5. & 15. Energetics and Thermochemistry Flashcards
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Endothermic v. Exothermic
Endothermic: energy from surroundings to system (positive integer)
Exothermic: energy from system to surroundings (negative energy)
Define ΔH
ΔH is defined as the heat transferred by a closed system during a chemical reaction
Standard enthalpy change of a reaction
ΔH⦵_298 is measured at 25°C/298K, 100 kPa with all species in their standard state
Standard enthalpy of formation
Written as ΔH_f°
The energy change upon the formation of 1mol of a substance from its constituent elements in their standard state (at standard conditions).
As it requires the formation of 1 mol of substance, ratios can be shown in fractions for other substances
Calculating the heat of a reaction, q
q = mcΔT = -ΔH*n(moles)
Enthalpy change of combustion
ΔH_c⦵ is the energy liberated under the complete combustion of 1 mol of a substance
Def Hess’s law
regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes. This law is a manifestation that enthalpy is a state function. For molecules
Bond enthalpy | Bond disassociation enthalpy
the amount of energy required to break 1 mol of bonds in gaseous covalent molecules under standard conditions - endothermic process (positive integer).
Calculating for 1: enthalpy of formation and 2: bond enthalpy
1: ∆H°reaction=∑(∆H_f°products) - ∑(∆H_f°reactants)
2: ∆H = ∑(BE bonds broken)- ∑(BE bonds formed)
1st law of thermochemistry
conservation of energy
The Barn-Haber cycle
Pathway for forming ionic compounds does not matter, ΔH_final - ΔH_initial will always be the same. All but one of the 6 H° are known
Types of enthalpy changes
(standard) Change of reaction standard Bond enthalpy standard Change of formation standard change of atomisation standard electron affinity standard first ionisation energy standard lattice enthalpy standard change of solution standard change of hydration standard change of solvation
