Chapter 2 Section Chapter 2 Section 11

ENERGYENERGY

EnergyEnergy

• The ability to do work or cause The ability to do work or cause change.change. (the first part is more a (the first part is more a physics idea and the second a physics idea and the second a chemistry)chemistry)– Example: Energy is released in the Example: Energy is released in the

explosive reaction that occurs between explosive reaction that occurs between hydrogen and oxygen to form waterhydrogen and oxygen to form water

Physical ChangesPhysical Changes

• Review from CH. 1Review from CH. 1::• A change of matter from one form to A change of matter from one form to

another without a change in chemical another without a change in chemical properties.properties.

• Examples: Ice melting and water boilingExamples: Ice melting and water boiling– It’s still water (meaning the identity has It’s still water (meaning the identity has

not changed 2 hydrogen atoms are bonded not changed 2 hydrogen atoms are bonded to 1 oxygen atom)! Only the physical state to 1 oxygen atom)! Only the physical state has changed!has changed!

Chemical ChangeChemical Change

• A change that occurs when one or A change that occurs when one or more substances change into an more substances change into an entirely entirely newnew substance with substance with differentdifferent properties. properties.

• When a new substance is formed you When a new substance is formed you have a chemical change which is have a chemical change which is also a chemical reaction.also a chemical reaction.

Every Change in Matter Every Change in Matter involves a Change in Energyinvolves a Change in Energy

• AllAll physical and chemical changes involve a physical and chemical changes involve a change in energychange in energy

• Evaporation – the change of a substance Evaporation – the change of a substance from a liquid to a gas. This change requires from a liquid to a gas. This change requires an an INPUTINPUT of energy. of energy.

• Condensation – when a vapor turns into a Condensation – when a vapor turns into a liquid. The change requires a liquid. The change requires a releaserelease of of energy. energy. – When hydrogen and oxygen react to form water When hydrogen and oxygen react to form water

(chemical change) there is an explosion that (chemical change) there is an explosion that signifies that energy is being released. signifies that energy is being released.

Reaction Types based on Reaction Types based on energyenergy

• We can classify reactions as We can classify reactions as endothermicendothermic or or exothermicexothermic depending upon whether it absorbs depending upon whether it absorbs or releases energy.or releases energy.

Endothermic ProcessEndothermic Process

• Any change in matter in which energy is Any change in matter in which energy is absorbed.absorbed.

• Examples: Melting of ice, boiling of Examples: Melting of ice, boiling of water (physical changes)water (physical changes)

• Examples: barium hydroxide + Examples: barium hydroxide + ammonium nitrate (chemical change)ammonium nitrate (chemical change)

• https://www.youtube.com/watch?https://www.youtube.com/watch?v=MyAzjSdc3Fcv=MyAzjSdc3Fc

Both require an input of energyBoth require an input of energy

Exothermic reactionExothermic reaction

• Any change in matter in which energy Any change in matter in which energy is RELEASED.is RELEASED.

• Examples: Freezing water, Examples: Freezing water, condensation of water vapor (physical condensation of water vapor (physical changes)changes)

• Example: Hydrogen + Oxygen Example: Hydrogen + Oxygen (chemical change)(chemical change)

https://www.youtube.com/watch?https://www.youtube.com/watch?v=9_Nk_5Ui8Pov=9_Nk_5Ui8Po

Law of Conservation of Law of Conservation of EnergyEnergy

• During any physical or chemical During any physical or chemical change, the total quantity of energy change, the total quantity of energy remains constants.remains constants.

• Energy cannot be created or Energy cannot be created or destroyed but can be changed from destroyed but can be changed from one form to anotherone form to another

• Pg.41 Figure 4Pg.41 Figure 4

Energy is often Energy is often transferredtransferred

• SYSTEM – All the components that are SYSTEM – All the components that are being studied at any given time.being studied at any given time.

• SURROUNDINGS – Everything outside the SURROUNDINGS – Everything outside the system.system.

• Energy is often transferred back and forth Energy is often transferred back and forth between a system and its surroundingsbetween a system and its surroundings

• Exothermic process – Transfer of energy Exothermic process – Transfer of energy from the system to the surroundingsfrom the system to the surroundings

• Endothermic process – Transfer of energy Endothermic process – Transfer of energy from the surroundings to the systemfrom the surroundings to the system

Energy can be transferred Energy can be transferred in Different formsin Different forms

• Energy, light, mechanical, heat, Energy, light, mechanical, heat, electrical, sound are different forms electrical, sound are different forms of energy.of energy.

• Thus, energy can be transferred in Thus, energy can be transferred in different forms.different forms.

• Photosynthesis – light energy is Photosynthesis – light energy is transferred from the sun to green transferred from the sun to green plants.plants.

HEATHEAT

• The energy The energy transferred between objects transferred between objects that are at different that are at different temperaturestemperatures..

• Do you remember the direction heat Do you remember the direction heat always flows?always flows?– Warmer object to a cooler objectWarmer object to a cooler object

THE MOST COMMON TRANSFERS OF THE MOST COMMON TRANSFERS OF ENERGY IN CHEMISTRY ARE THOSE THAT ENERGY IN CHEMISTRY ARE THOSE THAT INVOLVE HEATINVOLVE HEAT..

Energy can be released or Energy can be released or absorbed as heatabsorbed as heat

• If energy is released during a reaction - If energy is released during a reaction - exothermicexothermic– Ammonium Nitrate being ignitedAmmonium Nitrate being ignited

http://www.youtube.com/watch?http://www.youtube.com/watch?v=vYKbBavXriY&safe=activev=vYKbBavXriY&safe=active

• If energy is absorbed during a reaction – If energy is absorbed during a reaction – the reaction is classified as endothermicthe reaction is classified as endothermic– Baking bread Baking bread

Heat is different from Heat is different from TemperatureTemperature

• A measurement of the average A measurement of the average kinetic energy of the random motion kinetic energy of the random motion of particles in a substance.of particles in a substance.

• Think of heat as the energy that is Think of heat as the energy that is transferred from a stove to water transferred from a stove to water due to a temperature difference.due to a temperature difference.

Temperature is Expressed Temperature is Expressed Using Different ScalesUsing Different Scales

• Fahrenheit scale is not used in chemistryFahrenheit scale is not used in chemistry• The SI unit for Temperature is the The SI unit for Temperature is the

KelvinKelvin,K,K• Zero on the Celsius scale is the freezing Zero on the Celsius scale is the freezing

point for waterpoint for water• Zero on the Kelvin scale is Zero on the Kelvin scale is absolute zero,absolute zero,

the temperature at which the minimum the temperature at which the minimum average kinetic energies of all particles average kinetic energies of all particles occur.occur.

Converting between the Converting between the Celsius and Kelvin scaleCelsius and Kelvin scale

• t(t(oo F) = (t ( F) = (t (o o C) x 1.8) + 32 C) x 1.8) + 32 converting celsius to converting celsius to fahrenheit fahrenheit

• t(t(ooC) = (t(C) = (t(ooF) – 32) / 1.8 F) – 32) / 1.8 converting fahrenheit to celsius converting fahrenheit to celsius

• t(t(oo C) = t (K) -273.15 K C) = t (K) -273.15 K converting Kelvin to celsiusconverting Kelvin to celsius

• t(K) = t (t(K) = t (oo C) + 273.15 C) + 273.15 ooC C converting celsius to Kelvinconverting celsius to Kelvin

Problems: 25 Problems: 25 ooC to KC to K

300 K to 300 K to ooCC

Specific HeatSpecific Heat

• The quantity of heat required to raise one The quantity of heat required to raise one gram of a substance by one degree celsius.gram of a substance by one degree celsius.

• Metals tend to have low specific heats (What Metals tend to have low specific heats (What does having a low specific heat mean?)does having a low specific heat mean?)

• Water has a high specific heat! (Why does a Water has a high specific heat! (Why does a watched pot never boil?)watched pot never boil?)

• Specific heat is a characteristic property Specific heat is a characteristic property which means every substance has its own which means every substance has its own unique specific heat!unique specific heat!

Specific Heat EquationSpecific Heat Equation

•Cp =Cp = q . q .

m m .. T T

Cp = specific heat (J/ gCp = specific heat (J/ gooC) (K)C) (K)

q = heat energy (J)q = heat energy (J)

m = mass (g)m = mass (g)

T = change in temp (T = change in temp (ooC or K)C or K)

Specific Heat EquationSpecific Heat Equation

Cp

q

m T