Enthalpy And Heat Problems 2u692u

1) (a) Under what conditions will the enthalpy change of a process equal the amount of heat transferred into or out of the system? (b) During a constantpressure process the system absorbs heat from the surroundings. Does the enthalpy of the system increase or decrease during the process? 2) The decomposition of zinc carbonate, ZnCO3 (s), into zinc oxide, ZnO (s), and CO2 (g) at constant pressure requires the addition of 71.5 kJ of heat per mole of ZnCO3. (a) Write a balanced thermochemical equation for the reaction. (b) Draw an enthalpy level diagram for the reaction. 3) Without referring to tables, predict which of the following has the higher enthalpy in each case: (a) 1 mol CO2 (s) or 1 mol CO2 (g) at the same temperature, (b) 2 mol of hydrogen atoms or 1 mol of H2, (c) 1 mol H2 (g) and 0.5 mol O2 (g) at 25ºC or 1 mol H2O (g) at 25ºC, (d) 1 mol N2 (g) at 100ºC or 1 mol N2 (g) at 300ºC. 4) Consider the following reaction: CH3OH (g)  CO (g) + 2H2 (g)

ΔH = +90.7 kJ

(a) Is heat absorbed or evolved in the course of this reaction? (b) Calculate the amount of heat transferred when 45.0g of CH3OH (g) is decomposed by this reaction at constant pressure. (c) For a given sample of CH3OH, the enthalpy change on reaction is 18.5 kJ. How many grams of hydrogen gas are produced? (d) What is the value of ΔH for the reverse of the previous reaction? How many kilojoules of heat are released when 27.0g of CO (g) reacts completely with H2 (g) to form CH3OH (g) at constant pressure? 5) One way of forming small quantities of oxygen gas in the laboratory is to heat KClO3: 2 KClO3 (s)  2 KCl (s) + 3 O2 (g)

ΔH = -89.4 kJ

For this reaction, calculate ΔH for the formation of: (a) 0.855 mol of O2 (b) 10.75g of KCl. (c) The decomposition of KClO3 proceeds spontaneously when it is heated. Do you think that the reverse reaction, the formation of KClO3 from KCl and O2, is likely to be feasible under ordinary conditions? Explain your answer.

6) (a) Which substance in the table below requires the smallest amount of energy to increase the temperature of 50.0g of that substance by 10K? (b) Calculate the energy needed for this temperature change. Specific heat of Some Substances at 298K Elements Compounds Substance Specific Heat Substance Specific Heat (J/g•K) (J/g•K) N2 (g)

1.04

H2O (l)

4.18

Al (s)

0.90

CH4 (g)

2.20

Fe (s)

0.45

CO2 (g)

0.84

Hg (l)

0.14

CaCO3 (s)

0.82

7) The specific heat of ethylene glycol is 0.450 J/g•K. How many J of heat are necessary to raise the temperature of 62.0g of ethylene glycol from 15.2ºC to 40.8ºC? 8) (a) When a 3.88-g sample of solid ammonium nitrate dissolves in 60.0g of water in a coffee-cup calorimeter, the temperature drops from 23.0ºC to 18.4ºC. Calculate ΔH (in kJ/mol NH4NO3) for the solution process NH4NO3 (s)  NH4+ (aq) + NO3- (aq) Assume that the specific heat of the solution is the same as that of pure water. (b) Is this process endothermic or exothermic? 9) In an experiment, 0.60 g of propane (C3H8) was completely burned in air. The heat evolved raised the temperature of 100 g of water by 64.9°C. Use this data to calculate the enthalpy of combustion (∆Hcomb) of propane (the specific heat capacity of water is 4.18 J g-1 K-1).