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Exam 9: Thermochemistry-Energy Changes in Chemical Reactions

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Question 1

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Suppose 23.00 g ammonium chloride (53.491 g/mol)dissolves in water in a calorimeter to form 100.00 g of solution.The temperature decreases by 10.75°C.Calculate the $\Delta$ H of solution of NH₄Cl in kJ/mol.Assume the specific heats of the solution and of the empty calorimeter are 4.18 J/(g . °C)and 0.170 kJ/°C,respectively. NH₄Cl(s) $\to$ NH₄⁺(aq)+ Cl^-(g) $\quad$$\Delta$ H = ? kJ/mol

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H_rxn<...

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Question 2

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Which of the following substances,a-d,will release the most energy during combustion in air?

A) 1 mol of C₆H₁₄
B) 1 mol of C₂H₆
C) 1 g of C₆H₁₄
D) 1 g of C₂H₆
E) Cannot answer without more information.

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A hot cup of coffee (the system)produces 70 kJ as it cools.What is the change in energy of the system? Of the surroundings? Of the universe? Include signs in your answers.

In an experiment,10.0 g of ice at -20.0°C is converted into steam with a temperature of 110.0°C.How much energy is required for this process? [ $\Delta$ H_vap = 2260 J/g; $\Delta$ H_fus = 334 J/g; c_p (ice) = 2.06 J/(g . °C) ; c_p(water) = 4.18 J/(g . °C) ; C_p(steam) = 1.99 J/(g . °C) ]

Which of the following regarding state and path functions is false?

A gas contained in a cylinder with a movable piston does 554 J of expansion work on the surroundings while gaining 977 J of thermal energy.What is the change in internal energy of the gas?

A gas contained in a cylinder with a movable piston undergoes the following two changes: (a)It does 2260 J of work on the surroundings while absorbing 4230 J of heat.(b)1950 J of work is done on the gas while it releases 3920 J of heat.What is its total change of internal energy?

A 550 g solution at 27.27°C is produced by mixing 0.800 mol hydrocyanic acid with 0.800 mol potassium hydroxide (40.00 g/mol) in water initially at 23.20°C.Calculate the molar enthalpy of neutralization of HCN with KOH.Assume that the heat capacity of the solution is 4.18 J/(g . ^oC) .

When 10.5 g KBr(s) dissolves in 125 g of water at 24.2°C,the final temperature of the solution comes to 21.1°C.Assume that the heat capacity of the solution is roughly 4.18 J/(g . °C) .What is the molar enthalpy of solution of potassium bromide (119 g/mol) ?

In an experiment,138 g of graphite is heated to 90.0°C and then quickly transferred to 150.0 g of water in a calorimeter at 25.0°C.The heat capacity of the calorimeter with the water is 157 J/°C.The final temperature comes to 50.0°C.What is the approximate specific heat capacity of graphite? [c_p(water) = 4.18 J/(g . °C) ]

Motor fuels are mixtures of hydrocarbons,but they can be characterized by their average properties.Gasoline (114 g/mol average,d = 0.69 g/mL)and diesel fuel (226 g/mol average,d = 0.85 g/mL)have average enthalpies of combustion of 5650 kJ/mol and 10,200 kJ/mol,respectively.Calculate the fuel density (kJ/mL)of these compounds,and determine how many times more valuable diesel fuel is in terms of its energy content as measured by fuel density.

In terms of the enthalpy of formation,which of the following compounds is the most unstable compared to its elements under standard conditions?

The following diagrams illustrate the flow of energy (q) and work (w) in different processes.Which one is definitely an exothermic process?

Suppose 3.03 kg of asphalt at 58.0°C are quickly transferred to a large water bath containing 10.0 kg of water at 18.0°C.The final equilibrium temperature reached is 20.5°C.What is the heat capacity of asphalt? Assume heat is exchanged only between the water and the asphalt.[c_p,water = 4.18 J/(g . °C)]

In addition to identifying the reactants and products,a thermochemical reaction equation provides __________

In a steam engine,steam in a cylinder expands against a piston exerting 10.00 atm of external pressure.The volume of the cylinder increases by 10.00 L and simultaneously the steam cools,losing 3.000 $\times$ 10³ kJ of energy to the surroundings.What is the change in energy of the steam? (101.3 J = 1 L . atm)

The basal metabolic rate (BMR)is the minimum energy intake required to maintain a body that is awake and at rest.While the rate varies with age,gender,health,and body mass,a reasonable value is 1600 Cal per day.Walking can require 2.5 times the BMR.Using this information,calculate the energy expended by someone walking at a speed of 3.0 mph and determine how far she could walk with the energy from a quarter-pound hamburger (125 g)that has a food value of 3.6 Cal/g.

Given the standard enthalpies of formation for the following substances,determine the change in enthalpy for the combustion of 1.0 mol of butane. Substance

Calculate the enthalpy of reaction for the conversion of iron(II)oxide in carbon monoxide to iron metal and carbon dioxide using the following reaction information.

In an experiment,110.0 g of iron is heated to 78.0°C and then quickly transferred to 100.0 g of water in a calorimeter at 18.0°C.The final temperature comes to 24.0°C.What is the approximate heat capacity of the calorimeter? [c_p(iron) = 0.450 J/(g . °C) ; c_p(water) = 4.18 J/(g . °C) ]