9.1 Enthalpy Changes, 9.2 Measuring Enthalpy Changes, 9.3 Bond Enthalpies & 9.4 Hess' Law and Enthalpy Cycles

Question 1

What is enthalpy (H)?

Answer 1

• A measure of the heat energy in a chemical system

Question 2

What is activation energy?

Answer 2

• The minimum amount of energy required for a reaction to take place

Question 3

How do we measure enthalpy change?

Answer 3

• Under standard conditions

Question 4

What are the standard conditions?

Answer 4

• Pressure: 100 kPa
• Temperature: 298 K
• Standard states (physical states under these conditions)

Question 5

What is standard enthalpy change, and its symbol?

Answer 5

• The enthalpy change of a reaction when measured under standard conditions
• ΔH⦵

Question 6

What is standard enthalpy change of reaction, and its symbol?

Answer 6

• 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
• ΔrH⦵

Question 7

How can the standard enthalpy change of reaction differ?

Answer 7

• If you have the same chemical equation, but with each chemical shown to have twice the molar quantity, the standard enthalpy change of reaction also doubles, etc.

Question 8

What is the standard enthalpy change of formation, and its symbol? Required.

Answer 8

• The enthalpy change 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 states
• ΔfH⦵

Question 9

What is the standard enthalpy change of combustion, and its symbol? Required.

Answer 9

• The enthalpy change that takes place when one mole of a substance is reacted completely with oxygen under standard conditions, with all reactants and products in their standard states
• ΔcH⦵

Question 10

What is the standard enthalpy change of neutralisation, and its symbol? Required.

Answer 10

• The enthalpy 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
• ΔneutH⦵

Question 11

What is the formula for specific heat capacity, and what do each of the symbols stand for? Include units.

Answer 11

• q = mcΔT
• q: heat energy (J)
• m: the mass that changes temperature (g)
• c: the specific heat capacity of the substance that changes temperature
• ΔT: change in temperature measured

Question 12

What is average bond enthalpy? Required.

Answer 12

• When 1 mole of bonds of gaseous covalent bonds are broken

Question 13

Write a symbol equation showing the bond enthalpy of Cl2.

Answer 13

• Cl2 (g) -> 2Cl (g)

Question 14

What are 2 limitations of average bond enthalpies?

Answer 14

• The actual bond enthalpy varies according to the chemical environment of the bond (which other bonds the atoms are involved in)
• Average bond enthalpy is calculated using the actual bond enthalpies from different chemical environments

Question 15

Which processes are endothermic and exothermic?

Answer 15

• Bond breaking is endothermic
• Bond making is exothermic

Question 16

What is Hess’ law?

Answer 16

• The overall enthalpy change of a reaction is independent of the route it takes

Question 17

How is the enthalpy change of combustion used to indirectly calculate the enthalpy change of a reaction?

Answer 17

• The products from the combustion of the reactants/ products are written at the bottom
• The enthalpy change of combustion of the reactants and the products are used to calculate the enthalpy change

Question 18

How is the enthalpy change of formation used to indirectly calculate the enthalpy change of a reaction?

Answer 18

• The elements needed to form the reactants/ products are written at the bottom
• The enthalpy change of formation for the reactants and the products are used to calculate the enthalpy change