Chapter 10: EnergyChapter 10: Energy

Honors ChemistryHonors Chemistry

What is energy?What is energy?

The ability to The ability to do work or do work or

produce heatproduce heat

But first…But first…• State functionState function: a property that : a property that

changes changes independently of its pathindependently of its path• Real life examples:Real life examples:

• DisplacementDisplacement• TanningTanning

• Chemistry examples:Chemistry examples:• Changes in Changes in energyenergy• Changes in Changes in temperaturetemperature

∆∆X = XX = Xfinalfinal – X – Xinitialinitial

Two general classes of Two general classes of energy:energy:

• Potential energy: • Energy that has the potential of

being converted to other forms of energy (like kinetic energy) and do work in the process

• Think:• Water behind a dam• Raised object• Two charges separated in space

• Kinetic energy: • Energy due to motion

Law of Conservation of Law of Conservation of EnergyEnergy• Energy can neither be created nor

destroyed• It can only be converted from one form to

another

Thermodynamics!Thermodynamics!• The study of energy• Law of Conservation of Energy is AKA the First

Law of Thermodynamics:

The energy of the universe is The energy of the universe is constantconstant!!

Types of energy…Types of energy…• Radiant energy: comes from the sun

and is earth’s primary energy source• Thermal energy: energy associated

with the random motion of atoms and molecules

• Chemical energy: energy stored in chemical bonds

• Nuclear energy: energy stored in the neutrons and protons of an atom

• Electrical energy: potential energy associated with the forces between charged particles

• Light energy

Hot! Hot! Hot!Hot! Hot! Hot!

• Heat: energy transferred from a hotter object to a colder one• Heat flows due to

temperature differences• No temperature difference

= no heat flow

• Symbolized by q (or Q)

More on heat…More on heat…

• What was heat again?!?• Your answer:

• Heat cannot be measured directly• Changes due to heat can be

measured• How do you think we do this?!?

Temperature…Temperature…

100°C

Hi TE

25°C

Med TE

0°C

Lo TE

• Temperature is a measure of the thermal energy of something

• Thermal energy is how much random movement there is

• Temperature ≠ thermal energy• But the greater the

thermal energy, the higher the temperature

Defining a system and its Defining a system and its surroundings…surroundings…• System: part of

the universe being studied• Usually reactants

and products

• Surroundings: everything else in the universe Surroundings:

Everything elseSystem:

Reactants and Products in flask

Thermochemistry is the study of heat change in chemical reactions

openmass & energyExchange:

closedenergy

isolatednothing

SYSTEMSURROUNDINGS

System:

Exothermic vs. Exothermic vs. Endothemric ProcessesEndothemric Processes

Exothermic• Process that gives of

energy• Energy flows out of

the system

• 2H2 (g) + O2 (g) 2H2O (l) + Energy

• H2O (g) H2O (l) + Energy

Endothermic• Process that requires

energy• Energy flows into the

system

• Energy + 2HgO (s) 2Hg (l) + O2 (g)

• Energy + H2O (s) H2O (l)

Exothermic Reaction

• Reactants have more potential energy than products• Potential energy is stored

in chemical bonds

• Amount of energy released by system (c)

Endothermic Reaction

• Reactants have less potential energy than products

• Amount of energy absorbed by the system (c)

Exothermic Endothermic

Reactants

Reactants

2H2 (g) + O2 (g) 2H2O (l) + Energy

Products

Products

Energy + 2HgO (s) 2Hg (l) + O2 (g)

• ∆E = Efinal – Einitial

• ∆E = Eproducts – Ereactants

• ∆E = (–) number

Exothermic Endothermic

Reactants

Reactants

2H2 (g) + O2 (g) 2H2O (l) + Energy

Products

Products

Energy + 2HgO (s) 2Hg (l) + O2 (g)

• ∆E = Efinal – Einitial

• ∆E = Eproducts – Ereactants

• ∆E = (+) number

Measuring Energy Changes

• Calorie: amount of energy (heat) required to raise 1g of H2O by 1C • Unit = cal

• Kilocalorie: 1000 calories • Used to measure energy

content of food• Unit = kcal or Cal

More units of energy…

• Joule (SI unit of energy)• Unit = J

• 1 cal = 4.184 joules

Energy required to change temperature depends on…

1. Amount of substance being heated (grams)

2. Temperature change (°C or K)3. Material that is being heated

• Different materials respond differently to transfer of heat• Know: 1 cal, or 4.184 J, of energy raises the

temperature of 1 g of H2O by 1°C• But… 4.184 J of energy raises the temperature

of 1 g of silver metal by 17°C!!• Think: wooden vs. metal spoon!

Specific Heat Capacity

• Amount of energy (heat) required to change the temperature of 1g of a substance by 1C

• Unit = J/g °C = Jg-1 °C-1

• Symbolized by “s” (“c” in other books?!?)

• See table on page 279…

Heat requirements…

•Q = m x s x T •Q = amount of energy (heat)

required to heat up or cool down a sample

•m = mass of sample (g)•s = specific heat capacity of

sampleT = change in temperature

desired