thermochemistry. what is energy? energy is the ability to do work or produce heat. the law of...
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Chapter 15Thermochemistry
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What is energy? Energy is the ability to do work or produce
heat. The Law of Conservation of Energy:
◦ This law states that can not be created or destroyed only transferred.
Two types of energy:◦ Kinetic◦ Potential
Energy
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Heat is energy transferred from a warmer object to a cooler object.
Heat is represented mathematically as q The amount of heat energy required to raise
the temperature of one gram of pure water by one degree Celsius is called…
a calorie The SI unit for energy is the Joule 1 Joule = 0.2390 calories 1 calorie = 4.184 Joules
Heat
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A breakfast of cereal, orange juice, and milk contains 230 Calories. Convert this amount of energy in to Joules.
9.6 x 105 J
Problems
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Glucose is a simple sugar found in fruit. Burning 1.00 g of glucose releases 15.6 kJ of energy. How many Calories are released?
3.73 Calories
An fruit and oatmeal bar contains 142 Calories. Convert this energy to Joules
5.94 x 105
A chemical reaction releases 86.5 kJ of heat. How many Calories are released?
20.7 Calories
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The specific heat of any substance is the amount of heat required to raise the temperature of one gram of that substance by 1 degree Celsius.
The specific heat of water is: 1 cal/g-oC 4.184 J/g-oC The specific heat of concrete is 0.84 J/g-oC This is why in the summer concrete gets hot
and water says cool.
Specific Heat
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The specific heat of a substance can be used to calculate the heat energy absorbed or given off when that substance changes temperature.
q = (C)(m)(ΔT) remember ΔT = Tfinal – Tinitial Calculate the heat absorbed by a 5 x 103 g
block of concrete when its temperature is raised from 20 oC to 26 oC.
25,000 J or 25 kJ
Using Specific Heat
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The temperature of a sample of iron with a mass of 10.0 g changed from 50.4 oC to 25.0 oC and released 114 J. What is the specific heat of iron?
0.449 J/g-oC
If the temperature of 34.4 g of ethanol increase from 25.0 oC to 78.8 oC, how much heat has been absorbed buy the ethenol. (C for ethanol is 2.44 J/g-oC)
4.52 x 103 J
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A 155 g sample of an unknown substance was heated from 25.0 oC to 40.0 oC. In the process, the substance absorbed 5696 J of energy. What is the specific heat of the substance?
2.45 J/g-oC
A 38.8 g piece of metal alloy absorbs 181 J as its temperature increases from 25 oC to 36 oC. What is the alloy’s specific heat?
0.424 J/g-oC
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Thermochemistry is the study of heat changes during chemical reactions or phase changes.
When studying thermochemistry we look at two things:
System◦ The system is the specific part of the universe
that we are studying. Surroundings
◦ The surroundings are everything else in the universe.
Thermochemistry
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Enthalpy is defined as the heat content of a system at constant pressure.
The change in enthalpy for a reaction is called the enthalpy (heat) of reaction.
ΔHrxn ΔHrxn = Hproducts – Hreactnats
Enthalpy and Enthalpy Change
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A thermochemical equation is a balanced equation that includes the physical states of all reactants and products and the enthalpy change.
4 Fe(s) + 3 O2(g) 2 Fe2O3(s) ΔH = -1625 kJ
NH4NO3(s) NH4+(aq) + NO3
- ΔH = 27 kJ
Thermochemical Equations
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Hess’s Law states that if you can add two or more equations to produce a final equation for a reaction than the sum of the enthalpy changes of the individual reactions is the enthalpy change of the overall reaction.
Hess’s Law
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Calculate ΔH for the reaction 2 H2O2(l) 2 H2O(l) + O2(g)
2 H2(g) + O2(g) 2 H2O(l) ΔH = -572 kJ
H2(g) + O2(g) H2O2(l) ΔH = -188 kJ
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Use equations (a) and (b) to determine ΔH for the following reaction:
2 CO(g) + 2 NO(g) 2 CO2(g) + N2(g)
a) 2 CO(g) + O2(g) 2 CO2(g) ΔH = -566.0 kJ
b) N2(g) + O2(g) 2NO(g) ΔH = -180.6 kJ
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ΔH for the following reaction is -1789 kJ. Use equation (a) to determine ΔH for reaction (b).
4 Al(s) + 3 MnO2(s) 2 Al2O3(s) + 3 Mn(s)
a) 4 Al(s) + 3 O2 2 Al2O3(s) ΔH = -3352 kJ
b) Mn(s) + O2(g) MnO2 ΔH = ?
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The enthalpy of formation for any reaction is defined as the heat change when all reactants are in their elemental form and only one mole of product is produced.
S(s) + 3 F2(g) SF6 ΔHof = -1220 kJ
Sometimes we need to use fractional coefficients.
H2(g) + F2(g) HF ΔHof = -273 kJ
Enthalpy of Formation
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We can use the enthalpy of formation for components of a reaction to calculate the total enthalpy change of the reaction (ΔHrxn).
H2S(g) + 4 F2(g) 2 HF(g) + SF6(g)
a) ½ H2(g) + ½ F2(g) HF ΔHof = -273 kJ
b) S(s) + 3 F2(g) SF6 ΔHof = -1220 kJ
c) H2(g) + S(s) H2S(g) ΔHof = -21 kJ
Using Enthalpy of Formation
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Determine ΔH for CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l)
Using: ΔHo
f(CO2) = -394 kJ ΔHo
f(H2O) = -286 kJ ΔHo
f(CH4) = -75 kJ ΔHo
f(O2) = 0 kJ
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When things rust the reaction taking place is: 4 Fe(s) + 3 O2(g) 2 Fe2O3(s) ΔH = -1625 kJ
Any physical or chemical that occurs with no outside intervention is a spontaneous process.
If we reverse the above reaction: 2 Fe2O3(s) 3 O2(g) + 4 Fe(s) ΔH = 1625 kJ
This process would be non-spontaneous
Reaction Spontaneity
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Entropy is a measure of the number of possible ways a system can be configured.
If we have a piece of paper cut into 8 different sections there would be 56 different ways we could arrange them. (8 x 7)
If we cut the paper into 16 different pieces there would be 240 different ways we could arrange them. (16 x 15)
We have increased the papers entropy.
Entropy
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The second law of thermodynamics states that a spontaneous reaction will always occur in such a way that entropy increases.
Remember that the change in enthalpy (ΔH) is defined as:
Hproducts – Hreactants
Similarly: ΔS = Sproducts – Sreactants
If ΔS is positive the entropy of the system is increasing. If ΔS is negative the entropy of the system is
decreasing.
The Second Law Of Thermodynamics
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• Phase Changes:◦ When a phase change occurs from a more
ordered state to a less ordered state ΔS will be positive.
◦ Solid Liquid ΔS > 0
◦ When a phase change occurs from a less ordered state to a more ordered state ΔS will be negative.
◦ Gas Liquid ΔS < 0• Dissolving a gas into a solvent always
results in a decrease in entropy.
Predicting Entropy Changes
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Assuming no change in physical state, entropy increases when the number of moles of products is greater than the number of moles of reactants.◦ 2 SO3(g) 2 SO2(g) + O2(g)
Entropy increases when a solute dissolves in a solvent.◦ NaCl(s) Na+(aq) + Cl-(aq)
Entropy increases as temperature increases.
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Predict the sign of ΔS for each of the following chemical of physical processes
ClF(g) + F2(g) ClF3(g) ΔS =
NH3(g) NH3(aq) ΔS =
Entropy has the units Joules/Kelvin
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Named after physicist J. Willard Gibbs, free energy is the maximum amount of energy available during a chemical reaction.
Gibbs Free Energy Equation: ΔG = ΔH – TΔS
When a reaction occurs at standard conditions (298 K and 1atm)
ΔGo = ΔHo - ΔSo
Gibbs Free Energy
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ΔG = ΔH – TΔS
A reaction where ΔH is negative and ΔS is positive will always be spontaneous.
N2(g) + 3 H2(g) 2 NH3(g) ΔH = -91.8 kJ ΔS = -197 J/k
ΔG = ΔG = -33.1 kJ
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ΔH ΔS ΔG Reaction Spontaneity
ΔG = ΔH - TΔS
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For a process ΔH is 145 kJ and ΔS is 322 J/K. Calculate ΔG for this reaction at 298 K. Is it spontaneous?