Chapter 4 StaxChem Thermochemistry
Calorie (C)
. The Calorie (with a capital C), or large calorie, commonly used in quantifying food energy content, is a kilocalorie.
How much heat is produced by the combustion of 125 g of acetylene? formula: C2H2
125 g x 1mole/ 26 grams x −1301.1 kj/1 mole Answer: 6.25 × 103 kJ
When 100 mL of 0.200 M NaCl(aq) and 100 mL of 0.200 M AgNO3(aq), both at 21.9 °C, are mixed in a coffee cup calorimeter, the temperature increases to 23.5 °C as solid AgCl forms. How much heat is produced by this precipitation reaction? What assumptions did you make to determine your value?
: 1.31 × 103 J; assume no heat is absorbed by the calorimeter, no heat is exchanged between the calorimeter and its surroundings, and that the specific heat and mass of the solution are the same as those for water
calorimeter heat produced by the reaction is absorbed by the solution, ? the heat required is absorbed from the thermal energy of the solution?
A calorimeter is a device used to measure the amount of heat involved in a chemical or physical process. For example, when an exothermic reaction occurs in solution in a calorimeter, the heat produced by the reaction is absorbed by the solution, which increases its temperature. When an endothermic reaction occurs, the heat required is absorbed from the thermal energy of the solution, which decreases its temperature
exothermic
A change that releases heat is called an exothermic process
bomb calorimeter
A different type of calorimeter that operates at constant volume, colloquially known as a bomb calorimeter, is used to measure the energy produced by reactions that yield large amounts of heat and gaseous products, such as combustion reactions. (The term "bomb" comes from the observation that these reactions can be vigorous enough to resemble explosions that would damage other calorimeters.)
kilojoule 1 cal= _____ joules
A kilojoule (kJ) is 1000 joules. 1 calorie has been set to equal 4.184 joules
endothermic
A reaction or change that absorbs heat is an endothermic process.
A piece of unknown metal weighs 217 g. When the metal piece absorbs 1.43 kJ of heat, its temperature increases from 24.5 °C to 39.1 °C. Determine the specific heat of this metal, and predict its identity.
Answer: c = 0.45 J/g °C; the metal is likely to be iron 1430J = (specific heat ) × (217 g) × (14.6'c) 3168.2 .451
energy
Energy can be defined as the capacity to supply heat or do work.
One calorie=
One calorie (cal) = exactly 4.184 joules, and one Calorie (note the capitalization) = 1000 cal, or 1 kcal. (This is approximately the amount of energy needed to heat 1 kg of water by 1 °C.)
A 360-g piece of rebar (a steel rod used for reinforcing concrete) is dropped into 425 mL of water at 24.0 °C. The final temperature of the water was measured as 42.7 °C. Calculate the initial temperature of the piece of rebar. Assume the specific heat of steel is approximately the same as that for iron (Table 5.1), and that all heat transfer occurs between the rebar and the water (there is no heat exchange with the surroundings).
The temperature of the water increases from 24.0 °C to 42.7 °C, so the water absorbs heat. That heat came from the piece of rebar, which initially was at a higher temperature. Assuming that all heat transfer was between the rebar and the water, with no heat "lost" to the surroundings, then heat given off by rebar = −heat taken in by water, or: qrebar = −qwater Since we know how heat is related to other measurable quantities, we have: (c × m × ΔT)rebar = −(c × m × ΔT)water
Thermal energy
Thermal energy is kinetic energy associated with the random motion of atoms and molecules.
specific heat
amount heat required to raise temp of 1 gram material by 1'C intrinsic property-doesn't matter on how much material that you have c = q /mΔT
heat capacity
amount heat required to raise the temp of a object by 1'c this is an extrensic property (matters how much of the substance that you have)
chemical thermodynamics
chemical thermodynamics, the science that deals with the relationships between heat, work, and other forms of energy in the context of chemical and physical processes
energy is transferred out of a system when
energy is transferred out of a system when heat is lost from the system, or when the system does work on the surroundings
expansion work
A type of work called expansion work (or pressure-volume work) occurs when a system pushes back the surroundings against are straining pressure, or when the surroundings compress the system
When 0.963 g of benzene, C6H6, is burned in a bomb calorimeter, the temperature of the calorimeter increases by 8.39 °C. The bomb has a heat capacity of 784 J/°C and is submerged in 925 mL of water. How much heat was produced by the combustion of the glucose sample?
Answer: 39.0 kJ qrxn = −(qwater + qbomb) −[(4.184 J/g °C) × (925 ml g) × (35.6 °C − 23.8 °C) + 893 J/°C × (35.6 °C − 23.8 °C)]
Standard enthalpy of combustion (ΔHC °)
Standard enthalpy of combustion (ΔHC °) is the enthalpy change when 1 mole of a substance burns (combines vigorously with oxygen) under standard state conditions; it is sometimes called "heat of combustion
first law of thermodynamics,
it shows that the internal energy of a system changes through heat flow into or out of the system
. If heat flows into the system,
(or work is done on the system,) its internal energy increases, ΔU < 0.
heat
Heat (q) is the transfer of thermal energy between two bodies at different temperatures Heat flow (a redundant term, but one commonly used) increases the thermal energy of one body and decreases the thermal energy of the other.
heat absorbed by calorimeter is often--- what happens to heat during a chemical reaction heat produced or consumed in the reaction must be equal to
The amount of heat absorbed by the calorimeter is often small enough that we can neglect it (though not for highly accurate measurements, as discussed later), and the calorimeter minimizes energy exchange with the surroundings. Because energy is neither created nor destroyed during a chemical reaction, there is no overall energy change during the reaction. The heat produced or consumed in the reaction (the "system"), qreaction, plus the heat absorbed or lost by the solution (the "surroundings"), qsolution, must add up to zero: qreaction + qsolution = 0
When one substance is converted into another, there is always an associated conversion
When one substance is converted into another, there is always an associated conversion of one form of energy into another. Heat is usually released or absorbed, but sometimes the conversion involves light, electrical energy, or some other form of energy. For example, chemical energy (a type of potential energy) is stored in the molecules that compose gasoline.
When thermal energy is lost, the
When thermal energy is lost, the intensities of these motions decrease and the kinetic energy falls.
enthalpy
enthalpy(H) to describe the thermodynamics of chemical and physical processes. Enthalpy is defined as the sum of a system's internal energy (U) and the mathematical product of its pressure (P) and volume (V): H =U+PV
A piece of unknown metal weighs 348 g. When the metal piece absorbs 6.64 kJ of heat, its temperature increases from 22.4 °C to 43.6 °C. Determine the specific heat of this metal (which might provide a clue to its identity).
q/(mass of substance) × (temperature change) = (specific heat ) 6640J/(348 g) × (21.2'c) = (specific heat ) 6640J/7377.6 = 0.900 J/g °C Comparing this value with the values in Table 5.1, this value matches the specific heat of aluminum, which suggests that the unknown metal may be aluminum
Bomb Calorimetry When 3.12 g of glucose, C6H12O6, is burned in a bomb calorimeter, the temperature of the calorimeter increases from 23.8 °C to 35.6 °C. The calorimeter contains 775 g of water, and the bomb itself has a heat capacity of 893 J/°C. How much heat was produced by the combustion of the glucose sample?
qrxn = −(qwater + qbomb) = −[(4.184 J/g °C) × (775 g) × (35.6 °C − 23.8 °C) + 893 J/°C × (35.6 °C − 23.8 °C)] This reaction released 48.7 kJ of heat when 3.12 g of glucose was burned.
When solid ammonium nitrate dissolves in water, the solution becomes cold. This is the basis for an "instant ice pack" (Figure 5.16). When 3.21 g of solid NH4NO3 dissolves in 50.0 g of water at 24.9 °C in a calorimeter, the temperature decreases to 20.3 °C. Calculate the value of q for this reaction and explain the meaning of its arithmetic sign. State any assumptions that you made.
qrxn = −qsoln = −(c × m × ΔT) soln = −[(4.184 J/g °C) × (53.2 g) × (20.3 °C − 24.9 °C)] = −[(4.184 J/g °C) × (53.2 g) × (−4.6 °C)] +1.0 × 103 J = +1.0 kJ The positive sign for q indicates that the dissolution is an endothermic process.
thermochemistry
science concerned with the amount of heat absorbed or released during chemical and physical changes—an area called thermochemistry
When a 3.00-g sample of KCl was added to 3.00 × 102 g of water in a coffee cup calorimeter, the temperature decreased by 1.05 °C. How much heat is involved in the dissolution of the KCl? What assumptions did you make?
(4.184)(303g)(1.05) 1.33 kJ; assume that the calorimeter prevents heat transfer between the solution and its external environment (including the calorimeter itself) and that the specific heat of the solution is the same as that for water
surroundings
(the other components of the measurement apparatus that serve to either provide heat to the system or absorb heat from the system).
If heat flows out of the system,
(work is done by the system), its internal energy decreases, ΔU > 0.
A gummy bear contains 2.67g sucrose, C12H22O11. When it reacts with 7.19g 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)
2.67 g sucrose x 1 mole/321.14 =.008314 43.7/.008314 ΔH =−5960kJ page 259
: potential energy kinetic energy
: potential energy, the energy an object has because of its relative position, composition, or condition, and kinetic energy, the energy that an object possesses because of its motion. Water at the top of a waterfall or dam has potential energy because of its position; when it flows downward through generators, it has kinetic energy that can be used to do work and produce electricity in a hydroelectric plant
standard enthalpy of formation
A standard enthalpy of formation is an enthalpy change for a reaction in which exactly 1 mole of a pure substance is formed from free elements in their most stable states under standard state conditions.
How much heat, in joules, must be added to a 5.00 × 102 -g iron skillet to increase its temperature from 25°C to 250 °C? The specific heat of iron is 0.451 J/g °C.
Answer: 5.05 × 104 J 0.451 J/g °C x 500grams x 225'c 5.0737 x 10 4
a system can undergo a change and energy can----
As a system undergoes a change, its internal energy can change, and energy can be transferred from the system to the surroundings, or from the surroundings to the system.
energy can do ______ but cant________
Energy can be converted from one form into another, but all of the energy present before a change occurs always exists in some form after the change is completed. This observation is expressed in the law of conservation of energy: during a chemical or physical change, energy can be neither created nor destroyed, although it can be changed in form
Energy is transferred into a system when it
Energy is transferred into a system when it absorbs heat (q) from the surroundings or when the surroundings do work (w) on the system.
enthalpy is a state function
Enthalpy values for specific substances cannot be measured directly; only enthalpy changes for chemical or physical processes can be determined. it doesn't matter how it got their just the start and end values
the enthalpy change (ΔH) equation is:
For processes that take place at constant pressure (a common condition for many chemical and physical changes), the enthalpy change (ΔH)is: ΔH =ΔU+PΔV PΔV represents work (w), namely, expansion or pressure-volume work u= changes in interna energy
When 0.0500mol of HCl(aq) reacts with 0.0500 mol of NaOH(aq) to form 0.0500 mol of NaCl(aq), 2.9kJ of heat are produced. What isΔH, the enthalpy change, per mole of acid reacting, for the acid-base reaction run under the conditions described inExample 5.5? HCl(aq)+NaOH(aq) ⟶ NaCl(aq)+H2O(l
For the reaction of 0.0500 mol acid (HCl), q = −2.9 kJ. This ratio −2.9kJ /0.0500mol HCl can be used as a conversion factor to find the heat produced when 1 mole of HCl reacts: ΔH =1 mol HCl × −2.9kJ/ 0.0500 mol HCl =−58kJ The enthalpy change when 1mole of HCl reacts is−58kJ.Since that is the number of moles in the chemical equation, we write the thermochemical equation as: HCl(aq)+NaOH(aq) ⟶ NaCl(aq)+H2O(l) ΔH =−58k
calorie
Historically, energy was measured in units of calories (cal). A calorie is the amount of energy required to raise one gram of water by 1 degree C (1 kelvin). However, this quantity depends on the atmospheric pressure and the starting temperature of the water.
gains thermal energy loses thermal energy
If a substance gains thermal energy, its temperature increases, its final temperature is higher than its initial temperature, Tfinal − Tinitial has a positive value, and the value of q is positive. If a substance loses thermal energy, its temperature decreases, the final temperature is lower than the initial temperature, Tfinal − Tinitial has a negative value, and the value of q is negative.
how to determine the amount of heat, q, entering or leaving the substance
If we know the mass of a substance and its specific heat, we can determine the amount of heat, q, entering or leaving the substance by measuring the temperature change before and after the heat is gained or lost: q = (specific heat ) × (mass of substance) × (temperature change) q = c × m × ΔT = c × m × (Tfinal − Tinitial) c is the specific heat of the substance, m is its mass, and ΔT (which is read "delta T") is the temperature change, Tfinal − Tinitial
state function
Internal energy is a type of quantity known as a state function (or state variable), whereas heat and work are not state functions. The value of a state function depends only on the state that a system is in, and not on how that state is reached
metal heat capacity water heat capacity
Liquid water has a relatively high specific heat (about 4.2 J/g °C); most metals have much lower specific heats (usually less than 1 J/g °C).
what happens to substances as their temperature increases and decreases
Most substances expand as their temperature increases and contract as their temperature decreases. This property can be used to measure temperature changes, The operation of many thermometers depends on the expansion and contraction of substances in response to temperature changes
When 50.0 mL of 0.10 M HCl(aq) and 50.0 mL of 0.10 M NaOH(aq), both at 22.0 °C, are added to a coffee cup calorimeter, the temperature of the mixture reaches a maximum of 28.9 °C. What is the approximate amount of heat produced by this reaction? HCl(aq) + NaOH(aq) ⟶ NaCl(aq) + H2 O(l)
Next, we know that the heat absorbed by the solution depends on its specific heat, mass, and temperature change: qsolution = (c × m × ΔT) solution To proceed with this calculation, we need to make a few more reasonable assumptions or approximations. Since the solution is aqueous, we can proceed as if it were water in terms of its specific heat and mass values. The density of water is approximately 1.0 g/mL, so 100.0 mL has a mass of about 1.0 × 102g (two significant figures). The specific heat of water is approximately 4.18 J/g °C, so we use that for the specific heat of the solution. Substituting these values gives:
calorimetry
One technique we can use to measure the amount of heat involved in a chemical or physical process is known as calorimetry. Calorimetry is used to measure amounts of heat transferred to or from a substance. To do so, the heat is exchanged with a calibrated object (calorimeter). The change in temperature of the measuring part of the calorimeter is converted into the amount of heat (since the previous calibration was used to establish its heat capacity)
work
One type of work (w) is the process of causing matter to move against an opposing force. For example, we do work when we inflate a bicycle tire—we move matter (the air in the pump) against the opposing force of the air already in the tire.
As Figure 5.21 suggests, 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.
Starting with a known amount (1.00 L of isooctane), we can perform conversions between units until we arrive at the desired amount of heat or energy. The enthalpy of combustion of is o octane provides one of the necessary conversions.Table 5.2gives this value as −5460 kJ per 1 mole of isooctane (C8H18).
Substances act as reservoirs of energy, meaning
Substances act as reservoirs of energy, meaning that energy can be added to them or removed from them. Energy is stored in a substance when the kinetic energy of it s atoms or molecules is raised.
how does heat flow
Suppose we initially have a high temperature (and high thermal energy) substance (H) and a low temperature (and low thermal energy) substance (L). The atoms and molecules in H have a higher average KE than those in L. If we place substance H in contact with substance L, the thermal energy will flow spontaneously from substance H to substance L. The temperature of substance H will decrease, as will the average KE of its molecules; the temperature of substance L will increase, along with the average KE of its molecules. Heat flow will continue until the two substances are at the same temperature
how I figured out equation:
THings need to know-Since the solution is aqueous, we can proceed as if it were water in terms of its specific heat and mass values. heat value-4.184J Mass- total mass (50ml + 50 ml) (now convert it into grams: 100ml x 1l/1000 x 1000 grams/1 L = 100 grams) (4.184J) (100grams) (28.9-22= 6.9) 2886.96 2.88 x 10 3
temperature measures
Temperature is a quantitative measure of "hot" or "cold." When the atoms and molecules in an object are moving or vibrating quickly, they have a higher average kinetic energy (KE), and we say that the object is "hot." When the atoms and molecules are moving slowly, they have lower KE, and we say that the object is "cold"
joule
The SI unit of heat, work, and energy is the joule. A joule (J) is defined as the amount of energy used when a force of 1 newton moves an object 1 meter
A 248-g piece of copper initially at 314 °C is dropped into 390 mL of water initially at 22.6 °C. Assuming that all heat transfer occurs between the copper and the water, calculate the final temperature.
The final temperature (reached by both copper and water) is 38.8 °C.
The greater kinetic energy may be
The greater kinetic energy may be in the form of increased translations (travel or straight-line motions), vibrations, or rotations of the atoms or molecules.
heat capacity
The heat capacity (C) of a body of matter is the quantity of heat (q) it absorbs or releases when it experiences a temperature change (ΔT) of 1 degree Celsius (or equivalently, 1 kelvin): C =q/ΔT Heat capacity is determined by both the type and amount of substance that absorbs or releases heat. It is therefore an extensive property—its value is proportional to the amount of the substance.
The total of all possible kinds of energy present in a substance is called
The total of all possible kinds of energy present in a substance is called the internal energy (U), sometimes symbolized as E.
There are two ways to determine the amount of heat involved in a chemical change
There are two ways to determine the amount of heat involved in a chemical change: measure it experimentally, or calculate it from other experimentally determined enthalpy changes.
amount of heat consumed in a reaction is ---- to the amount of hear lost by solution
This means that the amount of heat produced or consumed in the reaction equals the amount of heat absorbed or lost by the solution: qreaction = −qsolution
system
a system (the substance or substances undergoing the chemical or physical change)
A flask containing 8.0 × 102 g of water is heated, and the temperature of the water increases from 21 °C to 85 °C. How much heat did the water absorb?
q = (specific heat ) × (mass of substance) × (temperature change) (4.184J) x800g x (85-21=64) The specific heat of water is 4.184 J/g °C, so to heat 1 g of water by 1 °C requires 4.184 J. We note that since 4.184 J is required to heat 1 g of water by 1 °C, we will need 800 times as much to heat 800 g of water by 1 °C. Finally, we observe that since 4.184 J are required to heat 1 g of water by 1 °C, we will need 64 times as much to heat it by 65 °C (that is, from 21 °C to 85 °C). This can be summarized using the equation: q = c × m × ΔT = c × m × (Tfina − Tinitial) = (4.184 J/g °C) × (800 g) × (85 − 20) °C = 220,000 J ( = 210 kJ) Because the temperature increased, the water absorbed heat and q is positive
The relationship between internal energy, heat, and work can be represented by the equation:
ΔU =q+w q( heat + heat flow into system, - heat flow out of system) w (work + if done on system, - if done by the system) u (internal energy) delta-changes
When1.42gofironreactswith1.80gofchlorine,3.22gofFeCl2(s)and8.60kJofheatisproduced.What is the enthalpy change for the reaction when 1 mole of FeCl2(s) is produced?
Answer: ΔH= −338 kJ
enthalpy equation
Chemists usually perform experiments under normal atmospheric conditions, at constant external pressure with q = ΔH, which makes enthalpy the most convenient choice for determining heat.
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 reacting for the reaction: Zn(s)+2HCl(aq) ⟶ ZnCl2(aq)+H2(g)
ΔH= −153 kJ 1.34 g Zn x 1mole/65.38 =.02 moles 3.14/.02 = 153
