ENERGETICS AND THERMOCHEMISTRY
ENERGY
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All chemical reactions are accompanied by energy changes
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Energy is the measure of the ability to do work
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Hat is a form of energy which is transferred as a result of difference in temperature in disorder of the behavior of particles
ENTROPY - Degree of disorderness
When entropy (s) value is +, spontaneous reaction is possible
"NEUTRALIZATION AND COMBUSTION ARE EXOTHERMIC"
TEMPERATURE – Measure of the average kinetic energy of a particle
Temperature increase depends upon:
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Mass of object
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Nature of substance
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Amount of heat added
ENTHALPY – Total heat content of a chemical substance (at constant pressure)
Internal energy stored in reactants.
The energy required to break one mole of chemical bonds in gaseous state.
Bond breaking is endothermic
SYSTEM = Area of interest
SURROUNDING = Everything else
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OPEN SYSTEM – Energy and matter
CLOSED SYSTEM – Energy
Gas exchange with surrounding
BEN
MEX
Bond making is exothermic
Chemical reactions that involve the transfer of heat between the system and surrounding are either exothermic or endothermic:
STANDARD ENTHALPY CHANGE OF COMBUSTION
The enthalpy change when 1 mole of a substance is burned in excess oxygen under standard conditions. The reactants and products must be in standard states.
e.g. S + O2 -> SO2 (Enthalpy change of combustion of sulfer)
STANDARD ENTHALPY CHANGE OF FORMATION
The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions. The reactants and products must be in their standard conditions. The reactants and products must be in their standard states.
e.g. C + O2 -> CO2 (Enthalpy change of formation of carbon dioxide)
STANDARD ENTHALPY CHANGE OF REACTION
The enthalpy change when the amounts of reactants shown in the equation react to give products under standard conditions. The reactants and products must be in their standard states.
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STANDARD STATE:
Temp: 298 K
Pressure: 101.3 kPa
Conc: 1 mol dm-3
AMOUNT OF HEAT ABSORBED/PRODUCED DEPENDS UPON:
- Temperature of reaction
- Nature of reactants and products
- Concentration of reactants
- State of reactants and products
CALORIMETRY! Use everything of water
Why are there differences in values?
- Not all heat produced by combustion is transferred to water
Some used by material of calorimeter and some to surroundings
- Complete combustion unlikely due to limited supply of oxygen
HESSE'S LAW - The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.
(used for those chemical reactions that cannot be derived directly from the lab)
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