Unit 1: Section 6 - Thermodynamics Flashcards
What is enthalpy change?
The heat energy transferred in a reaction at constant pressure
What is enthalpy change of formation?
ΔfH - the enthalpy change when 1 mole of a substance is formed from its constituent elements with all substances in their standard states - exothermic for most substances
2Na(s) + 1/2O2(g) -> Na2O(s)
What is enthalpy of combustion?
ΔcH - When 1 mole of a substance undergoes complete combustion in oxygen with all substances in their standard states - exothermic
H2 (g) + 1/2O2(g) -> H2O(l)
What is enthalpy of neutralisation?
ΔneutH - when 1 mole of water is formed in a reaction betweeen an acid and alkali under standard conditions - exothermic
1/2H2SO4(aq) + NaOH(aq) -> 1/2Na2SO4(aq) + H2O(l)
What is ionisation enthalpy?
ΔieH
1st ionisation enthalpy - enthalpy change whe each atom in 1 mole of gaseous atoms loses 1 electron to form 1 mole of gaseous 1+ ions - endothermic
Mg(g) -> Mg+(g) +e-
2nd ionisation enthalpy - enthalpy change when each ion in 1 mole of gasous 1+ ions loses 1 electron to form 1 mole of gaseous 2+ ions - endothermic
Mg+(g) -> Mg2+ +(g) +e-
What is electron affinity?
ΔeaH
1st electron affinity - enthalpy change when each atom in 1 mole of gaseous atoms gains 1 electron to form 1 mole of gaseous 1- ions - exothermic for many non-metals
O(g) + e- -> O-(g)
2nd electron affinity - enthalpy change when each ion in 1 mole of gasous 1- ions gains 1 electron to form 1 mole of gasoues 2- ions - endothermic
O-(g) + e- -> O2-(g)
What is enthalpy of atomisation?
ΔatH - when 1 mole of gaseous atoms is produced from an element in its standard state - endothermic
1/2I2(s) -> I(g)
What is Hydration enthalpy?
ΔhydH
When 1 mole of gaseous atoms dissolve in water (become hydrated) - exothermic
Mg2+(g) + aq -> Mg2+(aq)
What is enthalpy of solution?
ΔsolH
When 1 mole of an ionic solid disolves in an amount of water large enough so that the dissolved ions are well separated and don’t interact with each other - varies
MgCl2(s) + aq -> Mg2+(aq) + 2Cl-(aq)
What is bond dissociation enthalpy?
ΔdisH
When 1 mole of covalent bonds is broken in the gaseous state - endothermic
I2(g) -> 2I (g)
What is lattice enthalpy of formation?
ΔLEFH
When 1 mole of a solid ionic compound is formed from its consistuent ions in the gas phase - exothermic
Mg2+(g) + 2Cl-(g) -> MgCl2(s)
What is lattice enthalpy of dissociation?
When 1 mole of a solid ionic compound is broken up into its constituent ions in the gas phase - endothermic
MgCl2(s) -> Mg2+(g) + 2Cl-(g)
What is enthalpy of vaporisation?
ΔvapH
Whe 1 mole of a liquid is turned into a gas - endothermic
H2O(l) -> H2O(g)
What is enthalpy of fusion?
ΔfusH
When 1 mole of a solid is turned into a liquid - endothermic
Mg(s) -> Mg(l)
What are Born-Haber cycles used for?
Used to calculate lattice enthalpies
What is the perfect ionic model of a lattice?
Assumes that all the ions are spherical, and have their charge evenly distributed around them
Why is the perfect ionic model often not accurate?
- Ions are not perfectly spherical
- Polarisation often occurs when small positive ions or large negative ions are involved
- Some lattices are not regular and the crystal structure can differ
What does covalent character show?
When you find the lattice enthalpy experimentally, the value that you often get is not the theoretical lattice enthalpy, this is evidence that most ionic compounds have some covalent character - the positive and negative ions in a lattice aren’t exactly spherical, positive ions polarise neighbouring negative ions to different extents, and the more polarisation there is, the more covalent the bonding will be
What characteristics does covalent character affect?
- Solubility - makes the compound less soluble
- General conductivity
What 2 things happen when a solid ionic lattice dissolves?
- The bonds between the ions break to give free ions - endothermic
- Bonds between the ions and the water are made - exothermic
The enthalpy change of solution is the overall effect on the enthalpy of these 2 things
When do substances dissolve?
Generally only dissolve if the energy released is roughly the same, or greater than the energy taken in , so soluble substances tend to have exothermic enthalpies of solution
What is entropy?
A measure of the number of ways that particles can be arranged and the number of ways the energy can be shared out between the particles in a system
The more disordered the particles are, the higher the entropy is - a large, positive value of entropy shows a high level of disorder
What affects entropy?
Physical state - solids have the lowest entropy as they just vibrate about a fixed position but gases have the highest entropy as they have the most disordered arrangements, so they have the highest entropy
More particles - more ways they can be arranged and the more energy there is
What makes a substance more stable?
- Substances tend towards disorder - they’re more energtically stable when there’s more disorder
- This is why some reactions are feasible even when the enthalpy change is endothermic
How can you calculate the entropy change for a reaction?
ΔS = Sproducts - Sreactants
What is the standard entropy of a substance?
The entropy of 1 mole of that substance under standard conditions
What is free energy change?
ΔG, a measure used to predict whether a reaction is feasible
If ΔG is negative or equal to 0 then the reaction might happen by itslef
How do you calculate free energy?
ΔG = ΔH - TΔS
Make sure the units are all the same
How does temperature affect the feasibility of reactions?
The table - summary for temperature and Gibbs free energy
How to calculate the temperature at which a reaction becomes feasible?
- When ΔG=0 a reaction is just feasible
- Rearrange the equation to T=ΔH/ΔS
- In an exam question write down the og equation and the replacement of ΔG with 0 to gain all marks
What affects lattice enthalpy?
Size of the ion - the bigger the ion, the lower the magnitude of lattice enthalpy
Charge - higher the magnitude of charge, the bigger the magnitude of lattice enthalpy
When is ΔH and ΔS favourable?
ΔH - as negative as possible
ΔS - as positive as possible
How can you increases the lattice enthalpy of a compound? Why does this increase it?
- Smaller ions - the charge centres will be closer together
- Increased charge - there will be a greater electrostatic force of attraction between the oppositely charged ions
- Increasing the charge on the anion has a much smaller effect than increasign the charge on the cation, since increasing anion charge also has the effect of increasing ionic size
How can Born-Haber cycles be used to see if compounds could theoretically exist?
- Use known data to predict certain values of theoretical compounds, and then see if these compounds would be thermodynamically stable
- Was used to predict the existence of the first noble gas containing compound
What does sponatneous and feasible mean?
If a reaction is spontatneous and feasible, it will take place of its own accord; does not take account of rate of reaction
What are the limitations of using G as an indicator of whether a reaction will occur?
- Gibbs free energy only indicates if a reaction is feasible
- It does not take into account the rate of reaction (the kinetics of the reaction)
- In reality, many reactions that are feasible at a certain temperature have a rate of reaction that is so slow that effectively no reaction is occuring
