CHM-112 Ch. 6: Thermochemistry
A 1.5 g iron nail is heated to 95 degrees celsius and placed into a beaker of water. Calculate the heat gains by the water if the final equilibrium temperature is 57.8 degrees celsius. The specific heat capacity of iron is 0.449 J/ g x *C and the specific heat of water is 4.18 J / g x *C.
+ 25 J
A 3.54 g piece of aluminum is heated to 96.2 degrees celsius and allowed to cool to room temperature, 22.5 degrees celsius. Calculate the heat in kJ associated with the cooling process. Specific heat of Al is 0.902 J/ g x *C.
-.236 kJ
A cylinder is equipped with a piston expands against an external pressure of 1.58 atm. If the initial volume is 0.485 L and the final volume is 1.245 L, how much work is done?
-122 J
Explain why the bomb calorimeter exemplifies the First Law of Thermodynamics
The loss of heat energy from the sample becomes the gain in heat energy of the water bath.
Calorimetry
measure the thermal energy exchanged between the reaction (defined as the system) and the surroundings by observing the change in temperature of the surroundings
Fossil Fuels
cannot be replenished, their emissions contribute to air pollution, acid rain, and global warming
Endothermic Reaction
chemical reaction that is accompanied by the absorption of heat
1 atm L
101.3 J
For a system that gains thermal energy and does work on the surroundings:
Heat (q) is positive. work (w) is negative
Thermal Energy Transfer
High to low until equilibrium is reached, the amount of heat gained by one object is the same amount lost by the other (q metal = - q water)
The quantity of heat required to raise the temperature of 1 mole of a substance by 1 degree celsius.
Molar Heat Capacity
energy and matter
can be interchanged, but not destroyed-make up the physical universe
Intensive Property
dependent upon the kind of material absorbing heat (specific heat capacity, molar heat capacity)
A 55.0 g Al block initially at 27.5*C absorbs 725 J of heat. What is the final temperature of the Al block? Cs = .903
42.1 *C
The addition of hydrochloric acid to silver nitrate solution precipitates silver chloride: AgNo3 (aq) + HCl (aq) --> AgCl(s) + HNo3(aq). When 50.0 mL of 0.100 M AgNo3 is combined with 50.0 mL of 0.100 Hal in a coffee cup calorimeter, the temperature changes from 23.40 *C to 24.40 *C. Calculate the change in enthalpy fir the reaction as written. (1.00 g/mL as density of solution, Cs = 4.18 J/ g x *C)
50.0 mL , 0.100 M AgNo3 50.0 mL, 0.100 M HCl 100 mL = total mL = 100 g, 0.00500 mol each of AgNo3, HCl -68 jK/mol
If the system absorbs 559 J of heat and does 488 J of work during the expansion, the value of E is:
71 J
A bag of chocolate candy has 220 Cal. How much energy is this in kJ?
920 kJ
If a chemical equation is multiplied by some factor, then 𝚫Hrxn is also multiplied by the same factor.
A + 2B --> C ΔH1 2A+4B --> 2C ΔH2 =2*ΔH1
If a chemical equation is reversed, then 𝚫Hrxn changes sign.
A + 2B --> C ΔH1 C --> A + 2B ΔH2 = -ΔH1
If a chemical equation can be expressed as the sum of a series of steps, then 𝚫Hrxn for the overall equation is the sum of the heats of reaction for each step.
A + 2B > C ΔH1 +C --> 2D ΔH2 = A + 2B --> 2D ΔH3 =ΔH1 +ΔH
Temperature
A measure of thermal energy within a sample of matter
Adiabatic
heat does not enter/leave the system, Change in internal energy is equal to work
Internal Energy
the sum of the kinetic and potential energies of all of the particles that compose the system, considered a state function
Kinetic Energy
Energy associated with the motion of an object
Nuclear Energy
Potential energy in the nucleus of atoms
How much work is done when a container expands 2.55 L against an external pressure of 1.50 atm?
-387 J
Calculate the internal energy for a system that does 422 J of work and loses 227 J of energy as heat.
-649 J
When fuel is burned in a cylinder equipped with a piston, the volume expands from 0.255 L to 1.45 L against an external atmosphere of 1.02 atm. In addition, 875 J is emitted as heat. What is the change in internal energy for the burning of the fuel?
-998
External Pressure Kept Constant:
-Wgas = External Pressure x Change in Volume = -P x Change in Volume > expanding volume system doing negative work
Calorie (Cal)
1 Cal = 1kcal = 1000 cal = 4184 J
50.0 g of water at 22 degrees celsius is mixed with 125 g of water initially at 36 degrees celsius. What is the final temperature of water after mixing, assuming no heat is lost to the surroundings?
32 *C
Which of the following statements best explains why we calculated +CV but then used -CV in the above example.
Because of a change in direction of the flow of heat energy being considered,we were adhering to the First Law and conserving energy, we redefined the system from the calorimeter to the reaction.
Writing Formation Reaction: CO2 example
C + O2 --> CO (g) -elements must be in their standard state -change in Hf* = 0 -C (s, graphite) + O2 (g) --> CO2 (g) -the equation must be balance, but the coefficient of the product component must be 1
Under isochoric conditions:
Change in internal energy = Change in temperature Change in internal energy = q + w = q + 0 = 0
What does a bomb calorimeter measure?
Change in internal energy, volume is constant. Heat capacity of calorimeter is the amount of heat absorbed by the calorimeter for each degree rise in temperature and is called the calorimeter constant: Ccal (kJ / *C).
Energy that is associated with relative positions of electrons and nuclei in atoms and molecules:
Chemical Energy
Renewable Energy
Collected from renewable resources, which are naturally replenished on a human timescale, such as sunlight, wind, rain, tides, waves, hydroelectric power, wind power, and geothermal heat
Isochoric
Constant Volume, Change in internal energy is equal to q
Find the specific heat capacity of the substance composing the rock and determine whether the value is consistent with the rock being pure gold. (m = 4.7g, q = 57.2J, Ti = 25*C, Tf = 57*C, Cs = .128 for gold)
Cs = 0.38 J / g * C* > not gold, mixture of copper and silver
Potential Energy
Energy associated with the position or composition of an object
Chemical Energy
Energy associated with the relative positions of electrons or nuclei in atoms or molecules, considered a form of potential energy
Thermal Energy
Energy associated with the temperature of an object, considered a type of kinetic energy since it arises from the motion of atoms or molecules in a substance
Law of Conservation of Energy
Energy cannot be created nor destroyed, can be transferred from one object to another and it can assume different forms
Energy Exchange
Energy is transferred between the system and its surroundings, if the system loses energy the surrounding gains the same amount of energy and vice versa
Internal Energy Reaction
Energy of Products - Energy of Reactants
Standard Enthalpy Formation
Enthalpy change for the reaction forming 1 mole of pure compound from its constituent elements -the elements must be in their standard states -xhange in Hf* for a pure element in standard state is 0 kJ / mol
Standard Enthalpy Change
Enthalpy change when all reactants and products are in their standard states
Surroundings
Everything else with which the system can exchange energy
The condensation of water
Exothermic
Internal Energy Formula
Final Energy - Initial Energy
Pressure (external)
Force / Area
Energy Flow
If the reactants have a higher internal energy than the products, ΔEsys is negative and energy flows out of the system into the surroundings. If the reactants have a lower internal energy than the products, ΔEsys is positive and energy flows into the system from the surroundings.
Calorimetry at Constant Volume
If we use a system where there is no change in volume, W = -P x Change in Volume = 0. The internal energy of the system is equivalent to the heat of the system because there is no work being done. Cannot observe the temperature changes of the individual chemicals involved in a reaction, so instead we measure the temperature changes in the surroundings. ( q system = -q surroundings)
1/2mv^2
KE: kg*m^2/s^2 = 1 joule (SI units)
Heat/Thermal Energy
Kinetic Energy associated with molecular motion
Electrical Energy
Kinetic energy associated with the flow of electric charge
Light/Radiant Energy
Kinetic energy associated with transitions in an atom
Identical amounts of heat are applied to 50 g of lead, silver, and copper, all with the initial temperature of 25 degrees celsius. Which block will have the largest increase in temperature?
Lead, lowest specific heat, largest temperature increase since it requires less energy to raise one gram by one degree of celsius.
System
Material or process which we are studying the energy changes within
Find the change in enthalpy reactions for: N20 (g) + NO2 (g) --> 3NO (g). Use these reactions with know change in enthalpies: 2NO (g) + O2 (g) --> 2NO2 (g), change in H = -113.1 kJ N2 (g) + O2 (g) --> 2NO (g), change in H = 183.6 kJ N2O (g) --> 2N2 (g) + O2 (g), change in H = -163.2 kJ
N2O (g) + NO2 (g) --> 3NO(g), change in H = 157.6 kJ
In a coffee cup calorimetry, 50.0 mL of 0.100 M NaOH is added to 50.0 mL of 0.100 M HCl. The temperature of the solution increased by 14.9 *C. Calculate the change in enthalpy for the reaction. Use 1.00 g/mL as the density of the solution and 4.18 J/ g x *C as the specific heat capacity.
NaOH (aq) + HCl (aq0) = NaCl + H20 Change in H = q rxn / # mol = - q cc / # mol = (- 100.0 g x 4.18 J / g x *C x 14.9 *C) / 0.00500 mol = -1250 kJ / mol
Pressure-Volume Work
PV work is caused by a volume change against an external pressure, when gasses expand, the change in volume is positive but the system is doing work on the surrounds so Wgas is negative.
An endothermic reaction consists of what characteristics?
Postive change in enthalpy, absorbs heat from surroundings, feels cold to the touch
Electrocstatic PE b/w Irons
Q1 Q2 / d
Extensive Property
Specific heat, depends on the amount of matter being heated
Thermodynamics
Study of energy and its conversions, governs every process that involves change
Bomb Calorimetry
Surrounding area, usually made of a sealed, insulated container filled with water q surrounding = q calorimeter = -q system used for combustion reactions
sweat evaporating from skin
Sweat evaporating from skin cools the skin and is therefore endothermic, with a positive ΔH. The skin must supply heat to the perspiration in order for it to continue to evaporate.
calorie (cal)
The amount of energy required to raise one gram of water by one degree of celsius, 1 cal = 4.184 J
Energy
The capacity to do work
Hess's Law
The change for a stepwise process is the sum of the enthalpy changes, thregardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes.
C3H8(g) + 5 O2(g) ¡ 3 CO2(g) + 4 H2O(g) ΔHrxn = -2044 kJ
The equation tells us that when 1 mol of C3H8 reacts with 5 mol of O2 to form 3 mol of CO2 and 4 mol of H2O, 2044 kJ of heat is emitted.
Heat
The flow of energy caused by a temperature difference, way an object or set of objects exchanges energy
Heat Capacity
The proportionality constant between heat and the change in temperature, a measure of the system's ability to absorb thermal energy without undergoing a large change in temperature, quantity of heat required to change its temperature by 1 °C, the higher the heat capacity of a system, the smaller the change in tem- Elements perature for a given amount of absorbed heat.
Work
The result of a force acting through a distance,way an object or set of objects exchanges energy, what an object or set of object possesses
Thermochemistry
The study of relationships between chemistry and energy.
Change of Internal Energy
The sum of the heat (q) transferred and work (w) done
First Law of Thermodynamics
The total energy of the universe is constant, In other words, since energy is neither created nor destroyed, and since the universe does not exchange energy with anything else, its energy content does not change.
Summarizing Enthalpy
The value of ΔH for a chemical reaction is the amount of heat absorbed or evolved in the reaction under conditions of constant pressure. An endothermic reaction has a positive ΔH and absorbs heat from the surroundings. An endothermic reaction feels cold to the touch. An exothermic reaction has a negative ΔH and gives off heat to the surroundings. An exothermic reaction feels warm to the touch.
Acid rain is one of the problems associated with combustion of fossil fuels.
True
Cars that run on hydrogen fuel cells are environmentally friendly.
True
The burning of fossil fuels contributes to global warming.
True
The more energy produced per kg of carbon dioxide produced, the better the fuel.
True
The specific heat capacity (c) multiplied by mass (m) equals heat capacity (C).
True
The specific heat capacity (c) of non-metals is typically greater than that of metals.
True
A material with a smaller specific heat capacity will experience a greater temperature change than a material with a greater specific heat capacity per unit of heat.
True : q= mcΔT ⇒ c ∝ 1 / ΔT . So smaller c corresponds to a greater ΔT.
Energy Use and the Environment
US-each person uses over 10^5 kWh of energy per year most comes from the combustion of fossil fuels
Sun
Unlimited supply of energy
water freezing in a freezer
Water freezing in a freezer releases heat and is therefore exothermic, with a negative ΔH. The refrigeration system in the freezer must remove this heat for the water to continue to freeze.
Relationships Include The Change in Enthalpy Reaction
When reaction is multiplied by a factor, the change in enthalpy reaction is multiplied by the same factor (b/c the change in enthalpy reaction is extensive). If the reaction is reversed then the sign of the change in enthalpy is changed (=x - 1)
wood burning in a fire
Wood burning in a fire releases heat and is therefore exothermic, with a negative ΔH.
PE equation
molecular position : m g h
Exothermic Reaction
of a reaction or process accompanied by the release of heat
Standard Conditions
pure gas at exactly 1 atm pressure, pure solid or liquid in its most stable form at exactly 1 atm pressure and temperature 25 *C, substance in a solution with concentration 1 M
If there is a change in temperature during the acquisition of pressure and volume data of a gas:
q = -w + change in internal energy
Heat Capacity Formula
q = C × ∆T (C = heat capacity), C=q / ΔT =J / °C
Heat (J)
q = m * Cs * (Tf - Ti)
A constant volume calorimeter (bomb calorimeter) was calibrated by performing in it a reaction in which 5.23 kJ of heat energy was released, causing the temperature of 0.250 L of water to rise by 7.33 °C. What is the heat capacity Cv of the calorimeter?
q cal / change in T = 5.23 kJ / 7.33 *C = 0.714 kJ/*C
Calorimetry at Constant Pressure
reactions done in aqueous solutions are open to the atmosphere, calorimeter is often nested foam cups containing the solution [ q reaction = -q solution = -(mass solution x Cs solution x change in temperature)
Specific Heat Capacity (Cs)
the amount of heat required to raise the temperature of 1 gram of the substance by 1 degree of celsius. The units of specific heat capacity (also called specific heat) are J / g * C*
Molar Heat Capacity
the amount of heat required to raise the temperature of one mole of a substance one degree of celsius
State Function
the sum of the kinetic and potential energies of all of the particles that compose the system (initial and end points, not dependent on the process)
