KEY
1
Energy Changes in Chemical Reactions
Name - _______________
1.) You can think of this reaction, CH
4
+ 2 O
2
CO
2
+ 2 H
2
O , as occurring in two steps (it doesn’t
go this way, but it is convenient to think of it this way).
Step 1 - CH
4
+ 2 O
2
C + 4 H + 4 O (the reactants are broken down to individual atoms)
Step 2 - C + 4 H + 4 O CO
2
+ 2 H
2
O (the individual atoms are assembled into products)
a.) Does step 1 absorb or give off energy? absorb
b.) Does step 2 absorb or give off energy? give off
c.) Since the overall reaction is exothermic, which step involves more energy, step 1 or 2?
Step 2
2.) The energy needed to break an H-Cl bond is 432. 2 HCl + 432 H
2
+ Cl
2
How many kilojoules of energy are given off in the following reaction? Why?
H
2
+ Cl
2
HCl + _432_ kJ
Answer – The two reactions are the exact opposite of each other.
3.) Is the burning of wood exothermic or endothermic?
Answer – exothermic as heat (energy) is produced.
4.) Is the melting of sugar exothermic or endothermic?
Answer – endothermic as heat (energy) is absorbed by the sugar to change phases.
5.) A beaker becomes warm when a reaction occurs in it. Are the chemicals in the beaker gaining or losing
energy? Is the reaction endothermic or exothermic?
Answer – losing energy. Exothermic as energy is released (lost).
6.) Which contains more energy in an endothermic reaction, the reactants or products?
Answer – products. The reactants gain energy to become high energy products.
