8: Energetics I Flashcards
(13 cards)
Enthalpy change (ΔH) definition
the heat energy change in a reaction at constant pressure
Enthalpy change units?
KJ mol-1
Standard conditions?
Elements in their standard states
100kPa
298K
Standard enthalpy change of a reaction (ΔH°r) definition
the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions
Standard enthalpy change of formation (ΔH°f) definition
the enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions
Standard enthalpy of combustion ( ΔH°c) definition
the enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions
Standard enthalpy of neutralisation ( ΔneutH°) definition
the enthalpy change when an acid and an alkali react together to form 1 mole of water, under standard conditions
Enthalpy change equation?
From calorimetry
q = mcΔT
q - the heat lost or gained - same as the enthalpy change if the pressure is constant
m - mass of water in the calorimeter
c - specific heat capacity of water (4.18)
Units of enthalpy change equation?
q - Joules
m - grams
c - J g-1 K-1
Hess’s Law
the total enthalpy change of a reaction is always the same, no matter which route taken
Bond enthalpy definition
the amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase
Why are bond enthalpies always positive?
Bond breaking is always endothermic
How to calculate enthalpy change of a reaction using mean bond enthalpies?
Enthalpy change of a reaction = sum of bond enthalpies of reactants - sum of bond enthalpies of products
Breaking bonds - making bonds
