2.1- Energetics

Question 0

Difference between system and surroundings?

Answer 0

The system is the chemicals and the surroundings is everything outside the chemicals

Question 1

Define Enthalpy change?

Answer 1

The amount of heat energy taken in or given out during any change in a system provided the pressure is constant

Question 2

What is an exothermic change?

Answer 2

• Energy is transferred from the system (chemicals) to the surroundings
• The products have less energy than the reactants
• Delta H is negative

Question 3

Examples of exothermic reactions?

Answer 3

• Combustion of fuels

- Oxidation of carbohydrates such as glucose in respiration

Question 4

What is an endothermic reaction?

Answer 4

Energy is transferred from the surroundings to the system (chemicals)

They require an input of heat energy (eg Thermal decomposition)

Products have more energy than the reactants

Delta H is positive

Question 5

Define standard Enthalpy change of formation?

Answer 5

Enthalpy change when 1 mole of the compound is formed from it’s elements under standard conditions (298K and 100kPa), all reactants and products being in their standard states

Question 6

Define the standard Enthalpy change of combustion?

Answer 6

The Enthalpy change when 1 mole of a substance is combusted completely in oxygen under standard conditions (298K and 100kPa), all reactants and products being in their standard states

Question 7

What are standard conditions?

Answer 7

100kPa pressure

298K (25°C)

Solutions at 1moldm3

Standard state

Question 8

Energy change equation?

Answer 8

Q=mc(DT)
Q= energy change (J)
M= mass of solution (g)
C= specific heat capacity (J/g/k)
DT= temperature change (K)

Question 9

Errors in simple calorimetry?

Answer 9

Heat transfer to surroundings

Approximation of specific heat capacity of solution (assuming all solutions have SHC of water)

Neglecting SHC of calorimeter (heat absorbed by apparatus)

Reaction/dissolving may be slow

Density of solution is taken to be the same as water

Question 10

Molar Enthalpy change formula?

Answer 10

DH = Q/n

DH= molar Enthalpy change
Q= energy change (kJ)
n= no. of moles (mol)

Question 11

Errors in measuring combustion enthalpies?

Answer 11

Heat loss from calorimeter

Incomplete combustion of fuel

Incomplete transfer of heat

Evaporation of fuel after weighing

SHC of calorimeter not included

Measurements not carried out in standard conditions (H2O is a gas not liquid in this experiment)

Question 12

What is Hess’s Law?

Answer 12

Total Enthalpy change for a reaction is independent of the route by which the chemical change takes place

Question 13

Bond making and breaking, exothermic or endothermic?

Answer 13

Bond breaking is endothermic

Bond formation is exothermic

Question 14

Define mean bond energy?

Answer 14

Enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules