enthalpy change of reaction Flashcards
what is the definition of standard enthalpy change of reaction, ΔHrθ?
the standard enthalpy change of reaction, ΔHrθ, is the enthalpy change when molar quantities of reactants as stated in the equation react together under standard conditions of 298K and 1 bar
what is the defintion of standard enthalpy change of formation, ΔHfθ?
the standard enthalpy change of formation of a substance, ΔHfθ, is the enthalpy change when 1 mole of the substance is formed form its constituent elements in their standard states at 298K and 1 bar
what is the standard enthalpy change of elements in their standard states at 298K & 1 bar?
ZERO
what does it mean when the ΔHfθ of a compound is positive?
it means that the compound is energetically less stable than its constituent elements
what does it mean when the ΔHfθ of a compound is negative?
it means that the compound is energetically more stable than its constituent elements
what can ΔHfθ be used to predict?
it can be used to predict how readily a compound is formed form its constituent elements
what is the formula used to calculate ΔHrθ using ΔHfθ of the reactants & products?
ΔHrθ = ∑mΔHfθ (products) - ∑nΔHfθ (reactants)
what is the definition of standard enthalpy change of combustion, ΔHcθ?
the standard enthalpy change of combustion of a substance, ΔHcθ, is the heat energy evolved when 1 mole of the substance is completely burnt in excess oxygen at 298K & 1 bar
are combustion reactions always endothermic or exothermic?
ALWAYS EXOTHERMIC
what is the formula used to calculate the ΔHrθ using the ΔHcθ of the reactants & products?
ΔHrθ = ∑mΔHcθ (reactants) - ∑nΔHcθ (products)
what is the definition of standard enthalpy change of neutralisation, ΔHnθ
the standard enthalpy change of neutralisation, ΔHnθ, is the enthalpy change when an acid & a base react under infinitely dilute conditions to form 1 mole of water at 298K & 1 bar
is the standard enthalpy change of neutralisation usually exothermic or endothermic?
USUALLY EXOTHERMIC
why is the enthalpy change of neutralisation for weak acid/base with strong base/acid less exothermic than the enthalpy change of neutralisation for strong acid & strong base?
- unlike strong acids & bases, weak acids & bases are only partially dissociated in aqueous solution, hence, part of the heat evolved from the neutralisation process is taken in to further dissociate the weak acid or base for neutralisation to continue taking place
what is the definition of the standard enthalpy change of atomisation, ΔHatomθ?
the standard enthalpy change of atomisation, ΔHatomθ, is the heat energy required when 1 mole of free gaseous atoms is formed from the element in its standard state at 298K & 1 bar
is the standard enthalpy change of atomisation always exothermic or endothermic?
ALWAYS ENDOTHERMIC
