4 - Energetics

Question 1

Define enthalpy change. (1)

Answer 1

Heat energy change measured under conditions of constant pressure.

Question 2

What is an exothermic change?

Answer 2

Energy is transferred from the system (chemicals) to the surroundings. (negative change)

Question 3

What is an endothermic change?

Answer 3

Energy is transferred from the surroundings to the system (chemicals). (positive change)

Question 4

Define the standard enthalpy change of formation. (2)

Answer 4

The enthalpy change when 1 mole of the compound is formed from its constituent elements under standard conditions, all reactants and products being in their standard states.

Question 5

Define the standard enthalpy of combustion. (2)

Answer 5

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

Question 6

Define the standard enthalpy of combustion. (2)

Answer 6

The enthalpy change when one mole of a substance is combusted completely in oxygen under standard conditions, all reactants and products being in their standard states.

Question 7

What are standard conditions?

Answer 7

100kPa
298K
1M

Question 8

Give the equation including specific heat capacity.

Answer 8

q = mc x delta T

Question 9

What is Q?

Give units.

Answer 9

Energy change
Joules

Question 10

Why can errors occur in calorimetry. (6)

Answer 10

• Energy loss from calorimeter
• Incomplete combustion of fuel
• Incomplete transfer of energy
• Evaportation of fuel after weighing
• Heat capacity of calorimeter not included
• Measurements not carried out under standard conditions as H2O is gas not liquid in this.

Question 11

Define Hess’s Law.

Answer 11

The total enthalpy change for a reaction is independent of the route by which the chemical change takes place.

Question 12

Define mean bond enthalpy. (2)

Answer 12

The mean enthalpy required to break the covalent bond into gaseous atoms, averaged over different molecules.

Question 13

Why can data book enthalpies differ from experimental ones.

Answer 13

Data book value derived from different compounds

Question 14

Describe and explain an exponential relationship. (3)

Answer 14

Exponential relationship
Increasing temperature increases kinetic energy of particles so many more particles have required activation energy
Time for iodine to be used decreases

Question 15

When drawing a Buchner funnel what conditions must be met?

Answer 15

Filter paper labelled
Bung
Buchner Flash labelled

Question 16

How is the energy change of a reaction found?

Answer 16

Sum of reactant bond enthalpies - sum of product bond enthalpies

Question 17

suggest two disdavantages of using a glass beaker on a tripod and gauze rather than a calorimeter. (2)

Answer 17

• glass Poorer conductor (so not all energy transferred)
• tripod and gauze reduce heat transfer

Question 18

When given two reactants, what should you do when finding the moles of reactant for a specific heat capacity question?

Answer 18

Only use 1

Question 19

Why is enthalpy of formation of CO difficult to find?

Answer 19

Difficult to prevent C reacting with O2 to form some CO2