Exothermic and Endothermic Reactions

Exothermic and Endothermic Reactions

Exothermic Reactions

• An exothermic reaction transfers thermal energy (heat) to the surroundings, causing the temperature of the surroundings to increase.
• Examples:
  • Combustion
  • Many oxidation reactions
  • Neutralization
• Everyday examples:
  • Self-heating cans (e.g., for coffee)
  • Hand warmers

Endothermic Reactions

• An endothermic reaction takes in thermal energy (heat) from the surroundings, causing the temperature of the surroundings to decrease.
• Examples:
  • Thermal decomposition
  • Reaction of citric acid and sodium hydrogen carbonate
• Everyday examples:
  • Some sports injury packs

Distinguishing Between Exothermic and Endothermic Reactions

• EXOthermic: Energy EXits into the surroundings.
• ENdothermic: Energy ENters from the surroundings.

Reaction Pathway Diagrams

• Reaction pathway diagrams illustrate whether a reaction is exothermic or endothermic.
• Diagram components:
  • Reactants and products
  • Activation energy ($E_a$)
  • Overall enthalpy change ($\Delta H$)
• The diagram starts at the reactants and ends at the products.

Exothermic Reaction Pathway Diagram

• Reactants are at a higher energy level than the products, indicating energy has been released into the surroundings.
• The energy change of the reaction is negative.

Endothermic Reaction Pathway Diagram

• Reactants are at a lower energy level than the products, indicating energy has entered the system from the surroundings.
• The energy change of the reaction is positive.

Enthalpy Change ($\Delta H$) (Extended Only)

• The enthalpy change ($\Delta H$) of a reaction is the transfer of thermal energy during a reaction.
• Exothermic reaction: $\Delta H$ is negative.
• Endothermic reaction: $\Delta H$ is positive.

Activation Energy ($E_a$) (Extended Only)

• The activation energy ($E_a$) is the minimum amount of energy that colliding particles need to have in order to react.
• For chemical reactions to occur, particles must collide successfully with enough energy.

Drawing and Labeling Reaction Pathway Diagrams (Extended Only)

• Components to include:
  • (a) Reactants
  • (b) Products
  • (c) Enthalpy change of the reaction, $\Delta H$
  • (d) Activation energy, $E_a$
• Activation energy ($E_a$) is shown on the diagram with an arrow, starting at the energy of the reactants and extending to the maximum energy reached in the reaction (the peak of the pathway).
• Enthalpy change ($\Delta H$) is shown with an arrow from the energy of the reactants to the energy of the products.
• The y-axis is labeled 'Energy' and the x-axis is labeled 'Progress of reaction.'
• A curved line connects the reactants and products, showing the change of energy over the course of the reaction.

Exothermic Reactions

• Reactants and products are labeled on horizontal lines, with reactants at a higher energy level than the products.
• The overall enthalpy change, $\Delta H$, is negative, indicated by an arrow pointing downwards from the energy level of the reactants to the products.

Endothermic Reactions

• Reactants and products are labeled on horizontal lines, with reactants at a lower energy level than the products.
• The overall enthalpy change, $\Delta H$, is positive, indicated by an arrow pointing upwards from the energy level of the reactants to the products.

Bond Breaking and Bond Making (Extended Only)

• Bond breaking is an endothermic process.
• Bond making is an exothermic process.

Explanation of Enthalpy Change

• For a chemical reaction to occur:
  • Bonds must be broken in the reactants.
  • Bonds are made in the products.
• Breaking bonds requires energy input from the surroundings (endothermic).
• Making bonds releases energy into the surroundings (exothermic).
• The enthalpy change of a reaction is negative (exothermic) when: Energy released making bonds > Energy taken in breaking bonds.
• The enthalpy change of a reaction is positive (endothermic) when: Energy taken in breaking bonds > Energy released making bonds.

Calculating Enthalpy Change Using Bond Energies (Extended Only)

• Steps:
  1. Add up the bond energies for every bond in the reactants – totaling the ‘energy in’.
  2. Add up the bond energies for every bond in the products – totaling the ‘energy out’.
  3. Use the formula: $\Delta H$ = energy in – energy out

Example 1

Calculate the enthalpy change of the following reaction and state whether it is exothermic or endothermic:
$CH<em>3CH</em>3 + Cl<em>2 \rightarrow CH</em>3CH_2Cl + HCl$

Bond Energies (kJ/mol):

• C-H: 413
• C-C: 346
• Cl-Cl: 240
• C-Cl: 327
• H-Cl: 428

1. Draw out the molecules to show every bond.
2. Highlight/label and count every bond.

• Reactants (Energy in):
  • (6 x C-H) + (1 x C-C) + (1 x Cl-Cl) = (6 x 413) + (1 x 346) + (1 x 240) = 3064 kJ/mol
• Products (Energy out):
  • (5 x C-H) + (1 x C-C) + (1 x C-Cl) + (1 x H-Cl) = (5 x 413) + (1 x 346) + (1 x 327) + (1 x 428) = 3166 kJ/mol

1. Calculate the enthalpy change:

• $\Delta H$ = 3064 - 3166 = -102 kJ/mol
• The reaction is exothermic because the enthalpy change is negative.

Example 2

Calculate the enthalpy change of the following reaction and state whether it is exothermic or endothermic:

$2 H<em>2O \rightarrow 2H</em>2 + O_2$

Bond Energies (kJ/mol):

• O-H: 464
• H-H: 436
• O=O: 498

1. Draw out the molecules to show every bond.
2. Highlight/label and count every bond.

• Reactants (Energy in):
  • (4 x O-H) = (4 x 464) = 1856 kJ/mol
• Products (Energy out):
  • (2 x H-H) + (1 x O=O) = (2 x 436) + (1 x 498) = 1370 kJ/mol

1. Calculate the enthalpy change:

• $\Delta H$ = 1856 - 1370 = +486 kJ/mol

• The reaction is endothermic because the enthalpy change is positive.

• Note: Bond energy calculations may require rearranging the enthalpy change formula to calculate the bond energy of a specific bond.

Example 3

The enthalpy change of the following reaction is -808 kJ/mol. Using the given bond energies, calculate the bond energy of the O-H bond.

$CH<em>4 + 2 O</em>2 \rightarrow CO<em>2 + 2H</em>2O$

Bond Energies (kJ/mol):

• C-H: 413
• C=O: 800
• O=O: 498
• O-H: ?

1. Draw out the molecules and highlight each type of bond.
2. Add up the bond energies for reactants and products.

• Reactants (energy in):
  • (4 x C-H) + (2 x O=O) = (4 x 413) + (2 x 498) = 2648 kJ/mol
• Products (energy out):
  • (2 x C=O) + (4 x O-H) = (2 x 800) + (4 x ?) =

1. Rearrange the enthalpy change formula to calculate the missing bond energy.

• $\Delta H$ = energy in – energy out
• [ (4 x C-H) + (2 x O=O) ] – [ (2 x C=O) + (4 x O-H) ] = -808 kJ/mol
• [ (4 x 413) + (2 x 498) ] – [ (2 x 800) + (4 x ?) ] = -808 kJ/mol
• 2648 – (1600 + 4 x ?) = -808 kJ/mol
• 1048 - (4 x ?) = -808 kJ/mol
• Rearrange to isolate the unknown:
  • 1048 + 808 = (4 x ?)
  • 1856 = (4 x ?)
  • ? = O-H bond energy = 1856 ÷ 4 = 464 kJ/mol