Download Chapter-1 Energy and Change

Energy and Change
In all chemical and physical changes there is a change in energy. In all chemical changes, chemical
bonds are broken or formed. Energy is required to break a chemical bond (just as energy is required to
stretch a spring until it breaks). Conversely, forming a chemical bond releases energy. Virtually all
chemical reactions absorb or release energy because bond formation seldom exactly balances bond
breaking in the reaction.
The concept of energy:
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the usual definition of energy: the ability to cause change or do work.
o work is moving an object against an opposing force
o work = force × distance
o SI unit of work or energy: the joule (J)
two basic forms of energy
o potential energy: energy of position or composition
 examples
 boulder on a ledge
 chemical bonds
o kinetic energy: energy of motion
 examples
 landslide
 waves
why is the concept of energy useful?
o if something is isolated from everything else, its total energy never changes (Law of
Conservation of Energy)
o this allows seemingly unrelated behaviors of the system to be connected
o example: the pendulum
Two things energy is NOT
o some sort of invisible fluid
o something which can be measured directly
Energy and Change:
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Every change in a substance (physical or chemical) is accompanied by a change in energy.
Chemical changes are accompanied by greater energy changes than physical changes.
During a chemical reaction, one or more substances (reactants) are changed into one or more
new substances (products).
2Na + Cl2  2NaCl
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Reactants: substances that exist before the chemical change begins. Reactants are given on the
left-hand side of “yield ()” sign).
A + B  AB
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Products: new substance or substances produced as a result of a chemical reaction. Products are
given on the right hand side of “yield” sign.
A + B  AB
Exothermic Change:
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chemical reaction (or physical change) that releases heat.
A + B  AB + heat
4Fe(s) + 3O2(g)  2Fe2O3(s)
∆H = -1625 kJ
water  ice; steam  water
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∆H is called the change in enthalpy and is better known as the heat of reaction.
When ∆H is given in a reaction it is called a thermochemical reaction.
Thermochemical reactions are balanced chemical equations that include the physical states of
all reactants and products and the energy change. If energy (∆H) is positive (a reactant), the
reaction is endothermic but if energy (∆H) is negative (a product), the reaction is exothermic.
Endothermic:
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chemical reactions (or physical change that absorb energy.
A + B + heat  AB
H2(g) + S(s)  H2S(g)
∆H = 33.0 kJ
ice  water
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Activation Energy:
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initial input of energy required to get a reaction going.
1. Energy increases going from reactants to the top of the curve.
2. At the top of the curve:
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reactant bonds have been broken (activated complex/transition state)
activation energy: energy required to break reactant bonds; the minimum energy
required for the reaction to occur.
activation energy (bond energy): always endothermic (always positive)
catalysts work by lowering activation energy
From the top of the curve to the products, energy decreases. Energy is given off as bonds are formed
(energy of formation).
4. Bond formation is always exothermic.
5. The difference in energy between bond energy and energy of formation determines whether the
overall reaction is exothermic or endothermic.
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For an exothermic reaction, ∆H is negative.
For an endothermic reaction, ∆H is positive.
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Catalysts speed up a chemical reaction by lowering the activation energy. Chemical reactions can be
catalyzed by specialized proteins known as enzymes. Catalysis of biochemical reactions in the cell is
important because of the very low reaction rates of the uncatalyzed reactions.
Temperature Conversions
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Fahrenheit = 1.8oC + 32
Celsius = oF - 32
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Celsius (centigrade) is the most common for scientific work. Water boils at 100oC, 212oF
Kelvin = oC + 273
Absolute zero is “0” reading on the Kelvin Scale and represents the point where the average
kinetic energy of a substance is zero.
Heat and Temperature
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Heat and temperature are thought to be the same, but they are not.
Temperature is not energy.
Heat is energy.
Temperature is a measure of the average kinetic energy of the molecules in a sample of matter.
Temperature is directly proportional to kinetic energy. The higher the temperature the greater
the kinetic energy.
Any two substances having the same temperature also have the same average kinetic energy.
Heat is the total amount of energy possessed by the molecules in a sample of matter.
This total energy is the sum of both the kinetic energy and the potential energy.
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Heat Transfer
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Temperature measurement relies on heat transfer from higher concentrations of heat to lower
concentrations of heat.
In order for heat to flow or transfer:
There has to be a temperature difference. Energy only flows as heat if there is a temperature
difference.
Energy as heat flows from a higher temperature to a lower temperature.
The greater or larger the difference in temperature, the faster the energy flows.
Types of Heat Transfer:
Conduction is the transfer of energy through matter by direct contact of particles. Conduction takes
place primarily in solids but may also occur in liquids and gases to a lesser extent.
• Substances that allow heat to travel through are called conductors and those that do not are
called insulators.
If you pick up the handle of a cast iron frying pan from a hot stove you’ll experience
conduction! The heat reaches your hand via conduction from the burner to the bottom of the
pan through the metal handle to your hand.
Convection is the transfer of energy by the bulk movement of the heated substance primarily by a
liquid or gas (such as air). Hot air rises, cool air descends. Circulatory air motion due to warmer
air rising and cooler air falling is a common mechanism by which thermal energy is transferred.
Radiant heat transfer occurs between objects that are not touching. The transfer of energy in the
form of electromagnetic waves (radio waves, microwaves, x-rays, infrared waves, etc.) through
space (vacuum).
• Shiny and light colored materials reflect energy, dull and dark colored materials absorb energy.
• Matter must be present for conduction or convection to take place, but not radiation.
• The sun heating the earth is an example of radiant heat transfer. The sun warms the earth
without warming the space between the sun and the earth. The sun radiates heat to the roof,
which in turn radiates heat down toward the ceiling.
Heat and Its Measurement:
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When you have two samples of matter at different temperatures, heat will flow from higher to
lower concentrations (hot  cold) until the temperatures are equal.
Three factors determine the quantity of heat gained or lost during a temperature change:
The quantity (mass) of the matter changing temperature,
The nature (type) of matter changing temperature (specific heat), and
The size of the temperature change.
T = change in temperature
T = final temperature – initial temperature
T = Tf - Ti
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Specific Heat (Cp): the amount of heat required to raise the temperature of 1g of a substance
1oC. Specific heat is an intensive physical property and varies for each substance.
Joule (J): is the SI unit of heat energy and all other forms of energy.
Calorie (cal): non SI unit of energy defined as the amount of heat required to raise the
temperature of 1g of water one degree centigrade:
1 cal  4.2 joules;
1 C (food Calorie) = 1,000 cal or 1 kcal;
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Calorimeter: a device used to measure heat gained or lost during a chemical reaction. The
measurement of heat gained and lost is called calorimetry.
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To Calculate Heat Gained or Lost:
• q = mcpT
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cp = _q_ = _J_
mT g oC
q = amount of heat transferred (gained or lost)
m = mass of substance
Cp (or s) = specific heat
T = change in temperature = Tfinal – Tinitial = Tf - Ti
Substance (at 25oC and 1 atm)
Specific heat (J/g K)
ice
2.09
water
4.18
steam
1.86
sodium
1.23
aluminum
0.9
iron
0.45
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