Topic 8- Energetics I

Question 1

Give 2 reasons why reactions may be classified as examples of combustion.

Answer 1

Oxygen is a reactant.

Reactions are exothermic.

Question 2

Why is this reaction not a standard enthalpy change?

CH4 + 2O2 –> CO2 + 2H2O

Answer 2

This isn’t a standard enthalpy change of formation as there is more than one product.

Question 3

What symbol is enthalpy given? Therefore what is the symbol for enthalpy change?

Answer 3

H

ΔH

Question 4

A negative enthalpy change ………… heat energy

Answer 4

Gives out

Question 5

A positive enthalpy change ………….. heat energy

Answer 5

Absorbs

Question 6

What are the “standard conditions” for bond enthalpy?

Answer 6

25 degrees/ 298K

100kPa

Question 7

What is the standard enthalpy change of combustion?

Answer 7

The enthalpy change when 1 mole of a substance in its standard state burns completely in O2 under standard conditions.

Question 8

What is the standard enthalpy change of formation of a compound

Answer 8

The enthalpy change when 1 mole of a compound is formed from its constituent elements under standard conditions.

Question 9

State Hess’ Law.

Answer 9

The total enthalpy change of a reaction is the same no matter the route taken.

Question 10

Define enthalpy change

Answer 10

the heat energy change in a reaction at constant pressure (in kJ mol-1)

Question 11

The less enthalpy a substance has the more ……. it is

Answer 11

The less enthalpy a substance has the more stable it is.

Question 12

Combustion is ………..

Answer 12

Exothermic

Question 13

thermal decomposition is ……..

Answer 13

Endothermic

Question 14

What is the standard enthalpy change of reaction?

Answer 14

The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions.

Question 15

What is the standard enthalpy change of neutralisation?

Answer 15

The enthalpy change when an acid and an alkali react together, under standard conditions to form 1 mole of water.

Question 16

To find enthalpy changes of combustion you need a….

Answer 16

Calorimeter

Question 17

Give the 2 equations for calculating enthalpy changes

Answer 17

Q=mcΔT
Q= heat lost of gained (j) m=mass of water c=specific heat capactity (4.18J g-1 K-1) ΔT= temperature change of water/solution (K)
Q/mols = kJ mol -1

Question 18

Bond breaking is an ……….. process

Answer 18

Bond breaking is an endothermic process

Question 19

Bond making is an …………. process

Answer 19

Bond forming is an exothermic process,

Question 20

What is bond enthalpy?

Answer 20

The amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase.

Question 21

The less enthalpy a substance has the more …… it is

Answer 21

The less enthalpy a substance has the more STABLE it is

Question 22

Bond making is

Answer 22

exothermic

Question 23

Bond breaking is

Answer 23

endothermic

Question 24

298K =…. “c

Answer 24

25”c

Question 25

What do enthalpy level diagrams show vs reaction profiles?

Answer 25

Enthalpy level=overall change of reaction

Reaction profile= Enthalpy changes during a reaction

Question 26

What does calorimetry mean?

Answer 26

measuring heat changes

Question 27

what does a colorimeter do?

Answer 27

finds the enthalpy change of combustion by measuring temperature change of some water