enthalpy Flashcards
define enthalpy
the measure of the heat energy in a chemical system
how do you calculate enthalpy change?
enthalpy change = H(products) - H(reactants)
define conservation of energy
energy cannot be created or destroyed
exothermic change
heat out of the system to the surroundings (negative enthalpy change)
endothermic change
heat into the system from the surroundings (positive enthalpy change)
activation energy
the minimum energy required for a reaction to take place
standard conditions
100kPa
298K
1 mol dm-3
standard state
the physical state of a substance under standard conditions (RTP)
enthalpy changes of reaction
the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions
enthalpy of formation
enthalpy change when 1 mole of a compound is formed from its constituent elements under standard conditions with everything in its standard state
enthalpy of combustion
enthalpy change when 1 mole of fuel reacts completely with oxygen under standard conditions with everything in its standard state
enthalpy of neutralisation
the energy change that accompanies the reaction of an acid by a base to form one mole of H2O (l) under standard conditions with everything in its standard state
what is the equation used in calculating the enthalpy changes from experimental results?
q = mc deltaT
where:
q is heat energy in Joules
m is mass of substance in g (/volume)
c is specific heat capacity of water (4.18)
delta T is the change in temperature
then use enthalpy change= -q / n
where n is moles of substance
average bond enthalpy
the breaking of 1 mol of bonds in gaseous molecules
enthalpy changes associated with the breaking/making of bonds (endo/exo?)
breaking- endothermic
making- exothermic
(‘bendo mexo’)
why may an actual bond enthalpy differ from its average value?
the bond enthalpy differs depending on the environment that the bond is in
determining the indirect enthalpy change using enthalpy changes of formation
reactants ——-> products
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elements
arrow pointing away from elements
determining the indirect enthalpy change using enthalpy changes of combustion
reactants ——-> products
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combustion products
arrow pointing towards combustion products
