Exo- and Endothermic Reactions

1
Exo- and Endothermic Reactions
Exothermic reaction A chemical reaction that
results in the net production of energy.
reactants products
energy
2
In total, how many atoms are represented by the
following schematic for a chemical reaction?
Chemical Reactions and Equations CHECK YOUR
ANSWER

• A. 2
• 5
• 6
• 12

products
Explanation The atoms of the reactants are the
SAME atoms of the products, except in a different
configuration.
3
Exo- and Endothermic Reactions

• Exothermic reaction A chemical reaction that
  results in the net production of energy.
• reactants products
  energy

4
Exo- and Endothermic Reactions

• Exothermic reaction A chemical reaction that
  results in the net production of energy.
• reactants products
  energy
• Endothermic reaction A chemical reaction in
  which there is a net consumption of energy.
• energy reactants
  products

5
Exo- and Endothermic Reactions
6
Use the bond energies below to determine whether
the following reaction is exothermic or
endothermic
Exo- and Endothermic Reactions CHECK YOUR
NEIGHBOR
H-H (bond energy 436 kJ/mol) Cl-Cl (bond energy
243 kJ/mol) H-Cl (bond energy 431 kJ/mol)
H2 Cl2 ? 2 HCl

• Exothermic with more than 50 kJ of energy
  released.
• Endothermic with more than 50 kJ of energy
  absorbed.
• Endothermic with less than 50 kJ of energy
  absorbed.
• Exothermic with less than 50 kJ of energy
  released.

7
Use the bond energies below to determine whether
the following reaction is exothermic or
endothermic
Exo- and Endothermic Reactions CHECK YOUR ANSWER
H-H (bond energy 436 kJ/mol) Cl-Cl (bond energy
243 kJ/mol) H-Cl (bond energy 431 kJ/mol)
H2 Cl2 ? 2 HCl

• Exothermic with more than 50 kJ of energy
  released.
• Endothermic with more than 50 kJ of energy
  absorbed.
• Endothermic with less than 50 kJ of energy
  absorbed.
• Exothermic with less than 50 kJ of energy
  released.

8
Use the bond energies below to determine whether
the following reaction is exothermic or
endothermic
Exo- and Endothermic Reactions CHECK YOUR ANSWER
H-H (bond energy 436 kJ/mol) Cl-Cl (bond energy
243 kJ/mol) H-Cl (bond energy 431 kJ/mol)
H2 Cl2 ? 2 HCl
431 kJ/mol
436 kJ/mol
243 kJ/mol
431 kJ/mol

• Exothermic with more than 50 kJ of energy
  released.
• Endothermic with more than 50 kJ of energy
  absorbed.
• Endothermic with less than 50 kJ of energy
  absorbed.
• Exothermic with less than 50 kJ of energy
  released.

9
Use the bond energies below to determine whether
the following reaction is exothermic or
endothermic
Exo- and Endothermic Reactions CHECK YOUR ANSWER
H-H (bond energy 436 kJ/mol) Cl-Cl (bond energy
243 kJ/mol) H-Cl (bond energy 431 kJ/mol)
H2 Cl2 ? 2 HCl
431 kJ/mol
436 kJ/mol
243 kJ/mol
431 kJ/mol

• Exothermic with more than 50 kJ of energy
  released.
• Endothermic with more than 50 kJ of energy
  absorbed.
• Endothermic with less than 50 kJ of energy
  absorbed.
• Exothermic with less than 50 kJ of energy
  released.

10
Use the bond energies below to determine whether
the following reaction is exothermic or
endothermic
Exo- and Endothermic Reactions CHECK YOUR ANSWER
H-H (bond energy 436 kJ/mol) Cl-Cl (bond energy
243 kJ/mol) H-Cl (bond energy 431 kJ/mol)
H2 Cl2 ? 2 HCl
431 kJ/mol
436 kJ/mol
243 kJ/mol
431 kJ/mol
679 kJ/mol (absorbed)
862 kJ/mol (released)

• Exothermic with more than 50 kJ of energy
  released.
• Endothermic with more than 50 kJ of energy
  absorbed.
• Endothermic with less than 50 kJ of energy
  absorbed.
• Exothermic with less than 50 kJ of energy
  released.

11
Use the bond energies below to determine whether
the following reaction is exothermic or
endothermic
Exo- and Endothermic Reactions CHECK YOUR ANSWER
H-H (bond energy 436 kJ/mol) Cl-Cl (bond energy
243 kJ/mol) H-Cl (bond energy 431 kJ/mol)
H2 Cl2 ? 2 HCl
431 kJ/mol
436 kJ/mol
243 kJ/mol
431 kJ/mol
D 862 679 183 kJ/mol released
679 kJ/mol (absorbed)
862 kJ/mol (released)

• Exothermic with more than 50 kJ of energy
  released.
• Endothermic with more than 50 kJ of energy
  absorbed.
• Endothermic with less than 50 kJ of energy
  absorbed.
• Exothermic with less than 50 kJ of energy
  released.

12
Entropy and Chemical Reactions

• Exothermic reactions tend to be
  self-sustaining because they lead to large
  increases in entropy.
• Endothermic reactions
• tend to require the continual
• input of energy.

Example A campfire
Example Photosynthesis
13
Which type of chemical reaction leads to a
greater dispersal of energy?
Entropy and Chemical Reactions CHECK YOUR ANSWER
Exothermic
Endothermic