Energetics/Thermochemistry
Outline for Energetics
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1. endothermic and exothermic reactions
2. units of energy
3. specific heat, and molar heat
4. molar heats of combustion
5. heats or enthalpies of formation ∆Hf
6. Hess’s Law
7. Bond enthalpies (juniors only)
Endothermic and exothermic
reactions
• Endothermic
– Reactions which absorb energy or have
energy added in order to occur.
– Products have more stored energy than
reactants
– Products are less stable than reactants
– Writing the reactions
• kJ are written on the reactant side
• ∆H is positive
EXAMPLE OF AN ENDOTHERMIC
REACTION
Endothermic and exothermic
reactions continued
• Exothermic
– Reactions which release energy
– Products have less stored energy than the
reactants
– Products are more stable than the reactants
– Writing the reactions:
• The kJ are written on the products side
• ∆H is negative
EXAMPLE OF AN EXOTHERMIC
REACTION
Writing endothermic or exothermic
reactions
• Endothermic
• Example:
kJ +2H2O2H2 + O2
The kJ are written on the
reactant side.
2H2O2H2 + O2 ;∆H=kJ
The ∆H is positive.
• Exothermic
• Example:
2H2 + O2  2H2O + kJ
The kJ are written on the
product side.
2H2 + O2  2H2O;
∆H = - kJ The ∆H is
negative.
Enthalpy or change in heat
• Symbol for enthalpy is ∆H.
• - ∆H means energy is released such as
with an exothermic reaction.
• + ∆H means energy is used or absorbed
such as with an endothermic reaction.
Units of Energy
• Joule (J) (the unit used in the math)
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1newton.meter2/second2
Is the SI unit for energy
4.18J=1 calorie
1000J = 1 kilojoule (kJ)
• calorie (c)
– energy required to raise the temperature of 1 gram of
water by 1◦C
– 1000calories = 1kilocalorie (kcal) or 1 food calorie
(Calorie)
– 0.24cal = 1 joule
Bomb calorimeter
Math with energetics
Specific and molar heat
The heating or cooling of a substance
1)Heat Capacity- the amount of energy a substance can
absorb before its temperature is increased. General
equation is:
C = heat absorbed/increase in temp
a) Molar heat capacity: the energy required to raise the
temp of 1 mole of a substance by 1◦C. (units =J/mol·C)
b) Specific Heat: the energy required to raise the
temperature of 1 gram of a substance by 1◦C
symbol= cp units= J/g°C
examples: 4.18J/gC for H2O, 0.45J/gC for Fe,
0.71J/gC for carbon.
Equation for Specific heat
• Equation using specific heat
q= cp x m x ∆T
where:
cp is specific heat
m=mass in grams
∆T= change in temp.
q= energy in joules
Problems with specific heat
• Example 1: Find the energy needed to
raise the temp of 5.00x102ml of water from
20. C to 100. C. Assume no energy is lost
to the surroundings.
• Q=cpx m x ∆T Q=?, m= 5.0x102g (1g=1ml
for water), ∆T =100-20= 80 C, cp=4.18J/gC
• Substitute into the equation:
• Q= 4.18J/g C x 5.0x102g x 80. C
• Q= 1.7x105J or 170kJ
Problems with specific heat
• Example 2: A 20.0 g metal sample is
heated to 200 C and then dropped into
100.ml of water. Both the metal sample
and the water ended up with a final temp
of 20 C. Find the metal’s specific heat if
the water was 15 C before the metal was
placed into it.
Example 2 continued
In order to find the cp of the metal, there is:
m= 20.0g. ∆T= 200-20= 180 C. but Q=?, and cp=?
There are too many variables. However, the metal released
its heat into the water so we can find the Q by finding the
energy that went to heat the water.
Step 1: Find the energy to heat the water:
Q= ?, m= 100g H2O, cp= 4.18J/gC, ∆T= 20-15C= 5C
Q= 4.18J/gC x100g x 5C Q= 2090 J or 2.1 x 103J
Step 2: Find the specifc heat of the metal:
Q= 2090J, m= 20.0g, cp = ? ∆T= 200-20=180C
2090J= cp x 20.0g x 180C, cp= 0.58J/gC
Heats or Enthalpies of Formation
∆Hf◦
• What is a ∆Hf ◦?
– Used to calculate the energy involved in a reaction with out
experimenting.
– It is the energy content for one mole of a compound.
– It is the energy involved in making (forming):
• one mole of a compound
• From its simplest elements
• At 25 C and 1 atm.
– These values are used to determine the ∆H for a reaction.
– Writing the equation for a ∆Hf
examples; H2O(g): H2 + ½ O2 H2O;
∆Hf =- 242kj/mole This value is from an appendix
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◦
Bond Enthalpies
Bond Enthalpies
1.
Energy is required to break bonds, energy is released
when making bonds.
*Exothermic: more energy was released when making
bonds in the products than what energy was absorbed
to break bonds in the reactants.
*Endothermic: more energy was required to break the
bonds in the reactants than what energy was released
when making bonds in the products.
Bond Enthalpies(continued)
2. Bond Enthalpies:
• The average energy required to break a covalent bonds.
-@ 25 degrees Celsius, 1 ATM, always work w/ gases
-compound is turned into single gaseous atoms, not its
simplest, stable form in nature
Ex: C-H(g)C+H
413 KJ/mol
O2(g)O+O
495 KJ/mol
• Single bonds require less energy to break than double
bonds, < triple bonds.
• EXAMPLES: WRITE ON BOARD
Molar Heats of Combustion
• 1.Combustion reactions:
a) requirements for combustion
i) fuel
ii) ignition
iii) O2
b) products of complete combustion
i) energy & light
ii) stable compounds, each w/ oxygen in it
CCO2, HH2O, S SO2
Piston in the internal combustion
engine
continued
c) Writing Combustion Reactions
CH 4(g)+2O2  CO2(g)+3H2O(g)+KJ
2. Incomplete Combustion:
a) Why this happens: i) not enough O2
ii) not enough time
iii) not enough surface area
b) products made from incomplete combustion:
CO, ash, soot as well as CO2 and H2O