A-LEVEL Chemistry: 3.1.4: Energetics (ChemRevise)

Question 1

What is ‘Enthalpy Change’?

Answer 1

Enthalpy Change is the Amount of Heat Energy Taken or Given Out During any Change in a System, Provided the Pressure is Constant.

Question 2

If Enthalpy Change Occurs, then Energy is Transferred Between the ___ & the ___.

Answer 2

System, Surroundings.

Question 3

Explain what happens in an ‘Exothermic Enthalpy Change’.

Answer 3

Energy is Transferred from the System to the Surroundings.
The Products have Less Energy than the Reactants.

Question 4

Explain what happens in an ‘Endothermic Enthalpy Change’.

Answer 4

Energy is Transferred from the Surroundings to the System.
They Require an Input of Heat Energy (eg Thermal Decomposition).
The Products have More Energy than the Reactants.

Question 5

What is the Symbol for Change in Enthalpy?

Answer 5

∆H

Question 6

What is the Enthalpy Change for an Exothermic Reaction?

Answer 6

∆H is Negative.

Question 7

What is the Enthalpy Change for an Endothermic Reaction?

Answer 7

∆H is Positive.

Question 8

Common Oxidation Exothermic Processes are… (2)

Answer 8

-The Combustion of Fuels

-Oxidation of Carbohydrates (eg Glucose in Respiration)

Question 9

What is the ‘Standard Enthalpy Change of Formation’?

Answer 9

The Standard Enthalpy Change of Formation of a Compound is the Enthalpy Change when 1 Mole of the Compound is Formed from its Elements, Under Standard Conditions, & with all Reactants & Products in their Standard States.
∆fH

Question 10

What is the Symbol for Standard Enthalpy Change of Formation?

Answer 10

∆fH

Question 11

What is the ‘Standard Enthalpy Change of Combustion’?

Answer 11

The Standard Enthalpy Change of Combustion of a Substance is Defined as the Enthalpy Change that Occurs when 1 Mole of a Substance is Combusted Completely in Oxygen, Under Standard Conditions, & with all Reactants & Products in their Standard States.
∆cH

Question 12

What is the Symbol for Standard Enthalpy Change of Combustion?

Answer 12

∆cH

Question 13

What Will Incomplete Combustion Lead to?

Answer 13

Soot (Carbon), Carbon Monoxide, & Water.
It Will be Less Exothermic than Complete Combustion.

Question 14

Enthalpy Changes are normally quoted at Standard Conditions: (4)

Answer 14

• 100kpa
• 298K (Room Temperature(25’C)
• Solutions at 1moldm^-3

-All Substances in their Standard State.

Question 15

Energy Change = …

Answer 15

Mass * Specific Heat Capacity * Temperature Change

q = mc∆t

Question 16

How do you Calculate the Enthalpy Change (∆H) from Experimental Data? (3)

Answer 16

• Using q=mc∆t, Calculate the Energy Change for Quantities Used.
• Work Out the Number of Moles of Reactants Used.
• Divide q by the Number of Moles of Reactants Used.

Question 17

Enthalpies of Combustion Can be Calculated by Using Calorimetry. Fuel is Burned, & the Flame is Used to Heat Up Water in a Metal Cup. What are the Errors in this Method? (6)

Answer 17

-Energy Loss from Calorimeter.

-Incomplete Combustion of Fuel.

-Incomplete Transfer of Energy.

-Evaporation of Fuel After Weighing.

-SHC of Calorimeter Not Included.

-Measurements Not Carried Out Under Standard Conditions, as H2O is Gas, Not Liquid, in this Experiment.

Question 18

What is ‘Hess’s Law’?

Answer 18

Hess’s Law States that: ‘Total Enthalpy Change for a Reaction is Independent of the Route by which the Chemical Change Takes Place.’

It is a Version of the Law of Conservation of Energy.

Question 19

On an Energy Level Diagram, the Arrows Can Show…

Answer 19

The Different Routes a Reaction Can Proceed by.

Question 20

Often, Hess’s Law Cycles are Used to Measure the Enthalpy Change for a Reaction that Cannot be Measured Directly by…

Answer 20

Experiments.

Question 21

What is ‘Mean Bond Energy’?

Answer 21

The Mean Bond Energy is the Enthalpy Needed to Break the Covalent Bond into Gaseous Atoms, Averaged over Different Molecules.

Question 22

Why do we Use Values of Mean Bond Energy?

Answer 22

Because every Single Bond in a Compound has a Slightly Different Bond Energy.

Question 23

In an Exothermic Reaction, the Sum of the Bonds of the Reactant Molecules Will be ___ than the Sum of the Bonds of the Product Molecules.

Answer 23

Less.

Question 24

In an Endothermic Reaction, the Sum of the Bonds of the Product Molecules Will be ___ than the Sum of the Bonds of the Reactant Molecules.

Answer 24

Greater.

Question 25

(In general, if all Substances are Gasses):

Enthalpy Change = …

Answer 25

Total of Bond Energies Broken - Total of Bond Energies Formed

Question 26

There is a ___ ___ in the Enthalpies of Combustion for Successive Members of a ___ ___. Why is this?

Answer 26

Constant Rise, Homologous Series.
Because of the Increase in the Number of Carbon Atoms.