3.6 Enthalpy Changes

Question 1

What do enthalpy changes measure?

Answer 1

Measure the changes in energy during any chemical or physical change (energy and enthalpy at a level can be treated as being the same at a level)

Question 2

What is the principle of conservation of energy

Answer 2

Energy can’t be created or destroyed p, only converted from one form to another

Question 3

What is Hess’ law?

Answer 3

The energy change if any chemical reaction is the same regardless of the route taken

Question 4

What do you measure to calculate the energy change of a chemical reaction?

Answer 4

The energy given off in a reaction as it = the energy decrease of the chemicals in the reaction

Question 5

What is an exit heroic reaction?

Answer 5

A reaction that gives out energy as heat, leaving less chemical energy in the products than was present in the reactants

Question 6

What is an endothermic reaction?

Answer 6

A reaction that takes in energy as heat, leaving more chemical energy in the products than was present in the reactants at the start

Question 7

What is the enthalpy change of reaction?

Answer 7

The change in the chemical energy present in the substances during a chemical reaction(🔺H)

Question 8

Describe Hess’ law (energy cycles)

Answer 8

If a reaction can take place by more than one route, then the total energy change for each route will be the same

Question 9

What is important to note about the arrows in energy cycles ?

Answer 9

If you go against the direction of the arrow e.g A

Question 10

Which simple Calculation could you do it calculate the enthalpy change of reaction when you are given formation data only

Answer 10

Enthalpy change of reaction = 🔺fH (for all products) - 🔺fH (for all reactants)

Question 11

What should you do if there are two molecules of a substance In an enthalpy cycle?

Answer 11

Ensure that you double the 🔺H value of the substance (2x enthalpy of formation of the substance)

Question 12

What does the little circle symbol stand for after 🔺H?

Answer 12

It shows that it is the enthalpy change of a reaction under standard conditions

Question 13

What are the standard conditions?

Answer 13

Temperature = 298k (25 degrees c)
Concentration = 1 mol dm-3 for solutions 
Pressure = 101 kpa or 1 atm

Question 14

What is the standard state?

Answer 14

The physical state of a substance under standard conditions such as oxygen gas, liquid water and NaCl solid

Question 15

Define standard enthalpy change of reaction
⚫️
(🔺H )

Answer 15

The enthalpy change that occurs in a reaction between molar quantities in their stands,from states under standard conditions

Question 16

What is the standard enthalpy change of atomisation?
⚫️
(🔺atH )

Answer 16

The enthalpy change that occurs when one mole of atoms of an element in the gas phase are formed from the element in its standard state under standard conditions
E.g Na(s) —> Na(g) or 1/2Cl2(g) —> Cl(g) or 1/4P4(s) —> P(g)

Question 17

What is the standard enthalpy change of lattice formation?
⚫️
(🔺lattH )

Answer 17

The enthalpy change that occurs when one mole of an ionic compound is formed from ions of the elements in the gas phase
E.g Na+ (g) + Cl-(g)—> NaCl(s) or Ca2+ (g) + 2Cl-(g) —> CaCl2(s)

Question 18

What is also important to consider when thinking about the enthalpy change of lattice formation?

Answer 18

You may also see the enthalpy or breaking which is the reverse process, the energy change that occurs when one mole of an ionic compounds is split into ions of the elements in the gas phase.
E.g NaCl(s)—> Na+(g) + Cl-(g)

Question 19

What is the standard enthalpy change of hydration?
⚫️
(🔺hydH )

Answer 19

The enthalpy change that occurs when one mole of an ionic compound in solution is formed from ions of the element in the gas phase.
E.g Na+(g) + Cl-(g) + (aq) —> NaCl (aq) or
Ca2+(g) + 2Cl-(g) + (aq) —>CaCl2(aq)

Question 20

What is electron affinity

Answer 20

The enthalpy change that occurs when one mole of gaseous negative ions are formed from gaseous atoms of a substance by gaining one electron
E.g Cl(g) + e- —> Cl-(g) or O(g) + e- —> O-(g)

Question 21

What is ionisation energy

Answer 21

The enthalpy change that occurs when one mole of gaseous positive ions are formed from gaseous atoms of a substance by losing an electron
E.g Na(g) —> Na+(g) + e- or Cu(g) —> Cu2+(g) + 2e-

Question 22

Briefly describe an ionic substance

Answer 22

A lattice of positivity and negatively charged ions held together by electrostatic forces

Question 23

What just happen in order to allow an ionic substance to dissolve

Answer 23

The forces between the ions must be broken (to do this they must be replaced by other forces).
The water molecules are dipolar and :: the oxygen atoms surround the positively charged ions and the hydrogen atoms surround the negatively charged ions, :: the ions become hydrated

Question 24

What is the standard enthalpy change of a solution?

Answer 24

The enthalpy change that occurs when one mole of a substance dissolved completely in a solvent under standard conditions to form a solution.
It is the sum of the enthalpy of lattice breaking and the enthalpy of hydration
M+X-(s) —> M+(g) + X-(g) —> M+(aq) + X-(aq)

Question 25

Is the enthalpy of lattice breaking endothermic/exothermic?

Is the enthalpy of hydration endothermic/exothermic?

Answer 25

Lattice breaking = endothermic

Hydration = exothermic

Question 26

When would a salt dissolve?

Answer 26

When the lattice of hydration is greater than the enthalpy of lattice breaking

Question 27

When will a salt NOT dissolve

Answer 27

When the enthalpy of lattice breaking is greater than the lattice of hydration, the salt will not usually dissolve

Question 28

Name the pattern between enthalpy and solubility

Answer 28

The more exothermic the total of these values the more soluble a salt is likely to be.

Question 29

What two factors effect both the enthalpy of lattice breaking and enthalpy of solution?
What does this mean?

Answer 29

Both are increased by the increasing charge on the ions and increased by decreasing size of the ions.
This means that it is difficult to predict solubility from first principles and so patterns in solubility are used to predict whether particular salts are soluble or not.

Question 30

What groups of compounds are soluble?

Answer 30

1) All group 1 compounds
2) All ammonium compounds
3) All metal nitrates

Question 31

What groups of compounds are insoluble except when the rules above say they are soluble?

Answer 31

1) All metal carbonates
2) All metal hydroxides
3) All lead compounds

Question 32

What are the 4 steps in a born-Harbour cycle?

Answer 32

1) enthalpy of atomisation (formation of gas phase atoms)
2) ionisation energy (formation of a cation)
3) formation of an anion
4) enthalpy lattice of formation

Question 33

Describe the first process of a born-harbour cycle, the enthaloy of atomisation

Answer 33

The energy required to form one mole of atoms of an element in the gas phase from the element in its standard state
E.g Na(s) —> Na(g) or 1/2Cl2(g) —> Cl(g)

Question 34

Describe the second process seen in born-harbour cycles, ionisation energy.

Answer 34

The formation of a metal cation requires the atom to lose one or more electrons, and this requires energy to be out into the atom
E.g I.E = Na(g) —> Na+(g) +e-
Process is always endothermic as you need energy to remove an election from an atom

Question 35

What is the link between the enthalpy of atomisation of a diatomic gas and bond enthalpy?

Answer 35

the enthalpy of atomisation of a diatomic gas is half the bond enthalpy

Question 36

What is the name of the scientists who invented the born-harbour cycles?

Answer 36

Max born and Fritz harbour

Question 37

Describe the third process in born-harbour cycles, the formation of anions.

Answer 37

When a non-metal atom gains an electron to form an anion with a charge of -1, the energy changes is called the electron affinity
Electron affinity= Cl(g) + e- —> Cl-(g)

Question 38

Describe the forth process in the born-harbour cycles, the enthaloy lattice of formation.

Answer 38

Lattice enthalpies represent the energy released when the positive and negative ions in an ionic compound come together to form a solid.
Na+(g) +Cl-(g) —> NaCl (s)

Question 39

Why is the lattice enthalpy of formation always exothermic?

Answer 39

Since the formation of cation is ALWAYS endothermic and the formation of anions is often endothermic there would not be any ionic compounds formed without any other energy to balance this out, hence it’s always exothermic .

Question 40

State the relationship between the exothermic enthalpy of lattice formation and the stability of the ionic compound.

Answer 40

The more exothermic the enthalpy is, the more stable the ionic compound.

Question 41

What MUST you include when constructing born-harbour cycles?

Answer 41

1) The physical states for every substance as the enthalpy for a substance is different in different physical states
2) the arrows are draw clearly pointing in a certain direction
3) make sure to multiply the 🔺H value by the value of molecules off there is more than one molecule (look at cycle on pg 66)

Question 42

State the link between a negative enthalpy change of formation of a compound and it’s stability

Answer 42

If the enthalpy change of formation of a compound in negative then energy has been lost as the compound is formed from its elements and hence the compound is more stable than its elements.
The more negative the enthalpy change of formation of a compound is the more stable it is.

Question 43

State the link between a postive enthalpy change of formation of a compound and it’s stability

Answer 43

If the enthalpty change of formation of a compound is positive then the compound is unstable as compared with the elements that make it up. These elements need energy to change into the compound so they can exist but don’t decompose as the process would be too slow.

Question 44

What should you note about discussing stability of compounds in terms of enthalpy of formation.

Answer 44

The stability is relative to the elements within the compound