Energetics
- Created by: Sophia Dowden
- Created on: 02-03-16 11:57
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- Energetics
- Conservation of energy law
- Energy cannot be created or destroyed. It can only be transformed from one form to another.
- Enthalpy
- Specific for heat content.
- The symbol for energy is H.
- The symbol for enthalpy change is ^H
- Endothermic reactions require an energy input, this means that ^H will be positive.
- Exothermic reactions release energy which means that ^H will be negative.
- The symbol for enthalpy change is ^H
- The symbol for energy is H.
- Specific for heat content.
- Standard Conditions
- Temperature of 298K (25 degrees Celsius)
- Pressure of 1atm
- Solutions of conc. 1moldm-3
- Substances in their standard/most stable states.
- Standard conditions are used to obtain values for standard enthalpy change.
- Standard molar enthalpy change of combustion:
- The enthalpy change when one mole of a substance burns completely in oxygen under standard conditions.
- Standard molar enthalpy change of formation:
- The enthalpy change when one mole of a compound forms from its elements in their standard states understand conditions.
- Standard molar enthalpy change of combustion:
- Determining enthalpy changes in aqueous solution:
- ^H = -mc^T n
- Enthalpy Change (KJ or J)
- m = total mass of the solution (g) or V for total volume of solution.
- c = Specific heat capacity of water = 4.18Jg-1 Degree Celsius or 4.18Jg-1K
- ^T = Temperature change (Degree Celsius or K)
- n = Number of moles
- ^H = -mc^T n
- Hess's Law
- The total energy change that occurs when a system changes from one state to another is independent of the route taken.
- Enthalpy Cycles
- A
- D
- C
- D
- C
- D
- A
- Bomb Calorimeters
- Accurate values of enthalpy changes of combustion can be obtained using a bomb calorimeter.
- The apparatus is specifically designed to avoid heat losses by surrounding the 'bomb' first with water and then an insulating air jacket.
- Heating the sample in excess oxygen at high pressure ensures that combustion goes to complete combustion.
- The apparatus is specifically designed to avoid heat losses by surrounding the 'bomb' first with water and then an insulating air jacket.
- Accurate values of enthalpy changes of combustion can be obtained using a bomb calorimeter.
- Conservation of energy law
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