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Chemical reactions usually have enthalpy change
Enthalpy change, H, is the heat energy transferred in a reaction, at constant pressure.
You write it like this, Ho to show that the elements were under standard states and that
measurements were made under standard conditions. Standard conditions are 100kPa (1 atm)
Reactions can be exothermic or endothermic
Endothermic reactions take in heat energy, (H is positive).
Temperature falls, thermal decomposition of calcium carbonate is endothermic.
Exothermic reactions give out heat energy, (H is negative.
Temperature goes up, oxidation is exothermic. Combustion of fuel like methane is also exothermic.
Reactions involve the making and breaking of bonds
In a chemical reaction, reactant bonds are broken and product bonds are formed. Energy is needed
to break bonds, so this is endothermic. Energy is released when bonds are broken, so this is
exothermic. The enthalpy change of a reaction is the overall effect of these two changes.
If the energy absorbed whilst making bonds is greater than the energy transferred to the
surroundings as bonds are made, then an endothermic reaction will occur.
Whereas, if the energy released on bond formation is greater than that absorbed through breaking
bonds then an exothermic reaction is observed.
Bond energy: This is the amount of energy required to break a covalent bond, it indicates the
strength of a bond. Values are always quoted as bond energy per mole, E.
Enthalpy change can be calculated using average bond enthalpies
In a chemical reaction, energy is absorbed when bonds are broken and released when bonds are
made. Differences in the energy released and absorbed is the overall enthalpy change for a
Overall enthalpy change for a reaction = Total energy absorbed Total energy released
Types of enthalpy change
Standard enthalpy change of formation, Hof, is the enthalpy change when 1 mole of a compound
is formed from its element in its standard states under standard conditions.
Standard enthalpy change of combustion, Hoc, is the enthalpy change when 1 mole of substance
completely reacts with oxygen in its standard state under standard condition.
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Standard enthalpy change of reaction, Hor, is the enthalpy change when a reaction occurs under
molar quantities shown in the equation in its standard state under standard conditions.
Measuring enthalpy changes in labs.
To measure the enthalpy change of a reaction, you need to know the number of moles in the
reacting substance and the change in temperature.…read more