Enthalpy Change
- Created by: Emily Cartwright
- Created on: 14-04-14 09:09
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- Enthalpy Changes
- Enthalpy (H) is a measure of the energy content of a system.
- We cannot measure the absolute value of the enthalpy of a real system but we can measure enthalpy changes that occur when a chemical of physical change takes place
- The unit of enthalpy change is kJ mol-1 and must always have a + or - sign to show whether the reaction is exothermic (-) or endothermic (+)
- We cannot measure the absolute value of the enthalpy of a real system but we can measure enthalpy changes that occur when a chemical of physical change takes place
- Processes can be classified as exothermic (heat given out) or endothermic (heat taken in)
- Exothermic: Temperature change is negative
- The reaction produces a lower energy state. The energy that is no longer locked up in the system is released as heat, so the temperature rises
- Many chemical reactions are exothermic. Some of the most important examples are oxidation reactions, e.g.
- The combustion of methane in air
- The oxidation of carbohydrates such as glucose in respiration
- Endothermic: Temperature change is positive
- The reaction produces a higher energy state. Energy needs to be taken in from the surroundings, so either the reaction must be actively heated or the temperature falls
- Examples include;
- The thermal decomposition of calcium carbonate
- Photosynthesis
- Exothermic: Temperature change is negative
- Standard Conditions
- Pressure; 1 atmosphere (100kPa)
- 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 reversible reaction has the same magnitude but the opposite sign
- Determining Enthalpy Changes Experimentally
- Heat Evolved = mC(delta)T
- m is the mass of the substance (in g) being heated up or cooled down. This is nearly always water
- C is the specific heat capacity of the substance being heated or cooled
- (delta)T is the temperature change
- Using mC(delta)T to find the enthalpy change
- Find heat change: mC(delta)T
- Convert units of heat change to kJ: Divide by 1000
- Find heat change per mol: Divide by mol reacting
- Decide on the sign of enthalpy change
- Find heat change per mol: Divide by mol reacting
- Convert units of heat change to kJ: Divide by 1000
- Find heat change: mC(delta)T
- Heat Evolved = mC(delta)T
- Enthalpy (H) is a measure of the energy content of a system.
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