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Chemistry - Module 3 - Physical Chemistry - Part 1 - Coggle Diagram
Chemistry - Module 3 - Physical Chemistry - Part 1
enthalpy changes
exothermic and endothermic reactions
exothermic reactions release energy and have negative enthalpy change
temperature of surroundings often increases
endothermic reactions absorb energy and have positive enthalpy change
temperature of surroundings often decreases
enthalpy profiles and activation energy
enthalpy profile diagrams show energy change during a reaction
activation energy is the minimum energy needed to begin breaking reactant bonds
lower enthalpy corresponds to greater stability of a substance
definition and units
enthalpy change is the heat energy transferred in a reaction at constant pressure
symbol _H and units are kJ mol-1
standard conditions for reported values are 298 K and 100 kPa
standard enthalpy notation indicates elements in their standard states
standard enthalpy values in data books are under standard conditions
bond energies and bond breaking/forming
bond breaking versus bond formation
breaking bonds requires energy so bond breaking is endothermic with positive enthalpy change
energy required per mole is bond dissociation enthalpy for bonds in the gas phase
forming bonds releases energy so bond formation is exothermic with negative enthalpy change
overall reaction enthalpy is total energy absorbed minus total energy released
bond dissociation enthalpies and averages
bond dissociation enthalpies refer to bond breaking in gaseous compounds to allow fair comparisons
data-book values are averages over many compounds and may not match a single molecule exactly
measuring enthalpy changes and calorimetry
principles of experimental measurement
to measure an enthalpy change experimentally you need moles reacting and the temperature change
experimental values often differ from data-book values due to heat losses and incomplete reactions
enthalpy change at constant pressure equals heat q measured in the experiment
calorimetry for combustion and solution reactions
for combustion of a flammable liquid, burn the fuel to heat a known mass of water and measure temperature change
not all heat is absorbed by water because apparatus and surroundings also heat up, or the fuel does not combust completely
for reactions in solution such as neutralisation, mix known quantities in an insulated container and measure temperature change
use q = mc (change in T) to calculate heat where c is the specific heat capacity for water
enthalpy calculations and Hess's law
standard enthalpy change types
standard enthalpy change of reaction, formation, combustion, and neutralisation are defined under standard conditions
Combustion is for burning 1 mole of a substance completely in oxygen
neutralisation is for forming 1 mole of water from acid and alkali
formation is for forming 1 mole of compound from elements in their standard states
Hess's law and using data-book values
Hess's law states total enthalpy change is independent of the reaction pathway
useful for calculating enthalpy changes not directly measurable
use standard enthalpies of formation or combustion from data books to compute reaction enthalpies
enthalpy of elements in their standard states is zero