Chapter 9 Enthalpy

Question 1

What is enthalpy?

Answer 1

A measure of heat energy in a chemical system.

Question 2

What different energy profiles can a reaction have?

Answer 2

Exothermic or endothermic

Question 3

What is an exothermic reaction and what would a graph of enthalpy against time look like?

Answer 3

Energy transferred from the system to the surroundings by releasing heat.
The enthalpy change is negative
The temperature increases

Reactants have a higher enthalpy than the products (downward arrow for energy change on a graph)

Question 4

What is an endothermic reaction and what would a graph of enthalpy against time look like?

Answer 4

Energy transferred from the surroundings to the system by taking in heat.
The enthalpy change is positive
The temperature decreases

Reactants have a lower enthalpy than the products (upward arrow for energy change on a graph)

Question 5

What is activation energy?

Answer 5

The minimum energy required for a reaction to take place

Question 6

What does activation energy look like on an enthalpy change graph?

Answer 6

Activation is a bulge on a enthalpy profile diagram.
On an exothermic profile the bulge is higher than the reactants
On an endothermic profile the bulge is higher than the products

Question 7

What are the standard conditions and states for enthalpy change reactions?

Answer 7

Standard conditions are:
100kPa of pressure
25C or 298K temperature
1 mol/dm3 concentration of solutions

Standard states is the natural physical state of a substance under standard condition e.g. water is a liquid

Question 8

What is enthalpy change of reaction?

Answer 8

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states

Delta r H

Question 9

What is the enthalpy change of formation?

Answer 9

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states

Delta f H

All enthalpy changes of formation for elements have a value of 0kJ/mol because there is no change

Question 10

What is the enthalpy change of combustion?

Answer 10

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states

Delta c H

Question 11

What is the enthalpy change of neutralisation?

Answer 11

The enthalpy change that accompanies the reaction of an acid by a base to form one mole of H2O under standard conditions, with all reactants and products in their standard states

Question 12

What equation enables you to determine the enthalpy changes directly from appropriate experimental results?

Answer 12

Q=mc(delta)T

Q is heat energy in kJ
m is mass in g
c is the specific heat capacity of water in J/g/K
(Delta)T is the change in temperature

Question 13

What does the term ‘average bond enthalpy’ mean?

Answer 13

The energy required to break one mole of a specified type of bond in a gaseous molecule

Question 14

What is an exothermic reaction in terms of bond breaking and making?

Answer 14

Energy is released when bonds form

Bond making is exothermic as the enthalpy change is positive

Question 15

What is an endothermic reaction in terms of bond breaking and making?

Answer 15

Energy is required to break bonds

Bond breaking is endothermic so the enthalpy change is positive

Question 16

How can average bond enthalpies be used to calculate the enthalpy change of a reaction?

Answer 16

1. Calculate the sum of the bond enthalpy of all the bonds in the reactants
2. Calculate the sum of the bond enthalpy of all the bonds in the products
3. Enthalpy change of reaction is = bond enthalpy of reactants - bond enthalpy of products
4. If it is positive it is an endothermic reaction if it is negative it an exothermic reaction

Question 17

What are the limitations of using average bond enthalpies?

Answer 17

Actual bond enthalpy can vary depending on the chemical environment of the bond.
The reaction conditions that your classroom experiment occurred under may not be exactly the same as standard conditions

Question 18

What is Hess’ law?

Don’t need to know definition but is useful

Answer 18

If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route.

Question 19

What is the equation for enthalpy change of reaction following Hess law using enthalpy change of formation values?

Answer 19

(Delta)rH= sum((Delta)fH of products) - sum((Delta)fH of reactants)

Question 20

What is the equation for enthalpy change of reaction following Hess law using enthalpy change of combustion values?

Answer 20

(Delta)rH = sum((Delta)cH of reactants) - sum((Delta)cH of products)

Question 21

How can unfamiliar enthalpy change cycles be calculated?

Answer 21

In an exam you may be given a table of values of enthalpy change for several equations accompanied by a long reaction cycle.

Match the equations to the energies, using the direction of the arrows (equations) you can add or subtract the energy values to find a enthalpy change of reaction via a different route