Enthalpy and Entropy

Question 1

Define lattice enthalpy

Answer 1

Lattice enthalpy is the formation of 1 mole of ionic lattice its gaseous ions under standard condition

Question 2

Write an equation for lattice enthalpy

Answer 2

WRITE ONE

Question 3

why can we not measure lattice enthalpy directly

Answer 3

• Because it is impossible to form one mole of an ionic solid from its gaseous ions experimentally

Question 4

How do lattice enthalpy values indicate the relative strength of ionic bonds

Answer 4

• More exothermic lattice enthalpy values mean stronger ionic bonds (stronger electrostatic attraction)
• More exothermic lattice enthalpy mean higher melting and boiling points as more energy is required to overcome the interactions present
• The most exothermic lattice enthalpies arise when ions are small and have large charges as the charges cause large electrostatic forces and smaller ions can get closer together

Question 5

Why do ionic substances have strong ionic bonds and very high melting points

Answer 5

• When oppositely charged ions attract one another forming a giant ionic lattice there is a huge lowering of energy through very strong attraction, so although the amount of initial energy required to form ions is large the lowering of the energy on forming the lattice more than compensates for this

Question 6

What are the key features of a Born Haber cycle

Answer 6

• A continuous cycle is formed that can start at the elements and end at the elements
• It includes one step that shows the formation of one mole of the solid ionic lattice from gaseous ions which corresponds to the lattice enthalpy
• The remaining step shows intermediate changes that correspond to key enthalpy changes that can be measured
• The lattice enthalpy can be calculated by applying Hess’ law. If a reaction can take place via more than one route and the initial and final conditions are the same the total enthalpy change for each route is the same

Question 7

Draw a Born Haber cycle

Answer 7

Draw one

Question 8

define enthalpy change of formation

Answer 8

• This is the energy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions
• Usually exothermic as bonds are formed
• WRITE AN EQAUTION

Question 9

define ionisation energy

Answer 9

• This is the energy change that takes place when an electron is taken from the outer shell of 1 mole of gaseous atoms and this forms one mole of gaseous 1+ ions is formed under standard conditions in standard form
• Endothermic process because the electron being lost has to overcome attraction from the nucleus in order to leave the atom
• WRITE AN EQAUTION

Question 10

Define second ionisation energy

Answer 10

• This is when one mole of gaseous 2+ ions are formed from one mole of gaseous 1+ ions
• Endothermic process because the electron being lost has to overcome attraction from the nucleus
• WRITE AN EQAUTION

Question 11

Define enthalpy change of atomisation

Answer 11

• This is the energy change that takes place when one mole of gaseous atoms is formed from its element in its standard state
• Its always an endothermic process because bonds have to be broken
• WRITE AN EQAUTION

Question 12

Define electron affinity

Answer 12

• Electron affinity is the opposite of ionisation energy as it is the addition of electrons
• One mole of gaseous 1- ions is formed from gaseous atoms
• This is an exothermic process because the electron is attracted into the outer shell of an atom by the nucleus
• WRITE AN EQAUTION

Question 13

Define second electron affinity

Answer 13

• One mole of gaseous 2- ions is formed from gaseous 1- ions
• This is an endothermic process because the electron is repelled by the 1- ion, it has to overcome this repulsion
• WRITE AN EQAUTION

Question 14

Define enthalpy change of solution

Answer 14

• This is the enthalpy change that takes place when one mole of a solute is completely dissolved in water under standard conditions
• WRITE AN EQUATION

Question 15

Define enthalpy change of hydration

Answer 15

• This is the enthalpy change that takes place when dissolving one mole of gaseous ions in water
• WRITE AN EQUATION

Question 16

CONSTRUCT AN ENTHALPY CHANGE OF SOLUTION BORN HABER CYCLE

Answer 16

CONSTRUCUT AN ENTHALPY CHANGE OF SOLUTION BORN HABER CYCLE

Question 17

Qualitative explanation of the effect of ionic charge and ionic radius on the exothermic value of a lattice enthalpy can the enthalpy change of hydration

Answer 17

• Size of the ions if the ions are smaller they can get closer together so will attract one another more strongly, more charge dense
• Charges of the ions the ions with higher charges cause greater electrostatic attraction and more exothermic lattice enthalpy values
• Ionic bond strength
• They will attract one another more strongly and give rise to more exothermic lattice enthalpy values

Question 18

What happens when a solid dissolves

Answer 18

• Ionic lattice breaks down and the free ions become part of the solution
• A change in enthalpy occurs when this overall process happen, the enthalpy change is known as the standard enthalpy change of solution
• Can be endothermic or exothermic,
• The substance that is dissolved is known as the solute and the substance that it dissolves in is known as the solvent

Question 19

what happens when the ionic lattice breaks down

Answer 19

• The processes are identical but the reverse of one another
• Lattice enthalpy has a negative sign and is exothermic
• The breakdown of the ionic lattice has a positive sign and is endothermic

Question 20

Describe hydration

Answer 20

• Once the ionic lattice has broken down to its ions they have to become part of the solution
• The ions can do this if the solvent (water) can interact with them in similar ways to the bonding lattice therefore the ionic solids are able to dissolve in polar solvents such as water
  During hydration
  The positive ions will be attracted to the slight negative oxygen in water molecules
  The negative ions will be attracted to the slightly positive hydrogens in the water molecules
  Water molecules will surround the ions
  The enthalpy change occurs when ions become hydrated, energy is released and new bonds are forms between ions and water molecules

Question 21

What dictates the magnitude of enthalpy of hydration

Answer 21

• Size of ions – as size increases the enthalpy of hydrations becomes less exothermic, as size decreases charge increases and the enthalpy change of hydration becomes more exothermic
• Charges on the ions

Question 22

What is entropy

Answer 22

Explanation that entropy is a measure of the dispersal of energy in a system which is greater, the more disordered a system

Question 23

What is the standard entropy of a substance

Answer 23

• The standard entropy of a substance is the entropy content of one mole of the substance under standard conditions

Question 24

What is the standard entropy change of a reaction

Answer 24

• The standard entropy change of reaction is the entropy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions with all reactants and products being in their standard states

Question 25

What is entropy measured in

Answer 25

JK-1mol-1

Question 26

Describe the two things that you need to know about entropy

Answer 26

• Entropy is always a positive number above 0

- At 0 k entropy is zero for perfect crystals

Question 27

Most substances are thermodynamically stable…..

Answer 27

at the lowest energy state, this would correspond to a low entropy, however entropy tends to increase, liquid, water naturally evaporate into gaseous water increasing entropy, heat energy spreads out from hot objects increasing entropy, salt particles dissolve in water increasing entropy

Question 28

Ways to think about entropy

Answer 28

• Entropy is more spreading out of energy = higher entropy
• More random arrangements of particles= higher entropy
• Entropy can change during the course of a chemical process or reaction
• It is possible for entropy to decrease

Question 29

Explanation of the difference in magnitude of the entropy of a system

Answer 29

• Of solids – solid turning to liquid results in products that allow more disorder, this means that the ions can spread out and the positions of the ions are more disordered than within the lattice therefore entropy increases
• Liquids
• gases – if number of gas molecules increase during a reaction the entropy increases, if the number of gas molecules decrease during a reaction the entropy decreases
• Temperature – entropy increases with increasing temperature, as particles at higher temperatures have higher energy and move more, the arrangement of particles at higher temperatures becomes more random

Question 30

Calculation of the entropy change of a system and related quantities for a reaction given the entropies of the reactants and products

Answer 30

• Equation = S = sum of products – sum of reactants
• if a change makes a system more random then S is positive
• if a change makes the system more ordered then S is negative

Question 31

what is free energy change

Answer 31

this is the balance between enthalpy, entropy and temperature for a process

• G = H - TS
• a process can take place spontaneously when G is less than O

Question 32

for a process to happen spontaneously …

Answer 32

• entropy must increase, but there are examples of reactions that happen spontaneously that seem to lower entropy for example a metal rusting in oxygen or a gas condensing
• need to consider the entropy change of both the system and the surroundings

Question 33

how to calculate the total change in entropy

Answer 33

total = system + surroundings

• for spontaneous changes to occur the total change in entropy must be positive
• reactions that have a decrease in entropy can occur spontaneously if the change in entropy of the surroundings is positive enough to make the total change in entropy positive

Question 34

describe free energy

Answer 34

• changes in entropy are linked with the energy changes that occur during a reaction or change
• entropy is a measure of the dispersal of energy a reaction that would have a positive entropy change still may only be spontaneous at certain temperatures
• reaction and processes have to be considered in terms of enthalpy changes, total entropy changes and temperature

Question 35

large increases in entropy….

Answer 35

will cause decreases in G becasue teh term -TS will become larger

Question 36

large negative values for H…

Answer 36

will result in more negative values for G

Question 37

exothermic reactions are generally…

Answer 37

spontaneous, the negative value of H is usually still able to make G negative even if the entropy change is positive

Question 38

endothermic changes are only …

Answer 38

spontaneous if the entropy is positive and the temperature change is high enough to make TS large and positive

Question 39

limitations of using G to predict feasibility of reactions..

Answer 39

• calculating the value of free energy G gives a theoretical answer for whether a reaction or process will react spontaneously
• if a reaction has a negative value of G this doesnt mean that it will go onto react it depends on other kinetic factors such as..
• the reaction may have a high activation energy, energy needs to be supplied to overcome this
• the rate of reaction may be extremely slow
• equally the reaction that have positive values of G are considered not feasible, they can be made to take place by changing the temperature of the reaction