1.4 - Energetics

Question 1

What energy change is breaking bonds associated with?

Answer 1

Endothermic Reaction - Energy is taken in.

Question 2

What energy change is making bonds associated with?

Answer 2

Exothermic Reaction - Energy is released.

Question 3

What are some uses of thermochemistry

Answer 3

Measuring and comparing the energy values of fuels.
Calculating the energy requirements for industrial processes.
Working out the theoretical amount of energy released/taken in, in a reaction.
Predicting whether a reaction will take place or not.

Question 4

What is an endothermic reaction?

Answer 4

It has an overall positive enthalpy change.
Energy in breaking bonds > Energy out making bonds.

Question 5

What is an exothermic reaction?

Answer 5

It has an overall negative enthalpy change.
Energy in breaking bonds < Energy out making bonds.

Question 6

If a reversible reaction is endothermic one way, what type of reaction is the other way?

Answer 6

Exothermic

Question 7

Examples of Exothermic Reactions.

Answer 7

• Combustion
• Neutralisation

Question 8

Examples of Endothermic Reactions.

Answer 8

• Photosynthesis
• Decomposition

Question 9

Define enthalpy change and give the symbol used to represent it.

Answer 9

Energy change of a system at a constant speed. It is represented by the symbol ‘ΔH’.

Question 10

What are the standard conditions.

Answer 10

100kPa / 1 atm pressure
298K / 25*C Temperature

Question 11

What does ‘in standard state’ mean?

Answer 11

• The state an element/coumpound exists at in standard conditions (100kPa, 298K).

Question 12

Define standard enthalpy of formation.

Answer 12

• The enthalpy change when one mole of a compound is formed from it constituent elements in standard conditions (100kPa, 298K), with rectants and prodcts in their standard states.

Question 13

Define standard enthalpy change of combustion.

Answer 13

• The enthalpy change when one mole of a substance is burnt completely in oxygen in standard conditions (100kPa, 298K), with reactants and products in their standard states.

Question 14

What is the difference between heat and temperature?

Answer 14

• Heat is the sum of al particles’ energy, therefore it is affected by the amount of substance; temperature is related to the mean kinetix energy of the particles in a system =, so is independant of the number of particles present.

Question 15

How can you calculate enthalpy change from experimental data?

Answer 15

Use the equation:
q = mc∆T
Where:
- m = the mass of the substance being heated.
- c = specific heat capacity
- ∆T = teperature change.

Question 16

What is a flame colorimeter; how does it differ to a simple calorimter?

Answer 16

• Reduces heat lost to the sorrounding to give more accurae results; has a spiral chimney made of copper, an enclosed flame and the fuel is burnt in pure oxygen, not air.

Question 17

What is Hess’s Law?

Answer 17

States that the enthalpy change for a reaction is the same regardless of teh route taken.

Question 18

Waht is the enthalpy change of an element?

Answer 18

The enthalpy change of all elements in their standard states (the states in which they exist at 100kPa and 298K) is defined as 0.

Question 19

Define the bond dissociation enthalpy.

Answer 19

The enthalpy change required to break a covalent bond, with all species in the gaseous state; differs for the same bond type in different molecules.

Question 20

Define mean bond enthalpy.

Answer 20

Average value (across different chemical environments) for the bond dissociation enthalpy of a given bond.

Question 21

Why may experimental methods for enthalpy determination not be very accurate?

Answer 21

• Heat is lost to the surroundings.
• Not in standard conditions.
• Reaction may not go to completion.

Question 22

Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?

Answer 22

Bond enthalpies are a mean for the same bond across different molecules; standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate.