Energetics

Question 1

What is enthalpy?

Answer 1

A measure of the heat content of a substance. (H)

Question 2

What is enthalpy changes?

Answer 2

Change in heat content at constant pressure under standard conditions (ΔH)
Standard conditions (H°) - 100kPa and 298K

Question 3

What is thermochemistry?

Answer 3

Chemical reactions involve an energy change.
Overall change will result in either:
more energy put in to break bonds than is given out (endothermic)
More energy produced by forming bonds than to break bonds (exothermic)

Question 4

Describe an endothermic reaction

Answer 4

Change in energy is absorbed from the surroundings by the reactants to form the products.
Enthalpy of products increases (ΔH+)
More energy needed to break the bonds

Question 5

Describe an exothermic reaction.

Answer 5

Change in energy is released to the surroundings by the reactants to form the products.
Enthalpy of products decrease (ΔH-)
More energy needed to form the products.

Question 6

What is activation energy?

Answer 6

Minimum amount of energy needed to start a reaction. (Acts as a barrier to spontaneous change)

Question 7

What is the standard enthalpy change of reaction? - include the symbol

Answer 7

The enthalpy change for a reaction with the quantities shown in the chemical reactions. Value should always be quoted along with the equation - ΔH°r

Question 8

What is the standard enthalpy change of formation? - include the symbol

Answer 8

Enthalpy change when 1 mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions (ΔH°f).

Question 9

Why is the enthalpy of formation of an element 0?

Answer 9

By definition.

Question 10

What is the standard enthalpy change of combustion? - include the symbol

Answer 10

Enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions (ΔH°c).