic
What is standard enthalpy change of formation?
Enthalpy change when 1 mole of substance is formed from its constituent elements with all reactants and products in standard states under standard conditions
What is standard enthalpy change of combustion?
Enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions
How is enthalpy change calculated in terms of the sum of bonds broken and formed?
sum of bonds broken - sum of bonds formed
What is the equation for enthalpy of formation of sodium oxide?
2Na (s) + 1/2 O₂ (g) → Na₂O (s)
What is the ΔH sign for enthalpy of formation?
Positive or negative
What is the ΔH sign for enthalpy of combustion?
Negative
What is the equation for the enthalpy of combustion of hydrogen?
H₂ (g) + 1/2 O₂ (g) → H₂O (l)
What is ionisation enthalpy also known as?
First/second etc. ionisation energy
What is first ionisation energy?
Enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ions
What is second ionisation energy?
Enthalpy change when each ion in one mole of gaseous 1+ ions loses one electron to form one mole of gaseous 2+ ions
What is first electron affinity?
Enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions
What is second electron affinity?
Enthalpy change when each ion in one mole of gaseous 1- gains one electron to form one mole of gaseous 2- ions
What is the ΔH sign for ionisation enthalpy?
Positive
What is the ΔH sign for first electron affinity?
Negative
What is the ΔH sign for second electron affinity?
Positive
What is the equation for enthalpy of first ionisation energy of magnesium?
Mg (g) → Mg⁺ (g) + e⁻
What is the equation for enthalpy of second ionisation energy of magnesium?
Mg⁺ (g) → Mg²⁺ (g) + e⁻
What is the equation for first electron affinity of oxygen?
O (g) + e⁻ → O⁻(g)
What is the equation for second electron affinity of oxygen?
O⁻ (g) + e⁻ → O²⁻ (g)
What is bond dissociation?
Enthalpy change when one mole of covalent bonds is broken in the gaseous state
What is enthalpy of atomisation of an element?
Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state
What is enthalpy of atomisation of a compound?
Enthalpy change when gaseous atoms are produced from one mole of a compound in its standard state
What is lattice enthalpy of formation?
Enthalpy change when one mole of a solid ionic compound is formed from into its constituent ions in the gas phase
What is lattice enthalpy of dissociation?
Enthalpy change when one mole of a solid ionic compound is broken up into its constituent ions in the gas phase
What is hydration enthalpy?
Enthalpy change when one mole of gaseous ions become hydrated (dissolved in water)
What is enthalpy of solution?
Enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other
What is the ΔH sign for bond dissociation?
Positive
What is the ΔH sign for enthalpy of atomisation of an element?
Positive
What is the ΔH sign for enthalpy of atomisation of a compound?
Positive
What is the ΔH sign for lattice enthalpy of formation?
Positive
What is the ΔH sign for lattice enthalpy of dissociation?
Negative
What is the ΔH sign for hydration enthalpy?
Negative
What is the ΔH sign for enthalpy of solution?
Positive or negative
What is the equation for the bond dissociation of iodine?
I₂ (g) → 2I (g)
What is the equation for the enthalpy of atomisation of iodine?
1/2 I₂ (g) → I (g)
What is the equation for the enthalpy of atomisation of methane?
CH₄ (g) → C (g) + 4H (g)
What is the equation for the lattice enthalpy of formation of magnesium chloride?
Mg²⁺ (g) + 2Cl⁻ (g) → MgCl₂ (s)
What is the equation for the lattice enthalpy of dissociation of magnesium chloride?
MgCl₂ (s) → Mg²⁺ (g) + 2Cl⁻ (g)
What is the equation for the hydration enthalpy of magnesium ions?
Mg²⁺ (g) → Mg²⁺ (aq)
What is the equation for the enthalpy of solution of sodium chloride?
NaCl (s) → Na⁺ (aq) + Cl⁻ (aq)
What can 3 stages can reactions involving the formation of ionic compounds be broken into?
- Formation of free gaseous atoms from the elements in their standard states (enthalpy of atomisation)
- Addition or removal of electrons to form ions (enthalpy of ionisation/electron affinity)
- Attraction of ions to form the ionic compound (enthalpy of lattice formation)
What can be used to calculate the enthalpy change in the formation of ionic compounds?
Born-Haber cycles
What principles are applied in a Born-Haber cycle?
Hess’s Law
In a Born-Haber cycle, which way do the arrows for endo and exothermic changes go?
- endothermic changes go up
* exothermic changes go down
In Born-Haber cycles, how can the enthalpy changes required to form free gaseous atoms be obtained?
From the atomisation enthalpies or bond dissociation enthalpies
Why is first electron affinity always exothermic?
The electron is attracted by the nuclear charge
Why are second and third electron affinities always endothermic?
An electron is being added to an already negative ion (which repel)
In Born-Haber cycles, which way are lattice enthalpies of formation drawn?
Downwards as they are exothermic
In Born-Haber cycles, which way are lattice enthalpies of dissociation drawn?
Upwards as they are endothermic
In a Born-Haber cycle, what should be done if the enthalpy of lattice dissociation is given instead of lattice formation?
Turn the arrow around and this value will be subtracted from the rest of the alternative route
In a Born-Haber cycle, what should be done if the bond dissociation of Cl2 was given instead of enthalpy of atomisation of Cl?
Divide the value by 2 as in bond dissociation 2 moles of Cl are formed
In a Born-Haber cycle, why is the lattice enthalpy usually shown as lattice enthalpy of formation (with a negative value)?
So that an equation can be written where the enthalpy of formation of the ionic compound equals the sum of all the other enthalpy changes
If there is stronger attraction in an ionic compound, what will happen to the lattice formation enthalpy?
It will increase
What is the lattice formation enthalpy dependent on?
- charge on ions
* size of ions
What effect does a higher charge on ions have on lattice formation?
Increase
What effect does smaller ions have on lattice formation?
Increase
What is lattice enthalpy found in Born-Haber cycles often called?
The experimental value
What is the perfect ionic model?
Both ions are spheres with no polarisation
What does a difference between theoretical and experimental values of lattice enthalpy show?
The compounds are not perfectly ionic
Why is the perfect ionic model not realistic?
In reality the positive ion will attract the outer electrons of the negative ion - this leads to polarisation
What does a bigger difference between theoretical and experimental values of lattice enthalpy indicate?
The bigger difference, the more covalent character
For lattice enthalpy values, which is the experimental and which is the theoretical?
The experimental value is the one calculated by Born-Haber cycles
What does it mean, that the values for theoretical and experimental values of lattice enthalpy for NaCl are very close?
It is an ionic compound
What does it mean, that the values for theoretical and experimental values of lattice enthalpy for AgBr are very different?
It would be classed as ionic with covalent character
What does the magnitude of the lattice enthalpy indicate?
The overall strength of the ionic bonding
What does the difference between theoretical and experimental values of lattice enthalpy indicate?
The amount of covalent character
Why is water a polar molecule?
Because of the difference in electronegativity of the oxygen and hydrogen in the molecule
How do ions become hydrated?
- ions in an ionic lattice
- cations attracted to Oˢ⁻ and anions to Hˢ⁺
- distorts charge cloud of ions and reduces forces holding them together
- in solution, ions move from lattice and become surrounded by water molecules
When has a solid dissolved?
When the ions no longer interact with each other
When does enthalpy of hydration increase?
For smaller ions with an increasing charge - due to increased attraction from the nucleus
What 3 processes can dissolving an ionic compound be broken into?
- Breaking the ionic solid into gaseous ions → lattice dissociation
- Hydrating the positive ions → enthalpy of hydration of cations
- Hydrating the negative ions → enthalpy of hydration of anions
How can the processes for dissolving an ionic compound be shown?
In a Hess cycle or a Born-Haber cycle
When is a compound more likely to dissolve, in terms of enthalpy of solution?
When the enthalpy of solution is more exothermic
What equation can be used to calculate the enthalpy of solution?
ΔH solution = ΔH lattice dissociation + ΔH hydration
What are feasible/spontaneous reactions?
Ones that occur of their own accord - thermodynamically possible
What type of reaction are many feasible reactions?
Exothermic
When is it possible for an endothermic reaction to occur spontaneously?
When there is an increase in disorder and involve mixing or spreading out
What is entropy?
The degree of disorder in a system
What is the degree of disorder in a system?
Entropy
What is the symbol for entropy?
S
When will the value for entropy of a reaction be positive?
When the products are more disordered than the reactants
What happens to particles at 0K (absolute 0)?
Particles do not move as they have no energy
What happens to entropy as a substance melts?
Large jump - increasing value of entropy
What happens to entropy as a substance boils?
Very large jump - increasing value of entropy
What effect will dissolving a substance have on entropy?
Increase entropy
What effect will a reaction that produces a gas have on entropy?
Increase entropy
What factors can increase the entropy of a reaction?
- dissolving a substance
- reactions that produce a gas
- reactions that produce more moles of a product
What formula can be used to calculate the entropy change of a reaction?
ΔS = ΣS products - ΣS reactants
How can you see what entropy values are?
By looking them up in tables
When are chemical reactions favoured?
If there is an increase in entropy
What happens in terms of feasibility if ΔS is positive?
Likely to be feasible
What happens in terms of feasibility if ΔS is negative?
Unlikely to be feasible
Is entropy the only factor involved in the feasibility of reactions?
No
What does the chance that a reaction proceeds depend on?
A balance between enthalpy, entropy and temperature
What is Gibbs free energy a combination of?
Enthalpy, entropy and temperature
What equation shows the relationship between enthalpy, entropy and temperature?
ΔG = ΔH - TΔS
In ΔG = ΔH - TΔS, what does G stand for?
Gibbs free energy (kJmol⁻¹)
In ΔG = ΔH - TΔS, what does H stand for?
Enthalpy (kJmol⁻¹)
In ΔG = ΔH - TΔS, what does T stand for?
Temperature (K)
In ΔG = ΔH - TΔS, what does S stand for?
Entropy (JK⁻¹mol⁻¹)
In ΔG = ΔH - TΔS, what must be remembered for the units of S?
Needs to be divided by 1000
In ΔG = ΔH - TΔS, what does it mean if ΔG is less than or equal to zero?
The reaction is feasible, even if ΔH is positive
In ΔG = ΔH - TΔS, what does it mean if ΔG is positive?
The reaction is not feasible, even if ΔH is negative
Why might many endothermic reactions proceed spontaneously?
There is an increase in entropy
also become feasible when temperature is increased
Why might some exothermic reactions not proceed spontaneously?
Because there is a decrease in entropy
When does the effect of entropy become more important, and why?
Endothermic reactions because ΔG depends on TΔS
What is the critical temperature?
The temperature at which ΔG=O, i.e. the point at which the reaction is just feasible
What equation is used to calculate the critical temperature of a reaction?
T = ΔH/ΔS
