Energetics

Question 1

enthalpy defintion

Answer 1

Enthalpy is a measure of heat given out or taken in during a process.
When objects are heated they expand.
Enthalpy takes into account the energy used in expansion.
To measure energy, need to have a fixed volume so enthalpy is easier to measure.

Question 2

Endothermic reaction

Answer 2

Positive enthalpy change
Takes in thermal energy
Examples:
- Photosynthesis
- Cracking alkanes
- Melting ice
- Evaporating water
- Thermal decomposition

Question 3

Exothermic reaction

Answer 3

Negative enthalpy change
Gives out thermal energy 
Examples:
- Combustion
- Oxidation of carbohydrates
- Respiration
- Nuclear fission
- Neutralisation

Question 4

BENDO-MEXO

Answer 4

Bond-Breaking is ENDOthermic

Bond-Making is EXOthermic

Question 5

Standard conditions

Answer 5

298K
1 atm / 100 kPa
All elements in their standard states
Solutions at 1moldm-3

Question 6

Standard enthalpy of reaction

Answer 6

Enthalpy change when substances react under standard conditions in quantities given by the equation for the reaction

Question 7

Standard enthalpy of combustion

Answer 7

The enthalpy change when one mole of a substance is burned completely in excess oxygen with all reactants and products in their standard states under standard conditions

Question 8

Standard enthalpy of formation

Answer 8

The enthalpy change when one mole of a substance is formed from its constituent elements with all reactants and products in their standard states under standard conditions

Question 9

Calorimetry

Answer 9

Bomb calorimetry uses a bomb calorimeter to measure enthalpy changes of combustion.
Sample of compound is burned in sealed vessel and temp change is measured.
Inaccuracies: heat lost to surroundings, incomplete combustion, loss of some reactant before combustion due to evaporation

Question 10

Measuring enthalpy change

Answer 10

q = m × c × ΔT
If the pressure is constant, q = ΔcH
Assume solutions have same density as water 1gcm-3
To determine enthalpy change,
Calculate energy change
Work out moles of reactant used
Divide q by number of moles not in excess
Add sign and unit
Initial and final temps are inaccurate due to heat loss so measure temp loss after reaction is complete and extrapolate line to find value closer to true value.

Question 11

Definition hess law

Answer 11

Enthalpy change for a reaction is independent of the pathway whether it be direct or indirect.
Depends only on the initial and final states

Question 12

Conservation of energy

Answer 12

Energy cannot be created or destroyed, only transferred from one form to another

Question 13

Mean bond enthalpy

Answer 13

enthalpy change when one mole of covalent bonds is broken into gaseous atoms averaged over different molecules containing that bond
Mean bond enthalpies only valid in gas phase, using mean so won’t give perfect answer

Question 14

General points for bond enthalpies

Answer 14

Generally shorter covalent bond gives stronger bond
Triple covalent bond generally shorter than double which are shorter than single
Bond length increases down group as atomic radius increases
F-F bond enthalpy lower than expected as bond very short and there are repulsions between lone pairs of electrons on F atom so less energy required

Question 15

General points for combustion enthalpy

Answer 15

Constant rise in size of enthalpies as number of carbon atoms increases
Experimental enthalpies will be much lower than calculated due to significant heat loss. Incomplete combustion leads to less energy being released.