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 (speed) of the particles in a substance No such thing as COLD in chemistry there is only heat flowing out Heat and Energy both measured in Joules (J)

Energy (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

Exothermic and Endothermic 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

Exo and Endo (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

Exo and Endo (cont) When Al is added to CuSO4 the mixture heats up so it would be exothermic.

State Changes If the state change absorbs energy (takes in 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 ΔH = Hproducts – Hreactants

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) 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. ΔH can be written next to the reaction like in this exo reaction: 2 H2 (g) + O2 (g) → 2 H2O (l) ΔH = -572 kJ

Enthalpy (cont) ΔH can be written as part of the reaction For exo reactions the ΔH is written as a product because it is being released by the reaction 2 H2 (g) + O2 (g) → 2 H2O (l) + 572 kJ For endo the ΔH is written as a reactant because energy must be absorbed for it to react 2 H2O (l) + 572 kJ → 2 H2 (g) + O2 (g)

Enthalpy (cont) Identify the following processes as exothermic or endothermic: B2H6 (g) + 3 O2 (g) + 2147.5 kJ → B2O3 (s) + 3 H2O (l) Na (s) + ½ Cl2 (g) → NaCl (s) ΔH = -411 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