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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

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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

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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

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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

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Heating Curve for Water

https://www.youtube.com/watch?v=lTKl0Gpn5oQ

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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!

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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

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• 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.

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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!!)

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