7.2 Thermochemistry Objective 2

Essential Questions What is heat? What is an exothermic process? Endothermic? How can you use heat to tell which type it is? What is enthalpy? What is ΔH? How can you use ΔH to determine if a reaction is endothermic or exothermic?

Thermochemistry Thermochemistry is the study of changes in heat in chemical reactions

Energy Thermal energy is heat Heat is transfer of energy from a hotter object to a colder one

Heat Heat and temperature are NOT the same because temperature measures kinetic energy of a particles in a substance No such thing as COLD in chemistry-only heat flowing out Heat and Energy both measured in Joules (J)

Thermochemistry (cont) What does the law of conservation of energy say? Energy cannot be created nor destroyed If energy cannot be created nor destroyed then why is there heat? Energy is being transferred or changed

Thermochemistry (cont) Depending on how the heat/energy is being transferred a process is either exothermic or endothermic Exothermic process is when energy is being released as heat to the surroundings Endothermic process is when energy is being absorbed as heat from its surroundings

Thermochemistry (cont) If in an experiment if the temperature of the surroundings increases the reaction would be exothermic If in an experiment if the temperature of the surroundings decreases the reaction would be endothermic Example: When Al is added to CuSO4 the mixture heats up so it would be exo.

State Changes If the state change requires absorbing energy to occur it is endothermic If the state change releases energy (energy has to leave) than it is exothermic Example: melting would be endo, but freezing would be exo

Enthalpy Enthalpy (H) is used to measure the heat of a system (what you are studying) ΔH is the change in enthalpy which means the change in heat for the system

Enthalpy Diagrams (don’t need to write) Exothermic Reaction: CaO (s) + H2O (l) → Ca(OH)2 (s) ΔH = - 65.2 kJ These enthalpy diagrams show exothermic and endothermic processes: a) the reaction of calcium oxide and water and b) the decomposition of sodium bicarbonate. Identifying In which case is the enthalpy of the reactant(s) higher than that of the product(s)?

Enthalpy Diagrams (don’t need to write) Endothermic Reaction: 2 NaHCO3 (s) → Na2CO3 (s) + H2O (g) + CO2 (g) ΔH = 129 kJ These enthalpy diagrams show exothermic and endothermic processes: a) the reaction of calcium oxide and water and b) the decomposition of sodium bicarbonate. Identifying In which case is the enthalpy of the reactant(s) higher than that of the product(s)?

Enthalpy (cont) ΔH = Hproducts – Hreactants Exothermic reactions have a negative ΔH value because the reactants are higher in enthalpy than the products. Endothermic reactions have a positive ΔH value because the products have a higher enthalpy. When you reverse a reaction it reverses the sign of ΔH

Enthalpy (cont) ΔH can be written as part of the equation or next to the equation For exo reaction the ΔH value is written as a product because it is being produced For endo the ΔH value is written as a reactant or as a positive number after the equation

Enthalpy (cont) Identify the following processes as exothermic or endothermic: B2H6 (g) + 3 O2 (g) + 2147.5 kJ → B2O3 (s) + 3 H2O (l) H2 (g) + ½ O2 (g) → H2O (l) ΔH = -285.8 kJ CO2 (g) → C (s) + O2 (g) ΔH = +393.5 kJ 3C (s) + 4 H2 (g) → C3H8 (g) +103.85 kJ Sweat evaporating on your skin Water boiling on the stove CO2 freezing to form dry ice endo exo endo exo endo endo exo

Essential Questions What is heat? What is an exothermic process? Endothermic? How can you use heat to tell which type it is? What is enthalpy? What is ΔH? How can you use ΔH to determine if a reaction is endothermic or exothermic?

7.2 Tracked Assignment Worksheet