Download - Chemistry and Energy
How are they related?
Energy Encountered Daily
What is Energy?
Defined as the ability to do work or create heat.
Many types of energy Thermal Light Gravitational Kinetic Potential
Light Energy Review
How is light energy produced? Electrons release light energy when
they fall from a high energy level to a lower energy.
We’re now going to talk about energy released or used in a chemical reaction. Heat energy
Thermochemistry
The study of heat used or released in a chemical reaction.
Let’s investigate heat as it compares to temperature using the Heat vs. Temperature Handout
Specific Heat Calculations q = mCΔT
q = heat (J or cal or Cal)4.184 cal = 1 Joule1000 cal = 1 Cal (dietary calorie)m = mass (g)C = specific heat (J/g oC or cal/g ooC)ΔT = change in temperature (o C or K) = Tf - Ti
Specific Heat
Specific heat of water = 1 cal /g o C or = 4.184 J / goC
Specific heat of most metals = < 1 J / goC
Do metals heat slowly or quickly compared to water?
Do metals stay warm longer or shorter than water?
Practice Problem
How much energy is required to heat 120.0 g of water from 2.0 oC to 24.0oC?
q = mCΔTm= 120.0 gC = 4.184 J/goCΔT= (24.0 – 2.0)oC = 22.0oC
q = (120.0g)(4.184 J/goC)(22.0oC) =
Practice Problem
How much heat (in kJ) is given off when 85.0 g of lead cools from 200.0oC to 10.0 oC? (Specific heat of lead = 0.129 J/g oC)
q = mCΔTm = 85.0 gC = 0.129 J/g oCΔT = (10.0 – 200.0)oC = - 190.0oC
q = (85.0 g)(0.129 J/g oC)(- 190.0oC) = -
How Do Chemical Reactions Create Heat energy? Consider the combustion of gasoline
(octane)2 C8H18 +25 O2 16 CO2 +18 H2O
Potential Energy: Stored energy Potential energy is stored in the bonds of the
reactant s and the products When bonds are broken, the energy is available When produce bonds form, some energy is
used in these bonds The excess energy is released as heat
Kinetic Energy
Directly related to temperature
Is Heat Used or Released? Endothermic reactions used heat
from the surroundings Sweating Refrigeration
Exothermic heat releases heat to the surroundings Hot hands Combustion Exercise
Endothermic Reactions
Decrease in kinetic energy decrease in temperature heat will transfer from the environment to the system resulting in a cooler environment
Absorbs heat from its surrounding. The system gains heat Positive value for q H = q = 0 Hproducts Hreactants
Exothermic Reactions
Increase in kinetic energy increase in temperature of system heat released to the environment resulting in a hotter environment
Releases heat to its surroundings The system loses heat Negative value for q H = q = 0 Hproducts Hreactants
Enthalpy
Heat content for systems at constant pressure
Symbol is H Terms heat and enthalpy are used
interchangeably for this course H = q = m C T
Heat moves from ________ to ___________.
Law of Conservation of Energy
Energy is not lost or gained in a chemical reaction
In a chemical reaction potential energy is transferred to kinetic energy
Thermochemical Equations
An equation that includes the heat change
Example: write the thermochemical equation for this reaction CaO(s) + H2O(l) Ca(OH)2(s) H = -
65.2 kJ
CaO(s) + H2O(l) Ca(OH)2(s) + 65.2 kJ
Stoichiometry and Thermochemistry
Tin metal can be extracted from its oxide according to the following reaction:
SnO2(s) + 4NO2(g) + 2H2O(l) + 192 kJ Sn(s) + 4HNO3(aq)
How much energy will be required to extract 59.5 grams of tin?
How to solve
1. Use your stoichiometry2. Treat heat as a reactant or
productSnO2(s) + 4NO2(g) + 2H2O(l) +
192 kJ Sn(s) + 4HNO3(aq)59.5 g Sn 1 mol Sn 192 kJ 1 g Sn 1 mol Sn
If an Object feels hot, it means it is undergoing a change with a H that is:
a. positiveb. negativec. whether the object feels hot
or not is unrelated to its Hd. I don’t know
If the object feels hot, it means it is undergoing:
a. an exothermic reactionb. an endothermic reactionc. whether it feels hot or not is
unrelated to whether it is undergoing an exothermic or an endothermic change
How does ice melt?
Molar Heat of Fusion
Heat absorbed by one mole of a substance during melting
Constant temperature Hfus H2O(s) H2O(l) H = 6.01
kJ/mol
Molar Heat of Solidification Heat lost when 1 mole of a liquid
solidifies Temperature is constant Hsolid Hfus = -Hsolid
H2O(l) H2O(s) H = -6.01 kJ/mol
Molar Heat of Vaporization Heat needed to vaporize 1 mole
of a liquid Hvap H2O(l) H2O(g) Hvap = 40.7
kJ/mol
Molar Heat of Condensation Heat released when 1 mole of
vapor condenses Hcond
H2O(g) H2O(s) Hcond = -40.7 kJ/mol
Hvap = -Hcond
Phase Change Diagram for Water
Phase Change Diagram
The House that Heats Itself http://www.sciencefriday.com/videos/
watch/10007
Calorimetry
Method used to determine the heat involved in a physical or chemical change.
Relies on the law of conservation of energy
Calorimeter
Simple Calorimeter
Calorimetry Math Heat gained by the water = q Heat lost by the system = -q
mCT = q T = Tf –Ti , m = mass, C = specific heat
q gained by water = q lost by system q water = - q system mCT = -mCT(mass H2O)(spec. heat H2O)(T H2O) = -
(mass sys)(spec. heat sys)(T sys)
Standard Heat of Reaction Heat change for the equation as it is
writtenH = Hf(products) - Hf(reactants)
Standard Heats of Formation (Hf) Change in enthalpy when 1 mole
of the compound is formed from its elements in their standard states at 25oC and 101.3 kPa
Hess’s Law
A way to calculate the heat of a reaction that may be too slow or too fast to collect data from.
Add together several reactions that will result in the desired reaction. Add the ΔH for these reactions in the same way.
Htotal = Hproducts - Hreactants