Energy Relationships in Chemical Reactions

Energy Relationships in Chemical Reactions

What is heat? What is a state function? What is enthalpy? Is enthalpy a state function? What does this mean? How can we calculate this? How are the methods the same/different?

Thermochemical Equations Is H negative or positive? System absorbs heat Endothermic H > 0 H reactants <H products Enthalpy 6.01 kj are absorbed for every 1 mole of ice that melts at 0 0 C and 1 atm. H 2 O(s) H 2 O(l) H = 6.01 kj/mol

Thermochemical Equations Is H negative or positive? System gives off heat Exothermic H < 0 H reactants >H products Enthalpy 890.4 kj are released for every 1 mole of methane that is combusted at 25 0 C and 1 atm. CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) H = -890.4 kj/mol

What are the rules for enthalpy? 1. Writing a thermochemical equation 2. Direction of the equation (reactants vs. products) 3. Extensive property increasing or decreasing the amounts of products or reactants 4. State property one reaction or multiple reactions to an overall reaction

Key Definitions: Standard conditions Enthalpy of formation Enthalpy of combustion Enthalpy of solution Enthalpy of vaporization (heat of vaporization) Enthalpy of fusion (heat of fusion)

How do we measure change in enthalpy for a reaction? Direct method Indirect methods Using a series of reactions (Hess s Law) Using tables of data (enthalpy of formation)

Table 6.4, p. 199

Practice: What is the enthalpy of combustion for methane? What volume of methane at 1 atm and 25 o C is needed to heat 500 ml of water from 5 o C to 95 o C? If the current price of natural gas is $0.10 per cubic foot, (and we assume natural gas is all methane) how much does it cost to heat the water?

Practice Discussion activity, number 15, p. 40: What is the thermochemical equation for the molar enthalpy of solution of calcium chloride? Discussion activity, number 16, p. 42: What is the enthalpy of formation for dinitrogen pentoxide given: 2H g O g 2H O l H 571.5 kj 2 2 2 N O g H O l 2HNO l H 76.6 kj 2 5 2 3 1 3 1 N2 g O2 g H2 g HNO3 l H 174 kj 2 2 2

Practice Discussion activity, number 17, p. 40: Which substance(s) will have an enthalpy of formation equal to zero? a. O(g) b. O 2 (g) c. O 2 (l) d. H 2 O(l) e. OH (aq)

For the reaction energy diagram shown to the right, the reaction is A. exothermic with ΔH reactants > ΔH products. B. exothermic with ΔH reactants < ΔH products. C. endothermic with ΔH reactants > ΔH products. D. endothermic with ΔH reactants < ΔH products. Which reaction corresponds to the reaction for the enthalpy of formation for methanol, CH 3 OH(l)? A. 2CH 3 OH(l) + 3O 2 (g) 2CO 2 (g) + 4H 2 O(l) B. CH 3 OH(l) + 3 / 2 O 2 (g) CO 2 (g) + 2H 2 O(l) C. 2C(s, graphite) + O 2 (g) + 4H 2 (g) 2CH 3 OH(l) D. C(s, graphite) + ½ O 2 (g) + 2H 2 (g) CH 3 OH(l) Hess s Law states when reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. This is valid because change in enthalpy is A. extensive. B. a state function. C. a type of energy. D. equal to energy when no work is done.

What is the standard molar enthalpy of solution for sodium chloride if 8.00 g of sodium chloride decreases the temperature of 74.90 g of water by 1.81 o C? Assume the heat transfer occurs only between the water and the reaction. The specific heat of water is 4.18 J g 1 oc 1. A. 4140 kj mol 1 B. 567 kj mol 1 C. 4.14 kj mol 1 D. 0.567 kj mol 1