Chapter 22 - enthalpy and entropy Flashcards
Define lattice enthalpy
The enthalpy change that accompanies the formation of 1 mole of an ionic lattice from its gaseous ion`s under standard conditions of 298K and 100kPa
Define standard enthalpy change of formation
The enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions of 100kPa and 298K with each element in their standard states
Define standard enthalpy change of atomisation
The enthalpy change that takes place for the formation of 1 mole of an element in gaseous form from the element in its standard state under standard conditions of 100kPa and 298K
Define first ionisation energy
The energy required to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions
Define first electron affinity
The energy change that takes place when 1 electron is added to each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1- ions.
Define second ionisation energy
The enthalpy change required to remove 1 electron from each ion in 1 mole of 1+ ions to form 1 mole of 2+ ions
Define second electron affinity
The enthalpy change that occurs when 1 electron is added to each ion in 1 mole of gaseous 1- ions to form 1 mole of gaseous 2- ions
Is lattice enthalpy endothermic or exothermic? Why?
Exothermic because energy is given out when ionic bonds are formed from gaseous ions
Is the standard enthalpy change of atomisation endothermic or exothermic? Why?
Endothermic because bonds are broken to form gaseous atoms which requires energy
Is the standard enthalpy change of formation endothermic or exothermic?
It can be either but it is usually exothermic
Is first ionisation energy endothermic or exothermic? Why?
Endothermic because the electron being lost has to overcome attraction from the nucleus in order to leave the atom which requires energy
Is second ionisation energy endothermic or exothermic? Why?
Endothermic because the electron being lost has to overcome attraction from the nucleus in order to leave the atom which requires energy
Is first electron affinity endothermic or exothermic? Why?
Exothermic because the electron is being attracted into the outer shell of an atom by the nucleus
Is second electron affinity endothermic or exothermic? Why?
Endothermic because the electron is repelled by the 1- ions which has to be overcome which requires energy
What does lattice enthalpy indicate?
The strength of an ionic lattice
What does lattice enthalpy measure?
A measure of how strong an ionic bond is in a giant ionic lattice
What is a Born - Haber cycle?
An energy diagram used to calculate lattice enthalpies by taking an indirect route
List 2 factors that affect lattice enthalpy
size of ions
ionic charge
Explain how ionic charge influences lattice enthalpy
Across a period, ionic charge increases, meaning attraction between ions increases, and lattice enthalpy becomes more exothermic, as ionic bonds are stronger between the oppositely charged ions, and thus lattice enthalpy is increased
Explain how size of ions influences lattice enthalpy
Ionic size increases down a group
And the distance between ions in lattice increases as there are more shells
Therefore, the attraction between the ions decreases
And lattice enthalpy becomes less exothermic
The more negative the lattice enthalpy the ____ the melting point
Higher
Define enthalpy change of solution
The enthalpy change that takes place when 1 mole of a solute dissolves in a solvent under standard conditions of 298K and 100kPa
Give an equation showing enthalpy change of a solution
NaCl (s) + aq –> Na+ (aq) + Cl- (aq)
Give the experimental procedure to calculate enthalpy change of solution
- Weigh out sample using mass balance
- Measure out known volume of water using a measuring cylinder
- Measure the temperature of water using a thermometer
- Stir the mixture using the thermometer until all has dissolved and temperature no longer changes. Record this value
- Use q=mc. T to calculate energy in j
- Use enthalpy change of solution =(q/1000 - needs to be in kJmol-1) /n to calculate enthalpy change, where n = moles and q= energy
What are the stages of dissolving a solute in water?
The ionic lattice is broken down into gaseous ions
The ions then become hydrated so the ions bond with water molecules
Define enthalpy change of hydration
The enthalpy change that takes place when gaseous ions are dissolved in water to form 1 mole of aqueous ions under standard conditions of 298K and 100kPa
List 2 factors affecting enthalpy of hydration
size of ions
ionic charge
Explain how size of ions affects enthalpy change of hydration
Distance between ions increases as ionic size increases
Therefore, the attraction between the ions and water molecules decreases
And hydration enthalpy becomes less exothermic
Explain how ionic charge of the enthalpy change of hydration
The attraction between ions and water molecules increases as ionic charge increases
Hydration enthalpy therefore becomes more exothermic
What is entropy?
A measure of how dispersed energy is within a system
What does it mean when entropy is greater?
The dispersal of energy and the disorder of the molecules is greater
Does entropy increase or decrease over time?
increase, as energy will disperse over time
What are the units of entropy?
JK-1mol-1
Which has the greatest entropy? Solids, liquids or gases
gases, as they have the greatest dispersal of energy
At 0K how much entropy do perfect crystals have?
zero entropy
When a system changes, if the energy becomes —– then the entropy will increase, and the entropy change is —
more dispersed
positive
When a system changes, if the energy becomes —–then the entropy will decrease, and the entropy change is —–.
less dispersed/ more concentrated
negative
List 4 instances when entropy in a system increases
Melting
Boiling
Number of gaseous molecules increases
Dissolving a substance
Define standard entropy
the entropy of 1 mole of a substance under standard conditions of 100kPa and 298K
Give the equation to calculate entropy change in a system
Entropy change = the sum of the entropy of the products – the sum of the entropy of the reactants
What does it mean for a reaction to be feasible?
It will occur spontaneously
When is a reaction feasible (in terms of energy change)
A chemical system becomes more stable and its overall energy decreases
What is overall energy of a system known as?
Free energy change
What does the Gibbs equation calculate?
The equation to calculate free energy change
What is the Gibbs equation (give formula)
Free energy change Jmol-1 = enthalpy change kJmol-1 – entropy change at a specific temperature
How do you calculate entropy change at a specific temperature?
(temperature K x entropy change JK-1mol-1)
How do you convert from celcius to kelvin?`
ADD 273
For a reaction to be feasible the free energy change has to be less than ___
zero
If the enthalpy change is negative and entropy change is positive will the reaction be feasible?
Yes, because the free energy change is less than zero. -10 - +10 = -20
Remember, you are subtracting entropy, so if it is positive, a negative sign and a positive sign make a negative
If the enthalpy change is positive and the entropy change is negative will the reaction be feasible? why?
No, because free energy change will always be more than 0
10 - - 10 = 20
Remember, you are subtracting entropy, and a negative sign and negative sign make a positive
What other factors need to be considered when considering if a reaction will occur?
activation energy
rate of reaction
If the enthalpy change is negative and the entropy is negative, what conditions are needed in order for the reaction to be feasible? Why?
Low temperatures are needed, so that T^S is smaller than enthalpy
-10 –7 = -3
If the enthalpy change is positive and the entropy is positive what conditions are needed in order for the reaction to be feasible? Why?
High temperatures are needed, so that T^S is bigger than enthalpy, so that the overall reaction is negative
10 - +17 = -7
What is the enthalpy change of an endothermic reaction?
Positive
What is the enthalpy change of an exothermic reaction?
Negative
In order for an endothermic reaction to be feasible, what conditions are there?
T^S must be positive, and must be larger than ^H, so temperatures must be high so that the overall reaction is negative
10 - +17 = -7
- this makes sense with Le Chatelier so double check answers
In order for an exothermic reaction to be feasible, what conditions are there?
T^S must either be positive, or more less negative than ^H, so that the overall energy change is still negative.
Either
-10 - + 7 = -17
or
-10 - - 3 = -7
