8: Energetics I

Question 1

What are the standard conditions

Answer 1

• Pressure 100 kPa
• Temperature 298 K
• Standard states (solid, liquid or gas)

Question 2

Define the standard Enthalpy change of reaction and its sign

Answer 2

The Enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions (exo/endo)

Question 3

Define the standard Enthalpy change of formation and its sign

Answer 3

The Enthalpy change when one mole of compound is formed from its elements under standard conditions (exo/endo)

Question 4

Define the standard Enthalpy change of combustion and its sign

Answer 4

The Enthalpy change when one mole of substance is burnt in excess oxygen under standard conditions (exo)

Question 5

Define the standard Enthalpy change of neutralisation and its sign

Answer 5

The Enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions (exo)

Question 6

What is the standard Enthalpy of formation of an element in its standard state

Answer 6

0

Question 7

What are factors of exothermic reactions

Answer 7

• Heat energy is given off by the reactions to the surroundings
• Products have less energy than reactants
• Energy of system decreases
• Energy and temperature of surroundings increases
• Negative delta H
• Thermodynamically feasible
• Kinetically hindered

Question 8

What is the exothermic Enthalpy level diagram

Answer 8

Question 9

What are the factors of endothermic reactions

Answer 9

• Heat energy is absorbed by the reaction from the surroundings
• Products have more energy than the reactants
• Temperature and energy of the surroundings decreases
• Energy of the system increases
• Positive delta H

Question 10

What is the endothermic Enthalpy level diagram

Answer 10

Question 11

Describe the experiment measuring Enthalpy changes

Answer 11

• Add the reaction mixture to a polystyrene cup
• Place a plastic lid over the top
• Check the initial temperature
• Check the final temperature

Question 12

What is calorimetry

Answer 12

The measurement of Enthalpy changes in chemical reactions, done in calorimeters (e.g. polystyrene cup)

Question 13

What is the equation for heat transferred

Answer 13

Question 14

What is the equation for Enthalpy change

Answer 14

Question 15

What is Hess’s Law

Answer 15

The total Enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same

Question 16

What law of conservation of energy inspired Hess’s law

Answer 16

Energy cannot be created or destroyed, it can only change form (in a closed system the total amount of energy present is always constant)

Question 17

Why is Hess’s Law used

Answer 17

To calculate Enthalpy changes that can’t be found experimentally (e.g. using a calorimeter)

Question 18

How to calculate Hess’s Law

Answer 18

• Arrows drawn away/towards products or reactants based on the definition of the Enthalpy change it is calculating
• To calculate delta H, follow the arrows (e.g. -delta H1+delta H2)

Question 19

Define bond (dissociation) Enthalpy

Answer 19

The energy required to break a bond

Question 20

Is breaking bonds endothermic or exothermic

Answer 20

Endothermic

Question 21

Is making bonds endothermic or exothermic

Answer 21

Exothermic

Question 22

What happens to bond Enthalpy when the reaction is exothermic

Answer 22

• It means that more bonds are being made than broken
• Products are more stable than reactants

Question 23

What is the endothermic bond Enthalpy profile

Answer 23

• More bonds are being broken than made
• Products are less stable than reactants

Question 24

What is average bond Enthalpy

Answer 24

The energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds

Question 25

What is the equation for bond Enthalpy

Answer 25

• Remember definitions and moles