Enthalpies Flashcards
Energy changes and rules in Chemistry!!!
What is the Law of Conservation of Energy?
That energy can never be destroyed or created, but only transfered from one store to another…
What is Enthalpy?
The measure of Thermal Energy involved in a chemical reaction
How can we find the Enthalpy Change?
H(products) - H(reactants) = delta H
delta H will be positive when products having more energy than reactants….
and opposite goes the same……….
What happens during exothermic and endothermic reactions?
- EXO = Energy transfered from the sytem to surroundings, with TEMP INCREASE
- ENDO = Energy transfered to sytem from system to surroundings, with TEMP DECREASE.
What are the Standard Enthalpy Conditions?
- Pressure = 100 Kpa (1 atm)
- Temperature = 25.C (298K)
- Concentration = 1moldm-3
- Physical State = its state at RTP
What is the Standard Enthalpy Change of Reactions?
- Enthalpy change that accompanies a reaction in molar quantities shown in chemical equations, under standard conditions!
What is the Standard Enthalpy Change of Formation?
- Enthalpy change taking place when one mole of a compound is formed from its own elememts, under standar conditions
What is the Standard Enthalpy Change of Combustion?
- Enthaply change taking place when 1 mole of a compound is completely burned with oxygen, under standard conditions
What is the Standard Enthalpy Change of Neutralisation?
- Enthalpy change that accompanies a neutralisation reaction, with a base and an acid, to form 1 mole of liquid water, under standard conditions!
What is Hess’ Law?
“If a reaction can occur in 2 or multiple routes with the exact same initial and final conditions, the enthalpy change will remain the same for all routes!!
How can we construct enthalpy change for formation?
- Using arrow POINTING UP, labelling delta H 1,2 and 3 in correspondong directions
- Using Route 1 and Route 2 rule equations to find enthalpy changes
ROUTE 2 = -delta H2 +delta H3
How can we construct enthalpy change for combustion?
- Using arrow POINTING DOWN, labelling delta H 1,2 and 3 in correspondong directions
- Using Route 1 and Route 2 rule equations to find enthalpy changes
ROUTE 2 = delta H2 - delta H3
What is the 2 equations needed for Bond Enthalpy Calculations?
- Energy = Mass * SHC * Temp Change
- Enthalpy Change = Energy / Moles
- Water SHC: 4.18 j/kg/
Why may the enthalpy change of combustion be inaccurate at times?
- Heat lost to surroundings apart from water = more energy required to heat the same mass
- Incomplete combustion of methanol = methanol producing CO than CO2
- Evaporation of combustion chemical = if evaporated from the combustion vessel, lost mass of chemical unburned!
- Non-Standard conditions = official values are always under standard conditions, ensuring consistency and control variable!