Chapter 10 Energy Changes in Chemical Reactions

A combustion reaction known to generate x kJ of heat is measured in a constant volume (bomb) calorimeter. If the calorimeter is observed to change from Ti °C to Tf °C, what is the heat capacity of the entire calorimeter (in kJ/K)?

x/Tf-Ti

Which of the following equations correctly defines enthalpy change in terms of internal energy change (U) and gas expansion work (at constant pressure)?

ΔH = ΔU + PΔV

Match each thermodynamic symbol with its correct definition.

ΔHrxn ---> The change in enthalpy for a given reaction ΔHorxn ---> The enthalpy change of a reaction measured at the standard state ΔHof ---> The enthalpy change when 1 mol of a compound forms from its elements in their standard states

The reaction below has an enthalpy change equal to Q. Match the modified reactions below with their corresponding enthalpies in terms of Q. 2 A → B; ΔH = Q

-Q ---> B → 2A 1/2Q ---> A → 12B -2Q ---> 2B → 4A 3Q ---> 6A → 3B

Which of the following reactions is associated with the standard enthalpy of formation of NH3 (g)?

1/2N2 (g) + 3/2H2 (g) → NH3 (g)

The standard enthalpy of formation of a compound is the enthalpy change associated with the reaction that generates _____ mole(s) of that compound from its component _____ when all of the substances are under _____ conditions.

1; elements; standard

Calculate the heat capacity of a given calorimeter if combustion of a 1.000-gram sample of biphenyl, (C6H5)2, releases 21.78 kJ of heat together with a 2.94 °C temperature rise.

7.41 × 10^3 J/°C

Which of the following would result in work being done by the gas on its surroundings?

A heated gas is allowed to escape into the air. Gas in a piston expands, pushing the piston outward.

Select all the statements that correctly describe a state function.

A state function is a property that is independent of the path taken to achieve it. A state function depends on the initial state of the system.

Select all the statements that correctly describe how thermochemical equations may be manipulated.

Any exothermic reaction would become an endothermic reaction if its direction were reversed. When an equation is multiplied by a constant, the associated enthalpy change is also multiplied by that constant.

Select all of the statements that correctly describe bond enthalpies.

Bond enthalpies are always positive. Bond enthalpies represent the energy required to break bonds.

Which of the following reactions would have an enthalpy change equal to ΔHf°? Select all that apply.

C(graphite) + O2 (g) → CO2 (g) Fe (s) + 3232Cl2 (g) → FeCl3 (s)

_____ is the measurement of heat changes.

Calorimetry

Which of the following is the best definition of calorimetry?

Calorimetry is the measurement of heat changes that accompany physical or chemical processes.

True or false: A chemical reaction is observed to take place in a flask. When the reaction is complete, the flask feels colder than it was before the reaction took place. The reaction was exothermic.

False

True or false: To understand the total change that occurs in a state function, we must have a record of the conditions ("states") at all times between the initial and final states.

False

For a reaction that proceeds through a series of steps, ΔHoverall = ΔH1 + ΔH2 + ΔH3 + .... This is a restatement of _____ law.

Hess's

Which of the following substances would have a standard enthalpy of formation equal to 0 kJ/mol? Select all that apply.

Hg (l) Ar (g) H2 (g)

Ionic compounds differ significantly from molecular compounds. Select the all of the features that are typical of ionic compounds.

Ionic compounds have high melting points. Ionic substances form electrically conductive solutions when dissolved. Ionic substances tend to be solids at room temperature.

Which of the following options correctly represents the units for specific heat?

J/g*°C

Select the statements that correctly describe lattice energy.

Lattice energy is a measure of the total strength of interaction between ions in the solid. Lattice energy is the energy required to separate 1 mol of an ionic solid into gaseous ions.

Generate the equation A + 2B → 2D from a combination of the reactions provided. Reaction 1: A + 2B → 2C Reaction 2: C → D How must reactions 1 and 2 be combined to sum to the overall reaction?

Reaction 1 + 2(Reaction 2)

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

The formation of an ionic bond involves a number of different processes. If we consider the separate electron transfer processes involved in the formation of sodium chloride, which individual steps would require an input of energy? The overall process is represented by the equation shown below. Select all that apply. 2Na (s) + Cl2 (g) → 2NaCl (s)

Removal of an electron from Na Dissociation of Cl2 to form 2Cl Conversion of Na (s) → Na (g)

Match the effects of the following combinations of Reactions 1 and 2 with the reaction resulting from these combinations. Reaction 1: A → 2C Reaction 2: C → B

Rxn 1 + (-Rxn 2) ---> A + B → 3C Rxn 1 + 2(Rxn 2) ---> A → 2B (-Rxn 1) + (-Rxn 2) ---> B + C → A

Which of the following values are needed to solve for the specific heat of a heated unknown solid when it is placed in a coffee cup calorimeter?

The final temperature of the system The initial temperature of the solid The mass of the solid The initial temperature of the water The mass of the water in the cup

Select all the statements about the study of energy changes that are correct.

Thermochemistry is a branch of thermodynamics. One aspect of thermodynamics is the study of energy transfer in the form of heat.

Match each branch of study with the correct description.

Thermodynamics ---> The study of the interconversion of heat and other kinds of energy Thermochemistry ---> Focuses primarily on the exchange of heat in chemical systems

Which of the following relationships are correct regarding forces of attraction in molecular and ionic species?

Weak forces between molecules result in gaseous, liquid, or low-melting covalent compounds. The attractive forces between ions tend to be stronger than the attractive forces between adjacent molecules.

The formation of an ionic compound involves the transfer of electrons. The removal of one or more electrons from a metal is a process that _____ energy, whereas the addition of one or more electrons to a nonmetal _____ energy.

absorbs; releases

By definition, the standard enthalpy of formation of a pure _____ in its standard state is equal to 0 kJ/mol.

element

he specific heat (s), is the quantity of _____ required to change the temperature of _____ gram(s) of an object by 1 degree _____.

energy; 1; Celsius

When ΔH for a thermochemical equation is negative, that reaction is _____, while a positive value for ΔH is associated with an _____ reaction.

exothermic; endothermic

The standard enthalpy of _____ is the change in enthalpy associated with the chemical reaction that generates 1 mole of a substance from its constituent _____ in their standard states.

formation; elements

The total change in the internal energy of a system is the sum of the energy transferred as _____ and/or _____

heat; work

The amount of energy required to separate 1 mole of ions in a crystalline solid into separate gaseous ions is called _____ energy. This energy is a measure of the total strength of the _____ bonds in the solid.

lattice; ionic

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

pressure

Car engines operate based on the mechanical work done by expanding gases. To calculate the total amount of work done, the _____ of the surroundings is multiplied by the change in the _____ of the gas during expansion. This is known as _____ work.

pressure; volume; PV

Imagine a heated solid being placed into an insulated cup of water. Match the variables associated with constant pressure calorimetry with their equivalent expressions.

qsolid ---> ssolid × masssolid × ΔTsolid -qsolid ---> qwater ssolid ---> -qwatermasssolid×ΔTsolid

Hess's law states that the enthalpy change for an overall process (which can be broken into multiple steps) will be equal to the _____ of the enthalpy changes of its individual steps.

sum

Select the answer that best completes the following sentence:The bond enthalpy is ______.

the enthalpy change associated with breaking a particular bond in one mole of gaseous molecules

Select the answer that best completes the following sentence. The standard enthalpy of formation, ΔHfo, is ______.

the enthalpy change when one mole of a substance is formed from its constituent elements in their standard states