Chapter 9: Enthalpy

Question 1

What is enthalpy?

Answer 1

Enthalpy (H) is the measure of the heat energy in a chemical system (atoms, molecules or ions making yo the chemicals). It cannot be measured by the enthalpy change can)

Question 2

What is enthalpy change and how can it be calculated?

Answer 2

Enthalpy change (ΔH) is the difference of energy between the reactants and the products.
ΔH = H(products) - H(reactants)

Question 3

What is the law of conservation of energy?

Answer 3

The law of conservation of energy states that energy cannot be created or destroyed. It is transferred between the system and the surroundings.

Question 4

What is an endothermic change and what’s does it enthalpy profile look like?

Answer 4

Energy that is transferred from the surroundings into the system.
It’s energy profile starts with the reactants line at low enthalpy and end with the product line at a higher enthalpy
ΔH = positive.

Question 5

What is exothermic change?

Answer 5

Energy that is transferred from the system to the surroundings.
The energy profile starts with the reactant line at high enthalpy and ends with the product line at a lower enthalpy. ΔH = negative

Question 6

What is activation energy?

Answer 6

The activation energy (Ea) is the minimum amount of energy required for a reaction to takes place.

Question 7

What are the standard conditions?

Answer 7

Standard pressure = 100 kPa
Standard temperature = 298 K
Standard concentration = 1 mol dm-3 (only for solutions)

Question 8

What is the definition of the standard state?

Answer 8

The physical state of substance under standard conditions.

Question 9

What is the enthalpy change of reaction?

Answer 9

The enthalpy change of reaction (ΔrH) is the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactant and products in their standard state.
Mg (s) + 1/2 O2 (g) —> MgO(s)

Question 10

What is the enthalpy change of formation?

Answer 10

The standard enthalpy change of formation (ΔfH) is the enthalpy changes 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 state.
Mg (s) + 1/2 O2 (g) —> MgO (s)

Question 11

What is the standard enthalpy change of combustion?

Answer 11

The standard enthalpy change of combustion (ΔcH) is the enthalpy change that takes place when one mole of s substance reacts completely with oxygen under standard conditions, with all the reactants and products in their standard state.
C4H10 (g) + 61/2O2 (g) —> 4CO2 (g) + 5H2O (l)

Question 12

What is the enthalpy change of neutralisation?

Answer 12

The standard enthalpy change of neutralisation (ΔneutH) is the energy change that accompanies the reaction of an acid by a base to from one mole of H2O(l), under standard conditions with all reactants and products in their standard states.
H+ (aq) + OH- (aq) —> H2O (l)

Question 13

What is the energy change equation and units?

Answer 13

q = mcΔT
q = energy change (J)
m = mass(g) (use the material changing temperature)
c = specific heat capacity
ΔT = change in temperature

Question 14

How do you determine the enthalpy change of combustion (ΔcH)?

Answer 14

1. Calculate the energy change of the water in KJ
   q = mcΔT then /1000 to get KJ
2. Calculate the amount of mole of organic product burnt
   mole = mass / mr
3. Calculate ΔcH in KJ mol(-1)
   answer from 1 (q in KJ) / answer from step 2 (mole)

Question 15

Why is the experimental value of ΔcH inaccurate?

Answer 15

• heat loss to the surroundings other than the water
• there may be some incomplete combustion with CO and C being produced instead of CO2
• evaporation is methanol from the wick
• it may have been done in not standard conditions
  These will all lead to a value of ΔcH that is less exothermic then expected.

Question 16

How to determine the enthalpy change of reaction ΔrH?

Answer 16

1. Calculate the energy change in the solution in KJ
   q = mcΔT / 1000
2. Calculate the amount in mol of the reactant that limited (not in excess) that reacted
   mole = conc x vol/1000
3. Calculate ΔrH in KJ mol(-1)
   kJ from step 1 / mole form step 2
   Always exothermic

Question 17

How to determine the enthalpy change of neutralisation ΔneutH?

Answer 17

1. Calculate the energy change q in the solution in KJ Calculate the total volume of by adding together both solutions
   q = mcΔT / 1000
2. Calculate the amount in mole of the two solution that reacted
   mole of solution 1 = mole of solution 2
   mole = conc x vol /1000
3. Calculate ΔneutH in KJ mol (-1)
   q from step 1 / mole from step 2

Question 18

What is average bond enthalpy?

Answer 18

Average bond enthalpy is the energy required to break one mole of a specific type of bind in a gaseous molecule.
- always endothermic
- always + enthalpy value

Question 19

How can you calculate enthalpy change from average bond enthalpy?

Answer 19

ΔrH = total bond enthalpies in reactants - total bing enthalpies in products

Question 20

What are the limitation of average bond enthalpies?

Answer 20

• only average bond enthalpies are used, however the actual energy involved in braking and making individual bonds may be different as they may be in different environments.
• calculating average bond enthalpies need all the species to be gaseous molecule so calculated Δrah is not standard enthalpy change.

Question 21

What is Hess’ law?

Answer 21

If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route.

Question 22

What does the enthalpy cycle of formation look like?

Answer 22

The common elements of the reactant and product are at the bottom with the enthalpy arrows (ΔfH) pointing upward toward the product and reactants.

Question 23

What does the enthalpy cycle of combustion look like?

Answer 23

Combustion products (H20 and CO2) at the bottom with arrows (ΔcH) pointed downward from the reactant and products to the combustion products.