Chapter 5: Energetics Flashcards
Define and distinguish between heat and temperature
HEAT: form of energy that flows from something at a higher temperature to something at a lower temperature
TEMPERATURE: measure of the average amount of kinetic energy possessed by the particles
(greater heat = higher temperature)
Eg. A larger substance and a smaller substance may have the same temperature, but the larger substance will still contain more heat
Define system and surroundings
SYSTEM: chemical reaction
SURROUNDING: everything else in the universe
Energy flows between the system and the surrounding
Define exothermic and endothermic reactions
EXOTHERMIC REACTION: when heat is evolved from the system to the surroundings, causing the temperature of the surroundings to increase
ENDOTHERMIC REACTION: when heat is absorbed from the surroundings by the system, causing the surroundings to decrease in temperature
Define enthalpy (H), enthalpy change (ΔH) and draw the enthalpy level diagrams for exothermic and endothermic reactions
ENTHALPY (H): energy level possessed by the particles making up a substance (cannot be measured)
ENTHALPY CHANGE (ΔH): heat energy exchanged in a chemical reaction (can be measured)
Enthalpy level diagrams:
- y-axis: Enthalpy
- Exothermic reaction: ΔH<0 (products are more stable than the reactants)
- Endothermic reaction: ΔH>0 (reactants are more stable than the products)
Define standard enthalpy change (ΔH°)
STANDARD ENTHALPY CHANGE (ΔH°): enthalpy change measured at standard conditions (SATP)
SATP: 100kPa and 298K
ΔH° (298K)
Define standard enthalpy change of reaction (ΔHᵣ°)
Eg.
If: N₂ + 3H₂ –> 2NH₃ (ΔHᵣ° = -92 KJ mol⁻¹)
Then 1/2N₂ + 3/2H₂ –> NH₃ (ΔHᵣ° = ? KJ mol⁻¹)
STANDARD ENTHALPY CHANGE OF REACTION: enthalpy change when molar amounts of reactants (as shown in the stoichiometric equation) react together to form the products under standard conditions
Eg.
If: N₂ + 3H₂ –> 2NH₃ (ΔHᵣ° = -92 KJ mol⁻¹)
Then 1/2N₂ + 3/2H₂ –> NH₃ (ΔHᵣ° = -46 KJ mol⁻¹)
Define standard enthalpy change of combustion (ΔH꜀°)
STANDARD ENTHALPY CHANGE OF COMBUSTION (ΔH꜀°): heat evolved when 1 mole of a substance is completely burnt in oxygen (complete combustion) under standard conditions
ΔH꜀°<0 (combustion is exothermic)
Define Specific Heat Capacity (SHC)
SPECIFIC HEAT CAPACITY: amount of energy needed (J) to raise 1g of a substance by 1K
- Higher SHC: more difficult to heat
Eg. SHC of aluminium: 0.90 J g⁻¹ K⁻¹
- 0.90J supplied to 1 g of Al: temp increase by 1K
- 1.80J supplied to 1g of Al: temp increase by 2K
- 1.80J supplied to 2g of Al: temp increase by 1K
State the formula for heat evolved/absorbed
q = mcΔT
Explain how a flame calorimeter is used to calculate the enthalpy change of combustion (ΔH꜀°) of a fuel
Fuel (of unknown ΔH꜀°) is used to burn water (known mass, known SHC and measured ΔT) to find the heat evolved (q), which can then be used to calculate the ΔH꜀°
q = mcΔT
mass of water x 4.18 J g⁻¹ K⁻¹ x (Final temp of water - Initial temp of water)
= heat evolved (J)
ΔH꜀° = -q/n
n=no. of mols of fuel burned
SYSTEMATIC ERRORS:
1) Heat lost to the surroundings
- Not all the heat evolved from the burning of the fuel was used to heat the water
- Introduce draught shield to reduce convection currents around set-up
2) Heat lost to the calorimeter
- Some of the heat evolved from the burning of fuel was used to heat the calorimeter
- Determine SHC of calorimeter and subtract heat absorbed by the calorimeter from the total heat evolved
3) Complete combustion is assumed
- Incomplete combustion could have taken place
Explain how simple calorimetry is used to determine the enthalpy change of a solution
SIMPLE CALORIMETRY: known amounts of reagent are measured out and placed in a polystyrene cup, maximum and minimum temperature is noted, SHC and density is assumed to be the same as that of water
- Add volumes of both solutions together to use as m value (unless 1 of the reactants is solid –> mass is negligible)
- Take average of initial temperatures of both solutions (if different)
Define enthalpy change of neutralisation (ΔHₙ°)
ENTHALPY CHANGE OF NEUTRALISATION (ΔHₙ°): heat evolved when 1 mole of water molecules are formed from an acid reacting with an alkaline under standard conditions
ΔHₙ°<0
Define enthalpy change of solution (ΔHₛₒₗ°)
ENTHALPY CHANGE OF SOLUTION (ΔHₛₒₗ°): enthalpy change when 1 mole of solute is dissolved in excess solvent to form a solution of ‘infinite dilution’ under standard conditions
State Hess’s law and use it to find unknown ΔH values
Eg.
2C + O₂ –> 2CO (ΔH°= -222 KJ mol⁻¹)
C + O₂ –> CO₂ (ΔH°= -394 KJ mol⁻¹)
2CO + O₂ –> 2 CO₂ (ΔH°=?)
HESS’S LAW: the enthalpy change accompanying a chemical reaction is independent of the pathway between the initial and final states
*Add (-) sign to reverse direction
Define standard enthalpy change of formation (ΔHբ°)
STANDARD ENTHALPY CHANGE OF FORMATION (ΔHբ°): enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
ΔHᵣ° = ΣΔHբ° (products) - ΔHբ° (reactants)
ΔHբ° of elements in their standard state = 0
