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The following data compare the standard enthalpies and free energies of formation of some crystalline ionic substances and aqueous solutions of the substances:
Substance | ΔHOf (KJ / mol) | Δ GOf (KJ / mol) |
AgNO3(s) | −124.4 | −33.4 |
AgNO3(aq) | −101.7 | −34.2 |
MgSO4(S) | −1283.7 | -1169.6 |
MgSO4(aq) | −1374.8 | -1198.4 |
Write the formation reaction for AgNO3 (s). Based on this reaction, do you expect the entropy of the system to increase or decrease upon the formation of AgNO3 (s)?
(b) Use ΔHO f and ΔGO f of AgNO3 (S) to determine the entropy change upon formation of the substance. Is your answer consistent with your reasoning in part (a)?
(c ) Is dissolving AgNO3 in water an exothermic or endothermic process?
What about dissolving MgSO4 in water?
(d) For both AgNO3 and MgSO4, use the data to calculate the entropy change when the solid is dissolved in water.
(e) Discuss the results from part (d) with reference to material presented in this chapter and in the “A Closer Look” box on page 814.
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Chapter 5 Solutions
Chemistry: The Central Science (13th Edition)
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- 9.42 Why is enthalpy generally more useful than internal energy in the thermodynamics of real world systems?arrow_forwardThe decomposition of ozone, O3, to oxygen, O2, is an exothermic reaction. What is the sign of q? If you were to touch a flask in which ozone is decomposing to oxygen, would you expect the flask to feel warm or cool?arrow_forwardThe process of dissolving ammonium nitrate, NH4NO3, in water is an endothermic process. What is the sign of q? If you were to add some ammonium nitrate to water in a flask, would you expect the flask to feel warm or cool?arrow_forward
- The formation of aluminum oxide from its elements is highly exothermic. If 2.70 g Al metal is burned in pure O2 to give A12O3, calculate how much thermal energy is evolved in the process (at constant pressure).arrow_forwardUnder what circumstances is the heat of a process equal to the enthalpy change for the process?arrow_forwardGasohol, a mixture of gasoline and ethanol, C2H5OH, is used as automobile fuel. The alcohol releases energy in a combustion reaction with O2. C2H5OH(l)+3O2(g)2CO2(g)+3H2O(l) If 0.115 g ethanol evolves 3.62 kJ when burned at constant pressure, calculate the combustion enthalpy for ethanol.arrow_forward
- The thermochemical equation for the burning of methane, the main component of natural gas, is CH4(g)+2O2(g)CO2(g)+2H2O(l)H=890kJ (a) Is this reaction endothermic or exothermic? (b) What quantities of reactants and products are assumed if H = 890 kJ? (c) What is the enthalpy change when 1.00 g methane burns in an excess of oxygen?arrow_forwardWhen a gas expands, what is the sign of w? Why? When a gas contracts, what is the sign of w? Why? What are the signs of q and w for the process of boiling water?arrow_forwardCoal is used as a fuel in some electric-generating plants. Coal is a complex material, but for simplicity we may consider it to be a form of carbon. The energy that can be derived from a fuel is sometimes compared with the enthalpy of the combustion reaction: C(s)+O2(g)CO2(g) Calculate the standard enthalpy change for this reaction at 25C. Actually, only a fraction of the heat from this reaction is available to produce electric energy. In electric generating plants, this reaction is used to generate heat for a steam engine, which turns the generator. Basically the steam engine is a type of heat engine in which steam enters the engine at high temperature (Th), work is done, and the steam then exits at a lower temperature (Tl). The maximum fraction, f, of heat available to produce useful energy depends on the difference between these temperatures (expressed in kelvins), f = (Th Tl)/Th. What is the maximum heat energy available for useful work from the combustion of 1.00 mol of C(s) to CO2(g)? (Assume the value of H calculated at 25C for the heat obtained in the generator.) It is possible to consider more efficient ways to obtain useful energy from a fuel. For example, methane can be burned in a fuel cell to generate electricity directly. The maximum useful energy obtained in these cases is the maximum work, which equals the free-energy change. Calculate the standard free-energy change for the combustion of 1.00 mol of C(s) to CO2(g). Compare this value with the maximum obtained with the heat engine described here.arrow_forward
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