Enthalpy Change
- Created by: Emily Cartwright
- Created on: 28-05-14 09:48
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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
- The energy that is no longer locked up in the system is released as heat, so the temperature rises
- 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
- Energy needs to be taken in from the surroundings, so either the reaction must be actively heated or the temperatures
- In an exothermic reaction a lower energy state is produced
- 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
- Untitled
- Find heat change per mol: Divide by mol reacting
- Convert units of heat change to kJ: divide by 1000
- Find heat change (J) = mc(delta)t
- Heat evolved = mc(delta)t
- Enthalpy is a measure of the energy content of a system
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