Transcript Energy (download)

Energy, work and heat
All processes in nature involve
energy
Learning objectives
• Define heat and energy and differentiate among
different types of energy
• Describe main uses of energy nationally and globally
• Apply units of energy to simple calculations of
energy changes in chemical processes
• Define heat capacity and use in calculations of
energy consumption
• Describe exothermic and endothermic reactions
• Describe principle of entropy and the “heat tax” and
the limitations of heat engines
• Describe the main sources of energy
• Describe basic principles of the Greenhouse effect
and global warming
Heat and chemical reactions
• The burning candle is a chemical
reaction
• It releases energy which heats up the
air molecules
• Heat is the flow of energy due to a
temperature difference
Energy from chemical reactions
performs work
• In what way is a human like a car?
• It’s unreliable
• Both burn fuel to run
– Humans burn glucose
• C6H12O6 + O2 = CO2 + H2O + energy
– Cars burn petrol
• C8H18 + O2 = CO2 + H2O + energy
Where would we be without it?
We like to use it
Energy is capacity to do work
• Work is done in different ways
• Kinetic energy is energy due to motion
• Potential energy is energy due to
position or state
– Height
– Chemical
– Electrical
Energy is mercurial
• Processes convert
energy from one form
to another
– Falling down stairs
• (potential → kinetic →
pain)
– Chemical reaction
• (potential → heat/light)
– Battery
• (potential → electrical)
– Engine
• (potential →heat→
kinetic)
But it never goes away
• Energy is conserved in any process
– None is lost
– None is gained
– But it goes from one place to another
• Law of Conservation of Energy:
Energy is neither created nor destroyed in a chemical
reaction
• Also known as the First Law of Thermodynamics
• System and Surroundings
– The process under study is the system
– Everything else is the surroundings
The First Law says that
“perpetual motion” can’t be
• If a friend asks you
to invest in his new
free energy machine
– don’t
• Visit the museum of
unworkable
machines for a
history of conmen
and futile energy
ideas
Heat and work
• The industrial revolution was founded on
conversion of heat into mechanical motion
• Joule (1843) was first to recognize
connection
• Heat: energy in transit – molecular motion
– Calorie is energy required to raise temperature of
1 g water by 1ºC
• Work: force applied over a distance
– Joule is a force of 1 Newton applied for 1 meter
1 cal = 4.18 J
Heat engines
• Newcomen steam engine invented 1712
• Watt improves design 1760
• Carnot (1820) described the operation
of heat engines in abstract terms – the
Carnot cycle: the foundation of modern
thermodynamics
• All engines based on burning fuels are
heat engines
Rules of the road
• First law: energy is conserved
• Is that all there is to it?
• Heat engines convert heat into
mechanical work
• Question: Can all heat be converted
into work with 100 % efficiency?
• First Law says yes
• Second Law says No!
Limitations on heat engines: the
Carnot cycle and entropy
• Limit on efficiency of a heat engine is
ruled by temperature difference
• Entropy and energy dispersal
• The second law of thermodynamics
Entropy of the universe is always
increasing
• Processes only occur spontaneously
when energy becomes more dispersed
– spread out
Various spontaneities:
dispersal
• Matter disperses – gas fills a container,
two liquids mix
• Heat disperses – hot object cools on
cold surface
• Motion disperses – a ball stops
bouncing
• These processes never reverse
spontaneously
Socks and spontaneity
• Would you be stunned
if the tumble dryer
matched the socks?
• Okay, you never match
the socks anyway
• Chaos in the sock
drawer is natural
• The same principles
apply to chemical
change (sort of)
Consequences for efficiency
• All processes use some of the energy
in dispersal
• More energy is lost due to inefficiency
– friction, wind resistance etc.
Measuring energy: calories are
case sensitive
• calorie is the energy required to raise temperature of
1 g of water 1 degree C
• Calorie is the food version = 1,000 cal
– Raises temperature of 1 pint of water 3.8ºF
• Joule is SI unit derived from mechanical work: the
work done when a force of 1 newton is applied for 1
meter
1 cal = 4.18 J
Measurements of energy use
What’s watt?
•
Watts measure the rate of delivery of energy or
power
1 W = 1 J/s
How many Mars Bars to power a 100 W bulb for
one minute?
•
–
–
1 min = 60 s x 100 W = 6 kJ
6 kJ = 1.4 kcal = 1.4 Cal (just a nibble)
Common energy conversions
• 10 g of octane is burnt to produce 8500 J.
How much is that in calories?
• 1 cal = 4.18 J
Power consumption
• An air conditioner is rated at 1,500 W. How
many kWh are used per month if it operates 6
h per day?
• What is cost at $0.15 per kWh?
Energy: in or out?
• Do chemical processes always create
heat?
• If you think yes then how does a cold
pack work?
• Answer: some processes absorb heat
from the surroundings
Exo-thermic and endothermic
• H2 + O2 gives out heat – exothermic
• N2 + O2 absorbs heat - endothermic
Enthalpy and chemical reactions
• Enthalpy of reaction (ΔH) measures heat of
the reaction
CH4 + 2O2 = CO2 + 2H2O ΔH = -11.8 kcal/g
The sun as our energy source:
directly and indirectly
• Indirect:
– Solar radiation provided energy for fossil fuels
– Heats the air (wind power)
– Evaporation of water (hydro power)
• Direct:
– Solar panels
– Photovoltaic cells
• Nonsolar:
– Nuclear
– Geothermal
Energy sources
• 85 % comes from fossil fuels
• Fossil fuels are hydrocarbons
– Petroleum
– Coal
– Gas
• 15 % is everything else
– Nuclear (8)
– Hydro (3)
– Renewables (3)
Electrical generation
•
•
•
•
More than 70 % comes from fossil fuels
Heat of combustion boils water
Steam turns a turbine
Turbine generates electricity
Finitude
• Gambling on the “Other”: how to turn 3 %
into 85 % without hurting anybody