CHEM 1215 Reading 6B

553 J of heat are released when a piece of metal with mass 84 g cools from 72°C to 21°C. Calculate the specific heat of the metal in J/g⋅°C.

0.13 J/g⋅°C

What would the final temperature be of a 38.0-g sample of aluminum that absorbs 375 J of energy, if its initial temperature is 22.1oC? The specific heat of aluminum is 0.897 J/g⋅oC.

33.1oC

Which of the following statements correctly describe the thermochemical equation shown? Select all that apply. 2NO (g) + O2 (g) → 2NO2 (g); ΔH = -114.2 kJ

57.1 kJ of energy will be released for every 1 mol of NO2 (g) produced in this reaction. If 2 mol of NO (g) reacts with 1 mol of O2 (g), 114.2 kJ of energy will be released. The reverse reaction will be endothermic.

Which of the following statements correctly describe the thermochemical equation shown? Select all that apply. CH4 (g) + H2O (g) → CO (g) + 3H2 (g); ΔH = +206.1 kJ

68.7 kJ of energy will be absorbed for every 1 mol of H2 (g) produced in this reaction. The reaction is endothermic.

Which of the following options correctly describe the use of a constant-pressure calorimeter? Select all that apply.

A constant-pressure calorimeter is treated as an isolated system. The heat change in a constant-pressure calorimeter is calculated using the equation q = msΔt. A constant-pressure calorimeter can be used to measure energy changes for both exothermic and endothermic processes.

A 3.56-g piece of copper metal is heated from 20.5°C to 325°C. Calculate the heat absorbed (in kJ) by the metal if the specific heat of copper is 0.385 J/g⋅°C.

+0.417 kJ

Assume that the following thermochemical reaction is known: 2A → 3B; ΔH = -50 kJ If 9.0 mol of B are formed during this reaction, what was the enthalpy change?

-150 kJ

What will be the value of ΔH for the reaction below if all of the coefficients are multiplied by 2? N2(g) + 3H2(g) → 2NH3(g) ΔH = -92.4 kJ

-185 kJ

If an exothermic reaction heats a calorimeter with a total mass of 10 grams from 20°C to 30°C what is the total enthalpy change for the reaction? Assume that the specific heat capacity of the system is 2 J/g⋅K.

-200 J

A solid initially at 75°C is placed in an insulated water bath initially at 30°C. The temperature reaches 45°C. The change in temperature of the solid was _____ °C, and the change in temperature of the water bath was _____ °C. (Be careful with + and - signs.)

-30.°C; +15°C

0.050 mol of Substance A and 0.050 mol of Substance B are allowed to react in a well-insulated polystyrene cup calorimeter according to the balanced equation A(aq) + B(aq) → C(aq). The reaction causes 100. g of solution to be heated from 25.0 oC to 31.2oC. Calculate the molar enthalpy of the reaction, assuming the specific heat of the solution to be 4.18 J/g⋅°C.

-52 kJ/mol

A solid initially at 97°C is placed in an insulated water bath initially at 22°C. The bath temperature reaches 31°C. What is the temperature change of the solid?

-66 oC

The internal energy change for a reaction is related to enthalpy by equation ΔU = ΔH - PΔV. This equation tells us that ΔH is very close to ΔU if there is no volume change in the reaction. In which of the following situations would ΔH give a good approximation of ΔU? Select all that apply.

A reaction involving only solids and liquids A reaction in which Δn, the change in the number of moles of gases present, is zero

Which of the following options correctly describe a thermochemical equation? Select all that apply.

A thermochemical equation relates the stoichiometric information in a balanced equation to the energy change of the reaction. A thermochemical equation includes the enthalpy change for the reaction. A balanced equation must be specified when quoting ΔH for the reaction.

ΔH will be equal to ΔU if the total number of moles of does not change during a chemical reaction (discounting any small volume change caused by liquids or solids).

Blank 1: gas or gases

The enthalpy of reaction (ΔH) is defined as the change in enthalpy for a chemical process. If ΔH is positive, then the enthalpy of the products is than the enthalpy of the reactants.

Blank 1: greater, more, higher, bigger, or larger

In a constant-pressure calorimeter, the enthalpy change for a reaction can be determined by measuring the exchanged between the reaction and the water (or solvent) it occurs in.

Blank 1: heat or thermal energy

If a chemical reaction occurs at constant volume, the change in internal energy (ΔU) becomes equal to the lost or gained by the system.

Blank 1: heat, heat energy, or q

The specific heat of a substance is given the symbol , and is defined as the amount of required to raise the temperature of gram of substance by 1oC.

Blank 1: s Blank 2: heat, heat energy, thermal energy, or q Blank 3: 1 or one

A chemical equation that includes the enthalpy change in addition to the balanced equation is called a(n) equation.

Blank 1: thermochemical

Which of the following options correctly describe enthalpy? Select all that apply.

For a system at constant pressure and volume, the change in enthalpy is equal to the change in internal energy. The change in enthalpy for a process is a state function. The change in enthalpy is equal to the heat absorbed or released at constant pressure.

The units for specific heat (s) are ______.

J/g⋅oC.

Which of the following statements correctly describe the two reactions outlined below? Select all that apply. Reaction 1: A → B ΔH = +50 kJ Reaction 2: C → D ΔH = -100 kJ

Reaction 2 is exothermic. The surroundings near Reaction 1 will become colder as it proceeds. Reaction 2 releases twice as much energy as Reaction 1 absorbs.

Which of the following options correctly describe the use of and information provided by a constant-volume bomb calorimeter? Select all that apply.

The bomb calorimeter is often used to determine heat of combustion. The heat of combustion is given by qrxn = -CcalΔt.

A 20-g sample of iron has a heat capacity of 9.0 J/oC. Which of the following statements about the heat change of the sample are correct? Select all that apply.

The heat change can be calculated using q = CΔT, where C = 9.0 J/oC. 9.0 J of heat are required to raise the temperature of the sample by 1.0oC.

A 1.922-g sample of methanol (CH3OH; molar mass 32.04 g/mol) is burned in a constant-volume bomb calorimeter. The temperature of the water in the calorimeter rises from 23.51°C to 27.70°C. The heat capacity of the calorimeter plus water is given by 10.4 kJ/°C. Which of the following options correctly reflect the steps required to calculate the molar heat of combustion of methanol? Select all that apply.

The molar heat of combustion for methanol is -7.27 x 102 kJ/mol. qcal = +43.6 kJ

1.50 mol of Al reacts according to the balanced equation 4Al(s) + 3O2(g) → 2Al2O3(s); ΔH = -3340 kJ. Which statement about the enthalpy change of the reaction is correct?

The reaction releases 1250 kJ of energy.

Which of the following statements relating to specific heat are correct? Select all that apply.

The specific heat for a substance has the same value regardless of the size of the sample. The symbol for specific heat is s. The units of specific heat are J/g⋅oC.

32.0 g of ZnS reacts according to the balanced equation 2ZnS(s) + 3O2(g) → 2ZnO(s) + 2SO2(g); ΔH = -879 kJ. Which of the following options correctly reflect the information given? Select all that apply.

This reaction releases 144 kJ of energy. 15.8 g of O2 are required to react completely with the ZnS present. 0.328 mol of ZnS reacts.

True or false: If ΔH = -285.8 kJ for the reaction 2H2(g) + O2(g) → 2H2O(l), the value of ΔH for the reaction 2H2O(l) → 2H2(g) + O2(g) is equal to +285.8 kJ.

True

A constant-volume calorimeter, often called a ______ calorimeter, is typically used to determine heats of combustion.

bomb

The measurement of heat changes is called .

calorimetry

A constant-pressure calorimeter, often called a ______ calorimeter, is typically used to determine heats of reaction in solution.

coffee cup

The change in enthalpy (ΔH) is equal to the change in heat (q) when the system is at constant _____.

pressure

The enthalpy change for a reaction (ΔH) is given by H(_____) - H(_____).

products; reactants

Enthalpy (H) is defined by the equation H = U + PV. At constant pressure, the enthalpy change of a reaction (ΔH) is equal to ______.

q

For a chemical reaction at constant volume, the change in internal energy (ΔU) becomes equal to _____.

q

The heat change in a reaction can be calculated using the equation q = msΔt. Which of the following options correctly describe the variables in this equation? Select all that apply.

s is the specific heat of the substance. This equation can be modified to q = CΔt. Δt = tfinal - tinitial

When a balanced chemical equation is written to include the enthalpy change for the reaction, it is called a(n) ______ equation.

thermochemical

For a reaction at constant ______, the change in internal energy (ΔU) is equal to the heat q gained or lost by the system.

volume

Silver nitrate decomposes on heating according to the balanced equation 2AgNO3 (s) →Δ→Δ 2AgNO2(s) + O2 (g); ΔH = +157.3 kJ. Which of the following options correctly reflect the steps needed to calculate the internal energy change at 1 atm and 25°C? Select all that apply. (R = 8.314 J/K⋅mol)

Δ(PV) = 2.48 kJ ΔU = 154.8 kJ

A 0.800-g piece of Zn metal is added to an insulated coffee-cup calorimeter containing 50.0 g of an aqueous AgNO3 solution at a temperature of 19.25°C. The final temperature of the reaction mixture is recorded as 22.77°C. Select the correct calculations needed to find ΔH for the reaction shown below in kJ/mol of Zn. The heat capacity of the solution is 1.96 J/g⋅°C. (Assume that the reagents are present in the correct stoichiometric ratio.) 2Ag+(aq) + Zn(s) → Ag(s) + Zn2+(aq)

Δt = (22.77 - 19.25)°C −0.350kJ0.0122mol-0.350kJ0.0122mol = -28.7 kJ/mol of Zn 0.800 g Zn x 1molZn65.39gZn1molZn65.39�Zn = 0.0122 mol Zn qsoln = (50.0 + 0.800) x 1.96 x 3.52 = 350 J