Transcript saulvillalobos.files.wordpress.com

Thermochemistry
Nature of Energy
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Kinetic Energy
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Ek = ½mv2
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Potential Energy
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Ep = mgh
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m=mass (kg)
v=velocity (m/s)
g = gravity (9.8 m/s2)
h = height (m)
Chemical Energy
Units of Energy
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Joule (J) – 1 kg-m2/s2
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Small amount of energy so we use kJ
Calorie (cal) – amount of energy needed to
raise the temperature of 1 gram of water by 1
degree
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Normally uses Cal = 1000 cal = 1kcal
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1 cal = 4.184 J
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How many calories are there in 15 joules?
Transfer of Energy
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To understand energy transfer we must define
two quantities:
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System
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Surroundings
Energy transferred in two ways
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Work (w) – causes motion
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Closed vs Open
W=Fxd
Heat (q) – causes temperature change
Energy is the ability to do work or transfer heat.
First Law of Thermodynamics
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1st Law = Energy is
conserved.
Internal energy of a
system = sum of all
kinetic and potential
energy of the
components of the
system
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Internal Energy = E
We measure ΔE =
Efinal – Einitial
Heat and Work
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ΔE = q + w
q > 0 when heat is
added to the system
w > 0 when work is
done on the system
Endothermic
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q>0
Exothermic
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q<0
Heat, Work and Internal Energy
Energy Practice
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If hydrogen and oxygen gases are trapped in a
cylinder and ignited water is produced. As the
reaction occurs, the system loses 1150J of heat
to the surroundings. The reaction also causes a
piston to rise as the hot gases expand. The
expanding gas does 480J of work on the
surroundings as it pushes against the
atmosphere. What is the change in internal
energy?
Energy Practice
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Calculate the change in the internal energy of
the system for a process in which the system
absorbs 140J of heat from the surroundings and
does 85J of work on the surroundings.
Calculate ΔE and determine where the process
is exo- or endothermic for the following cases.
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A system releases 133kJ of heat to the
surroundings and does 39 kJ of work on the
surroundings
The system absorbs 77.5kJ of heat while doing
63.5kJ of work on the surroundings
State Functions
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State function = a
function whose value
only depends on its
present condition not
on the history of the
sample
State Functions
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ΔE = q + w
Enthalpy
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Usually the only kind
of work done by a
chemical reaction is
mechanical work
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Reactions normally
done at constant P
Zn(s) + 2H+(aq) → Zn2
+
(aq) + H2(g)
W = -PΔV
Constant Volume vs. Pressure
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ΔE = q + w
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W = -PΔV
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qv= ΔE
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qp = ΔE + PΔV
Enthalpy
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H = enthalpy
Enthalpy measures the heat flow in chemical
changes occurring at constant pressure
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H = E + PV
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ΔH = ΔE + PΔV
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ΔH = qp
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When is ΔH positive and when is it negative?
Exo- and Endothermic
Enthalpy Practice
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Indicate the sign of the enthalpy change, ΔH, in
each of the following processes carried out
under atmospheric pressure and indicate
whether the process is exo- or endothermic.
a) an Ice cube melts
b) 1g of butane (C4H10) of combusted in sufficient
oxygen to give complete combustion to CO2 and
H2O
c) a Bowling is dropped from a height of 8 ft into a
bucket sand
Enthalpy Practice
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Suppose we confine 1g of butane and sufficient
oxygen to completely combust it in a cylinder.
The cylinder is perfectly insulating, so no hear
can escape to the surrounding. A spark initiates
combustion of the butane, which forms carbon
dioxide and water vapor. If we used this
apparatus to measure enthalpy change in the
reaction, would the piston rise, fall, or stay the
same?
Hint write a balanced reaction
Enthalpies of Reaction
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ΔH = Hfinal – Hinitial
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Enthalpy of products – enthalpy of reactants
Heat of Reaction (ΔHrxn) = Enthalpy change of
the reaction
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2H2(g) + O2(g) → 2H2O(g) ΔH = -483.6 kJ
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Thermochemical equation
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Enthalpy Diagram
Enthalpy Guidelines
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Enthalpy is an extensive property
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CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ΔH = -890kJ
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1 mol of CH4 w/ 2 mol of O2 releases 890kJ
2 mol of CH4 w/ 4 mol of O2 releases 1780kJ
How much heat is released when 4.50g of
methane is burned in a constant pressure
system?
Concept Practice
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Hydrogen peroxide can decompose to water
and oxygen by the following reaction:
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2H2O2(l) → 2H2O(l) + O2(g) ΔH = -196kJ
Calculate the value of q when 5.00g of H2O2(l)
decomposes at constant pressure.
Enthalpy Guidelines
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The enthalpy change
for a reaction is equal
in magnitude but
opposite in sign to ΔH
for the reverse
reaction.
The enthalpy change
for a reaction
depends on the state
of the reactants and
products.
Concept Practice
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Consider the following reaction:
2Mg(s) + O2(g) → 2MgO(s) ΔH = -1204kJ
a) Is the reaction exo- or endothermic?
b) Calculate the amount of heat transferred when
2.4g of Mg(s) reacts at constant pressure.
c) How many grams of MgO are produced during an
enthalpy change of -96.0kJ?
d) How many kJ of heat are absorbed when 7.50g of
MgO(s) are decomposed into Mg(s) and O2(g) at
constant pressure?
Homework Set 1
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12, 17, 20, 21, 22, 25, 29, 34, 35, 41