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7.3 Energy & Reactions
Energy
Exothermic
Endothermic
kilojoules (kJ)
Reactions include the breaking and forming of bonds which requires ___________. Some reactions
absorb energy, these reactions are called Endothermic (meaning taking in Energy). Most reactions
include giving off energy, these reactions are called ______________.
ENERGY – The amount of Energy taken in or given off by a reaction is measured in kiloJoules. This
is a quantity of Energy.
ENDOTHERMIC – Reactions that require the absorption of Energy are Endothermic. The process
which a plant uses sunlight (Energy) to store food is endothermic. This is called photosynthesis.
Heating Mercury Oxide breaks the substance into the metal Mercury and the gas Oxygen (O 2). This requires
the absorption of heat energy.
2HgO + 182 kJ  2Hg + O2
Notice the Energy is on the side with the (REACTANTS or PRODUCTS). This tells us the reaction is
Endothermic.
USING THE NUMBERS
The value of energy stated in the reaction can be used to tell how much energy is needed to cause the
reaction. Here the reaction tells that 182 kJ are required to break up 2 moles of HgO. Use this information to
answer the following questions:
1. How much energy is required to break up 4 moles of HgO?
Set the Ideal Ratio = the Available
𝐻𝑔𝑂
𝐸𝑛𝑒𝑟𝑔𝑦
=
2
182
=
4
𝑥
Solving for x gives x = 364 kJ
2. How much energy is required to break up 8 moles of HgO? (728 kJ)
3. How much energy is required to break up 10 moles of HgO? (910 kJ)
4. How many moles of HgO can be broken up by 91 kJ of heat energy? (1 mole)
EXOTHERMIC – Reactions that release or give off Energy are Exothermic. Most reactions give off
heat. Burning fuels gives off heat. Here is the reaction for burning Natural Gas as in your kitchen
stove.
CH4+ 2O2 → CO2 + 2H2O + 890 kJ
Notice the Energy is on the side with the (REACTANTS or PRODUCTS). This tells us the reaction is Exothermic.
1. How many moles of Methane (CH4) are needed to create 890 kJ of Energy? ( 1 mole)
2. How many moles of O2 are needed to create 890 kJ of Energy? ( 2 moles)
3. How many moles of Methane (CH4) are needed to create 8900 kJ of Energy? (10 mole)
CH4
Energy
=
1
890
=
x
8900
Solving for x gives x = 10 moles
4. How many moles of Methane (CH4) are needed to create 4450 kJ of Energy? (5 moles)
5. How many moles of Methane (CH4) are needed to create 1335 kJ of Energy? ( 1.5 moles)
BURNING LONGER CARBON CHAINS
See if there is a trend between the energy created and
the number of carbons in the chain. Complete the
table and estimate the graph.
Name of
Substance
Methane
Ethane
Propane
Butane
Chemical
Formula
CH4
C2H6
C3H8
C4H10
# or
Energy
Carbons from 1
in Chain mole (kJ)
1
890
2
1560
2220
2874