Energetics Flashcards
(17 cards)
What is standard temperature?
298k / 25*C
What is standard pressure?
100KPa
What is standard concentration?
1 mol dm -3
What is standard enthalpy change of combustion?
The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions
What is the experiment for determining the standard enthalpy change of combustion of a liquid fuel?
Combust the fuel (in a spirit lamp),
Measure the mass of the water before and after it is heated by the fuel, also measure the temperature change,
How do you work out the thermal energy change of the water?
Use the equation q= m x c x deltaT
m= mass of water (g)
c= specific heat capacity of water (4.18)
Delta T = temperature change of the water (*C/K)
What do you do after working out the thermal energy change of the water?
Work out the number of moles of fuel combusted using moles= mass/mr
What is the last step to work out the enthalpy change of combustion?
Divide the thermal energy by the number of moles
What are 4 reasons why the calculated value could be different to the published value?
- If you leave the spirit burner uncapped then fuel could evaporate
- A lot of the heat energy released does not pass into the water (could go to calorimeter or the air)
- Its possible not all the fuel went under complete combustion
- The experiment may not have happened under standard conditions
Why do you have to give the enthalpy change a negative sign during combustion?
Because combustion is always exothermic so looses energy
What is Hess’s law?
That if a reaction can be carried out by two different pathways then the total enthalpy change for these two pathways will be the same, providing that the starting and final conditions are the same for both pathways
What is standard enthalpy change of formation?
The enthalpy change when one mole of a compound is formed from its elements under standard conditions
How do you use standard enthalpies of formation to calculate the enthalpy change of a reaction?
Create and enthalpy cycle where the products and reactants are broken down into their elements, then their enthalpies of formation are used to make the cycle, they are then added (make sure the arrows and signs are the right way) to find the overall enthalpy change
What is an average bond enthalpy?
The energy needed to break one mole of a specific bond in the gaseous state
How do you work out enthalpy change using average bond enthalpies?
Work out the enthalpies for each side then add them together, the reactants side should be positive (endothermic) and the products side negative (exothermic), so you should be taking the products from the reactants
What are the units in q=mc∆t
q= Joules (but often will need to convert to kJ for final answer)
m= grams
c= Jg-1K-1
∆T= Kelvin
Why do the values from mean bond enthalpies differ from those determined by Hess’ law
Because they are just the mean and not the actual difference,
