Chapter 9 - Enthalpy

Question 1

What is the law of the conservation of energy?

Answer 1

Energy cannot be created or destroyed, just converted from one form to another.

Question 2

Define enthalpy

Answer 2

The heat content that is stored in a chemical system.

Question 3

Define enthalpy change

Answer 3

The heat exchanged with the surroundings during a chemical reaction.
The difference between the enthalpy of the products and the reactants.

Question 4

Define exothermic

Answer 4

A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings.

Negative enthalpy change

Question 5

Define endothermic

Answer 5

A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat beaing taken out of the surroundings.

Positive enthalpy change

Question 6

Define activation energy

Answer 6

The minimum energy required for a reaction to take place.

Question 7

Define standard enthalpy change of reaction

Answer 7

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.

Question 8

Define standard enthalpy change of formation

Answer 8

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.

Specifically for turning elements into compounds
1 mol of product
Enthalpy change of formation for an element would just be zero (forming an element from its element)

Question 9

Define standard enthalpy change of combustion

Answer 9

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.

One mol of reactant
The products are the oxides of the elements within the substance

Question 10

What is the equation used to measure an energy change?

Answer 10

Q=mcΔT
Q - energy change with surroundings (J)
m - mass of surroundings (g)
c - specific heat capacity (J/g/K)
ΔT - change in temperature (K)

Question 11

How is enthalpy change worked out from the energy change?

Answer 11

ΔH = -Q/n
ΔH - enthalpy change (J/mol)
Q - energy change with surroundings (J)
n - number of moles (mol)

Question 12

Define standard enthalpy change of neutralisation

Answer 12

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 13

What are the causes for less energy being transferred than expected when working out ΔH(c)?

Answer 13

Heat loss to the surroundings
Incomplete combustion
Evaporation of the fuel from the wick
Non-standard conditions

Question 14

How can heat loss be accounted for using a graph of temperature against time?

Answer 14

Extrapolate the cooling curve back to when it was added.

Question 15

Enthalpy change of neutralisation always has the same value, what is it?

Answer 15

-57.5 kJ/mol

Question 16

Define average bond enthalpy

Answer 16

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

Question 17

What are some properties of bond enthalpies?

Answer 17

Energy is always required to break bonds
Bond enthalpies are always endothermic
Bond enthalpies always have a positive enthalpy value

Question 18

What kind of energy and enthalpy changes are bond formation?

Answer 18

Exothermic and releases energy

Question 19

What is Hess’ law?

Answer 19

If a reaction can take place by more than one route and the initial and final concentrations are the same, the total energy change is the same for each route.

Question 20

What are the standard conditions

Answer 20

Standard pressure (100kPa)
Standard temp (298K/25 degrees)
Standard conc (1mol/dm3- only with solutions)
Standard states- the physical state of a substance under standard conditions (state should be the same as what you would expect it to be at room temp+ pressure)

Question 21

Difference between avg and actual bond enthalpy

Answer 21

Avg is calculated from the actual bond enthalpies in diff chemical environments

Actual bond enthalpy can vary depending on the chemical environment of the bond

Question 22

How do you burn fuels

Answer 22

Question 23

How can you improve the experimental designs

Answer 23

Burn in plentiful oxygen
• Draft shield
• Add lid to beaker
• Place cap over wick when not
burning
• Reduce gap between burner
and beaker
• Use standard conditions
• Use 3 DP balance OR digital
thermometer