Exothermic & Endothermic REACTIONS Energy in Reactions

Energy in Reactions Some chemical reactions release heat to the surroundings These are called EXOTHERMIC reactions Heat EXITS the reaction Some chemical reactions absorb heat from the surroundings These are called ENDOTHERMIC reactions Heat ENTERS the reaction

Energy in Reactions All substances have a type of chemical potential: The total energy (J or kJ) contained in a system Chemical potential energy in a substance The symbol for enthalpy is H ΔH = Hproducts – Hreactants

Exothermic Reactions An exothermic reaction converts the enthalpy stored in a substance into HEAT which is released to the surroundings N2 + 3H2  2 NH3+ heat Has higher enthalpy Has lower enthalpy

Exothermic Enthalpy Diagram Products Reactants Enthalpy (H) Reaction Proceeds ΔH is Negative

Exothermic Reactions 1. The “heat term” written on the PRODUCTS side N2 + 3H2  2 NH3 + 46.2 kJ 2. ΔH written beside the equation is NEGATIVE. N2 + 3H2  2 NH3 ΔH = - 46.2 kJ

In Summary… Exothermic Reactions: ΔH is negative Products are lower than reactants on the enthalpy diagram Heat is released to the surroundings

Endothermic Reactions An endothermic reaction absorbs heat from the surroundings and converts into enthalpy 2C + 2 H2 + heat  C2H4 Has lower enthalpy Has higher enthalpy

Endothermic Enthalpy Diagram Products Reactants Enthalpy (H) Reaction Proceeds ΔH is POSITIVE

Endothermic Reactions 1. The “heat term” written on the REACTANTS side. 2 C + 2H2 + 52.3 kJ  C2H4 2. ΔH written beside the equation is POSITIVE. 2 C + 2 H2  C2H4 ΔH = + 52.3 kJ

In Summary… Endothermic Reactions: ΔH is positive Products are higher than reactants on the enthalpy diagram Heat is absorbed from the surroundings

Please collect the Endo/Exo Worksheet from the front of the room, finish this worksheet by next class Next Class: Begin Stoich In Class Work