Developing Fuels - Enthalpies Flashcards
Define ΔH°
An enthalpy change at standard conditions
Define standard conditions
Temp -> 298K or 25°C
Pressure -> 1 atmosphere
Concentration -> 1 mol dm-1
How do you convert between K and °C?
Add or subtract 273
Define ΔH°c of C5H12
The enthalpy change when 1 mole of pentane combusts in O2 completely at standard conditions
Define enthalpy change
The amount of energy absorbed or released during a reaction
Describe an experiment that could calulate the enthalpy change of methanol (or any other combustable liquid)
Using a clamp, suspend a copper can containing 100cm3 above a spirit burner.
Weigh the spirit burner, including the lid, and record the mass
Record the temperature of the water using a thermometer
Light the spirit burner, and heat the can until the water has raised 30°C
Extinguish the flame immediatley and reweigh the spirit burner
Calculate the enthalpy change by using E = mcΔt and ΔH°r = E / mol
What errors could occur during the experiment to calculate the enthalpy change of a solution?
incomplete combustion
loss of energy to surroundings
not at standard conditions
Describe how the errors for the enthalpy change experiment could be prevented
Use a windshield to prevent energy loss
Use a lid to prevent energy loss
Use insulation to prevent energy loss
What is Hess’s Law?
The enthalpy change of a process is independent of the pathway taken assuming that the initial and end conditions are the same
In an enthalpy cycle of combustion, do the arrows in Hess’s law go up or down?
Down
In an enthalpy cycle of formation, do the arrows in Hess’s law go up or down?
up
How is % error caculated for 2 separate readings?
2 x (accuracy of instrument / difference between measurements)
Is bond breaking endothermic or exothermic?
endothermic
Is bond forming endothermic or exothermic?
Exothermic
