5.3 Enthalpy
The word "enthalpy" translates as:
"to heat within"
How many atm are there in a bar of pressure?
0.987 atm
For chemists, the IUPAC standard state refers to materials under a pressure of ___ bar and solutions at ___ M, and does not specify a temperature.
1, 1
Given the following thermochemical equation, H₂(g) + ¹/₂O₂(g) → H₂O(l) ∆H = -286 kJ What would be the enthalpy change if the molar amounts were doubled?
2 x (H₂(g) + ¹/₂O₂(g) → H₂O(l) ∆H = -286 kJ) 2H₂(g) + O₂(g) → 2H₂O(l) ∆H = -572 kJ
What is the usual temperature (not standard) in standard state conditions?
298.15 K
Write the heat of formation reaction equation for C₂H₅OH(l)
2C(s, graphite) + 3H₂(g) +¹/₂O₂(g)→C₂H₅OH(l)
Write the heat of formation reaction equation for Na₂CO₃(s)
2Na(s) + C(s, graphite) + ³/₂O₂(g) → Na₂CO₃(s)
Write the heat of formation reaction equation for Ca₃(PO₄)₂(s)
3Ca(s) + ¹/₂P₄(s) + 4O₂(g) → Ca₃(PO₄)₂(s)
Write the heat of formation reaction equation for C₂H₅OC₂H₅(l)
4C(s, graphite) + 5H₂ + ¹/₂O₂(g) → C₂H₅OC₂H₅
Energy is transferred into room-temperature metal wire if it is immersed in hot water. The wire ______ from the water.
Absorbs heat
What is the combustion reaction for the burning of coal or graphite?
C(s) + O₂(g) → CO₂(g) ∆H°c = -393.5 kJ
Due to the formula ∆H = q, we can measure q with a:
Calorimeter
Since enthalpy values for specific substances cannot be measured directly; only enthalpy _____ for chemical or physical processes can be determined.
Changes
Thermochemistry is a branch of _______, the science that deals with the relationships between heat, work, and other forms of energy in the context of chemical and physical processes.
Chemical thermodynamics
Enthalpy changes are typically tabulated for reactions in which both the reactants and products are at the same:
Conditions
Enthalpy change is the heat released or absorbed by a system under _______ during a chemical or physical process.
Constant pressure
The formula for enthalpy change is ∆H = q. A subscript p is added to the q to represent:
Constant pressure
Standard enthalpy of formation values are especially useful for computing or predicting enthalpy changes for chemical reactions that are impractical or __________ to carry out.
Dangerous
Negative w represents work ____ the system
Done by
Positive w represents work ______ the system.
Done on
When you push down on a syringe, work is _______ the system.
Done on
A thermochemical reaction that is exothermic in one direction, is _______ in the opposite direction.
Endothermic
In the formula H = U + PV, what is H?
Enthalpy
The sum of a system's internal energy (U) and the mathematical product of its pressure (P) and volume (V) is a property chemist's use called:
Enthalpy
The heat given off when you operate a Bunsen burner is equal to the _______ of the methane combustion reaction that takes place, since it occurs at the essentially constant pressure of the atmosphere.
Enthalpy change
The heat released or absorbed by a system under constant pressure during a chemical or physical process is:
Enthalpy change
∆H = ∆U + P∆V is the formula for:
Enthalpy change
The _______ of ethanol, -1366.8 kJ/mole, is the amount of heat produced when one mole of ethanol undergoes complete combustion at 25 °C and 1 atmosphere pressure, yielding products also at 25 °C and 1 atm.
Enthalpy of combustion
What does the following equation represent? C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l) ∆H°₂₉₈=-1366.8 kJ
Enthalpy of combustion of ethanol
If a chemical or physical process is carried out at constant pressure with the only work done caused by expansion or contraction, then the heat flow (q) and enthalpy change (∆H) are:
Equal
H₂(g) + ¹/₂O₂(g) → H₂O(l) ∆H = -286 kJ A negative value of an enthalpy change, ∆H, indicates an _______ reaction.
Exothermic reaction.
Expansion work is work done as a system ______ against external pressure.
Expands or contracts
A type of work called _____ occurs when a system pushes back the surroundings against a restraining pressure, or when the surroundings compress the system.
Expansion work
An example of what kind of work occurs during the operation of an internal combustion engine? The reaction of gasoline and oxygen is exothermic. Some of this energy is given off as heat, and some does work pushing the piston in the cylinder. The substances involved in the reaction are the system, and the engine and the rest of the universe are the surroundings. The system loses energy by both heating and doing work on the surroundings, and its internal energy decreases.
Expansion work
Determine the enthalpy of formation, ∆H°f, FeCl₃(s) from the enthalpy changes of the following two-step process that occurs under standard state conditions: Fe(s) + Cl₂(g) → FeCl₂(s) ∆H° = -341.8 kJ FeCl₂(s) + ¹/₂Cl₂(g) → FeCl₃(s) ∆H° = -57.7 kJ
Fe(s) + Cl₂(g) → FeCl₂(s) ∆H° = -341.8 kJ FeCl₂(s) + ¹/₂Cl₂(g) → FeCl₃(s) ∆H° = -57.7 kJ ______________________________________________ Fe(s) + ¹/₂Cl₂(g) → FeCl₃(s) ∆H° = -399.5 kJ ∆H°f of FeCl₃ is -399.5 kJ
The equation ∆U = q + w is one version of the _______ of thermodynamics.
First law
Give two reasons why the following is a thermochemical equation: H₂(g) + ¹/₂O₂(g) → H₂O(l) ∆H = -286 kJ
First reason: The ∆H value following the equation shows us the amount of heat associated with the reaction Second reason: The coefficients represents moles and coefficient has to be one for a reactant or product.
Hydrogen gas, H₂, reacts explosively with gaseous chlorine, Cl₂, to form hydrogen chloride, HCl(g). What is the enthalpy change for the reaction of 1 mole of H₂(g) with 1 mole of Cl₂(g) if both the reactants and products are at standard conditions? The standard enthalpy of formation of HCl(g) is -92.3 kJ/mol.
For the reaction H₂(g) + Cl₂(g) → 2HCl(g) ∆H°₂₉₈ = -184.6 kJ
When 0.0500 mol of HCl(aq) reacts with 0.0500 mol of NaOH(aq) to form 0.0500 mol of NaCl(aq), 2.9 kJ of heat are produced. What is ∆H, the enthalpy change, per mole of acid reacting, for the acid-base reaction? HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) ∆H = -2.9 kJ
For the reaction of 0.0500 mol acid (HCl), q = -29 kJ ∆H = 1 mol HCl x -2.9 kJ/0.0500 mol HCl ∆H = -58 kJ
When 1.34 g Zn(s) reacts with 60.0 mL of 0.750 M HCl(aq), 3.14 kJ of heat are produced. Determine the enthalpy change per mole of zinc produced for the reaction: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
For the reaction of 1.34 g zinc, q = -3.14 kJ ∆H = 1 mol zinc x -3.14 kJ/(1.34 g/65.38 g mol⁻¹) ∆H = -153 kJ
In physics, work = _____ x _______.
Force, distance
Standard enthalpy of formation is an enthalpy change of a chemical reaction in which 1 mole of a pure substance is ______ from its elements in their most stable states under standard state conditions.
Formed
Enthalpies of combustion for many substances have been measured. Many readily available substances with large enthalpies of combustion are used as ______, including hydrogen, carbon (as coal or charcoal), and hydrocarbons, such as methane, propane, and the major components of gasoline.
Fuels
Write the formula for enthalpy.
H = U + PV
The standard enthalpy of combustion is sometimes called:
Heat of combustion
Thermochemistry is a branch of chemical thermodynamics, the science that deals with the relationships between _______ in the context of chemical and physical processes.
Heat, work, and other forms energy
What law allows one to calculate the heat involved in a reaction without having to perform an experiment?
Hess's law
What law states: if a process can be written as the sum of several stepwise processes, the enthalpy change of the total process equals the sum of the enthalpy changes of the various steps.
Hess's law
Compounds containing only hydrogen and carbon are known as:
Hydrocarbons
What kinds of compounds are the major components of fossil fuels?
Hydrocarbons
What is the combustion reaction of hydrogen?
H₂(g) + ¹/₂O₂(g) → H₂O(l) ∆H°c = -285.8 kJ
As a system undergoes a change, its ________ can change, and energy can be transferred from the system to the surroundings, or from the surroundings to the system.
Internal energy
Both processes, immersing a room-temperature metal wire in hot water or rapidly bending the wire back and forth, increase the _______ of the wire, which is reflected in an increase in the wire's temperature.
Internal energy
Energy stored in chemical bonds plus kinetic energy is called:
Internal energy
Enthalpy (H) is the sum of the system's _______ and the mathematical product of its pressure (P) and volume (V).
Internal energy
The formula H = U + PV represents the sum or a system's _______ and the mathematical product of its pressure and volume.
Internal energy
The total of all possible kinds of energy present in a substance is called the ________, sometimes symbolized as E.
Internal energy (U)
The equation ∆U = q + w represents the relationship between:
Internal energy, heat, and work
Positive q represents heat flow ____ the system.
Into
Enthalpy is a state function because:
It is derived from three state functions (U, P, and V)
Energy is stored in a substance when the _______ of its atoms or molecules is raised.
Kinetic energy
Internal energy is stored energy (e.g. chemical bonds) plus:
Kinetic energy
When thermal energy is lost, the intensities of the motions of atoms and molecules decrease and the _____ falls.
Kinetic energy
Standard enthalpy of formation values are especially useful for computing or predicting enthalpy changes for chemical reactions that are difficult to make:
Measurements
Since enthalpy change is an extensive property, we try to report a per-amount intensive value of ∆H, often "normalized" to a per- ______ state.
Mole
The heat produced by a reaction measured in a bomb calorimeter is ____ to ∆H because the closed, constant-volume metal container prevents expansion work from occurring.
Not equal
The enthalpy change for the reaction forming one mole of NO₂(g) is +33.2 kJ: ¹/₂N₂(g) + O₂(g) → NO₂(g) ∆H = +33.2 kJ What is the enthalpy change when 2 moles of NO₂(g) are formed?
N₂(g) + 2O₂(g) → 2NO₂(g) ∆H = +66.4 kJ
Calculate ∆H for the process: N₂(g) + 2O₂(g) → 2NO₂(g) From the following information: N₂(g) + O₂(g) → 2NO(g) ∆H = 180.5 kJ NO(g) + ¹/₂O₂(g) → NO₂(g) ∆H = -57.06 kJ
N₂(g) + O₂(g) → 2NO(g) ∆H = +180.5 kJ NO(g) + ¹/₂O₂(g) → NO₂(g) ∆H = -57.06 kJ ____________________________________________ N₂(g) + 2O₂(g) → 2NO₂(g) ∆H = +66.38 kJ Since we ended up with 2 moles of NO₂ then we multiply the ∆H = -57.06 x 2 = -114.12 kJ
Negative q represents heat flow ____ the system.
Out of
A property that is independent of ____ is a state function.
Path
Standard state is a set of _______ as accepted as common reference conditions for reporting thermodynamic properties; 1 bar of pressure, and solutions at 1 molar concentrations, usually at a temperature of 298.15 K.
Physical conditions
In a thermochemical equation, the enthalpy change of a reaction depends on the ______ of the reactants and products of the reaction (whether we have gases, liquids, solids, or aqueous solutions), so these must be shown.
Physical state
Examine the two equations below: H₂(g) + ¹/₂O₂(g) → H₂O(l) ∆H = -286 kJ H₂(g) + ¹/₂O₂(g) → H₂O(g) ∆H = -252 kJ Why are their enthalpy changes different?
Physical state of the reactants or products makes a difference. If gaseous water forms, less heat is released
In chemistry, expansion work = ____ x _____
Pressure, change in volume
Expansion work is also known as:
Pressure-volume work
Write the formula showing how P∆V represents expansion work.
P∆V = -w
The value of a state function depends only on the state that a system is in, and not how the state is:
Reached
Substances act as ______ of energy, meaning that energy can be added to them or removed from them.
Reservoirs
If the direction of a thermochemical equation is ______, the arithmetic sign of its ∆H is changed (a process that is endothermic in one direction and exothermic in the opposite direction).
Reversed
The greater kinetic energy of atoms or molecules may be in the form of increased translations (travel or straight-line motions), vibrations, or _______.
Rotations
The enthalpy change when 1 mole of a substance burns (combines vigorously with oxygen) under standard state conditions is known as:
Standard enthalpy of combustion
The symbol ∆H°c represents the:
Standard enthalpy of combustion
A ______ is an enthalpy change for a reaction in which exactly one mole of a pure substance is formed from free elements in their most stable states under standard state conditions.
Standard enthalpy of formation
The symbol ∆H°f represents the:
Standard enthalpy of formation
A commonly accepted set of conditions used as a reference point for the determination of properties under other different conditions is known as a:
Standard state
What does the superscripted "o" represent in the enthalpy change symbol?
Standard state
A property depending only on the state of a system, not the path taken to reach that state is called a:
State function
An example of a _______ is altitude or elevation. If you stand on the summit of Mt. Kilimanjaro, you are at an altitude of 5895 m, and it doesn't matter whether you hiked there or parachuted there. The distance you traveled to the top of Kilimanjaro is not a state function (distance walked depends on the path).
State function
Hess's law is valid because enthalpy is a _____: enthalpy changes depend only on where a chemical process starts and ends, but not on the path it takes from start to finish.
State function
The relationship between internal energy, heat, and work can be represented as ∆U = q + w. Internal energy is a type of quantity known as a _______, whereas heat and work are not.
State function
A gummy bear contains 2.67 g sucrose, C₁₂H₂₂O₁₁. When it reacts with 7.19 g potassium chlorate, KClO₃, 43.7 kJ of heat are produced. Determine the enthalpy change for the reaction. C₁₂H₂₂O₁₁(aq) + 8KClO₃(aq) → 12CO₂(g) + 11H₂O(l) + 8KCl(aq) Outline the steps needed to solve this problem.
Step 1: the question gives you the masses of the two reactants so it must be a limiting reactant problem. Step 2: calculate the moles of each reactant Step 3: multiply the moles of KClO₃ by the molar ratio of the two reactants, if the product is less than the calculated moles of sucrose, then sucrose is excess and KClO₃ is limiting Step 4: determine enthalpy change by multiplying 8 mol KClO₃ x q/mol KClO₃
The formula P∆V = -w shows the work on the:
Surroundings
What does the "298" represent in the symbol ∆H°₂₉₈?
The "298" represents the usual (but not technically standard) temperature, 298.15 K
Why are we going to have to convert the world to a hydrogen economy?
The combustion of hydrogen and oxygen yields water, not the powerful greenhouse gas, carbon dioxide.
H₂(g) + ¹/₂O₂(g) → H₂O(l) ∆H = -286 kJ If the reaction is reversed, what happens to the ∆H? H₂O(l) → H₂(g) + ¹/₂O₂(g) ∆H = ?
The ∆H sign is reversed and so: H₂O(l) → H₂(g) + ¹/₂O₂(g) ∆H = 286 kJ
In the formula for enthalpy, H = U + PV, the variable U is impossible to measure. Why?
To determine the internal energy of a system you would have to measure the kinetic energy of every entity
Internal energy (U) is the ______ of all possible kinds of energy present in a substance.
Total
Bomb calorimeters often measure changes in pressure so the formula ∆H = q cannot be used. True or False?
True
Enthalpy is a state function. True or False?
True
If we have values for the appropriate standard enthalpies of formation, we can determine the enthalpy change for any reaction. True or False?
True
In a thermochemical equation, if the coefficients are multiplied by some factor, the enthalpy change must be multiplied by the same factor, because ∆H is an extensive property. True or False?
True
It doesn't matter much if the standard state conditions of pressure is 1 bar or 1 atm because the ∆H values are essentially the same. True or False?
True
Many thermochemical tables list values with a standard state of 1 atm. True or False?
True
The enthalpy changes for many types of chemical and physical processes are available in the reference literature, including those for combustion reactions, phase transitions, and formation reactions. True or False?
True
When determining the ∆H for a chemical reaction, be sure to take both stoichiometry and limiting reactants into account. True or False?
True
When it comes down to it, the definition of work in chemistry (w=P∆V) is the same measure as the definition of work in physics (w=Force x distance). They both measure the same thing. True or False?
True
The symbol for internal energy is:
U, sometimes E
In the formula ∆H = ∆U + P∆V, why is there no ∆ in front of the P?
We assume the reaction occurs under constant pressure
Energy is transferred into room-temperature metal wire if you rapidly bend the wire back and forth, the wire becomes warmer because of the _______ done on it.
Work
Energy is transferred out of a system when heat is lost from the system, or when the system does ____ on the surroundings.
Work
The internal energy of a system can be changed by heat flow and:
Work
Energy is transferred into a system, when it absorbs heat (q) from the surroundings or when the surroundings do ______ on the system.
Work (w)
In the formula ∆H = ∆U + P∆V, the mathematical product P∆V represents ____.
Work, namely expansion work
When 1.42 g of iron reacts with 1.80 g of chlorine, 3.22 g of FeCl₂(s) and 8.60 kJ of heat is produced. What is the enthalpy change for the reaction when 1 mole of FeCl₂(s) is produced? Fe(s) + 2Cl(g) → FeCl₂(s)
mol FeCl₂ = 3.22 g/(55.85 g mol⁻¹ Fe + 2 x 35.45 g mol⁻¹) = 0.0254 mol FeCl₂ mol FeCl₂ produced = (1.42 g Fe/55.85 g mol⁻¹ Fe) x 1 mol FeCl₂/1 mol Fe = 0.02542 mol FeCl₂ mol FeCl₂ produced = (1.80 g Cl/35.45 g mol⁻¹ Cl) x 1 mol FeCl₂/ 2 mol Cl = 0.02538 mol FeCl₂ ∆H = 2 mol x -8.60 kJ/0.02538 mol Cl ∆H = -678 kJ (WRONG--I am checking on the correct answer ---338 kJ)
How much heat is produced by the combustion of 125 g of acetylene? One mole of acetylene yields -1301.1 kJ
mol acetylene = 125 g/26.036 g mol⁻¹ mol acetylene = 4.801 mol ∆H°c = -1301.1 kJ/1 mol x 4.801 mol ∆H°c = -6246.6 kJ
A gummy bear contains 2.67 g sucrose, C₁₂H₂₂O₁₁. When it reacts with 7.19 g potassium chlorate, KClO₃, 43.7 kJ of heat are produced. Determine the enthalpy change for the reaction. C₁₂H₂₂O₁₁(aq) + 8KClO₃(aq) → 12CO₂(g) + 11H₂O(l) + 8KCl(aq)
mol sucrose = 2.67 g x 1 mol/342.3 g mol⁻¹= 0.00780 mol mol KClO₃ = 7.19 g x 1 mol/ 122.5 g mol⁻¹ =0.0587 0.0587 mol KClO₃ x 1 mol sucrose/8 mol KClO₃ = 0.00734 sucrose is needed (less than the moles present [0.00780] so sucrose is in excess) ∆H = 8 mol x -43.7 kJ/0.0587 mol KClO₃ ∆H = -5960 kJ
Ozone, O₃(g), forms from oxygen, O₂(g), by an endothermic process. Ultraviolet radiation is the source of the energy that drives this reaction in the upper atmosphere. Assuming that both reactants and products of the reaction are in their standard states, determine the standard enthalpy of formation, ∆H°f of ozone from the following information: 3O₂(g) → 2O₃(g) ∆H°₂₉₈=+286 kJ
³/₂O₂(g) → O₃(g) For the formation of 2 mol of O₃(g), ∆H°₂₉₈=+286 kJ. This ratio, (286 kJ/2 mol O₃), can be used as a conversion factor to find the heat produced when 1 mole of O₃(g) is formed, which is the enthalpy of formation for O₃J(g): ∆H° for 1 mole of O₃(g)=1 mol O₃ x 286 kJ/2 mol O₃=143 kJ Therefore, ∆H°f [O₃(g)]=+143 kJ/mol
Given the following thermochemical equation, H₂(g) + ¹/₂O₂(g) → H₂O(l) ∆H = -286 kJ What would be the enthalpy change if the molar amounts were halved?
¹/₂ x (H₂(g) + ¹/₂O₂(g) → H₂O(l) ∆H = -286 kJ) ¹/₂H₂(g) + ¹/₄O₂(g) → ¹/₂H₂O(l) ∆H = -143 kJ
Given that: H₂(g) + Cl₂(g) → 2HCl(g) ∆H = -184.6 kJ What is the enthalpy change of: 2HCl(g) → H₂(g) + Cl₂(g) ∆H = ?
∆H = +184.6 kJ
∆H = q + w - w can be rewritten as:
∆H = q
Since ∆U is equal to q + w and P∆V is equal to -w, then ∆H = ∆U + P∆V can be rewritten as:
∆H = q + w - w
For processes that take place at constant pressure (a common condition for many chemical and physical changes), the enthalpy change is represented by the formula:
∆H = ∆U + P∆V
Write the enthalpy change symbol under standard state conditions.
∆H°
Write the symbol for standard enthalpy of combustion.
∆H°c
The combustion of gasoline is a highly exothermic process. Let us determine the approximate amount of heat produced by burning 1.00 L of gasoline, assuming the enthalpy of combustion of gasoline is the same as that of isooctane, a common component of gasoline. The density of isooctane is 0.692 g/mL. One mole of isooctane yields -5460 kJ.
∆H°c = (-5460 kJ/1 mol x 1 mol/114 g x 0.692 g/1 mL x 1000 mL/1 L) x 1.00 L ∆H°C = -3.31 x 10⁴ kJ
What symbol represents the standard enthalpy of formation?
∆H°f
What is the enthalpy change of ∆H°₂₉₈: C(s) + O₂(g) → CO₂(g) if the two intervening steps are: C(s) + ¹/₂O₂(g) → CO₂(g) ∆H°₂₉₈ =-111 kJ CO(g) + ¹/₂O₂(g) → CO₂(g) ∆H°₂₉₈ =-283 kJ
∆H°₂₉₈ = -394 kJ
What equation represents the relationship between internal energy, heat, and work?
∆U = q + w
