Energetics

Question 1

Definition of enthalpy change?

Answer 1

The change in heat energy at a constant pressure

Question 2

Whats the △H sign for exothermic?

Answer 2

Negative

Question 3

Are the bonds made or broken in an endothermic reaction?

Answer 3

Broken

Question 4

Definition for mean bond enthalpy?

Answer 4

Energy required to break one mole of a covalent bond into gaseous atoms, averaged over a range of different compounds

Question 5

Definition for standard enthalpy of formation?

Answer 5

When 1 mole of a substance is formed from its constituent elements under standard conditions with all reactants + products being in their standard states

Question 6

Why does the following reaction NOT show the standard enthalpy of formation of ammonia?

N2 + 3H2 —> 2NH3

Answer 6

2 moles of ammonia is formed

Question 7

Why is the enthalpy of formation of Na(g) zero?

Answer 7

Na is an element in its standard state

Question 8

Why is the enthalpy of formation of Na(l) not zero?

Answer 8

Na(l) is not in the standard state

Question 9

Definition of standard enthalpy of combustion?

Answer 9

When 1 mole of a substance is completely burnt in excess O2 under standard conditions and all reactants + products being in their standard states

Question 10

Why may the enthalpy of combustion of a compound be difficult to measure?

Answer 10

Incomplete combustion may occur

Question 11

Definition of Hess’ Law?

Answer 11

The enthalpy change for a chemical reaction is the same whatever route is taken from reactants to products

Question 12

Equations to calculate enthalpy change?

Answer 12

Q=mc△T
△H=Q(KJ)/n

Question 13

What does 1g equal to?

Answer 13

1cm3

Question 14

What moles do you use if there’s 2 solutions added together?

Answer 14

Limiting reagent - solution with lowest moles

Question 15

What assumption is made in calorimetry?

Answer 15

Energy transferred to water from the reacting
chemicals = Energy released in reaction

Question 16

What sources of errors are there in combustion calorimetry? (3)
Improvements? (3)

Answer 16

ERRORS
- Heat loss to surroundings
- Incomplete combustion of fuel
- Some fuel evaporates

IMPROVEMENTS
- Add a lid
- Insulate sides of calorimeter
- Reduce distance between flame + beaker

Question 17

What sources of errors are there in solution calorimetry? (2)
Improvements? (3)

Answer 17

ERRORS
- Heat loss to surroundings
- Incomplete reaction

IMPROVEMENTS
- Add a lid
- Insulate sides of calorimeter
- Stir solution whilst measuring △T

Question 18

Steps to measure enthalpy change using a cooling curve? (5)

Answer 18

Question 19

If the temperature increases in a reaction then what sign would the enthalpy of solution be?

Answer 19

Negative

Question 20

Suggest a change to students method of calculating enthalpy using same apparatus that would reduce uncertainty in temperature change? Why?

Answer 20

Use larger mass of solid so temperature change is greater

Question 21

Suggest why the enthalpy change for the thermal decomposition of solid AgNO2 is difficult to determine experimentally?

Answer 21

AgNO2 has to be heated so can’t measure the temperature change caused by reaction

Question 22

Mean bond enthalpy equation for HCl?

Answer 22

Question 23

Enthalpy of formation for H2O?

Answer 23

Question 24

Standard enthalpy of combustion for CH4?

Answer 24

Question 24

Mean bond enthalpy equation for O2?

Answer 24

Question 25

Suggest the most likely reason for a large difference between an experimental value and data book value?

Experimental value = -142 kJ mol-1
Data Book value = -310 kJ mol-1

Answer 25

heat loss to surroundings