Energetics

Question 1

What is thermochemistry?

Answer 1

the study of heat changes during chemical reactions.

Question 2

What are the principles of thermochemistry?

Answer 2

> When a chemical reaction takes place, chemical bonds break and new ones are formed.

> Energy must be put in to break bonds and energy is given out when bonds are formed. As a result energy change happens

> The overall change may result in energy being given out or taken in.

> At the end of the reaction, if energy has been given out, the reaction is exothermic.

> At the end of the reaction, if energy has been taken in, the reaction is endothermic.

> Always the case that a reaction that is endothermic in one direction is exothermic in the reverse direction.

Question 3

Give examples of exothermic reaction?

Answer 3

Reactions involving neutralising acid with alkali

Question 4

Give an example of an endothermic reaction?

Answer 4

The breakdown of limestone (calcium carbonate) to lime (calcium oxide) and carbon dioxide

endothermic reaction is heating of copper sulfate.

Question 5

What does endothermic mean?

Answer 5

Reactions that take in heat from their surroundings to keep the reaction going.

Question 6

What does exothermic reaction mean?

Answer 6

reactions give out heat as they proceed

Question 7

What factors affect the amount of energy given out in a reaction?

Answer 7

Depends on the quantity of reactants.

Question 8

What is the amount energy given off measured in?

Answer 8

Kilojoules per mole, kJ mo1- 1

Question 9

What is enthalpy change?

Answer 9

When you measure a heat change at constant pressure

Question 10

What is the standard conditions for measuring enthalpy change?

Answer 10

> Pressure of 100 kPa

> Temperature of 298K

Question 11

What is standard molar enthalpy of formation?

Answer 11

Is the enthalpy change when one mole of substance is formed from its constituent
elements under standard conditions. all reactants and products being in their standard states.

Question 12

What is Standard Molar Enthalpy Of Combustion?

Answer 12

Is the enthalpy change when one mole of substance is completely burnt in
oxygen under standard conditions, all reactants and products being in their standard states.

Question 13

What is specific heat capacity?

Answer 13

Is the amount or heat needed to raise the temperature of Jg of substance by I K

Question 14

What can a calorimeter be used for?

Answer 14

it can be used to approximate

Question 15

Why is the heating of Blue copper sulfate crystals a endothermic reaction in product to reactant side?

Answer 15

The water molecules are bonded to the copper sulfate - so in order to break these bonds and make white, anhydrous copper sulfate, heat energy must be supplied

Question 16

Why can thermochemistry be seen as important in real life?

Answer 16

it enables us to compare the efficiency of different fuels

Question 17

What does standard molar enthalpy change mean?

Answer 17

The general name for the enthalpy change for any reaction

Question 18

What is heat?

Answer 18

measure of the total energy of all the particles present in a given amount of substance

Question 19

What is temperature? `

Answer 19

related to the average kinetic energy of the particles in a system

Question 20

How can you measure enthalpy change?

Answer 20

By arranging the heat to be transferred into a particular mass of a substance, often water

You then find out:

1) mass of the substance that is being heated up or cooled down
2) work out the temperature change
3) work out the specific heat capacity of the substance.

Question 21

Formula for enthalpy change?

Answer 21

Mass of substance (m) x specific heat capacity (c) x temperature change

Question 22

Why is a flame calorimeter better than a simple calorimeter?

Answer 22

• This is because the spiral chimney is made of copper
• The flame is enclosed
• The fuel burns in pure oxygen, rather than air.

{which reduces heat loss}

Question 23

What is hess’s law?

Answer 23

Hess’s law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.

Question 24

Why are polyester cups best suited to be used for calorimetery?

Answer 24

They are good insulators (this reduces heat loss through their sides)

They have a low heat capacity so they absorb very little heat

Question 25

What are enthalpy diagrams used for?

Answer 25

To represent the enthalpy changes in chemical reactions - they show the energy (enthalpy) levels of the reactants and products of a chemical reaction on a vertical scale, so you can compare their energies

Question 26

What is the enthalpy of elements in their standard state?

Answer 26

0

Question 27

What is bond dissociation enthalpy?

Answer 27

As the enthalpy change required lO break a covalent bond with all species in the gaseous state.