Chapter 5: Energetics

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

SYSTEM: chemical reaction SURROUNDING: everything else in the universe Energy flows between the system and the surrounding

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

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)

STANDARD ENTHALPY CHANGE (ΔH°): enthalpy change measured at standard conditions (SATP) SATP: 100kPa and 298K ΔH° (298K)

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)

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)

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

q = mcΔT

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

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)

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

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

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

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

AVERAGE BOND ENTHALPY: average amount of energy required to break 1 mole of covalent bonds in a gaseous molecule under standard conditions - Bond enthalpy may differ in different molecules (especially those not of the gaseous state) • Bond breaking: Endothermic • Bond forming: Exothermic ΔHᵣ° = ΣBE (bonds broken) - ΣBE (bonds formed)

Enthalpy (y-axis ) vs Reaction coordinate (x-axis) • Include activation energy (Eₐ): minimum amount of energy colliding particles must possess for a collision to result in a chemical reaction