DF 3 - Energetics

Question 1

What is the meaning of the term ‘endothermic’?

Answer 1

Endothermic reactions are those that take energy in from the surroundings: the energy needed to break bonds is greater than the energy needed to make bonds in the reaction

Question 2

What is the meaning of the term ‘exothermic’?

Answer 2

Exothermic reactions are those that release energy into the surroundings: the energy needed to break the bonds is less than the energy needed to make the bonds in a reaction

Question 3

What are the standard conditions?

Answer 3

• A pressure of 100 kPa
• A temperature of 298 K (room temperature)
• All solutions at a concentration of 1 mol dm^-3 for electrode potentials

Question 4

Define the standard enthalpy change of reaction, ^rH0

Answer 4

The enthalpy change that occurs when species are reacted in the molar ratios as defined by a chemical equation. The reaction takes place under standard conditions with reactants and products in their standard conditions

Question 5

Define the standard enthalpy change of combustion, ^cH0

Answer 5

The enthalpy change that occurs when a species is reacted with one mole of oxygen under standard conditions with products and reactants in their standard states

Question 6

Define the standard enthalpy change of formation, ^fH0

Answer 6

The enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states under standard conditions

Question 7

Define the standard enthalpy change of neutralisation, ^neutH0

Answer 7

The enthalpy change that occurs when one mole of water is produced from a neutralisation reaction happens under standard conditions

Question 8

What is average bond enthalpy?

Answer 8

The energy needed to break one mole of the stated bond under standard conditions

Question 9

What relation does bond enthalpy have to bond strength?

Answer 9

The greater the bond enthalpy, the greater the bond strength. As the bond strength increases the length of the bond decreases

Question 10

Match the reaction processes to the thermal result:
- Bond breaking and bond making
- Exothermic and endothermic

Answer 10

Bond breaking = Endothermic
Bond making = Exothermic

Question 11

What does each term mean in q = mc^T?

Answer 11

q: the energy absorbed by the material (normally water) in J
m: the mass of material being used in g
c: the specific heat capacity of the material used (in data sheet c of water = 4.18 J g^-1 K^-1)
^T: the change in temperature of the material