1.6.3 Enthalpy Changes Flashcards
Enthalpy is
the total chemical energy inside a substance
An enthalpy change is represented by the symbol
ΔH (Δ= change; H = enthalpy)
A reaction is exothermic when the products have
less energy than the reactants
Heat energy is given off by the reaction to
the surroundings
The temperature of the environment
increases
The energy of the system
decreases
There is an enthalpy decrease during the reaction so ΔH is
negative
Exothermic reactions are what possible
thermodynamically possible (because the enthalpy of the reactants is higher than that of the products)
However, if the rate is too slow, the reaction may not occur, in this case the reaction is
kinetically controlled
The enthalpy change during an exothermic reaction
A reaction is endothermic when the products have
more energy than the reactants
Heat energy is absorbed by the reaction from
the surroundings
The temperature of the environment
decreases
The energy of the system
increases
There is an enthalpy increase during the reaction so ΔH is
positive
The enthalpy change during an endothermic reaction
the system is
the substances that are reacting (i.e. the reaction itself)
the system is the substances that are reacting (i.e. the reaction itself) and the surroundings is
everything else (e.g. the flask the reaction is taking place in)
To be able to compare the changes in enthalpy between reactions the pressure is
100 kPa
To be able to compare the changes in enthalpy between reactions the temperature is
298 K (25 oC)
To be able to compare the changes in enthalpy between reactions, each substance involved in the reaction is in its
standard physical state (solid, liquid or gas)
To show that a reaction has been carried out under standard conditions, the symbol
Ꝋ is used
ΔHꝊ =
the standard enthalpy change
Standard Enthalpy Change of reaction
the enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions
Standard Enthalpy Change of formation is
the enthalpy change when
one mole of a compound is formed from its elements under standard conditions
Standard Enthalpy Change of combustion is
the enthalpy change when
one mole of a substance is burnt in excess oxygen under standard conditions
Standard Enthalpy Change of neutralisation is
the enthalpy change when one mole of water is
formed by reacting an acid
and alkali under standard
conditions
Standard Enthalpy Change of reaction exo/endo
both
Standard Enthalpy Change of formation exo/endo
both
Standard Enthalpy Change of combustion exo/endo
exo
Standard Enthalpy Change of neutralisation exo/endo
exo
The ΔHfꝊ of an element in its standard state is
zero, for example, ΔHfꝊ of O2(g) is 0 kJ mol-1
