# Enthalpy Change

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• Enthalpy Changes
• Enthalpy is a measure of the energy content of a system
• We can measure enthalpy changes that occur when a chemical or physical change takes place
• Processes can be classified as exothermic (heat given out), or endothermic (heat taken in)
• In an exothermic reaction a lower energy state is produced
• The energy that is no longer locked up in the system is released as heat, so the temperature rises
• e.g. Combustion, oxidation reactions, respiration
• In an endothermic reaction a higher energy state is produced
• Energy needs to be taken in from the surroundings, so either the reaction must be actively heated or the temperatures
• e.g. Thermal decomposition of calcium carbonate, photosynthesis
• Standard conditions
• Pressure = 1 atmosphere
• Temperature = 25 degrees centigrade
• Enthalpy change of reaction
• Standard enthalpy change of reaction is the enthalpy change when the number of moles shown in the equation react to form the products , under standard conditions
• Enthalpy change of combustion
• Standard enthalpy change of combustion is the enthalpy change when 1 mole of a substance is burned completely in oxygen under standard conditions
• Enthalpy change of formation
• Standard enthalpy change of formation of a substance is the enthalpy change when 1 mole of a substance is formed from it's elements under standard conditions
• Reversible reactions
• If the enthalpy change for a reaction is known, then the enthalpy change for the reverse reaction has the same magnitude, but with the opposite sign
• Determining enthalpy changes experimentally
• Heat evolved = mc(delta)t
• m is the mass of substance heated up or cooled down
• C is the specific heat capacity
• delta t is the temperature change
• Summary of steps
• Find heat change (J) = mc(delta)t
• Convert units of heat change to kJ: divide by 1000
• Find heat change per mol: Divide by mol reacting
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