Enthalpy
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Define enthalpy
the measure of the heat content of a substance. measured in kilojoules per mole
can enthalpy be measured
no, however, a change in enthalpy can be
what are the standard conditions
pressure: 100kPa temp: 298K
define activation energy
the minimum energy required to initiate a reaction
what’s an enthalpy profile diagram and how can it show if a reaction is exothermic/endothermic?
its an enthalpy-reaction progress diagram. if the enthalpy of the products is higher than the reactants then energy has been absorbed making it an endothermic reaction. if its lower then energy was released so its an exothermic reaction
define enthalpy change of formation
enthalpy change when one mole of a substance is made from its constituent elements in there standard states under standard conditions
define enthalpy change of combustion
enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
define enthalpy change of neutralisation
enthalpy change when one mole of water is formed from an acid and alkali under standard conditions
define specific heat capacity
the energy required to raise 1kg of a substance by 1 degree Celsius (or by one kelvin would be the same value)
possible problems with calorimetry experiments
heat loss to the surroundings, container absorbing some of the heat the mass and SHC of water are used not the solution
what is Hess’s law
the enthalpy change for a reaction is the same regardless of the route taken
how can the change in enthalpy (ΔH) be calculated for a reaction if given the enthalpy change of formation
ΔH= enthalpy change of formation of the products - enthalpy change of formation of the reactants. this is due to Hess’s law
example of Hess cycle using ΔcH
Reactants ΔH –> products ΔcH ΔcH Oxides ΔcH of reactants to the oxides is the same as the ΔH of the reaction + ΔcH of the products to the oxides
why does oxygen not have an enthalpy change of combustion
oxygen can not be added to oxygen so it has no combustion reaction
define average bond enthalpy
the average enthalpy change when breaking one mole of a given type of bond in the molecules in a gaseous species
how can average bond enthalpy be used to calculate the enthalpy change of a reaction
add the average bond enthalpy for every bond in the reactants. take away the average bond enthalpy of every bond in the products
define ‘lattice enthalpy’
enthalpy change when 1 mole of an ionic lattice is formed from its constituent gaseous ions at an infinite separation so the ions don’t interact
define ‘enthalpy of atomisation’
the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state 1/2 Cl2 –> Cl
define ‘bond dissociation enthalpy’
the enthalpy change when one mole of a covalent bond is broken into two gaseous atoms
define ‘first ionisation enthalpy’
the enthalpy change when one moles of electrons is removed from 1 mole of gaseous atoms forming one moles of gaseous 1+ ions
define ‘electron affinity’
the enthalpy change when one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous 1- ions
define ‘enthalpy change of hydration’
the enthalpy change when one mole of gaseous ions become aqueous ions (always exothermic)
define ‘enthalpy change of solution’
the enthalpy change when one mole of an ionic solid dissolves in a large enough amount of water to ensure that the dissolved ions are well separated and do not interact