Exothermic and Endothermic Reactions  Section 5.1

The study of energy changes associated with chemical reactions The study of energy changes associated with chemical reactions Most reactions absorb or evolve energy Most reactions absorb or evolve energy This energy can be in the form of heat, light, or mechanical energy This energy can be in the form of heat, light, or mechanical energy Amount of energy is measured in joules, J, or usually kJ since the joule is a small amount Amount of energy is measured in joules, J, or usually kJ since the joule is a small amount Thermochemistry

Enthalpy is the total energy of a system, also known as heat content and abbreviated as “H” Enthalpy is the total energy of a system, also known as heat content and abbreviated as “H” Some of the energy of a system is stored as chemical potential energy in chemical bonds Some of the energy of a system is stored as chemical potential energy in chemical bonds Bonds are broken and formed during chemical reactions Bonds are broken and formed during chemical reactions Bonds broken require an energy input and bonds formed release energy Bonds broken require an energy input and bonds formed release energy Enthalpy

All reactions are accompanied by a change in the potential energy of the bonds All reactions are accompanied by a change in the potential energy of the bonds This is called the enthalpy change This is called the enthalpy change Enthalpy cannot be measured directly, but the enthalpy change can be measured Enthalpy cannot be measured directly, but the enthalpy change can be measured The symbol for the enthalpy change of reaction is Δ H and the units are kJ mol -1 The symbol for the enthalpy change of reaction is Δ H and the units are kJ mol -1 Enthalpy Change

Δ H is equal to the difference in enthalpy between the reactants and the products Δ H is equal to the difference in enthalpy between the reactants and the products This assumes that the reaction occurs with no change in temperature or pressure, or that these conditions are restored to their initial values This assumes that the reaction occurs with no change in temperature or pressure, or that these conditions are restored to their initial values If this is true, then the Δ H is equal to the change in the potential energy of the chemical bonds If this is true, then the Δ H is equal to the change in the potential energy of the chemical bonds More about Δ H

Sometimes a reaction occurs in which the enthalpy of the products is greater than the enthalpy of the reactants Sometimes a reaction occurs in which the enthalpy of the products is greater than the enthalpy of the reactants This means that the Δ H is positive This means that the Δ H is positive Heat energy was absorbed from the surroundings Heat energy was absorbed from the surroundings The surroundings either get colder, or heat is supplied by an external source The surroundings either get colder, or heat is supplied by an external source Reactants are more stable than the products as the bonds made are weaker than the bonds broken Reactants are more stable than the products as the bonds made are weaker than the bonds broken Endothermic Reactions

Sometimes a reaction occurs in which the enthalpy of the reactants is greater than the enthalpy of the products Sometimes a reaction occurs in which the enthalpy of the reactants is greater than the enthalpy of the products This means that Δ H is negative This means that Δ H is negative The chemical reaction releases heat energy to the surroundings The chemical reaction releases heat energy to the surroundings Products are more stable than the reactants as the bonds made are stronger than the bonds broken Products are more stable than the reactants as the bonds made are stronger than the bonds broken Exothermic Reactions

E a or A e E a or A e The minimum amount of energy required for a chemical reaction to occur The minimum amount of energy required for a chemical reaction to occur Must be overcome for the reaction to “go” Must be overcome for the reaction to “go” Activation Energy

Energy Level Diagrams

Abbreviated as Δ H ө Abbreviated as Δ H ө Has special conditions: Has special conditions: 1 atm of pressure, or kPa 1 atm of pressure, or kPa Solutions are at 1 mol dm -3 Solutions are at 1 mol dm -3 Temperature of 298 K (25 ° C) Temperature of 298 K (25 ° C) These conditions are known as thermochemical standard conditions These conditions are known as thermochemical standard conditions Standard Enthalpy Change

Combustion reactions are exothermic (you should know this!) Combustion reactions are exothermic (you should know this!) CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O ( l ) CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O ( l ) Acid-base neutralization reactions are also exothermic (more about those later, but you should remember: Acid-base neutralization reactions are also exothermic (more about those later, but you should remember: Do as you otta’ add acid to wata’ is due to this fact!) Do as you otta’ add acid to wata’ is due to this fact!) The LAB you did where you measured the change in enthalpy The LAB you did where you measured the change in enthalpy More about Exothermic Reactions

When a chemical reaction is written with an associated change in enthalpy: When a chemical reaction is written with an associated change in enthalpy: CH 4(g) + 2O 2(g) → CO 2(g) + 2H 2 O ( l) Δ H = -890 kJ mol -1 CH 4(g) + 2O 2(g) → CO 2(g) + 2H 2 O ( l) Δ H = -890 kJ mol -1 NH 4 NO 3(s) + aq → NH 4 + (aq) + NO 3 - (aq) Δ H = +25 kJ mol -1 NH 4 NO 3(s) + aq → NH 4 + (aq) + NO 3 - (aq) Δ H = +25 kJ mol -1 You can easily tell which reaction is exothermic and which is endothermic from the Δ H You can easily tell which reaction is exothermic and which is endothermic from the Δ H Thermochemical Equation