Ch. 10.4 and 11 Thermodynamics

Ch. 10.4 and 11ThermodynamicsYou must turn in your notes

Energy Definition: the capacity to do work, or to produce heat;

SI (metric) unit is Joule Non-SI unit is calorie

1 cal = 4.18 J 1 calorie = the amount of energy needed to raise 1

gram of water 1° C Heat - energy that is transferred from a warm

object to a cooler object; represented by “q” Temperature - a measure of the average kinetic

energy of an object

Specific heat/Change of Temp

Quantity of heat needed to raise 1 gram of a substance 1° C; unit is J/g°C FORMULA: q= c x m x ΔT q= heat (J) c = specific heat (J/g°C) m = mass (g) T = change in temp. (°C)

Specific Heat Practice Problem #1

1. Calculate the amount of heat in joules needed to warm 250. g of water from 25.0°C to 95.0°C. (c=4.184 J/°C g)

Practice Problem #2

How much heat is lost when 50.0 grams of Al is cooled from 130.0 °C to 62.0 °C? The specific heat of Al is 0.897 J/g°C

Change of state and heat

heat of fusion- amount of heat needed to melt 1 gram of a substance at its melting point

Hf copper = 205 J/g Hf water = 80 cal/g= 334 J/g q = mHf heat of vaporization- amount of heat needed to

boil 1 gram of a substance at its boiling point Hv water = 540 cal/g = 2260 J/g q= mHv

Heating/Cooling Curves

Change of Phase

Change of Temperature

Know which formula to use when!

Change of state Practice Problems

1. Calculate the amount of heat, in Joules, needed to melt 70.0g of copper at its melting point.

2. Calculate the heat required, in calories, to change 250g of water at 100°C to steam at 100°C.

3. Calculate the amount of heat needed to change 20g of ice at -10.0°C to water at 80.0°C.

Enthalpy (H) a measure of heat content of a system H = change in heat content that accompanies a

process Hrxn = Hfinal - Hinitial

Hrxn = Hproducts - Hreactants

** ΔHrxn can also be written as ΔHf, for heat of formation**

**Chemical systems in the world tend to achieve the lowest possible energy. Would this occur in an exothermic or an endothermic reaction?

Exothermic reactions

chemicals react and give off heat (feel hot); H is negative; products are more stable

4Fe + 3O2 2Fe2O3 + 1625 kJ

reactants

products

Endothermic reactions

chemicals need to absorb energy in order for the reaction to take place (feel cool); H is positive; reactants are more stable

27 kJ + NH4NO3 NH4+ + NO3

-

products

reactants

Practice problems #1

CO (g) + NO (g) CO2 (g) + N2 (g)

Practice #2

CH4 (g)+ O2 (g) CO2 (g)+ H2O (g)

Practice #3

N2 (g) + O2 (g) NO2 (g)

Entropy (S)

Measure of disorder or chaos in a system Law of disorder – states that things move

spontaneously in the direction of maximum chaos or disorder

ΔSsystem = SP - SR If ΔS is +, there is an increase in entropy. If ΔS is -, there is a decrease in entropy

Rules for disorder 1. Entropy increases as particles move apart.

Gas > Liquid > Solid I2 (s) I2 (g)

2. Entropy increases when you divide a substance into parts (when the total number of products > the total number of reactants)

2H2O 2H2 + O2

3. Entropy increases as temp increases b/c particles move faster. (unit for entropy is J/K mole)

4. Entropy increases when you dissolve a solid into a liquid. Entropy decreases when you dissolve a gas into a liquid.

Entropy Examples

Water (liquid) - water (solid)

KCl(s) KCl (l)

C(s) + O2 (g) CO2 (g)

Gibbs’ Free Energy (G)

Energy available to do work Relates enthalpy (H) and entropy (S),

using the equation: ΔG = ΔH – TΔS

ΔG = GP - GR

Still confused?

Try these online notes http://www.sciencegeek.net/Chemistry/Po

werpoint/Unit7/Unit7_files/frame.htm

Spontaneous Reactions

If ΔG is negative, the reaction will occur spontaneously.

If ΔG is positive, the reaction will NOT occur spontaneously.

Quick Spontaneous Chart

ΔH - + + -

ΔS + - + -

ΔG - + Depends on temp

Depends on temp

spontaneous Non-

spontaneous