Chapter 9 - Enthalpy

Question 1

What is enthalpy (H)?

Answer 1

The thermal energy stored in a chemical system.

Question 2

What is enthalpy change (ΔH)?

Answer 2

The heat energy change measured under conditions of constant pressure.

Question 3

What is an exothermic reaction?

Answer 3

A reaction in which the products have less energy than the reactants:

• Chemical energy is changed into thermal energy
• Chemicals lose energy
• Products have less energy than reactants.
• The energy lost by the chemicals is gained by the surroundings, which increase in temperature
• ΔH is negative.

Question 4

What is an endothermic reaction?

Answer 4

A reaction in which the the products have more energy than the reactants:

• Thermal energy is changed into chemical energy.
• Chemicals gain energy
• Products have more energy than reactants.
• The energy gained by the chemicals is lost by the surroundings, which decrease in temperature
• ΔH is positive.

Question 5

What are the standard conditions of standard enthalpy changes?

Answer 5

Pressure = 100kPa
Temperature = 298K

Question 6

What are standard states?

Answer 6

States which substances are in under standard conditions.

Question 7

What is the enthalpy change of formation?

Answer 7

The enthalpy change when one mole of a compound is formed from its elements.

Question 8

What is the enthalpy change of combustion?

Answer 8

The enthalpy change when one mole of a compound undergoes complete combustion.

Question 9

What is the enthalpy change of neutralisation?

Answer 9

The enthalpy change involved with the formation of one mole of water from neutralisation.

Question 10

What is the heat energy change equation?

Answer 10

q = mcΔT

Question 11

What is activation energy?

Answer 11

The minimum energy required for a reaction to take place.

Question 12

What is the average bond enthalpy?

Answer 12

The mean amount of energy required to break one mole of a specified type of covalent bond in a gaseous molecule.

Question 13

When more energy is needed to break bonds than is released when new bonds are formed, the overall energy change is…

Answer 13

…endothermic.

Question 14

When more energy is released in forming new bonds than is needed to break bonds, the overall energy change is…

Answer 14

…exothermic.

Question 15

TRUE or FALSE

An actual bond enthalpy may differ from the average value.

Answer 15

TRUE

Question 16

In general, ΔH =

Answer 16

H of products - H of reactants

Question 17

What is calorimetry?

Answer 17

The quantitative study of chemical energy in a chemical reaction.

Question 18

What are common errors and assumptions in calorimetry?

coffee cup and spirit burner calorimetry

Answer 18

• Heat loss to the surroundings
• Incomplete combustion
• Conditions not standard
• Some alcohol or water may evaporate

Question 19

What are method for reducing heat loss?

Answer 19

• Adding a lid

- Adding insulation around container

Question 20

Is bond breaking exothermic or endothermic?

Answer 20

Endothermic

Question 21

Is bond making exothermic or endothermic?

Answer 21

Exothermic

Question 22

ΔH =

in terms of bond enthalpy

Answer 22

ΔH = Σ(bond enthalpy of reactants) - Σ(bond enthalpy of products)

Question 23

What is Hess’ Law?

Answer 23

The enthalpy change of a reaction is independent of the route taken.

Question 24

What is calorimetry?

Answer 24

The quantitative study of chemical energy in a chemical reaction.

Question 25

How can you calculate enthalpy change in kJmol^1?

Answer 25

Calculate q / n

Then add + or - sign for ΔH

Question 26

What errors and assumption made in calorimetry?

Answer 26

• Heat loss to surroundings
• Incomplete combustion
• Conditions not standard
• Some alcohol or water may evaporate