Conservation of Energy Conservation of Energy

Hess' Law Enthalpy of Form ation

Enthalpy / Calorim etry

1st Law of T herm odynam ics

Energy

Energy

Energy is: Energy is:

• the ability (or capacity) of a the ability (or capacity) of a system to do work or supply (or system to do work or supply (or produce) heat.produce) heat.

The Nature of EnergyThe Nature of Energy

• Kinetic energy is the energy of motion:

• Potential energy is the energy an object possesses by virtue of its position.

• Potential energy can be converted into kinetic energy. Example: a bicyclist at the top of a hill.

2

2

1vmEk

Types of Potential Energy

• Gravitational - this is the most familiar. A rock poised to roll down a hill has potential energy. A ball thrown into the air gains more and more potential energy as it rises. The higher in the gravity field you go, the more potential energy you gain. Generally speaking, chemistry does not concern itself with the potential energy from gravity.

Types of Potential Energy

• Electrical - in certain materials, you can remove electrons from one area and send them to another. The area losing the electrons becomes more and more positive and the area gaining them becomes negative. The greater and greater the charge difference, the more energy is stored within the system. An example of this is a storm cloud about to "hurl" a lightning strike Earthwards.

Types of Potential Energy

Chemical - this is slightly more complex. Certain chemicals have bonds which require little energy to break. This energy must be put into the bond to break it. However, during the course of the chemical reaction, new bonds form which give off MORE energy than that which was put in. The positional aspect comes from first breaking bonds between atoms (which takes energy) and then rearranging the atoms in new positions to form new bonds (which gives off energy).

Types of Potential Energy

• Nuclear - the famous equation E = mc2 governs this source of potential energy. We can consider the mass itself to be potential energy, since it can be converted from a form not being used (while it is the mass), to kinetic energy. This type of potential energy is released (in measurable amounts) during radioactive decay, fission and fusion.

The Nature of EnergyThe Nature of Energy

• Units of Energy• SI Unit for energy is the joule,

J:

• sometimes the calorie is used instead of the joule:

• 1 cal = 4.184 J (exactly)• A nutritional Calorie:• 1 Cal = 1000 cal = 1 kcal

J 1s m kg 1

m/s 1kg 22

1

2

1

2-2

22

vmEk

Conservation of Energy Conservation of Energy TerminologyTerminology

• System: part of the universe we are interested in.• Surrounding: the rest of the universe.• Boundary: between system & surrounding.• Exothermic: energy released by system to surrounding.• Endothermic: energy absorbed by system from surr.• Work ( w ):the transfer of energy from one mechanical

system to another. It is always completely convertible to the lifting of a weight.

• Heat ( q ): transfer of energy between two objects

Always remember!!!!

• Energy can not be created nor destroyed.

The First Law of The First Law of ThermodynamicsThermodynamics

• Internal Energy• Internal Energy: total

energy of a system.• Involves translational,

rotational, vibrational motions.

• Change in internal energy,

initialfinal EEE

The First Law of The First Law of ThermodynamicsThermodynamics

• Energy cannot be created or destroyed.

• Energy of (system + surroundings) is constant.

• Any energy transferred from a system must be transferred to the surroundings (and vice versa).

• From the first law of thermodynamics:

wqE

The First Law of The First Law of ThermodynamicsThermodynamics

First Law of Thermodynamics

• Calculate the energy change for a system undergoing an exothermic process in which 15.4 kJ of heat flows and where 6.3 kJ of work is done on the system.

E = q + w

The First Law of The First Law of ThermodynamicsThermodynamics

• Exothermic and Endothermic Processes

• Endothermic: absorbs heat from the surroundings.• An endothermic reaction feels cold.

• Exothermic: transfers heat to the surroundings.• An exothermic reaction feels hot.

EnthalpyEnthalpy

• Chemical reactions can absorb or release heat.

• However, they also have the ability to do work.

• For example, when a gas is produced, then the gas produced can be used to push a piston, thus doing work.

• Zn(s) + 2H+(aq) Zn2+(aq) + H2(g)

• The work performed by the above reaction is called pressure-volume work.

• When the pressure is constant,

VPw

EnthalpyEnthalpy

• Enthalpy, H: Heat transferred between the system and surroundings carried out under constant pressure.

• If the process occurs at constant pressure,

VPE

PVEH

EnthalpyEnthalpy

• Since we know that

• We can write

• When change in H is positive, the system gains heat from the surroundings.

• When change in H is negative, the surroundings gain heat from the system.

VPw

P

P

P

q

VPVPq

VPwq

VPEH

)(

Enthalpy => Heat of ReactionEnthalpy => Heat of Reaction