C3- Energy Calculations Flashcards
Describe the practical method for measuring energy change from burning ethanol.
1) Measure 50cm3 of water using measuring cylinder and pour into copper calorimeter.
2) Measure start temperature of water and start mass of spirit burner containing fuel.
3) Heat water using flame from spirit burner.
4) Measure maximum temperature of water after spirit burner switched off.
5) Measure end mass of spirit burner with fuel.
Why should the water not be allowed to get higher than 100oC in the practical method for measuring energy change?
Because the energy will still be released by the fuel but the temperature will not get any higher than boiling point so the calculation will be wrong.
How can the amount of energy transferred to the water in the energy change experiment be calculated?
Q= m x c x 𝚫T
Q= change in heat energy.
m= mass of water (g).
c= specific heat capaticy of water (approx. 4.2 J/goC).
𝚫T= temperature change (oC).
How can energy change in kJ/mol be calculated from kJ/g?
Multiply the energy change in kJ/g by the relative formula mass of the substance.
What does this graph show?
An energy level diagram of an exothermic reaction.
What does this graph show?
Energy level diagram of an endothermic reaction.
What do catalysts do?
Catalysts provide an alternative reaction pathway with a lower activation energy so the rate of reaction increases.
What type of reaction is bond breaking?
Endothermic.
What type of reaction is bond making?
Exothermic.
Define bond energy.
The energy needed to break the bond between two atoms.
Give the method for bond energy calculations.
1) Add together all the bond energies for all the bonds in the reactants.
2) Add together all the bond energies for all the bonds in the products.
3) Calculate the energy change:
Energy in- Energy out.
Calculate the energy change.
H-C is 412.
C=C is 612.
H-H is 436.
C-C is 347.
1) (412 x 4) + 612 + 436= 2, 696.
2) (412 x 6) + 347= 2, 819.
3) 2696 - 2819= -123 kJ/mol.
This is an exothermic reaction.
