Chapter 9 Enthalpy

Question 1

What is ENTHALPY?

Answer 1

Enthalpy (H) is the measure of the heat energy in a chemical system.

The chemical system refers to the atoms, molecules, or ions making up the chemicals.

• (sometimes thought of as the energy stored within bonds)
• enthalpy cannot be measured but enthalpy changes can be.

Question 2

What is an enthalpy change?

Answer 2

In a chemical reaction, the reactants and products are likely to have different enthalpies.
The difference in the enthalpies = enthalpy change (∆ H)

∆ H = H (products) - H (reactants)
- can be positive OR negative, depending on whether the products contain more or less energy than the reactants

• can be determined by measuring the energy transfer between the system and the surroundings

Question 3

What is the law of conservation of energy?

Answer 3

Energy can not be created or destroyed.

Question 4

Energy transfer??

Answer 4

Energy transferred from the system to the surroundings = EXOthermic

Energy transferred from the surroundings to the system = ENDOthermic

Question 5

Exothermic?

Answer 5

Energy transferred from the system to the surroundings.
- temperature of the surroundings increases as they gain energy

∆ H is NEGATIVE

Question 6

Endothermic?

Answer 6

Energy transferred from surroundings into system.
- the temperature of the surroundings decrease as they lose energy

∆H is POSITIVE

Question 7

What is activation energy?

Answer 7

The minimum energy required for a reaction to take place.

• reactions with small activation energies take place very rapidly, because the energy needed to break bonds is readily available from the surroundings
• very large activation energies may present such a large energy barrier that a reaction may take place very slowly or not at all

Question 8

What are the standard conditions?

Answer 8

• enthalpy change = kJ/mol
• standard pressure = 100 kPa
• standard temp. = 298 K
• standard concentration = 1mol/dm^3
• standard state = data table

Question 9

What is a standard enthalpy change?

Answer 9

∆H (standard sign)
- enthalpy change under standard conditions

Question 10

Define standard enthalpy change of reaction?

Answer 10

The standard enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states.

Question 11

Define enthalpy change of formation?

Answer 11

The standard enthalpy change of formation is the enthalpy change that takes place when one mole of a compound if formed from its elements under standard conditions, with all reactants and products in their standard states.

Question 12

Define enthalpy change of combustion?

Answer 12

The standard enthalpy change of combustion is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.

Question 13

Define the standard enthalpy change of neutralisation?

Answer 13

The standard enthalpy change of neutralisation is the energy change that accompanies the reaction of an acid by a base to form one mole of H2O (l), under standard conditions, with all reactants and products in their standard states.

Involves the reaction of H+ (aq) with OH- (aq) to form 1 mole of H2O (l).

• the value of enthalpy change of neutralisation is the SAME for ALL neutralisation reactions.

Question 14

What is 0K in degrees Celsius?

Answer 14

• 273 degrees Celsius

Question 15

At what temperature does ice melt on the Kelvin scale?

Answer 15

273 K = (0 degrees Celsius)

Question 16

At what temperature does water boil at on the Kelvin scale?

Answer 16

373 K = (100 degrees Celsius)

Question 17

How do you calculate an energy change?

Answer 17

• mass (of surroundings) = GRAMS (g)
• specific heat capacity = Joules per gram per Kelvin
• temperature change = KELVIN (K)

Question 18

What is specific heat capacity?

Answer 18

The energy required to raise the temperature of 1g of a substance by 1K.

• (good conductors of heat have SMALL SHC values)
• (insulators of heat have LARGE SHC values)
• water SHC = 4.18 J/g/K

Question 19

Calculating an energy change formula??

Answer 19

Q = mc∆T

Q = heat energy = JOULES (J)

Question 20

Determination of enthalpy change of combustion PRACTICAL??

Answer 20

1) measure out water, pour in beaker, and record initial temp.

2) add methanol (alcohol) to the spirit burner and weigh the spirit burner

3) place the spirit burner under the beaker, light it, and burn the methanol (alcohol) whilst stirring the water (w/ thermometer)

4) after a few minutes extinguish the flame, and record the MAX temperature reached by the water

5) reweigh the spirit burner (assume the wick hasn’t been burnt)

Calculation…

1) calculate energy change
2) calculate amount in mol. of alcohol burnt
3) calculate standard enthalpy change of combustion (complete of 1 mol.)

Question 21

What is the accuracy of the experimental enthalpy change of combustion value?

Answer 21

Not accurate due to…

• HEAT LOSS to the surroundings other than water
• INCOMPLETE combustion of methanol
• EVAPORATION of alcohol from the wick
• NON-STANDARD conditions

(All but the last of these reasons would lead to the value of enthalpy change of combustion being LESS exothermic than expected).

Question 22

Determination of enthalpy change of reaction??

Answer 22

1) calculate energy change in the solution in kJ

2) calculate the amount (mol.) of reactant (not in excess) that reacted

3) calculate the enthalpy change of reaction (kJ/mol)

Question 23

Determination of enthalpy change of neutralisation??

Answer 23

1) calculate the energy change in the solution in kJ

2) calculate the amount (mol.) of acid and base that reacted

3) calculate the enthalpy change of reaction (kJ/mol)

Question 24

What is average bond enthalpy?

Answer 24

The energy required to break one mole of a specified type of bond in a gaseous molecule.

• energy is always required to break bonds
• bond enthalpies are always endothermic (+ POSITIVE value)

Question 25

What are the limitations of average bond enthalpies?

Answer 25

The actual bond enthalpy can vary depending on the chemical environment of the bond.

• it is calculated from the actual bond enthalpies in different chemical environments

Question 26

Bond breaking??

Answer 26

Energy is required to break bonds = ENDOTHERMIC

∆ H = POSITIVE

Question 27

Bond making??

Answer 27

Energy is released when bonds formed = EXOTHERMIC

∆ H = NEGATIVE

Question 28

What does the different between the energy required for bond breaking and the energy released in bond making determine?

Answer 28

Whether an overall reaction is EXO or ENDO

Question 29

How do you determine enthalpy changes from average bond enthalpies?

Answer 29

= ∑ (bond enthalpies in reactants) - ∑ (bond enthalpies in products)

Question 30

What is Hess’ Law?

Answer 30

If a reaction can take place by 2 routes, and the starting and finishing conditions are the same, then the total enthalpy change is the SAME for either route.

• comes from the idea of conservation of energy

Route 1 : A + B
Route 2 : C

A + B = C

Question 31

Indirect determination of enthalpy changes?? (Reaction)

Answer 31

You can work out the standard enthalpy change of any reaction from the standard enthalpy changes of formation of the reactants and products.

1) construct the enthalpy cycle between the reactants, products, and their elements (arrows)

2) add enthalpy change of formation value and calculate the unknown enthalpy change

Question 32

Indirect determination of enthalpy changes?? (Combustion)

Answer 32

‘It would be impossible to measure the enthalpy change of this reaction directly since C and H form so many compounds so the product would be formed alongside many others.’

• the enthalpy changes of combustion of the reactants and the product CAN be measured directly… so you use that.

1) construct the enthalpy cycle between reactants, products, and their common combustion products (CO2 and H2O)

2) add enthalpy change of combustion values and calculate unknown

Question 33

What are the 2 rules that can help with enthalpy cycle calculations?

Answer 33

(1) Using enthalpy changes of formation
= enthalpy change of reaction = enthalpy change of formation of products - enthalpy change of formation of reactants

(2) Using enthalpy changes of combustion
= enthalpy change of reaction = enthalpy change of combustion of reactants - enthalpy change of combustion of products