Energetics I

Question 1

What is meant by system?

Answer 1

The materials or chemicals being studied.

Question 2

What is meant by surroundings?

Answer 2

Everything around the system-in theory the whole universe (includes what you do the reaction in eg: test tube).

Question 3

What is a closed system?

Answer 3

A system where matter cannot be exchanged with the surroundings but energy can.

Question 4

What is enthalpy change?

Answer 4

The overall energy exchanged with the surroundings, when change happens at constant pressure and the final temperature in the same as the initial temperature.

Question 5

The symbol and units for enthalpy change are:

Answer 5

ΔH, kJmol^-1

Question 6

What are the axis labels for an energy level diagrams?

Answer 6

Energy on the y-axis and course of reaction on the x-axis.

Question 7

Equation for energy change:

Answer 7

E=mcθ

Where:
E is energy change in J.
m is mass of solution in g.
c is the specific heat capacity of water in Jg^-1K^-1.
θ is change in temperature in K.

Question 8

What is the specific heat capacity of water?

Answer 8

4.18 Jg^-1K^-1

Or 4.2 if stated in the question.

Question 9

Assumptions made when calculating energy change in a solution?

Answer 9

1g of solution is 1cm^3.
All energy is transferred to solution.
The solution has the same SHC as water.

Question 10

What apparatus do we use to obtain accurate enthalpy changes for combustion?

Answer 10

A bomb calorimeter.

Question 11

Why does a bomb calorimeter work well for calculating accurate enthalpy changes of combustion?

Answer 11

It is designed to ensure the sample burns completely with an adequate oxygen supply and energy losses are avoided. A higher pressure is used to ensure the sample burns completely.

Question 12

Values of enthalpy changes vary with what factors?

Answer 12

Temperature, pressure and concentration.

Question 13

Why do we have standard conditions for enthalpy changes?

Answer 13

To ensure that all enthalpy changes are measured evenly without varying temperature, pressure or concentration.

Question 14

Standard conditions are:

Answer 14

A pressure of 100 kPa (approx pressure of atm at sea level).
A stated temperature (usually 298K).
Substances in their most stable state at 100 kPa and stated temperature.
Solutions with a conc of 1 moldm^-3.

Question 15

In thermochemistry the states of substances are important so we must what when writing symbol equations?

Answer 15

Give state symbols.

Question 16

Which allotrope is used for carbon for standard enthalpy changes and why?

Answer 16

Graphite as it is more energetically stable at 298K.

Question 17

Define the standard enthalpy change of combustion.

Answer 17

The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions into standard products.

Question 18

Define the standard enthalpy change of formation.

Answer 18

One mole of compound is formed from its elements into standard states under standard conditions.

Question 19

What is the standard enthalpy change of neutralisation.

Answer 19

The enthalpy change when an acid and an alkali react to form one mole of water under standard conditions.

Question 20

State Hess’s Law.

Answer 20

The enthalpy change accompanying a chemical change is independent of the route by which chemical change occurs.

Question 21

Is bond breaking endothermic or exothermic?

Answer 21

Endothermic.

Question 22

Is bond making endothermic or exothermic?

Answer 22

Exothermic.

Question 23

What is meant by bond enthalpy?

Answer 23

The energy required to break all the bonds in 1 mole of a substance in the gaseous state.

Question 24

When calculating enthalpy changes by bond enthalpies what is the main assumption made?

Answer 24

That all bond enthalpies are the same as the average bond enthalpy for that bond.