# Energetics

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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.
• 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.
• 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
• 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
• 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.