8.6 The Thermodynamic Standard State

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1 8.6 The Thermodynamic Standard State The value of H reported for a reaction depends on the number of moles of reactants...or how much matter is contained in the system C 3 H 8 (g) + 5O 2 (g) > 3CO 2 (g) + 4H 2 O(g) H = kj 2 C 3 H 8 (g) + 10 O 2 (g) > 6 CO 2 (g) + 8 H 2 O(g) H = kj Also, the reactants and products must have their physical states specified [(g), (l), (s) ] since heat is required to change temperatures and states. For reaction of 1 mole of propane above, H = kj if the water is produced as a liquid. C 3 H 8 (g) + 5O 2 (g) > 3CO 2 (g) + 4H 2 O(l) H = kj Thus it is convenient to define a standard state... a pure substance in a specified state, normally its most stable form at 1 atm pressure, 25 o C and, if in solution, at a concentration of 1 mol/l. From now on we will be using measurements of H made under these standard state conditions as indicated by addition of a superscript o to the symbol of H. Thus, H o is the standard enthalpy of reaction. The first thermchemical equation on the previous side can now be written: C 3 H 8 (g) + 5O 2 (g) > 3CO 2 (g) + 4H 2 O(g) H o = kj 1

2 8.7 Enthalpies of Physical and Chemical Change Almost every change in a chemical system involves a change in enthalpy, H. The changes are either : physical (same compound different state) or chemical (different compound) changes. 1. Physical Changes [ Lab #2, part ll ] Each of these physical state changes require a change in enthalpy, so we can talk about the enthalpy of fusion ( H fusion ), the enthalpy of vaporization ( H vap ), or the enthalpy of sublimation ( H subl ). For physical changes in water: Melt ice H 2 O (s) ----> H 2 O (l) Hf usion = 6.01 kj/mol vaporize water H 2 O (l) ----> H 2 O (g) H vap = 40.7 kj/mol sublimation H 2 O (s) ----> H 2 O (g) H sub = kj/mol Draw and describe Figure 8.5 (textbook) in class [ on blackboard] If the reaction is reversed, the value has the same magnitude but is opposite in sign. H 2 O (l) ----> H 2 O (s) H = kJ/mol 2. Chemical Changes: Endothermic and Exothermic Reactions [ Lab #3] A reaction is endothermic if heat moves from the surroundings into the system during the reaction. H is positive. A reaction is exothermic if the system releases heat to the surroundings. H is negative. 2 Fe (s) + Al 2 O 3 (s) 2 Al (s) + Fe 2 O 3 (s) H o = kj 2 Al (s) + Fe 2 O 3 (s) 2 Fe (s) + Al 2 O 3 (s) H o = kj 2

3 Problem 8.8 (b) How much heat (in kj) is evolved or absorbed when 4.88g of barium hydroxide octahydrate react with ammonium chloride as seen in Solution to problem done in class on blackboard Ans: q (heat absorbed) = 1.24 kj Note: H is (+) Heat moves from surroundings system 8.8 Calorimetry and Heat Capacity We need to calculate the quantity of heat (q) flowing during a reaction. We can measure the amount of heat transferred during a chemical reaction using a device called a calorimeter. There are two types The first type is open to the atmosphere and consists of a well insulated reaction vessel, a stirrer, and a thermometer. The reaction vessel is open to the atmosphere (constant pressure, but V) and usually involves reactions in solution. The reaction is the system and the water is the surrounding. We measure q p to calculate H of the reaction. In the lab Exp. #2 and #3 You use a simple cup calorimeter 3

4 The second type is a bomb calorimeter which measures heat evolved during a combustion reaction. It is preformed at Constant Volume, therefore we are calculating E of the reaction of the reaction by measuring heat evolved (q v ). With both calorimeters we measure temperature change ( T) to calculate heat flow during a reaction (q reaction) The exact amount of heat absorbed ( or given off) is equal to the substances heat capacity (C) times the temperature change. q = C x T Heat Capacities of substances depends on its size and composition. Two types of heat capacities: a) Specific heat (units: J/ g. o C) - the amount of heat necessary to raise the temperature of 1 g of substance by 1 o C. i.e., H 2 O (l) is J/ g. o C q = (specific heat ) x (Mass) x T b) Molar heat capacity (C m ) (Units: J/ mol. o C) - amount of heat necessary to raise the temperature of 1 mol of substance by 1 o C i.e., Fe (s) is 25.1 J/ mol. o C q = (C m ) x (moles of substance) x T 4

5 Relation between Molar heat capacity (C m) and specific heat C m = specific heat x (molar mass) I.e., For water (l) C m (H 2 O) = J/ g. o C x g/mol = J/ mol. o C Problem 8.10 Assuming Coca-Cola has the same specific heat as water 4.18 J/(g C), calculate the amount of heat in kj transferred when one can (about 350 g) is cooled from 25 C to 3 C. Ans: q = -32 kj.. solution to problem done in class Problem 8.12 [ similar to lab #3, Part l] When 25.0 ml of 1.0 M H 2 SO 4 is added to 50.0 ml of 1.0 M NaOH at 25.0 C in a calorimeter, the temperature of the aqueous solution increases to 33.9 C. Assuming the specific heat of the solution is 4.18 J/(g C), that its density is 1.00 g/ml, and that the calorimeter itself absorbs a negligible amount of heat, calculate H (in kj) for the reaction: H 2 SO 4 (aq) + 2 NaOH (aq) 2 H 2 O (l) + Na 2 SO 4 (aq) Ans: -1.1 x 10 2 kj solution to problem done in class 5

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