Chemistry Unit 9 Thermochemistry IHS Henry

Exothermic Reactions

• “Feels” hot

• Heat energy is being released/produced

• ΔH Notation

  • Reaction → Products ΔH = (-) kJ/mol

• Energy Term Notation

  • Reaction → Products + E*

    * Will always be a positive value in ET notation

• (System) [-Q] → Surroundings [+Q]

Endothermic Reactions

• “Feels” cold

• Heat energy is being absorbed/required

• ΔH Notation

  • Reaction → Products ΔH = (+) kJ/mol

• Energy Term Notation

  • Reaction + E*→ Products*

    * Will always be a positive value in ET notation

• (System) [+Q] ← Surroundings [-Q]

Changes in Enthalpy

Enthalpy (ΔH)

• The amount of hear energy transferred during a reaction (kJ/mol)

• ΔH ≈ q* (products - reactants)

  • * ONLY TRUE UNDER CONSTANT PRESSURE

Activation Energy

• Minimum amount of energy required to create products

• Peak - Products

• Always a positive value

• E_a

Bond Enthalpy Equation

ΔH_{Bond E} = ΣH_Reactants + ΣH_Products

E.g.

ΣH_{R =} [1 mol x 120 kJ/mol] + [4 mol x 414 kJ/mol] = 1776 kJ

ΣH_P = [2 mol x -427 kJ/mol] + [3 mol x -283 kJ/mol] = -1703 kJ

1776 + (-1703) = 73 kJ

When chemical bonds of reactants are broken in a reaction

• Enthalpy of reactants = (+)

• ΔH_Reactants = (+)

• Endothermic process

When chemical bonds of products are formed in a reaction

• Enthalpy of products = (-)

• ΔH_Reactants = (-)

• Exothermic process

Thermochemistry Definition

Study of heat energy associated with physical transformations and/or chemical reactions

Heat Energy Properities

• Energy cannot be created or destroyed; only transferred/transformed

• Energy always flows from hot to cold