ENERGETICS Flashcards
Define enthalpy change
energy change per mole at constant pressure
What are the standards conditions
298K and 100kPa pressure
Define enthalpy of formation
enthalpy change when one mole of a substance is formed from its constituent elements in their standard states
Define enthalpy of combustion
enthalpy change where one mole of substance is completely burned in oxygen
Rules
-sulfur = S8 + phosphorus = P4
-halogens, hydrogen, oxygen, nitrogen = diatomic
-gaseous elements = H2, F2, CL2, O2, N2
-liquid elements = Hg and Br2
Define exothermic reaction
In an exothermic change energy is transferred from the system (chemicals) to the surroundings. The products have less energy than the reactants.
Common oxidation exothermic processes are the combustion of fuels and the oxidation of carbohydrates such as glucose in respiration.
Define endothermic reaction
In an endothermic change, energy is transferred from the surroundings to the system (chemicals).
require an input of heat energy e.g. thermal decomposition of calcium carbonate. The products have more energy than the reactants.
Equation that links energy change, temp change, shc and mass of water
Q=MC∆t
Write and equation that links enthalpy change, energy change and moles reacted
∆H = q/moles
Give 3 reasons why enthalpy changes of combustion calculated by calorimetry may not be accurate
-heat lost to surroundings
-evaporation of fuels
-incomplete combustion of fuels
State Hess’ Law
total enthalpy change for a process is independent of the route taken
equation to calculate enthalpy change of reaction from enthalpy changes of formation
E∆Hf products -E∆Hf reactants
equation to calculate enthalpy change of reaction from enthalpy changes of combustion
E∆Hc reactants -E∆Hc products
Define mean bond (disassociation) enthalpy
enthalpy change when one mole of covalent bonda in the gaseous state are completely broken
why are calculations from mean bond enthalpies often different from those calculated from Hess’ law (theoretical values)
bond enthalpies are a mean average taken over a range of compounds
