Chem 1200 Topic: Energy (Potential and Kinetic)(Exothermic and Endothermic)
For each transformation, determine the sign (+ or -) of the change in enthalpy (ΔH). P4(g) → 2 P2(g) P4(s) → P4(g) O2(g) → 2 O(g) 4 P(g) → P4(s) H2(g) → H2(s) C(diamond) → C(graphite)
(+) P4(g) → 2 P2(g) P4(s) → P4(g) O2(g) → 2 O(g) (-) 4 P(g) → P4(s) H2(g) → H2(s) C(diamond) → C(graphite) Note: A negative change in enthalpy means that the product of the reaction is lower energy than the starting material. Conversion to a higher-energy product gives a positive sign for the enthalpy change. Solid vs Gas When a substance goes from a solid state to a gaseous state, energy is required. So, ΔH will be positive. When a substance goes from a gaseous state to a solid state, energy is required. So, ΔH will be negative.
A gas cools, and the heat transferred in the process is 1550 J. At the same time it is compressed, and the work involved in compression is 640.0 J. Calculate the change in internal energy for the gas in joules.
-1550J+640J=-910J
Calculate ΔE for a chemical reaction that produces 830.0 kJ of heat but does no work on its surroundings. kJ
-830.0KJ
Conversion of 0.702 g of liquid water to steam at 100.0°C requires 1.56 kJ of heat. Calculate the molar enthalpy of evaporation of water at 100.0°C. _________________kJ/mol
0.702 g H2O @ 18.02g/m=0.0389 moles H2O molar enthalpy of evaporation:1.56 kJ / 0.0389 moles H2O = 40.102kJ/mol 40.102Kj / mol H2O
Calculate the kinetic energy of a 64.0 kg ice skater traveling at 6.00 m/s. kg·m2/s2
1150
A subcompact car with a mass of 1.10×103 kg and a loaded dump truck with a mass of 1.70×104 kg are traveling at the same speed. How many times more kinetic energy does the dump truck have than the car?
15.45
Calculate ΔE for a system that absorbs 816.0 kJ of heat from its surroundings and does 480.0 kJ of work on its surroundings. kJ
336KJ
When gasoline burns in a car engine, carbon dioxide and water vapor are produced. If these expanding gases do 4.4 L·atm of work on the pistons and the engine loses 330 J to the surroundings as heat, calculate the change in energy in Joules. Useful conversion factor: 101.3 J = 1 L⋅atm.
4.4Lxatm x 101.3J/1Lxatm= 445J -330J+-445J=-775J
state function
A function that depends only on the initial and final states of a system, not on the path in between.
closed system
A system in which no matter is allowed to enter or leave
isolated system
A system that can exchange neither energy nor matter with its surroundings.
Which of the following processes are exothermic, and which are endothermic? A. Ice cubes solidify in the freezer. B. Ice cubes in a frost-free freezer slowly lose mass. C. Dew forms on a lawn overnight.
A. Exothermic B. Endothermic C. Exothermic
Which of the following processes are exothermic, and which are endothermic? A. Molten aluminum solidifies. B. Rubbing alcohol evaporates from the skin. C. Fog forms over San Francisco Bay.
A. Exothermic B. Endothermic C. Exothermic
clothes dry on a clothesline A. endothermic B. exothermic
A. endothermic
ice cubes melt A. endothermic B. exothermic
A. endothermic
Endothermic
Absorbs heat(+) During an endothermic process, heat flows into the system from its surroundings (q > 0).
What happens to the internal energy of a liquid at its boiling point when it vaporizes?
As a result, the internal energy of the substance increases as it goes from a liquid to a gas.
For the heating curve shown below, select all the processes that occur as you move leftward from the gas state at Point A toward Point B. CondensationLiquid is heated. The solid is cooled. FreezingVaporizationDepositionLiquid is cooled.MeltingSolid is heated. Gas is heated. SublimationGas is cooled.
At A gas is present. Which is being cooled. Then in horizontal line, gas changes to liquid. [condensation] Then liquid is belling cooled. In 2nd horizontal line, liquid changes to solid and we reach point B [freezing] Answer: Liquid is cooled, Freezing, Gas is cooled, condensation
The following reactions take place in a cylinder equipped with a movable piston at atmospheric pressure. Which reactions will result in work being done on the surroundings? Assume the system returns to an initial temperature of 110°C. A. CH4(g)+2O2(g)---->CO2(g)+2H2O(g) B. C3H8(g)+5O2(g)---->3CO2(g)+4H2O(g) C. 2N2(g)+O2(g)---->2N2O(g)
B. C3H8(g)+5O2(g)---->3CO2(g)+4H2O(g) (A) In this reaction, 3 moles of gaseous reactants form 3 moles of gaseous products. The Δn for the reaction is 0, so this reaction does not do work on the surroundings. (B) In this reaction, 6 moles of gaseous reactants form 7 moles of gaseous products. The Δn for the reaction is +1, so this reaction does work on the surroundings. (C) In this reaction, 3 moles of gaseous reactants form 2 moles of gaseous products. The Δn for the reaction is -1, so this reaction does not work on the surroundings. Note: The moles of the reaction predicts the amount of work being done the surrounding.
What happens to the internal energy of a gas when it expands (with no heat flow)? A. It is impossible to determine without more information. B. The internal energy decreases with the expansion. C. The internal energy remains unchanged in the absence of heat flow. D. The internal energy increases with the expansion.
B. The internal energy decreases with the expansion.
propane burns in your grill A. endothermic B. exothermic
B. exothermic
Select the statements below that are true when an ice cube is placed in a glass of hot water. A.The energy lost by the ice cube will be equal to the energy gained by the water. B.qwater = -qice C.Thermal energy will be transferred from the water to the ice cube. D.qice = qwater E.Thermal energy will flow from the ice cube to the water. F.The energy gained by the ice cube will be equal to the energy lost by the water.
B. qwater = -qice C. Thermal energy will be transferred from the water to the ice cube. F. The energy gained by the ice cube will be equal to the energy lost by the water.
Which of the following processes are endothermic? A match burns. (The match is the system.) B. Rubbing alcohol feels cold on the skin. (The alcohol is the system.) C. molten metal solidifies. (The metal is the system.)
B.Rubbing alcohol feels cold on the skin. (The alcohol is the system.)
If a system loses heat AND does work on its surroundings, what are the signs of heat and work? HEAT WORK A + + B + − C − − D − +
C - - Since the system is losing energy from heat and losing it by doing work, we know the overall ΔE must be negative without having to know the exact values of q and w.
Frist Law of Thermodynamics
Energy cannot be created or destroyed
How are energy and work related?
Energy is the ability to do work and work is a transfer of energy
What is the energy change when 23.0 grams of water cools from 93.0 °C to 18.5 °C?
For water, the specific heat capacity (C) is approximately 4.18 J/g°C. Mass (m) = 15.0 grams Initial temperature (Ti) = 93.0°C Final temperature (Tf) = 14.5°C First, we calculate the temperature change (ΔT): ΔT=Tf−Ti=14.5°C−93.0°C=−78.5°C Now, plug in the values into the formula: q=m×C×ΔT q=15.0 g×4.18 J/g°C×(−78.5°C) q=−4,922 J
The enthalpy change for the explosion of ammonium nitrate with fuel oil is -7198 kJ for every 3 moles of NH4NO3. What is the enthalpy change for 1.0 mole of NH4NO3 in this reaction?
Given Data: ΔH for 3 moles of NH4NO3 = -7198 kJ Step 2 Asked- What is the enthalpy change for 1 mole of NH4NO3 in this reaction? Step 3 Now, In order to find ΔH for 1 mole of NH4NO3. ΔH for 3 moles of NH4NO3 = -7198 kJ ΔH for 1 mole of NH4NO3 = −7,1983 KJ=−2,399.33 KJ=−23.99×102 KJ
How much heat is needed to raise the temperature of 121 g of water from 39.0 °C to 90.0 °C?
Given:Mass of water (m) = 81.0g Initial temperature (T1) = 34°C Final temperature (T2) = 9.0× 101°C = 90°C Now let's calculate the change in temperature: ΔT=T2−T1 ΔT=(90°C)−(34°C) ΔT=56°C Using the equation: Q=m×c×ΔT Q=81.0g×4.18Jg·°C×56°C
Your hand is in contact with a hot stone. Heat,q Work,w Surroundings to the system: positive System to surroundings: negative
Heat,q Surroundings to the system: positive
The figure below shows portions of the heating curves for 2 moles of each of the three liquids: Compound Boiling Point (ºC) Chloroform 142 Water 100 Ethanol 78 Rank the liquids in order of their molar enthalpies of vaporization. Chloroform=Red Water=Blue Ethanol=Yellow
Highest 1. Water 2. Ethanol 3. Cholroform Lowest Note: The distance between the heating curve determines the molar enthalpies of vaporization. A longer distance indicates higher molar enthalpies of vaporization and a shorter distance has lower molar enthalpies of vaporization.
Describe two ways to increase the internal energy of a gas sample.
Increasing temperature adds energy to the gas molecules, increasing their motion and, decreasing the volume of the gas.
kinetic energy formula
KE = 1/2mv^2
What are the 8 state functions?
Mass, pressure, density, energy, temperature, volume, enthalpy, entropy
The following reactions are carried out at atmospheric pressure (external) inside the chamber of a piston. Will the piston plunger have moved out (expanded) or moved in (contracted) after the reaction has occurred and the system was allowed to return to its initial temperature of 110°C? N2(g)+3H2(g)--->2NH3(g) A. The piston expanded. B. The piston contracted. C. The piston is unchanged. C(s)+O2(g)--->CO2(g) A. The piston contracted. B. The piston expanded. C. The piston is unchanged. C2H5OH(g)+3O2(g)--->2CO2(g)+3H2O(g) A. The piston contracted. B. The piston expanded. C. The piston is unchanged.
N2(g)+3H2(g)--->2NH3(g)= B. The piston contracted. C(s)+O2(g)--->CO2(g)= C. The piston is unchanged. C2H5OH(g)+3O2(g)--->2CO2(g)+3H2O(g)= B. The piston expanded. 1. In this reaction, 4 moles of gaseous reactants form 2 moles of gaseous products. The Δn for the reaction is -2. The volume decreases and the piston moves down into the cylinder. 2. In this reaction, 1 mole of a gaseous reactant forms 1 mole of a gaseous product. The Δn for the reaction is 0. The volume stays the same and the piston does not move. 3. In this reaction, 3 moles of gaseous reactants form 5 moles of gaseous products. The Δn for the reaction is +2. The volume increases and the piston moves up the cylinder.
potential energy formula
PE=mgh (mass x gravity x height)
We define the system as the CaCl2 and water, and we define the immediate surroundings as the surface of the hot pack and the hands of the person holding it. With this in mind, please determine the signs qsurr, ΔHrxn, qsys
Positive=qsurr Negitive=ΔHrxn, qsys
Use average bond energies to estimate the value of ΔH for the reaction below. CH4(g)+2O2(g)CO2(g)+2H2O(g)
Reaction:- CH4(g) + 2O2(g) ----> CO2(g) + 2H2O(g) Bond broken:- 4 bonds of C-H and 2 bonds of O=O Bond formed :- 2 bonds of C=O and 4 bonds of O-H Bond. ∆H° (KJ/mol ) C-H 413 O=O 498 C=O 799 O-H 464 ∆H° = sum of bond broken of reactants - sum of bond formed of products = [4×∆H°(C-H)+2×∆H°(O=O)] -[2×∆H°(C=O)+ 4×∆H°(O-H)] = [(4×413)+(2×498)]-[(2×799)+(4×464)] = (1652+ 996)-(1598+1856) = 2648-3454 ∆H°= -806KJ ∆H°=−806KJ
Exothermic
Releases heat(-) During an exothermic process, heat flows out from the system into its surroundings (q < 0).
TRUE or FALSE: kinetic energy and potential energy both state functions?
TRUE: Kinetic energy and potential energy are known as Internal energy which is a state function
If 1495 J of heat is needed to raise the temperature of a 321 g sample of metal from 55.0°C to 66.0°C, what is the specific heat capacity of the metal?
The equation that relates these variables is q = mcsΔT where cs is the specific heat capacity. We know that q = 1495 J, m = 321 g, and ΔΔT = Tfinal - Tinitial or 66.0°C - 55.0°C. Therefore ΔΔT = 11.0°CSolveq = mcsΔΔT1495 J = 321 g ×× cs ×× 11.0°C cs = 0.423 J/g•°C
Which of the following descriptions and/or equations best represent the enthalpy change of a system? There may be more than one correct answer. A. The heat absorbed or released during a phase change or chemical reaction at constant volume B. ΔE = q + w C. The amount of energy required to raise the temperature of 1 mole of a substance by 1°C at constant pressure D. The heat absorbed or released during a phase change or chemical reaction at constant pressure E. The work done on or by a phase change or chemical reaction at constant pressure F. The sum of the internal energy and the pressure-volume product of a system G. ΔH = ΔE + PΔV H. H = E + PV
The heat absorbed or released during a phase change or chemical reaction at constant pressure ΔH = ΔE + PΔV
The heating curve for methanol has a number of interconnected lines. If the temperature is plotted on the y-axis, which line(s) are parallel to the x-axis? Select all that apply. A.the heating of solid methanol from -150°C to -94°CB.the melting of methanol at -94°CC.the heating of gaseous methanol from 65°C to 100°CD.the heating of liquid methanol from -94°C to 65°CE.the boiling of liquid methanol at 65°C
The lines that are parallel to x-axis are - the boiling of liquid methanol at 65 0C and the melting of methanol at -94 0C.
At 100.0°C the molar heat of vaporization of water is 40.67 kJ/mol. At 25.0°C, approximately room temperature, the molar heat of vaporization of water is 44.0 kJ/mol. During a strenuous workout, an athlete generates 1850 kJ of heat energy. What mass of water would have to evaporate from the athlete's skin to dissipate 1850kJ of heat in grams?
The molar heat of the vaporization of water is 44.0kJmole. Generated heat energy = 1850 kJ Mass of water =? We can use this equation to calculate the moles of water that will be vaporized Q=ΔHvap×n Where n is the moles of water Q is the heat transfer ΔHvap Is the molar heat of vaporization Rearrange the equation for the expression of n n=QΔHvap Putting the values in the expression of n n=1850kJ 44.0kJmol n=42.045moles Now, we can use the molar mass of water to calculate the mass of water mass of water=molarmass×moles Since, The molar mass of water = 18.015 g/mole mass of water=18.015gmol×42.045mol mass of water=757.53 Hence, 757.53 g of water will be vaporized.
Why is there a negative sign in front of the PΔV term in ΔE = q −PΔV?
The negative sign indicates that the system loses energy.
The temperature of a 100.0 g sample of water is raised from 30.0°C to 100.0°C. How much energy is required for the temperature change?
To calculate the energy required to heat water use the formula q=mc∆T substitute value in this equation we get the energy required to heat water We know the equation q=mc∆T Where, q= heat in J =? m= mass of water =100g c=specific heat capacity of water =4.184Jg.0C ∆T= change in temperature = Tfinal−Tinitial ∆T=100.00C−30.00 C∆T=70.00C Substitute the value in the above equation q=(100g)×(4.184Jg.0C)×(70.00C)q=29,288J
Atmospheric Research Atmospheric scientists often use balloons to carry their instruments aloft. How much P-V work does a gas in a balloon do on its surroundings at a constant pressure of 1.00 atm if the volume of gas triples from 1.250 ✕ 105 L to 1.050 ✕ 106 L? Express your answer in L ∙ atm and in joules (J).
W is the work done, P is the constant pressure, and ΔV is the volume change. P = 1.00 atm ΔV = V_final - V_initial = 1.050 ✕ 106 L - 1.250 ✕ 105 L. W=(1.00atm)*(1.050x10^6L-1.250x10^5L)=(1.00atm)x(9.25x10^5)=9.25x^L*atm.
If a sample of solid methanol is at -150°C and no heat has been added to the system, what are the coordinates of this initial point on the graph of the heating curve for methanol? A. (0, 150) B. (0, -150) C. (-150, 0) D. (150, 0)
When solid methanol is at,−150 a degree Celsius the starting point of the methanol heating curve is reached. Since no heat has been applied at this point, the substance is still in its solid state, signalling the beginning of the heating process that will move it through several stages. Explanation: The beginning point on the methanol heating curve corresponds to the solid phase at −150°C. The chemical is at its lowest energy state at this point because no heat has been applied. The temperature will progressively increase as heat is applied, and as the methanol absorbs energy it will change states (from solid to liquid to gas), producing a heating curve. Answer: The co-ordinates for the initial point are: (0,−150) To determine and monitor a methanol heating curve: Start with a starting point of solid phase, −150°C, with no additional heat. While measuring temperature changes, gradually apply heat. Observe temperature plateaus at phase changes, such as the change from solid to liquid. Keep going until the methanol turns into a gas. To build the heating curve, which displays phase shifts and temperature variations, plot temperature vs. time/heat input. Explanation: Track the temperature (ordinate) and time/heat input (abscissa) to determine the coordinates on a methanol heating curve. Start from the beginning, when the methanol is solid at −150°C. Keep an eye on temperature variations when heat is introduced. Temperature plateaus at phase transitions (such as solid to liquid); pay attention to these spots. Keep going until the methanol turns into a gas. To build the heating curve, plot temperature vs. time and heat input.
ΔE = q + w
When work is done ON a system, it increases the energy content of the system, and the sign of work is defined to be positive. When work is done BY a system, it decreases the energy content of the system, and the sign of work is defined to be negative. Notice that the sign used for heat and work is always with respect to the system. If the system is gaining energy, the sign will be positive. If the system is losing
A man lifts a 20 lb box from the ground to 1 m above the ground. Heat,q Work,w Surroundings to the system: positive System to surroundings: negative
Work,w Surroundings to the system: positive
A man lifts a 20 lb box from the ground to 1 m above the ground. Heat,q Work,w Surroundings to the system: positive System to surroundings: negative
Work,w System to surroundings: negative
exstensive property
a property that depends on the amount of matter in a sample
intensive property
a property that depends on the type of matter in a sample, not the amount of matter
opened system
a system that can exchange both matter and energy with the surrounding environment
If a system loses heat and has work done on it by the surroundings, what is the sign of ΔE? a. + b. − c. Cannot determine the sign without knowing the magnitudes of q and w.
c. Cannot determine the sign without knowing the magnitudes of q and w.
kinetic energy
energy of motion
Identify the variable associated with each quantity ΔH=ΔE+PΔV enthalpy change = pressure = volume change = internal energy change =
enthalpy change =ΔH pressure =P volume change =ΔV internal energy change =ΔE
exothermic phase changes
freezing, condensation, deposition
What happens to the internal energy of a gas when it expands (with no heat flow)?
its internal energy U decreases
Calorimetry
measurement of heat flow
endothermic phase changes
melting, vaporization, sublimation
Explain how there can be kinetic energy in a stationary ice cube
particles inside ice cubes can possess kinetic energy. Even though the overall movement of the particles is restricted due to the solid state of the ice, the particles still vibrate in place, which indicates the presence of kinetic energy.
calorimetry equation
q=mcΔT
What are the signs for heat and work for a gas that cools and, at the same time, is being compressed? q=+ w=− q=− w=+ q=− w=− q=+ w=+
q=− w=+
enthalpy change
the amount of energy absorbed by a system as heat during a process at constant pressure
Potential Energy (PE)
the energy stored in an object because of its position or composition
9.00 J of heat is added to 10.00 g of gold, which is initially at 25°C. The heat capacity of gold is 25.41 J/(mol • °C). What would be the final temperature of the gold? °C
use molar mass to find moles:(10.00 g Au) (1 mole Au / 197 grams) = 0.05076 moles of golddH = n C dT9.00J = (.05076 moles ) (25.41 J/ mol* degree C) (dT)dT = 6.98 Celsius rise in tempif initially at 25 C, a 6.98 Celsius rise in tempwill bring it up to 31.98 Celsius
A system receives 575 J of heat and delivers 875 J of work to its surroundings. What is the change in internal energy? ΔE = q + w
ΔE = 575J+-875J=-300J
What happens to the magnitude and sign of the enthalpy change when a process is reversed?
ΔH changes sign but keeps its absolute value. When the process is reversed, then the sign of ∆H changes, and becomes −∆H. So the absolute value of ∆H remains the same.
Which equation is used for calculating the enthalpy change of a reaction as estimated by bond enthalpies? ΔHrxn=ΣΔHbonds formed−ΣΔHbonds breaking ΔHrxn=ΣΔHbonds formed+ ΣΔHbonds breaking ΔHrxn=ΣΔHbonds breaking− ΣΔHbonds formed ΔHrxn=ΣΔHbonds breaking+ ΣΔHbonds formed
ΔHrxn=ΣΔHbonds breaking− ΣΔHbonds formed
Solid potassium hydroxide may be used to unclog sink drains. The dissolution of potassium hydroxide in water is quite exothermic. What are the signs of ΔHsys and qsurr? ΔHsys A. cannot be determined B. positive C. negative qsurr A. negative B. cannot be determined C. positive
ΔHsys= negative qsurr= positive Note: The system is the Soild potassium hydroxide and the surrounding is the water in the sink drain.
Gypsum is the common name for calcium sulfate dihydrate (CaSO4 • 2 H2O). When gypsum is heated to 150°C, it loses most of the water in its formula and forms plaster of Paris (CaSO4 • 0.5 H2O): 2(CaSO4•2H2O)(s)2(CaSO4•0.5H2O)(s)+3H2O(g) What is the sign of ΔH for making plaster of Paris from gypsum?
ΔΗ is positive because it requires energy to remove the waters of hydration.
