Enthalpy 3.9

Question 1

What is Enthalpy?

Answer 1

It is a measure of heat energy in a chemical system. Enthalpy is sometimes thought of the energy stored within bonds.

Question 2

What does the chemical system refer to?

Answer 2

The chemical system refers to the atoms, molecules or ions making up the chemical.

Question 3

Enthalpy cannot be measured but….

Answer 3

Enthalpy cannot be measured but Enthalpy changes can.

Question 4

What is an exothermic Enthalpy change?

Answer 4

The chemical system loses energy and surroundings gain energy.

Question 5

What is an endothermic enthalpy change?

Answer 5

Chemical system gains energy from the surroundings, temperature decreases.

Question 6

What is Standard Enthalpy Changes?

Answer 6

The enthalpy changes measured in standard conditions.

Question 7

Why is it important for enthalpy to be in a standard condition?

Answer 7

This is because enthalpy changes under different conditions and it offers comparison between different reactions.

Question 8

Standard Pressure

Answer 8

100kPa 1atm = 101 kPa

Question 9

Standard Temperature

Answer 9

298K = 25C

Question 10

Standard Concentration

Answer 10

1 mol dm-3

Question 11

Standard State

Answer 11

State of an element in room temperature and pressure

Question 12

What is Standard Enthalpy Change of Formation?

Answer 12

Is the Enthalpy Change that takes place when one mole of a compound is formed from its element in standard conditions, with all the reactants and products in their standard state.
Elements —> 1 mol

Question 13

What is the Standard Enthalpy Change of Combustion?

Answer 13

Is the Enthalpy change that takes place when one mole of a substance reacts completely with oxygen in standard conditions, with all the reactants and products in their standard state.

Question 14

What is the Standard Enthalpy of Neutralisation?

Answer 14

Is the enthalpy change that accompanies the reaction of an acid by a base to form one mole of H2O (l) under standard conditions, with all the reactants and products in their standard state.
Acid + Base ——> 1 mol

Question 15

Why is actual bond enthalpy different from average bond enthalpy?

Answer 15

Actual bond enthalpy is the energy required to break that bond in a specific compound. Average bond enthalpy is the mean energy required to break that type of bond in its gaseous state across a range of compounds.

Question 16

BENDO and MEXO

Answer 16

Breaking is endothermic and making is exothermic.

Question 17

Products - Reactants

Answer 17

Formation

Question 18

Reactants - Products

Answer 18

Combustion

Question 19

Arrows up

Answer 19

Formation

Question 20

Arrows down

Answer 20

Combustion

Question 21

Average bond enthalpy

Answer 21

The energy required to break one mole of a specific type of bond in a gaseous molecule across a range of compounds.

Question 22

What are the main sources of error in combustion experiments?

Answer 22

• Heat loss to the surroundings
• Incomplete Combustion
• Not in standard conditions
• Evaporation of alcohol from wick (use lid to prevent)

Question 23

What are ways to prevent heat loss in an combustion experiment?

Answer 23

• Replace beaker with polystyrene cup
• Distance between flame and beaker
• Volume of water

Question 24

Hess cycle equation

Answer 24

Enthalpy change = reactants - products , going in the opposite direction of the U means you have to flip the sign.

Question 25

What arrows should you use when different enthalpy changes are shown?

Answer 25

Use the arrows of the enthalpy change shown in the table/ the one provides

Question 26

Average bond enthalpy equation

Answer 26

reactants - products