Endothermic And Exothermic Reactions – part 2

Mr. Markus

Chemical bonds and Energy

• Chemical energy is the energy stored in the chemical bonds of a substance.
• Energy changes in chemical reactions are determined by changes that occur in chemical bonding.
• Chemical reactions involve the breaking of bonds in the reactants and the formation of bonds in the products.

Law of Conservation of Energy

• This law states that during any chemical reaction, energy is neither created nor destroyed, it is only changed form one form to another.
• In both endothermic and exothermic reactions, the total amount of energy before and after the reaction is the same

• Breaking bonds requires energy. It is endothermic.

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• Making new bonds gives out energy. It is exothermic.

Breaking bonds

• For example, take the reaction of propane that occurs in gas grills

C₃H₈ + 5O₂ 🡪 3CO₂ + 4H₂O + heat

• In order for the reaction to occur, the bonds between the carbon, hydrogen, and oxygen molecules must be broken.
• Breaking the bonds requires energy.
• This energy comes from igniting the gas grill. The energy needed to break the bonds is supplied when the igniter sparks.

Forming bonds

• In order to form the carbon dioxide and water, new bonds must be created.
• The formation of chemical bonds releases energy.
• The heat that comes from the gas grill comes from the formation of the new chemical bonds.

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Exothermic reactions

• A chemical reaction that releases energy to its surroundings.
• The energy released as the products form is greater than the energy needed to break the bonds in the reactants.
• An exothermic chemical equation is written with the heat as one of the products.

A + B 🡪 C + heat

This is a typical graph of an exothermic reaction with the products at a lower energy than the reactants.

Endothermic reactions

• A chemical reaction that absorbs energy from its surroundings.
• More energy is required to break the bonds in the reactants than is released by the formation of bonds in the products.
• In these reactions, heat is shown as one of the reactants

A + B + heat 🡪 C

This is a typical graph of an endothermic reaction with the products at a higher energy than the reactants.

Enthalpy => Heat of Reaction

Burning of Methane�CH₄ +2O₂🡪 CO₂ + 2H₂O

Ex: Burning of Methane�CH₄ + 2O₂🡪 CO₂ + 2H₂O

CH₄(g) + 2O₂(g) 🡪 CO₂(g) + 2H₂O(g)

The left side involves bond breaking and energy needs:

Four C – H bonds 4 x 435 kJ/mol = 1740 kJ/mol

Two O = O bonds 2 x 497 kJ/mol = 994 kJ/mol

total energy needed = 2734 kJ/mol

The right side involves bond making and gives out energy:

Two C = O bonds 2 x 803 kJ/mol = 1606 kJ/mol

Four O – H bonds 4 x 464 kJ/mol = 1856 kJ/mol

total energy given out = 3462 kJ/mol

The heat of reaction, ΔH is the energy change on going from reactants to products

ΔH = (energy needed to break bonds) – (energy given out when bonds form)

= 2734 – 3462

= -728 kJ/mol

Homework Questions

1. What happens to chemical bonds as a chemical reaction occurs?
2. Is the combustion of propane an endothermic or an exothermic reaction?
3. How do chemical reactions involve energy?
4. Is energy created during an exothermic reaction?
5. For the following reaction, what bonds are broken and what bonds are formed during the reaction? CH₄ + 2O₂ 🡪 CO₂ + 2H₂O