Chemistry II : Chapter 6: Energy Relationships in Chemical Reactions
Generate the equation A +n 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 options describes work in a chemical context?
The energy from a chemical process is directed in some way.
Thermodynamics is _____________.
The study of the interconversion between heat and other types of energy.
The study of heat changes in a chemical reaction is called_______.
Thermochemistry
A 0.800-g piece of Zn metal is added to an insulated coffee-cup calorimeter containing 50,0 g of an aqueous AgNO₂ 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 ∆H for the reaction shown below in kJ/mol 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)
a) 0.800 g Zn x 1mol Zn/ 65.39 g Zn = 0.0122 mol Zn b)∆t =(22.77 - 19.25)°c c)-0.350 kJ/0.0122 mol = -28.7 kJ/mol of Zn d)qsoln =(50.0+ 0.800) x 1.96 x 3.52 = 350 J
If there is a change in heat, Δq, which of the following expressions could be used to calculate this? Select all that apply.
a) q final, -q initial b) q products, q reactants
Use the equation for specific heat to determine the function needed to solve for the variables associated with temperature change. heat mass of substance change of temperature specific heat
a) s x mass x ∆t b) q -------- s x ∆t c) q --------- mass x s d) q ---------- mass x ∆t
CO (g) can be produced by the reaction of coal with steam, as shown by the balanced equation: H₂O (g) + C (s) → CO (g) + H₂ (g). Which of the following options correctly reflect the steps required to calculate ∆H for this reaction, given the information shown below? Select all that apply. Reaction 1: H₂ (g) + 1/2 O₂ (g) →H₂O (g); ∆H = -242.0 kJ Reaction 2: 2CO (g) → 2C (s) + O₂ (g) ; ∆H = + 221.0 kJ
a) ∆H = +131.5 kJ b) Reaction 2 must be reversed and divided by 2
The measurement of heat changes is called _______.
calorimetry
The first law of thermodynamics states that the total amount of ______________ in the universe remains constant.
energy
∆H will be equal to ∆U if the total number of moles of ______ does not change during a chemical reaction (discounting any small volume change caused by liquids or solids).
gases
If a chemical reaction occurs at constant volume, the change in internal energy (∆U) becomes equal to the _________ lost or gained by the system.
heat
The total change in the internal energy of a system is the sum of the energy transferred as _______ and/or _______.
heat work
If heat is transferred form a system to its surroundings do work on the system, the sign of q will be ________, and the sign of w will be_______.
negative; positive
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 substance are under_________ conditions.
one, elements, standard
The change in enthalpy (∆H) is equal to the change in heat (q) when the system is at constant _________.
pressure
The enthalpy change for a reaction (ΔΗ) is given by H (____)- H(_____).
products, reactants
The symbol________ always indicates heat (or thermal energy), which is transferred as a result of a difference in temperature between a system and its surroundings.
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 1°C.
s, heat, one
Define specific heat and heat capacity correctly and match each quantity correctly with its symbol
specific heat is: The amount of heat required to raise the temperature of 1 g of substance by 1° C. heat capacity is: The amount of heat required to raise the temperature of a given quantity of the substance by 1°C The symbol for specific heat is: s The symbol for heat capacity is: C
An element in its most stable state at a pressure of 1 atm is in its ___________ state, usually signified by the superscript "°".
standard
Heat (q) and work (w) are path-dependent quantities. They are therefore not________ functions
state
The energy change of the __________ is always equal in magnitude but opposite in sign to the energy change of the __________.
system, surroundings
A chemical equation that includes the enthalpy change in addition to the balanced equation is called a (n) __________ equation
thermochemical
A bomb calorimeter is used to measure the heat change for a process at constant________.
volume
A bomb calorimeter is used to measure the heat change for a process at constant___________.
volume
Work may be done in a chemical system by a change in _______ as a gaseous component of the system expands or contracts at constant_________.
volume pressure
Work may be done in a chemical system by a change in _____________ as a gaseous component of the system expands or contracts at constant___________.
volume, pressure
Energy is defined as the capacity to do ______.
work
Energy transferred when an object is moved by a force results in ___________ being done on that object.
work
Consider the following reactions: A------> 2B; ΔΗ=x kJ B------> C; ΔΗ= y kJ What would be the enthalpy change associated with the reaction below? A ----> 2 C
x+2y
Match each component of the equation shown with its correct description. ΔΗ°rxn = ΣnΔΗ°f( products)- ΣmΔΗ°f (reactants)
°- standard state ∆H°f - standard enthalpy of formation n and m- coefficients for reactants and products from balanced equation Σ- sum of
An unknown solid is in a freezer at constant temperature. It is then placed in a room-temperature water bath. Identify the sign (+/-) of the temperature change for the solid and for the water bath.
∆t solid - positive ∆t water bath - negative
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
Silver nitrate decomposes on heating according to the balanced equation ∆ 2AgNO₃n (s) → 2AgNO₂ (s) + O₂ (g); ∆H = + 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)
- ∆U=154.8 kJ -∆(PV)= 2.48 kJ
32.0 g of ZnS reacts according to the balanced equation 2ZnS (s) + 30₂ (g) →2ZnO (s) + 2SO₂ (g); ∆H = -879 kJ. Which of the following options correctly reflect the information given? Select all that apply.
-15.8 g of O₂ are required to react completely with the ZnS present. -This reaction releases 144 kJ of energy -0.328 mol of ZnS reacts.
Assume that the following thermochemical reaction is known: 2A -----> 3B; ∆H= -50 kJ If 9.0 mol of B are formed during this reaction, what the enthalpy change?
-150 kJ
Calculate the standard enthalpy change for the reaction SiO₂ (s) + 4HF (g) ----->SiF₄ (g) + 2H₂O (l) given the information in the table below. Compound ∆H°f (kJ/mol) HF (g) -273.0 H₂O(l) -285.8 SiF₄ (g) -1614.9 SiO2 (s) -910.9
-183.6 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 compressed gas expands form an initial volume of 0.200 L to a new volume of 0.700 L against a pressure of 1.00 atm. Which of the following options correctly reflect the amount of work done by the system? Select all that apply. (L∙ atm = 101.3 J)
-50.7 J -0.500 L ∙ atm
Which of the following options correctly describe a thermochemical equation? Select all that apply.
-A balanced equation must be specified when quoting ∆H for the reaction. - A thermochemical equation includes the enthalpy change for the reaction . -A thermochemical equation relates the stoichiometric information in a balanced equation to the energy change of the reaction.
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. -A constant-pressure calorimeter can be used to measure energy changes for both exothermic and endothermic processes. -The heat change in a constant-pressure calorimeter is calculated using the equation q=ms∆t.
Which of the following changes in energy would be assigned a negative value in a thermochemical equation? Select all that apply.
-A hot rock cools at sunset. -A compressed spring is allowed to uncoil.
The internal energy change for a reaction is related to enthalpy by equation ∆U = ∆H - P∆V. This equation tells us that ∆H is very close to ∆U if there is no volume change in the reaction. In which of the following situations would ∆H give a good approximation of ∆U? Select all that apply.
-A reaction in which ∆n, the change in the number of moles of gases present, is zero. -A reaction involving only solids and liquids.
Which of the following options correctly define a state function? Select all that apply.
-A state function is one that depends only on the initial and final states of the system. -A state function is a quantity that does not depend on the path taken to achieve it.
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 ∆H for the reaction.
Which of the following statements correctly describe exothermic and endothermic reactions? Select all that apply.
-An exothermic reaction is one that releases heat to its surroundings. -If the container of a reaction becomes colder during the reaction, the reaction is endothermic. - The reactants in and endothermic reaction are lower in energy than the products.
Why is ∆U ( change in internal energy) a state function although its components, q and w, are not?
-Changing the path taken for a process changes q and w by the same amount, so that ∆U still has the same value.
Match each type of energy to the correct definition. Radiant energy Thermal energy Chemical energy Potential energy
-Energy from the sun -Energy associated with the random motion of particles -Energy stored within the structural units of substances -Energy an object possesses due to its position
Which of the following statements correctly describe the two reactions outlined below? Select all that apply. Reaction 1: A ------> B ∆H = +50 kJ Reaction 2: C ------> D ∆H = -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
Which of the following options correctly describe enthalpy? Select all that apply.
-The change in enthalpy is equal to the heat absorbed or released at constant pressure. -The change in enthalpy for a process is a state function, -For a system at constant pressure and volume, the change in enthalpy is equal to ther change in internal energy.
Which of the following options correctly describe how the manipulation of a chemical equation will affect the quoted value of ∆Hrxn? Select all that apply.
-The magnitude of ∆H for the reverse reaction is the same as for the forward reaction. -If the coefficients of the balanced equation are multiplied by 2, the quoted value of ∆H will double
Hot coffee is poured into a mug. It slowly cools, releasing heat to the environment. If we consider the coffee to be the system, which of the following are part of the surroundings? Select all that apply.
-The mug -The air in the room -The table underneath the mug
Match each type of system with the correct definition. open system closed system isolated system
-exchanges both energy and mass its surroundings. -exchanges energy, but not mass, with its surroundings. -does not allow the exchange of energy of mass with its surroundings.
Internal energy consists of what two components?
-kinetic energy -potential energy
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.
-qcal = +43.6 kJ - The molar heat combustion for methanol is -7.27 x 10² kJ/mol
Which of the following are state functions? Select all that apply.
-volume -temperature -pressure
553 J of heat are released when a piece of metal with mass 84g cools from 72° C to 21°C. Calculate the specific heat of the metal in J/g∙ °C.
0.13 J/g ∙ ° C
Which of the following reaction is associated with the standard enthalpy of formation of NH₃ (g)?
1/2 N₂ (g) + 3/2 H₂ (g) → NH₃ (g)
Consider the following reactions: 2A → B; ∆H = x 2B → C; ∆H = y What is the total enthalpy change for the reaction 4A → C?
2x + y
Two equations must be manipulated in order to generate the overall equation A + 2B ------> D. Select the two chemical equations from below that can be arranged and added together to generate the overall equation. (Note: the equation(s) may need to be reversed or multiplied by a constant.)
A + B → C D → C + B
What is the net equation when the following reactions are summed? 2C( graphite) + 3H₂ (g) → C₂H₆ (g) C₂H₄ (g) → 2C (graphite) + 2H₂ (g)
C₂H₄ (g) + H₂ (g) → C₂H₆ (g)
True or false: If an ice cube is placed on a heat stove, heat transfer from the ice cube to the stove.
False
Which of the following reactions would have an enthalpy change equal to ∆H°f? Select all that apply.
Fe (s) + 3/2 Cl₂ (g) -----> FeCl₃ (s) C (graphite) + O₂ (g) ---->CO₂ (g)
Which of the following options correctly defines heat?
Heat is the transfer of thermal energy
For a reaction that proceeds through a series of steps, ∆H overall = ∆H₁ + ∆H₂ + ∆H₃ + .... 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.
H₂ (g) Ar (g) Hg (l)
