Chapter 17 Energy and Chemical change Pre-AP Chemistry BMHS Anahit Pivazyan.
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Transcript of Chapter 17 Energy and Chemical change Pre-AP Chemistry BMHS Anahit Pivazyan.
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Chapter 17“Energy and
Chemical change”
Pre-AP ChemistryBMHS
Anahit Pivazyan
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Section 17.1The Flow of Energy – Heat and Work
OBJECTIVES:
•Explain how energy, heat, and work are related.
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Section 17.1The Flow of Energy – Heat and Work
OBJECTIVES:
•Classify processes as either exothermic or endothermic.
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Section 17.1The Flow of Energy – Heat and Work
OBJECTIVES:
• Identify the units used to measure heat transfer.
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Section 17.1The Flow of Energy – Heat and Work
OBJECTIVES:
•Distinguish between heat capacity and specific heat capacity (also called simply specific heat).
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Energy Transformations “Thermochemistry” - concerned with
heat changes that occur during chemical reactions
Energy - capacity for doing work or supplying heat
• weightless, odorless, tasteless
• if within the chemical substances- called chemical potential energy
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Energy Transformations Gasoline contains a significant
amount of chemical potential energy Heat - represented by “q”, is energy
that transfers from one object to another, because of a temperature difference between them.
• only changes can be detected!
• flows from warmer cooler object
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Exothermic and Endothermic Processes
Essentially all chemical reactions and changes in physical state involve either:a) release of heat, orb) absorption of heat
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Exothermic and Endothermic Processes
In studying heat changes, think of defining these two parts:
• the system - the part of the universe on which you focus your attention
• the surroundings - includes everything else in the universe
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Exothermic and Endothermic Processes
Together, the system and it’s surroundings constitute the universe
Thermochemistry is concerned with the flow of heat from the system to it’s surroundings, and vice-versa.
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Exothermic and Endothermic Processes
The Law of Conservation of Energy states that in any chemical or physical process, energy is neither created nor destroyed.
• All the energy is accounted for as work, stored energy, or heat.
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Exothermic and Endothermic Processes Heat flowing into a system from it’s
surroundings:
• defined as positive
• q has a positive value
• called endothermic
–system gains heat (gets warmer) as the surroundings cool down
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Exothermic and Endothermic Processes
Heat flowing out of a system into it’s surroundings:
• defined as negative
• q has a negative value
• called exothermic
–system loses heat (gets cooler) as the surroundings heat up
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Exothermic and Endothermic Fig. 17.2, page 506 - on the left, the
system (the people) gain heat from it’s surroundings (the fire)
• this is endothermic (q is positive) On the right, the system (the body)
cools as perspiration evaporates, and heat flows to the surroundings
• this is exothermic (q is negative)
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Exothermic and Endothermic Every reaction has an energy
change associated with it Exothermic reactions release energy,
usually in the form of heat. Endothermic reactions absorb
energy Energy is stored in bonds between
atoms
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Units for Measuring Heat Flow
1) A calorie is defined as the quantity of heat needed to raise the temperature of 1 g of pure water 1 oC.
• Used except when referring to food• a Calorie, (written with a capital C),
always refers to the energy in food• 1 Calorie = 1 kilocalorie = 1000 cal.
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Units for Measuring Heat Flow2) The calorie is also related to the Joule,
the SI unit of heat and energy
• named after James Prescott Joule
• 4.184 J = 1 cal Heat Capacity - the amount of heat
needed to increase the temperature of an object exactly 1 oC
• Depends on both the object’s mass and its chemical composition
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Heat Capacity and Specific Heat Specific Heat Capacity
(abbreviated “C”) - the amount of heat it takes to raise the temperature of 1 gram of the substance by 1 oC
• often called simply “Specific Heat”• Note Table 17.1, page 508 (next slide)
Water has a HUGE value, when it is compared to other chemicals
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Table of Specific HeatsNote the tremendous difference in
Specific Heat.
Water’s value is
VERY HIGH.
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Heat Capacity and Specific Heat
For water, C = 4.18 J/(g oC) in Joules, and C = 1.00 cal/(g oC) in calories.
Thus, for water:
• it takes a long time to heat up, and
• it takes a long time to cool off! Water is used as a coolant!
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Heat Capacity and Specific Heat
To calculate, use the formula: q = mass (in grams) x T x C
heat is abbreviated as “q” T = change in temperature C = Specific Heat
• Units are either: J/(g oC) or cal/(g oC)
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Section 17.2Measuring and Expressing
Enthalpy Changes OBJECTIVES:
•Describe how calorimeters are used to measure heat flow.
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Section 17.2Measuring and Expressing
Enthalpy Changes OBJECTIVES:
•Construct thermochemical equations.
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Section 17.2Measuring and Expressing
Enthalpy Changes OBJECTIVES:
•Solve for enthalpy changes in chemical reactions by using heats of reaction.
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Calorimetry Calorimetry - the measurement of the
heat into or out of a system for chemical and physical processes.
• Based on the fact that the heat released = the heat absorbed
The device used to measure the absorption or release of heat in chemical or physical processes is called a “Calorimeter”
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Calorimetry Foam cups are excellent heat
insulators, and are commonly used as simple calorimeters under constant pressure.• See picture on the next slide
For systems at constant pressure, the “heat content” is the same as a property called Enthalpy (H) of the system
(They are good because they are well-insulated.)
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A foam cup calorimeter –
here, two cups are nestled
together for better
insulation
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Calorimetry Changes in enthalpy = H q = H These terms will be used
interchangeably in this textbook Thus, q = H = m x C x T H is negative for an exothermic
reaction H is positive for an endothermic
reaction
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Calorimetry Calorimetry experiments can be
performed at a constant volume using a device called a “bomb calorimeter” - a closed system
• Used by nutritionists to measure energy content of food
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A Bomb Calorimeter
A bomb calorimeter
http://www.chm.davidson.edu/ronutt/che115/Bomb/Bomb.htm
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C + O2 → CO2E
nerg
y
Reactants Products
C + O2
CO2
395kJ given off
+ 395 kJ
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Exothermic The products are lower in
energy than the reactants Thus, energy is released. ΔH = -395 kJ
• The negative sign does not mean negative energy, but instead that energy is lost.
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CaCO3 → CaO + CO2E
nerg
y
Reactants Products
CaCO3
CaO + CO2
176 kJ absorbed
CaCO3 + 176 kJ → CaO + CO2
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Endothermic The products are higher in
energy than the reactants Thus, energy is absorbed. ΔH = +176 kJ
• The positive sign means energy is absorbed
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Chemistry Happens in
MOLES An equation that includes energy is
called a thermochemical equation CH4 + 2O2 CO2 + 2H2O + 802.2 kJ
• 1 mole of CH4 releases 802.2 kJ of energy.
• When you make 802.2 kJ you also make 2 moles of water
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Thermochemical Equations The heat of reaction is the heat
change for the equation, exactly as written• The physical state of reactants
and products must also be given.
• Standard conditions (SC) for the reaction is 101.3 kPa (1 atm.) and 25 oC (different from STP)
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CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) + 802.2 kJ
If 10. 3 grams of CH4 are burned completely, how much heat will be produced?
10. 3 g CH4
16.05 g CH4
1 mol CH4
1 mol CH4
802.2 kJ
= 514 kJ
ΔH = -514 kJ, which means the heat is released for the reaction of 10.3 grams CH4
Ratio from balanced equation
1
Start with known valueConvert to moles Convert moles to desired unit
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Enthalpy The heat content a substance has at a
given temperature and pressure• Can’t be measured directly because
there is no set starting point The reactants start with a heat content The products end up with a heat content So we can measure how much enthalpy
changes
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Enthalpy Symbol is H Change in enthalpy is H (delta H) If heat is released, the heat content of
the products is lower
H is negative (exothermic) If heat is absorbed, the heat content
of the products is higher
H is positive (endothermic)
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Ene
rgy
Reactants Products
Change is down
ΔH is <0= Exothermic (heat is given off)
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Ene
rgy
Reactants Products
Change is upΔH is > 0
= Endothermic (heat is absorbed)
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Heat of Reaction The heat that is released or absorbed in a
chemical reaction Equivalent to H C + O2(g) CO2(g) + 393.5 kJ
C + O2(g) CO2(g) H = -393.5 kJ
In thermochemical equation, it is important to indicate the physical state
a) H2(g) + 1/2O2 (g) H2O(g) H = -241.8 kJ
b) H2(g) + 1/2O2 (g) H2O(l) H = -285.8 kJ
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Heat of Combustion The heat from the reaction that
completely burns 1 mole of a substance:
C + O2(g) CO2(g) + 393.5 kJ
C + O2(g) CO2(g) H = -393.5 kJ