Download Energy

Energy
Chapter 10
10.1 The Nature of Energy



Energy – the ability to do work or produce heat
Potential Energy – due to position or composition
Kinetic Energy – due to motion


Depends on the mass of the object (m) and its velocity (v)
KE = ½ mv2
The Law of Conservation of Energy


Energy can be
converted from one
form to another but can
be neither created nor
destroyed
The energy in the
universe is constant
Work

Work = Force x distance


W = Fd
Frictional Heating – 2 surfaces in contact with each
other

Depends on surface and force pushing the surfaces together
State Function

The property of the system that changes
independently of its pathway


The pathway is how you get there
Example



If you travel from Chicago to Denver what are
state functions?
The route you take to get there is your pathway, so
it is not a state function
Change in elevation doesn’t depend on how you
get there so it is a state function
10.2 Temperature and Heat

Temperature – Measure of the random motion of the
components of a substance

Heat – The flow of energy due to a difference in
temperature
10.3 Exothermic and Endothermic
Processes




System – part of universe
we are looking at
Surroundings – everything
else
Exothermic – energy flows
out of a system
Endothermic – energy
flows into a system


Where does energy as heat come from in
exothermic reactions?
It depends on the potential energy between the
products and reactants
10.4 Thermodynamics

Law of Conservation of Energy (a.k.a. The
First Law of Thermodynamics)


Energy can neither be created nor destroyed under
normal conditions
The energy of the universe is constant
E = internal energy







E is the sum of the kinetic energy and the potential
energy
Can be changed by the flow of work, heat, or both
∆ = change in; called “delta”
w = work
q = heat
∆E = q + w
Change in internal energy equals heat plus work

Thermodynamic quantities are made up of a
number that shows magnitude and a sign that
shows whether energy is flowing into the
system (endothermic = + ) or out of the system
(exothermic = - )
10.5 Measuring Energy Changes




calorie = amount of energy required to raise
the temperature of 1 gram of water by one
degree Celsius
1000 calories (1 kilocalorie) is what we refer
to as a “Calorie” with a capital C
1 calorie = 4.184 joules
1 cal = 4.184 J


To go from calories to joules multiply by 4.184
To go from joules to calories divide by 4.184
And now for a problem!






How much heat, in joules, is required to raise the
temperature of 7.40 g water from 29.0 °C to 46.0 °C?
We know we need 4.184 J of energy raise 1 g of
water 1 °C
We have 7.40 g of water so it will take 7.4 g x 4.184 J
to raise it 1 °C
We also need to raise the temperature 17 °C so 17.0
°C x 7.4 g x 4.184 J/ g x °C
So we need 526 J of energy
Now try this

Calculate the joules of energy required to heat 454 g of
water from 5.4 °C to 98.6 °C?



So we know that the amount of energy we
need to raise the temperature of a substance
depends on the amount of substance and the
change in temperature
But the substance also plays a big part
Specific Heat Capacity = the amount of energy
needed to raise the temperature of 1 g of a
substance 1 °C
Specific Heats






Liquid water = 4.184 J
Aluminum = 0.89 J
Gold = 0.13 J
This explains why certain things heat up faster
than others
The pot heats up faster than the water in it
The water in the pool is colder that the cement
around it
Now for another equation



The amount of energy required = the specific heat x
mass x change in temperature
Q = m x Cp x ∆T
Try this sample


A 1.6 g sample of metal that looks like gold requires 5.8
J of energy to change its temperature from 23 °C to 41
°C. Is the metal gold? (Hint – you are finding what s is
and comparing to what you know about gold’s specific
heat)
Answer = No; Gold’s s = 0.13 J/ g °C but this
substance has an s = 0.20 J / g °C
10.6 Thermochemistry (Enthalpy)


Enthalpy (symbol = H) is the same as the flow
of heat
∆Hp = heat
tells us it occurred under constant pressure
∆ means “change in”
So the enthalpy for a reaction at constant pressure
is the same as heat
 P


Calorimetry




Calorimeter = device used to determine the
heat associated with a chemical reaction
Reaction is run in calorimeter and temperature
change is observed
We can use calorimeter to find ∆H
Once we know ∆H for some reactions we can
use those to calculate ∆H for other reactions
10.7 Hess’s Law



The change in enthalpy for a given process is
independent of the pathway for the process (this
means it is a state function)
Hess’s Law states that the change in enthalpy from
reactants to products in a reaction is the same whether
it takes place in one step or a series of steps
N2 + 2O2 → 2NO2
∆H = 68 kJ
or



N2 + O2 → 2NO
∆H = 180 kJ
2NO + O2 → 2NO2
∆H = -112 kJ
So 180 kJ + (-112 kJ) = ∆H = 68 kJ
Characteristics of Enthalpy Changes

If a reaction is reversed, ∆H is reversed



Xe + 2F2 → XeF4
XeF4 → Xe + 2F2
∆H = -251 kJ
∆H = +251 kJ
Magnitude of ∆H is proportional to quantities
of reactants and products


Xe + 2F2 → XeF4
2(Xe + 2F2 → XeF4)
∆H = -251 kJ
∆H = -502 kJ
10.8 Quality versus Quantity of
Energy




One of the most important characteristics is
that it is conserved
Eventually all energy will take the form of heat
and spread evenly throughout the universe and
everything will be the same temperature
This means work won’t be able to be done and
universe will be dead; called “heat death”
We care more about what kind of energy
(quality) than the amount of energy (quantity)
10.9 Energy and Our World

Fossil Fuels formed by decaying products of
plants




Petroleum
Natural Gas
Coal
Greenhouse Effect – Visible light travels
through atmosphere, converted to infrared
radiation (heat) which is absorbed by certain
molecules, H20 and CO2 mainly, which radiate
it back to earth
10.10 Energy as a Driving Force

Energy Spread – in any given process,
concentrated energy is dispersed widely



Matter Spread – molecules of a substance are
spread out and occupy a larger volume


Happens with every exothermic reaction
When gas is burned, energy stored is dispersed
into surrounding air
Salt dissolves in water due to matter spread
These 2 processes are important driving forces
that cause events to occur
Entropy





Invented function that keeps track of disorder
Entropy (S) is a measure of disorder or
randomness
So a cube of ice has a a lower S value than
steam
Energy spread and Matter spread lead to
greater entropy
The entropy in the universe is always
increasing