9 - Enthalpy Flashcards
Enthalpy
Measure of the heat energy in a chemical system (the atoms, molecules or ions making up thr chemical)
Enthalpy change
Enegry of products - enegry of reactants
Exothermic profile
- delta H is negative
- more energy taken in from surroundings to break bonds, than energy released to surrounding when bonds are formed
- releases energy to surroundings
- temp of surrounding increase
Endothermic profile
- delta H is positive
- more energy released to surroundings when bonds form than energy taken in from surroundings when bonds break
- chemical system takes in heat energy
- surrounding decrease
Activation energy
Minimum energy required for reaction to start/take place.
The energy input required to break bonds acts as an energy barrier to the reaction
Standard conditions (phi)
- standard pressure of 100 KPA
- standard temp of 25 deg or 298 K
- standard concentration of solution of 1mol/dm3
- standard state at standard conditions
Standard Enthalpy chnage of Reaction
The Enthalpy change that accompanies a reaction of its molar quantities shown by the chemical equation, under standard conditions
Standard enthalpy change of formation
The enthalpy change that takes place when one mole of a compound is formed from its elements, under standard conditions
Standard Enthalpy Change of combustion
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen, under standard conditions
Standard Enthalpy Change of Neutralisation
The enthalpy change that accompanies a reaction of an acid by a base to form one mole of Water, under standard conditions
How to measure Enthalpy change
- Work out q= mct
- Work out amount of mole of the limiting reactant
- Work out energy in KJ/mol
Use density to work out mass = density (1) * vol provided
How is energy transfer lost/not efficient
- heat loss to surroundings other than the water to the beaker or surrounding air of the flame
- incomplete combustion of methanol forming soot and CO instead
- evaporation of methanol from the wick and so must be weighed immediately after flame out. Can also have cover to reduce error
- non standard conditions
How to minimise error
- use of draught screens and oxygen gas input would minimise error from heat loss and incomplete combustion
Measuring Enthalpy chnage of neutralisation
- Same q=mct but mass of both from volume and density combined
- Amount of mol reactant (use whatever in limit)
- Q/mol
Average bond Enthalpy
Energy required to break one mole of a specified type of bond in a gaseous molecule
Why is it average
Actual bond Enthalpy can vary depending on the chemical environment of the bond
Bond breaking
Endothermic
Bond forming
Exothermic
Enthalpy change from bond enthalpy
H = products - reactants
Limitations of using bond Enthalpy
- only average and may be differently calculated in different chemical environments
- also must be gaseous molecules and so might not be STANDARD change
Hess’ law
Allows Enthalpy chnage to be determined indirectly
Standard Enthalpy change from standard e change of formation
- Write down main reaction (A)
- Write intermediates which will be ELEMENTS IN NATURAL STATE AND SAME ON EITHER SIDE
- Arrows FROM elements to react + products
- write down enthalpy of products formed
- A = B+C (B will be reversed)
Standard Enthalpy change from standard e change of combustion
- Write down main reaction (A) e.g C + H2 or O2 if needed
- Write intermediates which will be CO2 + H20
- Arrows TO INTERMEDIATE FROM react + products
- write down enthalpy of elements starting with
- A = B+C (C will be reversed)
Will be able to tell to use formation or combustion by…
- if compounds given = formation
- if elements of C and H2 given = combustion
