Energy Chapter 10. 10.1 The Nature of Energy Energy – the ability to do work or produce heat Energy – the ability to do work or produce heat Potential.

EnergyEnergy

Chapter 10Chapter 10

10.1 The Nature of Energy10.1 The Nature of Energy Energy – the ability to do work or produce heatEnergy – the ability to do work or produce heat Potential Energy – due to position or compositionPotential Energy – due to position or composition Kinetic Energy – due to motionKinetic Energy – due to motion

Depends on the mass of the object (m) and its velocity (v)Depends on the mass of the object (m) and its velocity (v) KE = ½ mvKE = ½ mv22

The Law of Conservation of EnergyThe Law of Conservation of Energy

Energy can be Energy can be converted from one converted from one form to another but can form to another but can be neither created nor be neither created nor destroyeddestroyed

The energy in the The energy in the universe is constantuniverse is constant

WorkWork Work = Force x distanceWork = Force x distance

W = FdW = Fd Frictional Heating – 2 surfaces in contact with each Frictional Heating – 2 surfaces in contact with each

otherother Depends on surface and force pushing the surfaces togetherDepends on surface and force pushing the surfaces together

State FunctionState Function The property of the system that changes The property of the system that changes

independently of its pathwayindependently of its pathway The pathway is how you get thereThe pathway is how you get there

ExampleExample If you travel from Chicago to Denver what are If you travel from Chicago to Denver what are

state functions?state functions? The route you take to get there is your pathway, so The route you take to get there is your pathway, so

it is not a state functionit is not a state function Change in elevation doesn’t depend on how you Change in elevation doesn’t depend on how you

get there so it is a state functionget there so it is a state function

10.2 Temperature and Heat10.2 Temperature and Heat

Temperature – Measure of the random motion of the Temperature – Measure of the random motion of the components of a substancecomponents of a substance

Heat – The flow of energy due to a difference in Heat – The flow of energy due to a difference in temperaturetemperature

10.3 Exothermic and Endothermic 10.3 Exothermic and Endothermic ProcessesProcesses

System – part of universe System – part of universe we are looking atwe are looking at

Surroundings – everything Surroundings – everything elseelse

Exothermic – energy flows Exothermic – energy flows out of a systemout of a system

Endothermic – energy Endothermic – energy flows into a systemflows into a system

Where does energy as heat come from in Where does energy as heat come from in exothermic reactions?exothermic reactions?

It depends on the potential energy between the It depends on the potential energy between the products and reactantsproducts and reactants

10.4 Thermodynamics10.4 Thermodynamics

Law of Conservation of Energy (a.k.a. The Law of Conservation of Energy (a.k.a. The First Law of Thermodynamics)First Law of Thermodynamics) Energy can neither be created nor destroyed under Energy can neither be created nor destroyed under

normal conditionsnormal conditions The energy of the universe is constantThe energy of the universe is constant

E = internal energyE = internal energy

E is the sum of the kinetic energy and the potential E is the sum of the kinetic energy and the potential energyenergy

Can be changed by the flow of work, heat, or bothCan be changed by the flow of work, heat, or both ∆ ∆ = change in; called “delta”= change in; called “delta” w = workw = work q = heatq = heat ∆∆E = q + wE = q + w Change in internal energy equals heat plus workChange in internal energy equals heat plus work

Thermodynamic quantities are made up of a Thermodynamic quantities are made up of a number that shows magnitude and a sign that number that shows magnitude and a sign that shows whether energy is flowing into the shows whether energy is flowing into the system (endothermic = + ) or out of the system system (endothermic = + ) or out of the system (exothermic = - )(exothermic = - )

10.5 Measuring Energy Changes10.5 Measuring Energy Changes

calorie = amount of energy required to raise calorie = amount of energy required to raise the temperature of 1 gram of water by one the temperature of 1 gram of water by one degree Celsiusdegree Celsius

1000 calories (1 kilocalorie) is what we refer 1000 calories (1 kilocalorie) is what we refer to as a “Calorie” with a capital Cto as a “Calorie” with a capital C

1 calorie = 4.184 joules1 calorie = 4.184 joules 1 cal = 4.184 J1 cal = 4.184 J

To go from calories to joules multiply by 4.184To go from calories to joules multiply by 4.184 To go from joules to calories divide by 4.184To go from joules to calories divide by 4.184

And now for a problem!And now for a problem! How much heat, in joules, is required to raise the How much heat, in joules, is required to raise the

temperature of 7.40 g water from 29.0 temperature of 7.40 g water from 29.0 °C to 46.0 °C?°C to 46.0 °C? We know we need 4.184 J of energy raise 1 g of We know we need 4.184 J of energy raise 1 g of

water 1 °C water 1 °C We have 7.40 g of water so it will take 7.4 g x 4.184 J We have 7.40 g of water so it will take 7.4 g x 4.184 J

to raise it 1 °C to raise it 1 °C We also need to raise the temperature 17 °C so 17.0 We also need to raise the temperature 17 °C so 17.0

°C x 7.4 g x 4.184 J/ g x °C °C x 7.4 g x 4.184 J/ g x °C So we need 526 J of energySo we need 526 J of energy Now try thisNow try this

Calculate the joules of energy required to heat 454 g of Calculate the joules of energy required to heat 454 g of water from 5.4 °C to 98.6 °C?water from 5.4 °C to 98.6 °C?

So we know that the amount of energy we So we know that the amount of energy we need to raise the temperature of a substance need to raise the temperature of a substance depends on the amount of substance and the depends on the amount of substance and the change in temperaturechange in temperature

But the substance also plays a big partBut the substance also plays a big part Specific Heat Capacity = the amount of energy Specific Heat Capacity = the amount of energy

needed to raise the temperature of 1 g of a needed to raise the temperature of 1 g of a substance 1 substance 1 °C°C

Specific HeatsSpecific Heats

Liquid water = 4.184 JLiquid water = 4.184 J Aluminum = 0.89 JAluminum = 0.89 J Gold = 0.13 JGold = 0.13 J This explains why certain things heat up faster This explains why certain things heat up faster

than othersthan others The pot heats up faster than the water in itThe pot heats up faster than the water in it The water in the pool is colder that the cement The water in the pool is colder that the cement

around itaround it

Now for another equationNow for another equation The amount of energy required = the specific heat x The amount of energy required = the specific heat x

mass x change in temperaturemass x change in temperature Q = m x CQ = m x Cpp x x ∆∆TT Try this sampleTry this sample

A 1.6 g sample of metal that looks like gold requires 5.8 A 1.6 g sample of metal that looks like gold requires 5.8 J of energy to change its temperature from 23 J of energy to change its temperature from 23 °C to 41 °C to 41 °C. Is the metal gold? (Hint – you are finding what s is °C. Is the metal gold? (Hint – you are finding what s is and comparing to what you know about gold’s specific and comparing to what you know about gold’s specific heat)heat)

Answer = No; Gold’s s = 0.13 J/ g °C but this Answer = No; Gold’s s = 0.13 J/ g °C but this substance has an s = 0.20 J / g °C substance has an s = 0.20 J / g °C

10.6 Thermochemistry (Enthalpy)10.6 Thermochemistry (Enthalpy) Enthalpy (symbol = H) is the same as the flow Enthalpy (symbol = H) is the same as the flow

of heatof heat ∆∆HHpp = heat = heat

P P tells us it occurred under constant pressuretells us it occurred under constant pressure

∆ ∆ means “change in”means “change in” So the enthalpy for a reaction at constant pressure So the enthalpy for a reaction at constant pressure

is the same as heatis the same as heat

CalorimetryCalorimetry

Calorimeter = device used to determine the Calorimeter = device used to determine the heat associated with a chemical reactionheat associated with a chemical reaction

Reaction is run in calorimeter and temperature Reaction is run in calorimeter and temperature change is observedchange is observed

We can use calorimeter to find We can use calorimeter to find ∆H ∆H Once we know ∆H for some reactions we can Once we know ∆H for some reactions we can

use those to calculate ∆H for other reactionsuse those to calculate ∆H for other reactions

10.7 Hess’s Law10.7 Hess’s Law The change in enthalpy for a given process is The change in enthalpy for a given process is

independent of the pathway for the process (this independent of the pathway for the process (this means it is a state function)means it is a state function)

Hess’s Law states that the change in enthalpy from Hess’s Law states that the change in enthalpy from reactants to products in a reaction is the same whether reactants to products in a reaction is the same whether it takes place in one step or a series of stepsit takes place in one step or a series of steps

NN22 + 2O + 2O22 → 2NO→ 2NO22 ∆H = ∆H = 68 kJ68 kJoror NN22 + O + O22 → 2NO→ 2NO ∆H = 180 kJ∆H = 180 kJ 2NO + O2NO + O22 → 2NO → 2NO22 ∆H = -112 kJ ∆H = -112 kJ So 180 kJ + (-112 kJ) = ∆H = 68 kJSo 180 kJ + (-112 kJ) = ∆H = 68 kJ

Characteristics of Enthalpy ChangesCharacteristics of Enthalpy Changes

If a reaction is reversed, If a reaction is reversed, ∆H is reversed∆H is reversed Xe + 2FXe + 2F22 → XeF → XeF44 ∆H = -251 kJ ∆H = -251 kJ

XeFXeF4 4 → Xe + 2F→ Xe + 2F22 ∆H = +251 kJ ∆H = +251 kJ

Magnitude of ∆H is proportional to quantities Magnitude of ∆H is proportional to quantities of reactants and productsof reactants and products Xe + 2FXe + 2F22 → XeF → XeF44 ∆H = -251 kJ ∆H = -251 kJ

2(Xe + 2F2(Xe + 2F22 → XeF → XeF44)) ∆H = -502 kJ ∆H = -502 kJ

10.8 Quality versus Quantity of 10.8 Quality versus Quantity of EnergyEnergy

One of the most important characteristics is that One of the most important characteristics is that it is conservedit is conserved

Eventually all energy will take the form of heat Eventually all energy will take the form of heat and spread evenly throughout the universe and and spread evenly throughout the universe and everything will be the same temperatureeverything will be the same temperature

This means work won’t be able to be done and This means work won’t be able to be done and universe will be dead; called “heat death”universe will be dead; called “heat death”

We care more about what kind of energy We care more about what kind of energy (quality) than the amount of energy (quantity)(quality) than the amount of energy (quantity)

10.9 Energy and Our World10.9 Energy and Our World Fossil Fuels formed by decaying products of Fossil Fuels formed by decaying products of

plantsplants PetroleumPetroleum Natural GasNatural Gas CoalCoal

Greenhouse Effect – Visible light travels Greenhouse Effect – Visible light travels through atmosphere, converted to infrared through atmosphere, converted to infrared radiation (heat) which is absorbed by certain radiation (heat) which is absorbed by certain molecules, Hmolecules, H220 and CO0 and CO22 mainly, which radiate mainly, which radiate

it back to earthit back to earth

10.10 Energy as a Driving Force10.10 Energy as a Driving Force Energy Spread – in any given process, Energy Spread – in any given process,

concentrated energy is dispersed widelyconcentrated energy is dispersed widely Happens with every exothermic reactionHappens with every exothermic reaction When gas is burned, energy stored is dispersed When gas is burned, energy stored is dispersed

into surrounding airinto surrounding air Matter Spread – molecules of a substance are Matter Spread – molecules of a substance are

spread out and occupy a larger volumespread out and occupy a larger volume Salt dissolves in water due to matter spreadSalt dissolves in water due to matter spread

These 2 processes are important driving forces These 2 processes are important driving forces that cause events to occurthat cause events to occur

EntropyEntropy

Invented function that keeps track of disorderInvented function that keeps track of disorder Entropy (S) is a Entropy (S) is a measure of disorder or measure of disorder or

randomnessrandomness So a cube of ice has a a lower S value than So a cube of ice has a a lower S value than

steamsteam Energy spread and Matter spread lead to Energy spread and Matter spread lead to

greater entropygreater entropy The entropy in the universe is always The entropy in the universe is always

increasingincreasing