Unit 1- Energetics

Question 1

Define the Standard enthalpy change of a reaction

1.4.d

Answer 1

The energy transferred when the molar quantities of reactants as stated in the equation react under standar conditions

Question 2

Define the Standard enthalpy change of neutralisation

1.4.d

Answer 2

The enthalpy change when the amount of acids and alkali in the equation for the reaction neutralise eachother under standard conditions

Question 3

Define the Standard entalpy change of combustion

1.4.d

Answer 3

The enthalpy change when one mole of the substance bruns completely in oxygen under standard conditions

Question 4

Define the Standard enthalpy change of formation

1.4.d

Answer 4

The enthalpy change when one mole of the compound forms from its elements under standard conditions with the element and compound in a stable state

Question 5

Define the Standard enthalpy change of aomisation

1.4.d

Answer 5

The enthalpy change when one mole of gaseous atoms is formed from the element under standard conditions

Question 6

Define Hess’s law

1.4.e

Answer 6

Hess’s law states that the enthalpy change in converting reactants to products is the same regardless of the route taken provided their is no change in conditions

Question 7

How do you work out the standard enthalpy change of formation?

1.4.e

Answer 7

Use hess’s law and use the enthalpy of combustion as the routes

Question 8

How do you work out the standard enthalpy of reaction?

1.4.e

Answer 8

Use hess’s law and use the standard enthalpy changes of formation

Question 9

Define bond enthalpy

1.4.g

Answer 9

Energy required to break one mole of the bonds in a subsrance in the gaseous state

Question 10

Why do mean bond enthalpies vary from the experimental values?

1.4.g

Answer 10

• From variations in the strength of one kind of bond in different molecules
• When a reactant or product is not in the gaseous state

Question 11

What are the standard conditions?

1.4

Answer 11

• 1 atm pressure
• 298K/25 degrees ceclius temperature
• 1 mol/dm^3 concentration
• Most stable state

Question 12

What are the assumptions and errors in thermochemical experiments?

1.4.f

Answer 12

• All the heat energy from the flame heats the water
• Errors are loss of heat, flame disturbance and incomplete combustion

Question 13

What is the formula and units for energy transferred in a calorimeter?

1.4.f

Answer 13

Energy transferred = Mass x Specific heat capcitiy of water x temperature change

J = g x J/g/K x K

Question 14

What is the enthalpy change per mole?

1.4.d,e

Answer 14

Enthalpy change (J)/Mole (mol)

Units are J/mol