Video 6.1Q=mcΔT

Table I

0Exothermic reactions release heat and have negative values. 0 Example: When Carbon and Oxygen react they release

393.5kJ of heat per mole reacted.0Endothermic reactions absorb heat and have positive

values.0 Example: When Nitrogen and Oxygen react they absorb

182.6kJ of heat per mole.

Table I examples

0When C2H4 is formed, is heat released or absorbed?

0Table I: 2C + 2H2 C2H4 ΔH=52.4kJ

0When 2 moles of CH4 burn in oxygen, how much heat is released?

0Table I: CH4 + 2O2 CO2 + H2O ΔH=-890.4kJ

02CH4 2(-890.4) = -1780.8 KJ

Table I examples

0Reactions that release the most energy are the most stable. Which reaction becomes the most stable?

04Al + 3O2 2Al2O3 ΔH=-3351kJ

0Where did these values come from?

Calculating Heat of reactions

0q is the symbol for heat. 0If q is positive, the heat is endo.0If q is negative, the heat is exo.0q is measure in Joules, (J) or kilojoules (kJ).

0The heat of a reaction is based on:0 the mass of the substance0the temperature change it undergoes 0specific heat.

Specific Heat0Specific heat is the heat needed to raise the temperature of

one gram of a substance one degree Celsius.

To calculate heat:

q = mcΔT1. The temperature of 95.4g of

copper increases from 25 to 48C and absorbed 849J. Calculate copper’s specific heat.

849= (95.4) (x) (48-25)

Q = 0.387 J/gC

q = mcΔT

2. How much heat is needed to raise the temperature of 100g of water 50C?

Q = (100) (4.18) (50)

Q = 20900J or 20.90 KJ

q = mcΔT

3. If 600J are needed to heat 50g of water to 100C, what is the initial temperature?

97.13C

600 = (50) (4.18) (x)

X = 2.87

Video 6.2Review Gases, Liquids, and SolidsWith Phase Change Diagrams

Kinetic Molecular Theory of GasesKMT describes perfect gases:0Gases move in constant, random, straight-line paths.0Gases are separated by large distances, much larger than

their particle size. Therefore, gases volume is negligible. And gases are easily compressed.

0Gases do not have attractive or repulsive forces between molecules.

0Collisions between molecules can transfer energy but the total energy of the system is constant. This is called an elastic system.

Kinetic Molecular Theory of Gases

In summary, Perfect gases:

0Have no mass0Have no volume0Have no intermolecular forces

Kinetic Molecular Theory of Gases

0But we don’t have perfect gases. How do real gases deviate from ideal gases?0They have a volume, mass and small IMF under

high pressure and low temperature.0So, a real gas must be hot and under low pressure

to behave like an ideal gas.

Pressure0Gases exert a pressure on

surrounding substances because they are constantly moving and colliding with other surfaces.

0Only in a vacuum, where there are no molecules, there is no pressure.

0Gas pressure can be measured in atmospheres or kilopascals, according to reference table A.

Liquids

0No definite shape0Definite volume0Constant motion0No arrangement0Molecules are closer

together than a gas

Solids

0Definite shape0Definite volume0Constant vibration0Molecules are packed

tightly in a geometric (crystalline) pattern

Phase ChangesIdentify the phase change and if it’s

endothermic or exothermic:0Evaporation0Condensation0Melting 0Freezing0Sublimation0Deposition

Liquid to gas endothermic

Gas to liquid exothermic

Solid to liquid endothermic

Liquid to solid exothermic

gas to solid exothermic

Solid to gas endothermic

Thermochemistry

0The study of energy changes that occur in chemical reactions.

0 Kinetic energy refers to energy of motion. (Temperature)

0 Potential Energy refers to stored energy.

Phase Change Diagrams

Cooling Curve

B

E

D

C

A

F

Video 6.3q=mHf q=mHv

When can you use q=mcΔT?

0Only on the solid, liquid and gas only lines. (Where the temperature changes)

0So, what equations do we use if the temperature is not changing?

Two more equations from Table T

0Heat of vaporization: heat needed to change a substance from gas to liquid or liquid to gas.

q=mHv

0Heat of fusion: heat needed to change a substance from solid to liquid or liquid to solid.

q=mHf

0 If the IMF is strong, the heats of vaporization and fusion is high.

Q=mHv

1. Calculate the number of joules needed to vaporize 423g of H2O.

955, 980J or 955.98KJ

Q = (423) (2260)

Q=mHf

0How much heat is needed to melt ice at 0C if the sample weighs 255g?

85,170J or 85.17 KJ

Q = (255) (334)

For class

Measuring heat in the labYou can measure the heat of

physical and chemical changes in a calorimeter.

The calorimeter acts like a styrofoam cup, it insulates the reaction (doesn’t let the overall heat change).

Measuring heat in the lab0The heat released by the

reaction equals the heat absorbed by the water.

0You will measure the change in heat of the water using q=mcΔT.

Measuring heat in the lab0You will use a calorimeter

more like this. 0You must make sure you

always stir the solution to make the heat equal throughout the cup.

A student places a 68.4g piece of metal at 99.5C in a calorimeter filled with 103g of water at 25.2C. The temperature changes to 27.6C.

1. In terms of the metal, is the reaction endothermic or exothermic?

2. Calculate the heat absorbed by the water.3. Calculate the heat released by the metal.4. Calculate the specific heat of the metal.5. Using the following specific heats, determine the identity of

the metal and calculate the % error.Aluminum: 0.21 J/gCCopper: 0.090 J/gCGold: 0.030 J/gC

0http://www.sciencephoto.com/media/233708/view