Unit 3.5.1 - Thermodynamics Flashcards
What does ΔH stand for?
Enthalpy change
What is enthalpy change?
The heat energy transferred at a constant pressure.
Why is enthalpy often written as ΔHɵ?
To show the substances are in their standard states and the measurements were made under standard conditions.
What are the standard conditions for enthalpy change?
100 kPa pressure and 298 K
What value does exothermic reactions have for ΔH and why?
Negative because heat energy is given out so the system has lost energy.
What does value endothermic reactions have for ΔH and why?
Positive because heat energy is absorbed so the system has gained energy.
What is the lattice formation enthalpy?
The enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions under standard conditions.
What is lattice disassociation enthalpy?
The enthalpy change when one mole of a solid ionic compound is completely disassociated into gaseous ions under standard conditons.
What is enthalpy change of formation?
The enthalpy change when one mole of compound is formed form it’s elements in their standard states under standard conditions.
What is bond disassociation enthalpy
The enthalpy change when all the bonds of the same type in one mole of gaseous molecules are broken.
What is enthalpy change of atomisation of an element?
The enthalpy change when one mole of gaseous atoms is formed from an element in it’s standard state.
What is enthalpy change of atomisation of a compound?
The enthalpy change when one mole of a compound in it’s standard state is converted into gaseous atoms.
What is first ionisation enthalpy?
The enthalpy change when one mole of gaseous +1 ions is formed from one mole of gaseous atoms.
What is second ionisation enthalpy?
The enthalpy change when one mole of gaseous +2 ions is formed from one mole of gaseous +1 ions.
What is first electron affinity?
The enthalpy change when one mole of gaseous -1 ions is made from one mole of gaseous atoms.
What is second electron affinity?
The enthalpy change when one mole of gaseous -2 ions is made from one mole of gaseous -1 ions.
What is enthalpy of hydration?
The enthalpy change when one mole of aqueous ions is formed from gaseous ions.
What is enthalpy of solution?
The enthalpy change when one mole of a solute is dissolved in sufficient solvent that no further enthalpy change occurs on further dilution.
What type of bonds hoid ionic bond lattices together?
Ionic bonds.
Is energy required to break or form bonds?
Break
Is energy given out to break or form bonds?
Form
What are lattice enthalpies measures off?
Strength of ionic bond and how much energy they give out or take in.
Is lattice disassociation or lattice formation enthalpy negative and why?
Lattice formation enthalpy is negative because energy is given out when bonds are formed.
As lattice enthalpy can’t be measured directly, what do you have to use instead?
Born Haber cycles
What is Hess’s law?
The total enthalpy change of reaction is always the same no matter which route is taken.
What indirect route do you take to find lattice enthalpy of formation?
- electron affinity of - ion - ionisation enthalpy for + ion - atomisation enthalpies + enthalpy of formation.
How do Born Haber cycles for Group 2 elements differ from Group 1 elements?
You get a second ionisation enthalpy because of the +2 charge.
What model do you base theoretical lattice enthalpies on?
The purely ionic model of a lattice.
What does the purely ionic model assume?
All the ions are spherical and have their charge evenly distributed around them.
Are the experimental lattice enthalpies from doing a Born Haber cycle the same or different as the theoretical lattice enthalpies based on the purely ionic model?
Different
Why aren’t the theoretical values correct?
Ionic compounds usually have some covalent charactiers.
What is meant by covalent characters?
The positive and negative ions in a lattice aren’t usally spherical. Positive ions polarise neighbouring negative ions to different extents. The more polarisation there is the more covalent the bond will be.
If the experimental and theoretical lattice enthalpies for a compound are very different what does this show you?
The compound has a lot of covalent character.
If experimental and theoretical lattice enthalpies for a compound are very similar what does this show you?
The compound has very little covalent character.
What two things happen when a solid lattice dissolves in water?
- The bonds between the ions break. 2. Then the bonds between the ions and water are made.
Why can water form bonds with ions?
Oxygen is a lot more electronegative than hydrogen so it draws the bonding pair of electrons towards itself creating a dipole. Positive ions can form weak bonds with the partial negative charge on the oxygen. Negative ions can form weak bonds with the partial positive charge on the hydrogen ions.
What type of diagram can you use to find the enthalpy change of solution?
An enthalpy cycle
What indirect route can you do to find the enthalpy change of solution?
Lattice disassociation enthalpy + enthalpy hydration of + ion + enthalphy hydration of - ion.
What formula relates energy and enthalpy change?
Enthalpy change reaction = total energy absorbed - total energy released.
What is another way to write Enthalpy change reaction = total energy absorbed - total energy released.
Enthalpy change reaction - total enthalpies or bonds broken - total enthalpies of bonds formed.
What is mean bond enthalpy?
The energy required to break 1 mol of a given covalent bond averaged over a range of compounds.
Why are mean bond enthalpies never perfectly accurate?
it is only an average for a given type of bond in many different compounds and in various places within the compound.
What does entropy show you?
How much disorder there is.
What two things is entropy a measure of?
The number of ways that particles can be arranged and the number of ways that energy can be shared out between the particles.
What symbol represents entropy?
S
What are the three main things that affect entropy?
Physical state, dissolution and the number of particles.
How does physical state affect entropy?
Gas particles whizz around anywhere they like so they have the most random arrangement of particles so they have the highest entropy, solid particles just wobble about a fixed point so there isn’t much randomness so they have the lowest entropy.
What does dissolution involve?
Dissolving a solid.
How does dissolution affect entropy?
Dissolving a solid increases it’s entropy as the dissolved particles can move more freely increasing the number of ways the particles and their energy can be arranged.
How does the number of particles affect entropy?
More particles mean more entropy, the more particles you have the more ways they and their energy can be arranged.
What is a spontaneous change?
What that happens by itself, you don’t need to give it energy.
What is another name for a spontaneous change?
A feasible change.
Even though you have to supply energy for an endothermic reaction why are they sometimes spontaneous?
In some reactions the entropy increases such a lot that the reaction will happen by itself without you supplying energy.
Name two spontaneous endothermic reactions.
1, Evaporation of water. 2. The reaction of sodium hydrogen carbonate with hydrochloric acid.
Why is evaporation of water spontaneous?
You are going from a liquid to a gas so the entropy increases.
Why is the reaction between sodium hydrogen carbonate with hydrochloric acid spontaneous?
There are more particles in the products and CO2 is evolved which is a gas further increasing entropy.
For reactions to happen does entropy change have to be negative of positive?
Positive.
What two entropy changes are there during a reaction?
An entropy change between the reactants and products and an entropy change of the surroundings.
What is the entropy change of the system?
Entropy change between the reactants and the products.
ΔStotal =?
ΔSsystem + ΔSsurroundings
ΔSsystem = ?
Sproducts - Sreactants
ΔSsurroundings = ?
What are the units for ΔS?
JK-1mole -1
What is the symbol for free energy change?
ΔG
What is ΔG used to measure?
To predict whether a reaction is feasible.
What does ΔG have to be for a reaction to be feasible?
Less than or equal to 0
What does ΔG take account of?
The changes in enthalpy and entropy in a system
ΔG = ?
ΔH-TΔS
What are the units for ΔG and ΔH?
Jmole-1
Why might a reaction not always happen even though ΔG is -?
The reaction might have a really high activation energy or be so slow that you wouldn’t notice it happening at all.
What values of ΔH and ΔS will always be feasible at any temperature and why?
-ΔH and +ΔS, because ΔG will always be -
What values of ΔH and ΔS will not be feasible at any temperature and why?
+ΔH and -ΔS, because ΔG will always be +
What are the two values ΔH and ΔS which means a reaction is sometimes feasible and why?
- +ΔH and +ΔS then ΔG will be - for higher temperatures and so the reaction will be feasible at higher temperatures. 2. -ΔH and -ΔS then ΔG will be - for lower temperatures and so the reaction will be feasible for lower temperatures.
To find at what temperature a reaction become feasible what do you set ΔG = to?
0
What equation can you use to find temperature at which a reaction just becomes feasible?
