9.1 - Enthalpy Changes

Question 1

Define enthalpy

Answer 1

Measure of hat energy in a system (chemical)

Question 2

Define chemical system

Answer 2

Atoms, molecules or ions making up the chemicals

Question 3

Define enthalpy change

Answer 3

= H(products)-H(reactants)

It cannot be measured directly

Question 4

What is the law of conservation of energy?

Answer 4

Energy cannot be created or destroyed. It is always transferred

Question 5

Where is energy transferred between in a chemical reaction with a change in energy?

Answer 5

Between system and surrounding

Question 6

Define

i) System
ii) Surrounding
iii) Universe

Answer 6

i) Reactants & products
ii) Everything not in the chemical system
iii) System + surrounding

Question 7

What are the features of an exothermic reaction?

Answer 7

• More energy released when making bonds than taken in when breaking them
• Heat transferred to surroundings
• Energy loss in system, energy gain in surroundings
• Delta H is -ve
• Surrounding temperature increases

Question 8

What are the characteristics of an exothermic enthalpy profile diagram?

Answer 8

• Delta H arrow goes down (from reactant line to products)

- E(a) is from reactant line to top of peak

Question 9

What are features of an endothermic reaction?

Answer 9

• Energy transferred from surroundings to system
• Delta H is +ve
• Energy gain in system balanced by energy loss in surroundings
• Surrounding temperature decreases
• More energy taken in to break bonds than released to form bonds

Question 10

What are the characteristics of an endothermic enthalpy profile diagram?

Answer 10

• Reactants line lower than products
• Delta H arrow from reactants line UP to products line
• E(a) from reactants line up to peak of graph (NOT products line)

Question 11

Define activation energy.

Answer 11

• Bonds that exist between atoms and ions in reactants need to be broken for a reaction to occur
• E(a) is the minimum amount of energy input required to break these bonds and start a reaction

Question 12

What are standard enthalpy changes and what are the standard conditions?

Answer 12

Standard Enthalpy Changes: Enthalpy changes are measured under standard conditions

Standard Conditions:
100kPa
25 degrees C
Conc.: 1mol/dm3
(states: physical states of substance under standard conditions)

Question 13

Define enthalpy change of reaction.

Answer 13

Enthalpy change of a given reaction in the molar quantities stated in the reaction with all reactants and products in standard states
(State standard conditions)

Question 14

Define enthalpy change of formation.

Answer 14

Enthalpy change when 1 mole of a compound is formed from its constituent elements under standard conditions with all elements in standard states.
(State standard conditions)

Question 15

What is the enthalpy change of formation of an element?

Answer 15

0kg/mole as it is the formation of an element from the same element.

Question 16

Define enthalpy change of combustion.

Answer 16

Enthalpy change when 1 mol of a substance reacts completely with oxygen under standard conditions with all reactants and products in their standard states
(State standard conditions)

Question 17

Define enthalpy change of neutralisation.

Answer 17

Enthalpy change of the reaction of an acid with a base to from one mole of water
(Standard conditions and states)