3.1.8- Thermodynamics (PAPER 1) Flashcards
\What is standard enthalpy of formation? Endo or exo?
Exothermic
Enthalpy change when 1 mole of a compound is formed from its constituent elements in standard conditions, all reactants and products in standard states.
What is standard enthalpy of combustion? Endo or exo?
Exothermic
Enthalpy change when 1 mole of a compound burns completely with oxygen under standard conditions, all reactants and products in standard states.
What is standard enthalpy of neutralisation? Endo or exo?
Exothermic
Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions.
What is 1st Ionisation energy? Endo or exo?
Endothermic (requires energy)
Enthalpy change when 1 mole of gaseous electrons is removed from 1 mole of gaseous atoms, forming 1 mole of gaseous 1+ ions.
What is 2nd ionisation energy? Endo or exo?
Endothermic (larger than 1IE as more energy to remove e- from +ion)
Enthalpy change when 1 mole of gaseous electrons is removed from 1 mole of gaseous 1+ ions, forming 1 mole of gaseous 2+ ions.
What is 1st electron affinity? Endo or exo?
Exothermic- electron GAIN
Enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms, forming 1 mole of gaseous ions.
What is 2nd electron affinity? Endo or exo?
Endothermic- electron added to negative ion.
Enthalpy change when 1 mole of electrons is added to 1 mole of 1+ ions, forming 1 mole of 2+ ions.
What is standard enthalpy of atomisation? Endo or exo?
Endothermic- breaking bonds to form atom.
Enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state under standard conditions.
What is standard enthalpy of hydration? Endo or exo?
Exothermic
Enthalpy change when 1 mole of gaseous ions become hydrated/ dissolved in water.
What is standard enthalpy of solution?
Endo or exo?
Can be positive (endo) or negative (exo)
Enthalpy change when 1 mole of an ionic solid dissolves in enough water so dissolved ions are separated and do not interact with eachother (aq)
What is bond dissociation enthalpy?
Endo or exo?
Endothermic
Enthalpy change when 1 mole of covalent bonds are broken in the gaseous state.
What is the bond dissociation enthalpy the same as for diatomic molecules?
Bond dissociation enthalpy = 2x atomisation enthalpy.
Eg:
1/2Cl2 (g)—-> Cl2(g) = +120 (atomisation)
Cl2(g) —-> 2Cl (g) + 2(120) = +240 (dissociation)
What is mean bond enthalpy?
Endo or exo?
Endothermic
Enthalpy change to break the covalent bond into 2 gaseous atoms/ free radicals, averaged over different compounds.
What is lattice enthalpy of formation?
Endo or exo?
Exothermic
Enthalpy change when 1 mole of a solid ionic compound is formed from its constituent ions in the gas phase.
Applies specifically to ionic compounds with formation from gaseous ions-measure of strength of ionic bonds.
What is lattice enthalpy of dissociation?
Endo or exo?
Endothermic
Enthalpy change when 1 mole of a solid ionic compound is broken up into its constituent ions in the gas phase.
What is enthalpy of vaporisation?
Endo or exo?
Endothermic
Enthalpy change when 1 mole of a liquid is turned into a gas.
What is enthalpy of fusion?
Endo or exo?
Endothermic
Enthalpy change when 1 mole of a solid is turned into a liquid.
What does strength of lattice enthalpy depend on?
1) The size of the ions- larger ions have less negative lattice enthalpy as charge density decreases so attraction between ions decreases and less energy needed to pull them apart. ATOMIC RADIUS INCREASES DOWN A GROUP.
2) Charges of ions- Increased charge on cation (+ ion) or anion produces more attraction so higher charge density, bigger lattice enthalpy value.
What does enthalpy of hydration depend on?
1) The size of ions- smaller ions have greater charge density so stronger ion-dipole attractions between water molecules and the ions in the solution and more energy is released when they become hydrated and ΔHhydꝋ becomes more exothermic.
2) Size of charges on ions- enthalpy change more exothermic/negative for larger ionic charges, greater charge density resulting in stronger ion-dipole attractions between the water molecules and the ions in the solution so more energy released when they become hydrated.
What is the perfect ionic model?
Theoretical lattice enthalpies assume a perfect ionic model- ions
are 100% ionic and spherical and the attractions are purely electrostatic.
The charge is evenly distributed within the sphere/ion.
What are the differences between theoretical and Born Haber (experimental) lattice enthalpies?
The Born Haber lattice enthalpy= real experimental value.
When a compound shows covalent character, the theoretical and the born
Haber lattice enthalpies differ. More covalent character= bigger difference between the values.
What factors indicate higher covalent character?
-Positive ions that are small and/or highly charged are good at distorting- good at polarising negative ions.
-Negative ions that are large and/or highly charged are easier to distort- they are polarisable.
-Low solubility/ insoluble in water indicates some covalent character, melting points and electrical conductivity may not be as high as expected.
How to show covalent character?
Larger difference between experimental value from Born haber cycle and theoretical value indicates larger covalent character.
What does born-haber cycle show? Direct+indirect.
It shows the direct and indirect route to form an ionic compound from elements in standards states.
Direct route-involves one step/equation- shown by the enthalpy of formation. DOWN arrow as exothermic.
Indirect route- involves changing the elements from their standard states to their gaseous ions before forming the ionic compound.
Draw a born-haber cycle for the lattice enthalpy of formation of silver iodide:
1st electron affinity of iodine= -314kJmol-1
1st ionisation enthalpy of silver= +732kJmol-1
enthalpy of formation of silver iodide= -62kJmol-1
atomisation enthalpy of silver= +289kJmol-1
atomisation enthalpy of iodide= +107kJmol-1
Enthalpy of formation on bottom- down
Atomisation of silver then atomisation of iodide- up
IONISATION BEFORE AFFINITY
1st ionisation silver- up
1st electron affinity iodide- down
How are theoretical values for lattice enthalpy of formations calculated?
By a theoretical calculation that considers the size, charge and arrangement of ions in the lattice, assuming the structure is perfectly ionic.
How to know which compound is most thermodynamically stable- eg why is Calcium chloride CaCl2, not CaCl or CaCl3?
The largest exothermic value for enthalpy of formation is most thermodynamically stable.
How to work out enthalpy of solution?
1) Break solid ionic lattice into gaseous ions- lattice dissociation.
2) Dissolve gaseous ions in water- enthalpy of hydration.
Enthalpy of solution on top.
What is equation for enthalpy of solution using lattice formation?
Enthalpy of solution= -lattice formation + (sum of enthalpies of hydration)
What is equation for enthalpy of solution using lattice dissociation?
Enthalpy of solution= lattice dissociation + (sum of enthalpies of hydration)
What is entropy + its units?
A measure of disorder given by symbol S
Units- Jk-1mol-1
How does entropy change with state change?
Entropy at 0K is 0 as particles have no kinetic energy.
Entropy increases as temperature increases and as a substance changes from solid to liquid to gas.
Gases= highest entropy values as they have greatest freedom of movement
How is entropy increased?
A change in state from s to l or l to g.
An increase in the number of particles/ moles of gas.
What is equation for entropy change?
∑Δ S (products) – ∑ΔS (reactants)
What is diff between positive and negative entropy change?
Positive entropy change- increase in entropy, products have more disorder than reactants. POSITIVE ENTROPY = FEASIBLE
Negative entropy change- decrease in entropy, reactants have more disorder than products. NEGATIVE= NOT FEASIBLE
How does entropy link with feasibility of reactions?
A reaction can be spontaneous (feasible) even if it is unfavourable enthalpically. The reaction can be entropically favourable.
Eg- endothermic reactions are unfavourable as products have higher energy than reactants.
How may an unfavourable reaction still occur?
If changes in entropy overcome changes in enthalpy.
Some spontaneous reactions still occur even if endothermic and enthalpy change greater than 0, would be not feasible.
What is Gibbs free energy equation?
ΔG° = ΔH° - TΔS°
Gibbs free energy= enthalpy change (KJmol-1) - temperature (K) x entropy change (KJK-1mol-1)
How to tell if reaction is feasible?
If ΔG° is zero or negative.
Why may feasible reaction not occur?
The activation energy may be too high or rate of reaction too slow.
Feasibility with +ΔH and -ΔS?
Not feasible as ΔG positive.
Feasibility with -ΔH and +ΔS?
Feasible as ΔG negative.
Feasibility with -ΔH and -ΔS?
Feasible at low temp.
Feasibility with +ΔH and +ΔS?
Feasible at high temp.
How to calculate temperature when reaction is just feasible?
ΔG = 0
T = ΔH / ΔS
ALL IN KJ
How would you plot ΔG against T?
Temperature on x axis, ΔG on y axis.
Find ΔH- when T= 0, ΔG = ΔH
Find ΔS- gradient of line.
Limitations of Ellingham diagram?
Does not explain the rate of reaction.
Reactants and products are in equilibrium which is not often true.
