reaction sometimes used for H2S production: ⅛S8(s) + H2(g) → H2S(g) ∆Hrxn = -20.2 kJ (a) Is this an exothermic or endothermic reaction? (b) What is ∆Hrxn for the reverse reaction? (c) What is ∆H when 3.2 mol of S8 reacts? (d) What is ∆H when 20.0 g of S8 reacts?
Thermochemistry
Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.
Exergonic Reaction
The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.
1.Consider the following balanced thermochemical equationfor a reaction sometimes used for H2S production:
⅛S8(s) + H2(g) → H2S(g) ∆Hrxn = -20.2 kJ
(a) Is this an exothermic or endothermic reaction?
(b) What is ∆Hrxn for the reverse reaction?
(c) What is ∆H when 3.2 mol of S8 reacts?
(d) What is ∆H when 20.0 g of S8 reacts?
Liquid hydrogen peroxide, an oxidizing agent in many rocketfuel mixtures, releases oxygen gas on decomposition:
2H2O2(l) → 2H2O (l) + O2(g) ∆Hrxn = - 196. 1 kJ
How much heat is released when 732 kg of H2O2 decomposes?
Calculate ∆Hrxn for
Ca(s) + ½ O2(g) + Co2 (g) → CaCO3(s)
given the following set of reactions:
Ca(s) + ½ O2(g) →CaO(s) ∆H = -635.1 kJ
CaCO3(s) →CaO(s) + CO2(g) ∆H = 178.3 kJ
Write the balanced overall equation for the followingprocess (equation 3) and calculate ∆HoveralI.
(1) N2 (g) + O2(g) → 2NO(g) ∆H = 180.6 kJ
(2) 2NO(g) + O2(g) → 2NO2(g) ∆H = - 114.2 kJ
(3) ∆Hoverall=?
Calculate ∆H°rxn for each of the following:
(a) 2H2S(g) + 3O2(g) → 2SO2(g) + 2H2O (g)
(b) CH4(g) + CI2(g) → CCI4(l) + HCl(g) [unbalanced]
Copper(I) oxide can be oxidized to copper(II) oxide:
Cu2O(s) + ½ O2(g) →2CuO(s) ∆H°rxn = - 146.0 kJ
Given that ∆H°f of Cu2O(s) = -168.6 kJ/mol, what is ∆H°f of CuO(s)?
13. Stearic acid (C18H36O2) is a typical fatty acid, a moleculewith a long hydrocarbon chain and an organic acid group (COOH) at the end. It is used to make cosmetics, ointments,soaps, and candles and is found in animal tissue as part of manysaturated fats. In fact, when you eat meat, chances are that youare ingesting some fats that contain stearic acid.
(a) Write a balanced equation for the complete combustion ofstearic acid to gaseous products.
(b) Calculate ∆H°rxnfor this combustion (∆H°f = -948 kJ/mol).
(c) Calculate the heat (q) in kJ and kcal when 1.00 g of stearicacid is burned completely.
(d) The nutritional information for a candy bar states that oneserving contains 11.0 g of fat and 100 Calfrom fat (1 Cal =1 kcal). Is this information consistent with your answer in part (c)?
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