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# Thermodynamics – study of energy First law of thermodynamics Energy of the universe is constant - PowerPoint PPT Presentation

Thermodynamics – study of energy First law of thermodynamics Energy of the universe is constant Energy can not be created or destroyed. Energy is the ability to do work or produce heat. 2 types of Energy. Kinetic energy Energy of motion E = ½ mv 2. Potential energy

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• Thermodynamics – study of energy

• First law of thermodynamics

• Energy of the universe is constant

• Energy can not be created or destroyed

• Energy is the ability to do work or produce heat.

• 2 types of Energy

Kinetic energy

Energy of motion

E = ½ mv2

Potential energy

Energy of position

• Energy can not be created or destroyed but can be converted from one form to another

• Ex chemical energy (gas) can be converted to mechanical and thermal energy

Internal energy can be transferred by two types of energy flow:

• Heat (q) (q = quantity of heat)

• Work (w)

E = q + w

Change of energy in a

system

• common energy units for heat are the calorie and the joule.

• Calorie – the amount of energy (heat) required to raise the temperature of one gram of water 1oC.

• Joule – heat required to raise the temperature of 1 g of water by 0.24 K

1 calorie = 4.184 joules

• System – part of the universe on which we focus attention

• Surroundings – everything else in the universe

• Ex. Burning a match is the system releases heat to surroundings

Endothermic – energy flows into system

Exothermic – energy flows out of system

• C- Specific heat capacity energy required to change the mass of one gram of a substance by one degree Celsius. [joule/gram °C]

• Like energy storage bank

• C- Specific heat capacity energy required to change the mass of one gram of a substance by one degree Celsius. [joule/gram °C]

• Like energy storage bank

• Water has a high specific heat capacity that means it takes a lot of heat (joules) to increase the temperature of water compared to metals (water better at storing energy than metals

• To calculate the energy required for a reaction:

Q = C  m t

Q = C  m t

How much heat is required to warm 145 g of water from 25 *C to 65 *C

Q = m x C t

How much heat is required to warm 145 g of water from 25 *C to 65 *C the specific heat of liquid water is 4.184 J/g *C

Q = m x C t

Q = 145 g x 4.184 J x (65 *C - 25 *C)

g *C

Q = m x C t

How much heat is required to warm 145 g of water from 25 *C to 65 *C the specific heat of liquid water is 4.184 J/g *C

Q = C  m t

Q = 145 g x 4.184 J x (65 *C - 25 *C)

g *C

Q = 145 x 4.184 J x 40

Q = 24,267 joules

A quantity of water is heated from 20.0 *C to 48.3 *C by absorbing 623 Joules of heat energy, what is the mass of the water? Specific heat of water is 4.184 J/g *C

Q = C  m t

__Q__ = m

C x t

A quantity of water is heated from 20.0 *C to 48.3 *C by absorbing 623 Joules of heat energy, what is the mass of the water? Specific heat of water is 4.184 J/g *C

Q = C  m t

__Q__ = m

C x t

623 J = m

4.184 J /g *C x (48.3 *C - 20 *C)

A quantity of water is heated from 20.0 *C to 48.3 *C by absorbing 623 Joules of heat energy, what is the mass of the water? Specific heat of water is 4.184 J/g *C

Q = C  m t

__Q__ = m

C x t

623 J = m

4.184 J /g *C x (48.3 *C - 20 *C)

623 J = m

4.184 J /g *C x (28.3 *C)

5.26 g = m

In a household radiator a 25.5 g of steam at 100 *C condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g

Q = m Hvap

In a household radiator a 25.5 g of steam at 100 *C condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g

Q = m Hvap

Q = 25.5 g x 2260 J/g

Q = 57,630 J

Objectives condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g

• To consider the heat (enthalpy) of chemical reactions

• To understand Hess’s Law

A. Thermochemistry (Enthalpy) condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g

• Enthalpy, H– energy function

• At constant pressure H is equal to the energy that flows as heat.

Hp = heat = q

ΔH = q  (valid for constant pressure ONLY!)

A. Thermochemistry condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g

Calorimetry

• Enthalpy, H is measured using a calorimeter.

B. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Hess’s Law

• For a particular reaction, the change in enthalpy is the same whether the reaction takes place in one step or a series of steps.

• Example:

N2(g) + 2O2(g) 2NO2(g)H1 = 68 kJ

Objectives condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g

• To understand how the quality of energy changes as it is used

• To consider the energy resources of our world

• To understand energy as a driving force for natural processes

A. Quality Versus Quantity of Energy condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g

• When we use energy to do work we degrade its usefulness.

A. Quality Versus Quantity of Energy condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g

• Petroleum as energy

B. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy and Our World

• Fossil fuel – carbon based molecules from decomposing plants and animals

• Energy source for United States

B. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy and Our World

• Petroleum – thick liquids composed of mainly hydrocarbons

• Hydrocarbon – compound composed of C and H

B. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy and Our World

• Natural gas – gas composed of hydrocarbons

B. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy and Our World

• Coal – formed from the remains of plants under high pressure and heat over time

B. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy and Our World

• Effects of carbon dioxide on climate

• Greenhouse effect

B. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy and Our World

• Effects of carbon dioxide on climate

• Atmospheric CO2

• Controlled by water cycle

• Could increase temperature by 10oC

B. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy and Our World

• New energy sources

• Solar

• Nuclear

• Biomass

• Wind

• Synthetic fuels

C. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy as a Driving Force

• Natural processes occur in the direction that leads to an increase in the disorder of the universe.

• Example:

• Consider a gas trapped as shown

C. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy as a Driving Force

• What happens when the valve is opened?

C. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy as a Driving Force

• Two driving forces

C. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy as a Driving Force

• In a given process concentrated energy is dispersed widely.

• This happens in every exothermic process.

C. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy as a Driving Force

• Molecules of a substance spread out to occupy a larger volume.

• Processes are favored if they involve energy and matter spread.

C. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy as a Driving Force

• Entropy, S – function which keeps track of the tendency for the components of the universe to become disordered

C. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy as a Driving Force

• What happens to the disorder in the universe as energy and matter spread?

C. condenses (changes from gas to liquid) How much heat is released? Heat of vaporization is 2260 J/g Energy as a Driving Force

• Second law of thermodynamics

• The entropy of the universe is always increasing.