11

Energy and Energy and ThermochemistryThermochemistry

22

EnergyEnergy

The ability to do workThe ability to do work2 types2 types

Potential: stored energyPotential: stored energyKinetic: energy in motionKinetic: energy in motion

33

ThermochemistryThermochemistry

Changes of heat content and heat transferChanges of heat content and heat transferFollow Law of Conservation of EnergyFollow Law of Conservation of EnergyOr, 1Or, 1stst Law of Thermodynamics Law of Thermodynamics

Energy can neither be created nor destroyedEnergy can neither be created nor destroyed

44

Temperature & HeatTemperature & Heat

Heat not same as temperatureHeat not same as temperature Heat = energy transferred to one system by another Heat = energy transferred to one system by another

due to temperature differencedue to temperature difference Temperature = measure of heat energy content & Temperature = measure of heat energy content &

ability to transfer heatability to transfer heat ThermometerThermometer

Higher thermal energy, greater motion of Higher thermal energy, greater motion of constituentsconstituents

Sum of individual energies of constituents = total Sum of individual energies of constituents = total thermal energythermal energy

55

Systems and SurroundingsSystems and Surroundings

System = the object in questionSystem = the object in question Surrounding(s) = everything outside the systemSurrounding(s) = everything outside the system When both system and surrounding at same When both system and surrounding at same

temperature temperature thermal equilibrium thermal equilibrium When notWhen not

Heat transfer to surrounding = exothermicHeat transfer to surrounding = exothermic (you feel the heat) (you feel the heat) hot metal! hot metal! Heat transfer to system = endothermicHeat transfer to system = endothermic (you feel cold) (you feel cold) cold metal! cold metal!

66

Math!Math!

Joules (J) used for Joules (J) used for energy quantitiesenergy quantities But usually kJ (1000 J) But usually kJ (1000 J)

usedused Ye Royal Olde School Ye Royal Olde School

used calorie (cal)used calorie (cal) cal = amt of heat required cal = amt of heat required

to raise the temperature of to raise the temperature of 1.00 g of water by 11.00 g of water by 1CC

1 cal = 4.184 J (SI-unit)1 cal = 4.184 J (SI-unit) But…Calorie (Cal) = 1000 But…Calorie (Cal) = 1000

calcal Used in nutrition science Used in nutrition science

and on food labelsand on food labels

2

2

1 kg mJoule (J) =

s

77

Heat CapacityHeat Capacity

Specific heat capacitySpecific heat capacity Quantity of heat Quantity of heat

required to raise the required to raise the temp of 1 temp of 1 gramgram of any of any substance by 1 Ksubstance by 1 K

Molar heat capacityMolar heat capacity Quantity of heat Quantity of heat

required to raise the required to raise the temp of 1 temp of 1 molemole of any of any substance by 1 Ksubstance by 1 K

JC =

g K

4.184 Jspecific heat capacity of water =

g K

Jc =

mol K

75.4 Jmolar heat capacity of water =

mol K

88

Calculating heat transferCalculating heat transfer

FYIFYI Specific heat capacity Specific heat capacity

of metals is very lowof metals is very low < 1.000 J/(g< 1.000 J/(gK)K)

What does this tell us What does this tell us about heat transfer in about heat transfer in metals?metals?

Q = C m T

Q = transferred heat, m = mass of substance, T = temperature change

99

Let’s do an exampleLet’s do an example

In your backyard, you have a swimming In your backyard, you have a swimming pool that contains 5.19 x 10pool that contains 5.19 x 1033 kg of water. kg of water. How many kJ are required to raise the How many kJ are required to raise the temperature of this water from 7.2 temperature of this water from 7.2 °C to °C to 25.0 °C?25.0 °C?

1010

Example solvedExample solved

Trick: Trick: T in K = T in K = T in T in °C°C

6 8 5JQ = C m T = (4.184 ) (5.19 x 10 g) (298.2 K - 280.4 K) = 3.87 10 J = 3.87 10 kJ

g K

1111

Practice Practice

How many kJ are required to raise the How many kJ are required to raise the temperature of 25.8 g of quicksilver from temperature of 25.8 g of quicksilver from 22.5 22.5 °C to 28.0 °C? C°C to 28.0 °C? CHgHg = 0.1395 J/(g = 0.1395 J/(gK)K)

1212

SolutionSolution

3

T 28.0 C-22.5 C 5.5 C

J kJQ C m T (0.1395 ) 25.8g 5.5 C 20.J 20. 10 kJ

g K 1000J