3.1.4 Energetics

Question 1

Exothermic Reaction

Answer 1

release heat to surroundings, resulting in increase of temp of surrounding, products have lower enthalpy than reactants therefore deltaH is -ve
e.g. combustion, neutralization, displacement

Question 2

Endothermic Reaction

Answer 2

heat is absorbed from surroundings, temp of surrounding decrease, products have higher enthalpy than reactants, therefore, deltaH is +Ve
e.g. decomposition

Question 3

Standard Conditions

Answer 3

100kPa
293K
normal physical states

Question 4

Why are bond enthalpies only approximate?

Answer 4

it’s the average bond enthalpy measured compared to range of molecules/compounds

Question 5

Definition of Bond enthalpy

Answer 5

enthapy change for breaking 1 mole of particular bond in gaseous compound
averaged over range of compounds containing this bond

Question 6

Standard enthalpy of Formation

Answer 6

change in enthalpy when 1 mole of compound is formed from its elements in their standard states under standard conditions

Question 7

Standard enthalpy of Combustion

Answer 7

change in enthalpy when 1 mole of substance undergoes COMPLETE combustion in PURE oxygen at standard state under standard conditions

Question 8

equation to calculate delta H

Answer 8

mc(deltaT)/1000n
mass in kg
value in kJ/mol

Question 9

Reason for lower enthalpy change calculation in experiments (and other improvements)

Answer 9

• HEAT LOST TO ENVIRONMENT
• use more concentrated solutions to give larger temp change
• use temperature probe instead of thermometer
• repeat to disregard anomaly and improve accurate

Question 10

Hess’s Law

Answer 10

overall enthalpy change for a reaction is independent of the route the reaction takes

Question 11

Simple Calorimetry Practical

Answer 11

1) use thermometer or temp probe to record initial temp
2) weigh reactants
3) add reactants in
4) record temp every 30 seconds
5) plot graph and extrapolate back to find change in T

Question 12

Organic compound Calorimetry Practical

Answer 12

1) measure out 100cm3 of water into calorimeter help up by clamp, record initial temp
2) weight spirit burner
3) light spirit burner directly under calorimeter
4) unlit and record final temp
5) reweigh spirit burner, use change in mass to calculate moles reacted
6) use calorimetry equations to calculate enthalpy change