C7 - Energy Changes Flashcards
Why does the total energy remain the sane before and after a reaction?
Energy is neither created or destroyed.
What is an exothermic reaction?
A reaction where heat energy exits the system.
In an exothermic reaction the energy for ____ bonds is lower than _____.
Making
Breaking
Give two examples of an exothermic reaction.
Burning (combustion)
Freezing
What is an endothermic reaction?
A reactions where heat energy enters the system.
In an endothermic reaction the energy for ______ bonds is lower than _____.
Breaking
Making
Give two examples for an endothermic reaction.
Photosynthesis
Melting
What is bond energy?
Energy needed to break the bond between two atoms. Measured in KJ/mol.
What’s the equation for producing water?
2H2 + 02 —> 2H20
What is a more efficient use of energy from oxidising hydrogen?
Fuel cells
What is the anode equation from fuel cells?
2H2 + 4OH —> 4H2O + 4e-
What is the cathode equation for fuel cells?
02 + 4e- + 2H20 —> 4OH
What are advantages of fuel cells?
Only waste product is water.
Don’t need to be recharged.
Don’t pollute.
Very efficient.
Transfers energy directly.
Simple to construct.
What are disadvantages of fuel cells?
Hydrogen is highly flammable.
Sometimes produced for the cell by non-renewable sources.
Hydrogen is difficult to store.
Can have poisonous catalysts/ electrolysis.
