Thermochemistry Chemical Energy Reactions Thermochemistry Study of heat

Thermochemistry Chemical Energy & Reactions

Thermochemistry • Study of heat changes occuring during reactions – Potential • Stored energy due to chemical composition – Kinetic • is the energy of motion – Heat/thermal • Energy transferred from one substance to another due to temperature differences. • Always from hot ---> cold – Chemical • Energy stored within bonds of molecules (intramolecular) and between molecules (intermolecular) – Nuclear Energy • Change in the nucleus of the atom to form different elements

Energy and changes • System – Defined as the part of the universe on which we focus our attention. – The SURROUNDINGS include everything else in the universe.

Temperature vs Heat • Temperature – Measure of how hot or cold a substance is. – Higher T indicates higher molecular motion – Measured in o. C or K • Heat – Form of energy contained in a substance – Unit in J (Joule) or KJ (Kilojoule) – Compare T and heat content of a cup of coffee vs. iceberg – Quantity of heat in a substance depends on its: • Temperature • Mass • Specific Heat

• Enthalpy (H) – Energy content (KJ) of a substance – Excluding nuclear energy • Heat of Reaction ( H) – Amount of energy (KJ) released or absorbed during a chemical reaction or physical change • Heat of formation – Energy involved when a substance is produced • Heat of combustion – Energy released when a substance is burned

Types of changes • Molar heat of… – Refers to the amount of energy released or absorbed per mole of a substance involved in a chemical reaction. • Each type of change absorbs or releases energy. – Least in physical- most in nuclear (by far)

Exothermic reaction • A process that releases heat to the surroundings (becomes hot to touch) • Ex- combustion, condensation • Represented in equations by 2 H 2 + O 2 ---> 2 H 2 O + 340 k. J 2 H 2 + O 2 ---> 2 H 2 O H = -340 k. J • Negative sign denotes exothermic

Exothermic Graphical Representation • Products (P) have LESS energy or enthalpy content than reactants (R), since there is a net LOSS of energy to surroundings during reaction

Endothermic Reactions • Involve a net absorption of energy by the reactants. • Ex- Recharging a battery, evaporation • Represented by: N 2 + 2 O 2 + 67. 6 k. J --> 2 NO 2 N 2 + 2 O 2 ---> 2 NO 2 H = +67. 6 k. J • Positive sign indicates an endothermic reaction

Endothermic Reaction Graphical Representation • Products have GREATER enthalpy since energy is absorbed from the surroundings and stored in products

Specific Heat Capacity (c) • Amount of heat energy required to raise or lower the temperature of 1 g of a substance by 10 C. • Ex– – – H 2 O= 4. 19 J/go. C Iron = 0. 45 J/go. C Aluminum = 0. 89 J/go. C Copper = 0. 387 J/go. C Brass = 0. 38 J/go. C Zinc = 0. 39 J/go. C

How to find heat absorbed or lost? • Use Q = mc T • Where – m = mass of substance – Q = heat energy (KJ) – T = Change in temperature (Tf - Ti) – C = specific heat of substance

Q = mc T Example • A 30 g block of Aluminum at 25 OC absorbs 2. 508 k. J of energy. What was its final temperature? (c. Al = 0. 89 J/g 0 C) T = Q = 2508 J = 93 OC mc 30 x 0. 89 Tf = (Ti + T) = (25 + 93) = 118 o. C

Calorimeter • An instrument used to measure the amount of heat energy released or absorbed during a chemical reaction • If the reaction is EXOthermic, heat will be released into the water and the temperature of the H 2 O (TH 2 O) will rise • If the reaction is ENDOthermic, heat will be absorbed from the H 2 O and TH 2 O drops.

Example #1 • Julie dissolves 3. 7 g of Ca(OH)2 in 300 g of H 2 O at 15. 6 OC. The solution’s temperature rises to 21 OC. A) How much heat was released? B) What is Ca(OH)2’s molar heat of solution? A) Q=mc T m= mass of substance that absorbs the heat (water) NOT what released it (Ca(OH)2) Q= 300 x 4. 19 x 5. 4 = 6787. 8 J or 6. 79 k. J b) 3. 7 g x 1 mol/74 g = 0. 05 mol Ca(OH)2 Molar Heat = 1 mol x 6. 79 k. J/0. 05 mol = 135. 8 k. J

Example #2 • When 2. 4 g of CH 4 is burned in a calorimeter, the 850 m. L of H 2 O at 19 OC rises to 25. 3 OC. What is the molar heat of combustion of CH 4? Q = mc T = 850 x 4. 19 x 6. 3 = 22 437. 5 J or 22. 44 k. J/2. 4 g x 1 mol/16 g = 0. 15 mol CH 4 1 mol x 22. 44 k. J/0. 15 mol = 149. 6 k. J

“Stoichiometry” • Example #1 – 2 CO 2 + 43. 8 k. J --> 2 CO + O 2 – *43. 8 k. J/2 mol CO 2 or 43. 8 k. J/1 mol O 2 * – If 6. 7 L of CO 2 at 35 OC and 112 k. Pa is decomposed, how much heat energy is absorbed? n= PV = 112 x 6. 7 = 0. 293 mol CO 2 RT 8. 31 x 308 0. 293 mol x 43. 8 k. J 2 mol CO 2 = 6. 42 k. J

Example #2 • Using N 2 + 3 H 2 ---> 2 NH 3 H= -34. 2 k. J • If 22. 23 k. J of heat is released in a reaction, how many grams of H 2 must be reacted?

22. 23 k. J x 3 H 2 = 1. 95 mol H 2 34. 2 k. J 1. 95 mol H 2 x 2 g/mol = 3. 9 g of H 2 must be reacted

Heat Transfer • Heat always transfers from hot to cold • Heat will continue to transfer from the hot object to the cold object (as long as they are in contact) until the same final temperature is reached. • Heat LOST by hot = heat GAINED by cold • -Qlost hot = +Qgained cold • -(mh x Th x ch) = +(mc x Tc x cc)

Example • A 450 g sample of water at 80 OC is mixed with 360 g of water at 12 o. C. What is the final temperature of the mixture? -[450 g x (Tf - 80 OC) x 4. 19] = + [360 g x (Tf - 12 OC) x 4. 19] 4. 19’s cancel eachother out -450 Tf + 36000 = 360 Tf - 4320 -810 Tf = -40320 Tf = 49. 78 OC

Heat of neutralization problem • When 50 m. L of a 0. 25 M Na. OH solution is neutralized by 100 m. L of an HCl solution, the temperature of the mixture rises to 1. 5 OC. What is the molar heat of neutralization of Na. OH? (assume densities and specific heat of the solutions are same as water)

Solution • Heat released into “mixture”: Q=mc T = 150 g x 1. 5 OC x 4. 19 J/g. OC = 942. 8 J Mols of Na. OH neutralized : (molarity =mol/Vol) mol = molarity x Volume 0. 25 mol/L x 0. 05 L = 0. 0125 mol Na. OH Molar heat of neutralization of Na. OH: • 1 mol Na. OH x 942. 8 J/0. 0125 mol = 75420 J/mol or 75. 42 k. J/mol

Homework Read pages 653 -671 (Pay attention to graph on page 668) Questions within pages #1 -8 (due next class) Questions page 681 #13 -29