ch 15 acad.notebook
1
February 26, 2019
Ch. 15 - Energy and Chemical ChangeBIG IDEA:
Chemical reactions usually absorb or release energy.
I CAN:• Calculate the amount of heat absorbed or
released by a substance;• Calculate the change in enthalpy for a chemical
reaction;• Determine how changes in entropy and free
energy affect the spontaneity of reactions.
What is Energy?- the capacity to do work or produce heat- 2 basic forms: potential and kinetic
- cannot be seen, touched, smelled, massed- recognized by its affect on other things
- almost all the energy you use comes from chemical reactions
Law of Conservation of Energy• Energy may change from one form to another but the total quantity of energy does not change
Potential and Kinetic Energy in Chemical Systems
Chemical PE – depends on chemical composition, types of bonds and arrangement of atoms
KE – relates to motion of atoms and is proportional to temperature
Example: When gasoline (octane) is burned in a car's engine, some of the octane's stored energy is used to move the car but much of the potential energy is released as heat.
Measuring Heat Changes – Calorimetry
Heat (q) – energy that flows from a warmer object to a cooler object
Units of heat: joule (J) – SI unit of heat and
energycalorie (cal) – the amount of heat
required to raise the temperature of 1 gram of H2O by 1 °C
1 cal = 4.184 J
Calorie (with a capital C) – the nutritional calorie = 1000 cal or 1 kcal
ch 15 acad.notebook
2
February 26, 2019
1 cal = 4.184 J
1 kJ = 1000 J
1 Calorie = 1 kcal
1 kcal = 1000 cal
`
Example #1:
Given:
Find: ? calories and ? Joules
Example #2:An exothermic reaction releases 125J of heat. How many kcal are released?
Calculating Heat Released or Absorbed
q = m x c x ∆T
• q = heat absorbed or released• m = mass of substance in grams• c = specific heat of a substance (find in a table)• ∆T = Tfinal – Tinitial in °C
ch 15 acad.notebook
3
February 26, 2019
Example: A beaker with 75.0 g water at 20.0 C is heated to 95.0 C. How much heat is absorbed by the water?
A 2.50g nugget of pure gold absorbed 276 J of heat. What was the final temperature of the gold if the initial temperature was 25.0 C? The specific heat of gold is 0.129 J/(gxC)
Calorimetry Problems
Heat Lost by One Substance = Heat Gained by Another Substance
In lab, heat given off by cheese puff = heat gained by water
A calorimeter contains 195 g of water at 20.4 C. A 37.8 g sample of an unknown metal is heated to 133 C and placed into the water in the calorimeter. Heat flows from the metal to the water until both reach a final temperature of 24.6 C. What is the specific heat of the metal?
HEAT LOST BY METAL = HEAT GAINED BY WATER q (metal) = + q (water)
4 Fe (s) + 3 O2 (g) à 2 Fe2O3(s) + heatSystem – the reaction or process you are investigatingSurroundings – everything elseUniverse = System + Surroundings
To measure and study the heat changes that accompany reactions, chemists have defined a property called Enthalpy.Enthalpy (H) – the heat content of a system at constant pressureWe measure the change in enthalpy, ∆Hrxn which is the heat lost or gained by a system during a reaction. At constant pressure: q = ∆Hrxn
Thermochemistry the study of heat changes that accompany chemical reactions and phase changes
ch 15 acad.notebook
4
February 26, 2019
Thermochemical Equations
• 4 Fe (s) + 3 O2 (g) à 2 Fe2O3(s) ∆H=1625 kJ• Balanced Equation• Includes s, l or g• Includes ∆H, the enthalpy change.
• For the reverse rxn, ∆H is equal in magnitude but opposite in sign.
Exothermic Rxns
• Energy is released; a bondmaking process• “exo” – out of; “thermic” – heat; heat flows out of the system•• Ex: Heat pack rxn: heat flows from the heat pack (system) to your cold hands (part of the surroundings)• 4 Fe (s) + 3 O2(g) à 2 Fe2O3 (s) + HEAT
∆H = 1625 kJ∆H is always negative for an exothermic rxn
Endothermic Rxns
• Energy is required; a bondbreaking process• “endo” – into; heat flows into the system
• Ex: Cold Pack reaction: heat flows from the surroundings to the system (cold pack)• NH4NO3(s) + heat à NH4
+(aq) + NO3(aq) ∆Hrxn = +27 kJ
• ∆H is always positive for an endothermic rxn
1. Heat of Vaporization: H2O (l) à H2O (g) ∆Hvap = +40.7 kJ/mol
2. Heat of Fusion (melting) H2O (s) à H2O (l) ∆Hfus = +6.01 kJ/mol
Both of these phase changes ABSORB energy and are ENDOthermic.The reverse phase changes, condensation and freezing, RELEASE energy and are EXOthermic.
3. H2O (g) à H2O (l) ∆Hcond = 40.7 kJ/mol
4. H2O (l) à H2O (s) ∆Hsolid = 6.01 kJ/mol
Thermochemical Equations for Phase Changes
ch 15 acad.notebook
5
February 26, 2019
Heating Curve for Water
https://www.youtube.com/watch?v=lTKl0Gpn5oQ
ch 15 acad.notebook
6
February 26, 2019
15.4 Calculating the Enthalpy Change for a Rxn, ∆H°rxn ° = standard conditions: 1 atm, 25 °C
• ∆H°rxn = ∑ ∆Hf°(products) - ∑ ∆Hf°(reactants)
∆Hf° is the standard enthalpy of formationSee Table C-13, p. 921
What is ∆H°rxn ?H2O2(l) 4 H2O (l) + O2(g)
• Balance equation.• Find ∆Hf° in Table C-13 for each reactant and product.• Multiply each ∆Hf° by # of moles from balanced equation and solve. Remember: Products - Reactants!
ch 15 acad.notebook
7
February 26, 2019
15.5 Reaction SpontaneitySpontaneous Physical and Chemical
ProcessesA spontaneous process is a physical or
chemical change that occurs with no outside intervention.
• SPONTANEOUS PROCESSES:• A waterfall runs downhill• A lump of sugar dissolves in a cup of coffee• At 1 atm, water freezes below 0 0C and ice melts above 0 0C• Heat flows from a hotter object to a colder object• Iron exposed to oxygen and water forms rust
spontaneous
nonspontaneous
Does a decrease in enthalpy mean a reaction proceeds spontaneously?
Spontaneous reactions
We can’t decide whether a rxn is spontaneous solely on the basis of energy changes.
NH4NO3 (s) NH4+(aq) + NO3
(aq)
ENTROPY
Something other than enthalpy plays a role in determining whether or not a reaction occurs spontaneously. That something is called ENTROPY.
ENTROPY (S) is a measure of the randomness or disorder of the particles that make up a system. The greater the disorder of a system, the greater its entropy.
Spontaneous processes always increase the entropy of the universe.
Our slogan: "A messy bedroom is a good thing."
Predicting Changes in Entropy
ch 15 acad.notebook
8
February 26, 2019
• Entropy increases when there are more moles of gas products than gas reactants.2SO3(g) à 2SO2(g) + O2(g)
• As temp. increases, entropy increases because faster moving particles move more randomly.
How does the entropy of a system change for each of the following processes?
(a) Condensing water vapor
Randomness ________ Entropy _______
(b) Forming sucrose crystals from a supersaturated solution
Randomness ________ Entropy _______(c) Heating hydrogen gas from 600C to 800C
Randomness ________ Entropy _______
(d) Subliming dry ice
Randomness ________ Entropy _______
1. Imagine tossing two coins in the air.a. Predict the distribution of various combinations of heads and tails.b. What is the probability of the result being two heads?c. What is the most probable result?
Now imagine tossing three coins in the air.d. What is the probability of a three heads result?e. Which system has the highest entropy, the twocoin system or the threecoin
system?
Gibbs Free Energy –the energy available to do
useful work
∆G = ∆Hsys T∆Ssys
Gibbs free energy (G)
∆G < 0 The reaction is spontaneous in the forward direction.∆G > 0 The reaction is nonspontaneous as written. The reaction is spontaneous in the reverse direction.∆G = 0 The reaction is at equilibrium.
The sign of ∆G tells you if the reaction is spontaneous.
ch 15 acad.notebook
9
February 26, 2019
Gibb‛s Equation∆G = ∆Hsys -T∆Ssys
Example:
N2 (g) + 3 H2(g) à 2 NH3 (g)Is this rxn spontaneous?
Given: ∆H = -91.8 kJ, ∆S = -197 J/K and T=25 CCalculate: ∆G (WATCH UNITS!!)
ch 15 acad.notebook
10
February 26, 2019