enthalpy and entropy and lattice enthalpy Flashcards
Enthalpy change of formation definition?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
Lattice enthalpy of formation definition?
The enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions
Enthalpy change of 1st ionisation definition?
The enthalpy change when 1 mole of gaseous 1+ ions are made from 1 mole of gaseous atoms.
Describe ionic bonding?
-The bigger the charge on n ion the stronger the electrostatic attraction between ions.
-The smaller the ion the ion the stronger the electrostatic attraction between ions. Smaller ions can pack together closer and generally have a higher charge.
Why are born-haber cycles useful?
they are useful to calculate lattice enthalpies as it can’t be calculated directly from experiments.
what is lattice enthalpy?
measure of the strength of ionic bonding in a giant ionic lattice. Enthalpy change that accompanies the formation of one mole of an ionic compound formats gaseous ions under standard conditions. It is an exothermic change (negative value)
what is at the bottom of the born-haber cycle?
solid ionic compound
what is the definition for standard enthalpy of formation?
enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states. It is exothermic.
Are the exothermic arrows in a born-haber cycle upwards or downwards?
exothermic arrows are downwards
Are the endothermic arrows in a born-haber cycle upwards or downwards?
endothermic arrows are upwards
What is the definition for standard enthalpy of atomisation
The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions. It is endothermic as bonds are broken.
What is the definition for first ionisation energy?
enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions. Ionisation energies are endothermic because energy is required to overcome the attraction between a negative electron and the positive nucleus.
What is the definition for first electron affinity?
enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions. This is exothermic because the electron being added is attracted in towards the nucleus.
Describe the structure of a born-haber cycle?
1)first, enthalpy change of formation towards solid ionic compound
2) enthalpy change of atomisation x2
3)first ionisation energy
4)second ionisation energy
5)first electron affinity
lattice enthalpy of formation definition?
energy change when when one mole of a solid ionic lattice is formed from gaseous ions
Hydration enthalpy definition?
enthalpy change when one mole of gaseous ions are surrounded by water molecules
second electron affinity definition?
energy change when one mole of gaseous 2- ions is formed from gaseous 1- ions. It is endothermic.
lattice enthalpy of dissociation definition?
energy change when one mole of a solid ionic lattice is dissociated into gaseous ions
enthalpy change of solution definition?
energy change when one mole of solid completely dissolves to form aqueous ions
How do you calculate lattice enthalpy using the born-haber cycle?
-Go with the arrow, keep the sign the same
-Go against the arrow, change the sign
-Once all equations are balanced, change the enthalpy value according to the mole e.g if 2 moles in equation, multiply the enthalpy by 2. E.g if 1/2 moles in equation, divide enthalpy by 2.
what is the enthalpy change of solution?
the enthalpy change when 1 mole of an ionic substance is dissolved in the minimum amount of solvent to ensure no further enthalpy change is observed upon further dilution.
For a substance to dissolve what must occur?
-substance bonds break (endothermic)
-new bonds formed between the solvent and substance. (exothermic) In water, when bonds form between ions and water, the ions are hydrated.
Why do most ionic compounds dissolve in polar solvents like water?
-The delta positive hydrogen is attracted to the negative ions and the delta negative oxygen is attracted to positive ions. The structure breaks down.
-The water molecules surround the ions in a process called hydration
Describe hydration
New bonds formed must be the same strength or greater than those broken. If not then the substance is very unlikely to dissolve. Soluble substances tend to have exothermic enthalpies of solution for this reason.
Enthalpy of hydration definition?
enthalpy change when one mole of aqueous ions is made from one mole of gaseous ions
How do you calculate enthalpy change of solution?
-add lattice dissociation enthalpy with enthalpy of hydration
-go with the arrow, keep the sign the same. Go against the arrow, change the sign.
How does charge of ion affect enthalpy change of hydration?
ions with a higher charge attract water molecules more strongly as the electrostatic attraction is stronger. More energy is released when the bond is made which means they have a more exothermic enthalpy of hydration. The larger the charge, the greater the enthalpy of hydration.
How does size of ion affect enthalpy change of hydration?
The smaller ions have a higher charge density than larger ions. They can attract water molecules more strongly hence there is a more exothermic enthalpy of hydration. The smaller the ion, the greater the enthalpy of hydration.
what is the standard enthalpy change of solution?
enthalpy change that takes place when one mole of a solute dissolves in a solvent
what is entropy(s)?
-measure of disorder in a system
-entropy is the number of ways energy can be shared out between particles
-higher disorder, higher entropy
Describe entropy in states of matter?
-solids have lowest entropy as particles are arranged neatly in rows.
-liquids and gases are more disordered
what are the units for entropy?
JK-1mol-1
How does the number of particles affect entropy change?
if a reaction is in the same state but more moles are produced then entropy increases. There are more ways energy can be distributed.
Describe an entropy of 0
there would be no energy and all substances would have an entropy value of zero. The reaction is feasible
standard entropy definition
entropy of one mole of a substance under standard conditions
how do you calculate the entropy change of reaction?
(sum of standard entropies in products) - (sum of standard entropies in reactants)
how can a reaction occur?
if the products have a lower overall energy than the reactants. The term feasibility describes whether a reaction is able to happen and is energetically feasible (spontaneous).
what is free energy change (triangle G)
overall change in energy during a chemical reaction.
what two types of energy is make up fee energy change?
-enthalpy change (triangle H). This is the heat transfer between the chemical system and the surroundings.
-entropy change at the temperature of the reaction TtriangleS. This is the dispersal of energy within the chemical system itself.
what is the Gibb’ equation?
(triangle)G=(triangle)H - T(triangle)S
what does the symbolic Gibbs’ equation translate into?
free energy change(kJmol-1)= enthalpy change with surroundings(kJmol-1)- (temperature in K)(entropy change of system, JK-1mol-1)
For a reaction to be feasible?
-there must be a decrease in free energy. Free energy must be lower than zero.
In the Gibbs’ equation…
the entropy change value must be divided by 1000
If the free energy value is 0?
the system is in equilibrium
Even if the reaction is calculated to be feasible, why might you not observe a reaction?
activation energy is too high or the rate is very slow.
how do you calculate the temperature at which a reaction becomes just feasible?
when (triangle)G = 0…
T= (triangle)H/ (triangle)S
What reactions are enthalpically not favourable?
endothermic reactions
Can reactions that are enthalpically unfavourable still spontaneously react?
yes, if changes in entropy overcome changes in enthalpy
what makes a reaction entropically favourable?
-increasing moles
-change of state which increases entropy
how do you calculate enthalpy change (delta H)?
-calculate q(energy lost or gained) using q=mc(delta)T
-calculate moles
-divide q by moles
