Thermodynamics

X Unit 9

Energy: Basic Principles Thermodynamics – the study of energy

changes

Energy – the ability to do work or produce heat Note: Work is force acting over a

distance

Energy: Basic Principles

Kinetic Energy – energy of motion KE =

Potential Energy – energy due to position or composition

2

21mv

Law of Conservation of Energy A.k.a. first Law of Thermodynamics

Energy can be converted from one form to another but can’t be created or destroyed This means the total energy of the

universe is CONSTANT!

Heat vs. Temperature

Temperature – measure of the random motion of a substance Temperature is proportional to kinetic

energy (it is a measure of the average kinetic energy in a substance)

Heat (q) – flow of energy due to a temperature difference

System vs. Surroundings

A system is the part of the universe we are studying.

The surroundings are everything else outside of the system.

Direction of Heat Flow Heat transfer occurs when two objects are

at two different temperatures.

Eventually the two objects reach the same temperature At this point, we say that the system has

reached equilibrium.

Thermal Equilibrium Heat transfer

always occurs with heat flowing from the HOT object to the COLD object.

Exothermic vs. EndothermicExothermic process heat is transferred

from the system to the surroundings Heat is lost from the system

(temperature in system decreases)

Endothermic process heat transferred from the surroundings to the system Heat is added to the system

(temperature in system increases)

Exothermic ProcessProducts have lower energy

than reactants

Energyproducts < Energyreactants

Endothermic ProcessReactants have lower energy than products

Energyproducts > Energyreactants

Units of Energy Joule (J) is the SI unit of energy & heat

One kilojoule (kJ) = 1000 joules (J)

calorie (cal) = heat required to raise the temperature of 1.00 g of water by 1 °C

1 calorie = 4.18 J

Units of Energy Food is measured in Calories (also known

as kilocalories) instead of calories

1 Cal = 1 kcal = 1000 calories

Units of Energy3800 cal = __________ Cal = _________ J

Units of EnergyThe label on a cereal box indicates that 1 serving provides 250 Cal. What is the energy in kJ?

Heat TransferDirection and sign of heat flow – MEMORIZE!

ENDOTHERMIC: heat is added to the system & the temperature increases (+q)

EXOTHERMIC: heat is lost from the system (added to the surroundings) & the temperature in the system decreases (-q)

Specific Heat (Specific Heat Capacity) Specific Heat (C) - The quantity of heat

required to raise the temperature of one gram of a substance by 1 °C

Units: J/(g·°C) or J/(g·K) cal/(g·°C) or cal/(g·K)

Examples of Specific HeatAt the beach, which gets hotter, the sand or the water?

Higher specific heat means the substance takes longer to heat up & cool down!

Examples of Specific Heat Specific heat (C)= the heat required to

raise the temperature of 1 gram of a substance by 1 °C

Cwater = 4.184 J/(g°C)Csand = 0.664 J/(g°C)

Calculating Changes in Thermal E

q = mCΔTq = heat (J)m = mass (g)C = specific heat capacity, J/(g°C)ΔT = change in temperature, Tfinal – Tinitial

(°C or K)

***All units must match up!!!***

ExampleHow much heat in J is given off by a 75.0 g sample of pure aluminum when it cools from 84.0°C to 46.7°C? The specific heat of aluminum is 0.899 J/(g°C).

q = mCΔt

ExampleWhat is the specific heat of benzene if 3450 J of heat are added to a 150.0 g sample of benzene and its temperature increases from 22.5 °C to 35.8 °C?

q = mCΔt

Calorimetry Calorimetry: measurement of quantities of

heat A calorimeter is the device in which heat

is measured.

CalorimetryAssumptions:

Heat lost = -heat gained by the system

In a simple calorimeter, no heat is lost to the surroundings

Enthalpy Enthalpy (H)

The heat content of a reaction (chemical energy)

ΔH = change in enthalpy The amount of energy absorbed by or

lost from a system as heat during a chemical process at constant P

ΔH = ΔHfinal - ΔHinitial

Representation of Enthalpy as a Graph

Two Ways to Designate Thermochemical Equations

Endothermic:

a) H2 (g) + I2 (s) 2 HI (g) ΔH = 53.0 kJ

b) H2 (g) + I2 (s) + 53.0 kJ 2 HI (g)

Two Ways to Designate Thermochemical Equations

Exothermic:

a) ½ CH4 (g) + O2 (g) ½ CO2 (g) + H2O (l) ΔH = -445.2 kJ

b) ½ CH4(g) + O2(g) ½ CO2(g) + H2O(l) + 445 .2 kJ

Two Ways to Designate Thermochemical Equations

Note the meaning of the sign in ΔH in the equations above!!

Endothermic: ΔH = +Exothermic: ΔH = -