Chapter 9 Enthalpy

Question 1

What is enthalpy change?

Answer 1

The heat energy transferred in a reaction at constant pressure
△H = KJmol^-1

Question 2

What is an exothermic reaction?

Answer 2

Energy is transferred from the system to the surrounding
△H is negative
This causes the temperature of the surroundings to increase
Eg, combustion of methane
CH4 + 2O2 —-> CO2 + 2H2O

Question 3

What is an endothermic reaction?

Answer 3

Energy is transferred from the surroundings to the system
△H is positive
Temperature of the surroundings decreases
Eg, thermal decomposition of calcium carbonate
CaCO3 —-> CaO + CO2

Question 4

What is activation energy?

Answer 4

The minimum amount of energy required to start a reaction and overcome reactant bonds

Question 5

How is the stability of enthalpy affected?

Answer 5

The less enthalpy a substance has, the more stable it is

Question 6

State the standard conditions for temperature and pressure

Answer 6

298K
100KPa

Question 7

State the importance of using standard conditions

Answer 7

Using standard conditions means the enthalpy change results have not been affected by temperature of pressure

Question 8

What is Standard enthalpy change of a reaction?

Answer 8

The enthalpy change when a chemical reaction occurs under standard conditions in the stoichiometric ratios of the balanced equation

Question 9

What is the Standard enthalpy change of formation?

Answer 9

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions

Question 10

What is the standard enthalpy change of combustion?

Answer 10

The enthalpy change when one mole of a substance is completely combusted in oxygen under standard conditions

Question 11

What is Standard enthalpy change of neutralisation?

Answer 11

The enthalpy change when one mole of water is formed in the neutralisation reaction between an acid and alkali under standard conditions

Question 12

State how bonds change in an endothermic reaction

Answer 12

Energy is absorbed when bonds are broken, so bond breaking is endothermic

Question 13

State how bonds change in an exothermic reaction

Answer 13

Energy is released when bonds are formed, so bond making is exothermic

Question 14

What is bond dissociation enthalpy?

Answer 14

Amount of energy needed per mole to break gaseous compounds

Question 15

What is average bond enthalpy?

Answer 15

The energy required to break one mole of a covalent bond under gaseous conditions

Question 16

What is the equation used to find the standard enthalpy change of combustion?

Answer 16

Combustion = △ H Reactants - △ H Products

Question 17

What is the equation used to find the standard enthalpy change of formation?

Answer 17

Formation= △H Products - △H Reactants

Question 18

What is the equation used to find out the standard enthalpy change of neutralisation?

Answer 18

Q= mc △T
Q= energy in Joules
M= mass in grams (measuring the mass which changes temperature)
C= specific heat capacity
△T= change in temperature in degrees or kelvin

Find enthalpy change by = energy in KJ / moles

Question 19

Describe why the enthalpy change may not match the data books value

Answer 19

-Energy may be lost to the surroundings other than to the system
-Incomplete combustion
-Evaporation of volatile substances
-Non standard conditions

Question 20

What does Hess Law state?

Answer 20

Total enthalpy change of a reaction is always the same no matter which route is taken

Question 21

What is the average bond enthalpy equation?

Answer 21

Enthalpy change of reaction = energy in — energy out
Reactants — Products