Download Thermochemistry Thermochemistry is the science of

Thermochemistry
Thermochemistry is the science of relationships between heat and energy, which
is one area of thermodynamics - the study of energy and its transformations.
Energy is the potential or capacity to move matter.
Work = force x distance
Heat is the energy that is transferred from one object to another because of a
difference in temperature.
Kinetic Energy is the energy associated with an object by virtue of its motion.
Ek = 1/2 mν2
velocity, ν = distance/ time
SI Units for energy = kg x m2 / s2 = J (joule)
calorie - a non SI Unit of energy commonly used, is defined as the energy needed
to raise the temperature of one gram of water by one degree Celsius.
calorie = 4.184 J
Example: A pitcher throws a baseball on avergage 60 to 90 miles/hr. A regulation
baseball weighing 143 g is traveling 75 mile/ hr. What is its kinetic energy in
joules and calories?
Potential Energy is stored energy
EP = mgh
where m = mass, g = constant acceleration of gravity and h = height
Total Energy is the sum of the kinetic and potential energies plus its internal
energy.
Etot = Ek + Ep + U
Internal Energy, U = total energy = ∆E
∆E = Efinal - Einital
+ ∆E then Ef > Ei energy is gained from the surroundings
- ∆E then Ef < Ei energy is lost to the surroundings
Law of Conservation of Energy, 1st Law of Thermodynamics, says that the
energy may converted from one form to another, but the total quantity if energy
remains constant
Systems and Surroundings
System is the substance or substances
that are under study in which a change
occurs.
Surroundings are everything else in
the vicinity
Heat, q, is energy that flows in and out of a system because of a difference in
temperature between system and surroundings.
+q = heat is absorbed by the system
-q = heat is evolved or released by the system
The Heat of Reaction (at a given temperature) is the value of q required to return a
system to the given temperature at the completion of the reaction.
Exothermic process is a chemical reaction or a physical change in which heat is
evolved, q = -
Endothermic process is a chemical reaction or a physical change in which heat is
absorbed, q = +
Example: Ammonia burns in the presence of a palatium catalyst to give nitric
oxide, NO
4NH3(g) + 5O2(g)
4NO(g) + 6H2O(l)
In an experiment, 4 moles of NH3 is burned and evolves 1170 kJ of heat. Is the
reaction endothermic or exothermic? What is the value of q?
Endothermic
q=+
The reaction produces a
cold solution
Exothermic
q=The reaction produces a hot
solution and dissipates into the
surroundings
Enthalpy (H) is an extensive property of a substance that can be used to obtain the
heat absorbed or evolved in a chemical reaction under constant pressure.
Enthalpy is a state function, a property of a system that depends only on its present
state, which is determined by variables such as temperature and pressure, and is
independent of any previous history of the system.
∆H = qp heat added or lost by the system when the process occurs under constant
pressure
∆H = ∆Hfinal - ∆Hinitial
+ ∆H value - ∆Hfinal > ∆Hinitial gained heat
- ∆H value - ∆Hfinal < ∆Hinitial lost heat to surroundings
The ∆H for a reaction is equal in magnitude but opposite in sign to the ∆H for the
reverse reaction.
CO2(g) + 2H2O(g)
CH4(g) + 2O2(g)
CH4(g) + 2O2(g)
∆H = 802 kJ
CO2(g) + 2H2O(g) ∆H = -802 kJ
The ∆H for a reaction depends on the state of the reactants and products
H = U + PV
where U is the internal energy, P is the pressure, and V is volume.
A Thermochemical Equation is the chemical equation for a reaction (including
phase labels) in which the equation is given a molar interpretation, and the
enthalpy of reaction for these molar amounts is written directly after the equation.
N2 (g) + 3H2(g)
2NH3(g)
∆H = -91.8kJ
2N2 (g) + 6H2(g)
4NH3(g)
∆H = -184 kJ
2NH3(g)
N2 (g) + 3H2(g) ∆H = +91.8kJ
Calculating the Heat from Stoichiometry
the following equation? Assume the reaction happens at constant pressure.
N2 (g) + 3H2(g)
2NH3(g)
∆H = -91.8kJ
How much heat evolves when 10.0g of hydrazine reacts according to the following
reaction
2N2H4(l) + N2O4(l)
3N2(g) + 4H2O(g) ∆H = -1049 kJ
Heat is required to raise the temperature of a given amount of a substance, and the
quantity of heat depends on the temperature change.
Heat Capacity (C) is the quantity of heat needed to raise the temperature of the
sample of substance one degree Celsius.
q = C∆t
where ∆t = tf - ti
A piece of iron requires 6.70 J of heat to raise the temperature by one degree
Celsius. What is the quantity of heat required to raise the temperature of the piece
of iron from 25.0ºC to 35.0ºC?
Specific Heat is the quantity of heat required to raise the temperature of one gram
of a substance by one degree Celsius at constant pressure.
q = s
x
m
x
∆t
Calculate the heat absorbed by
15.0g of water to raise its
temperature from 20.0ºC to 50.0ºC
(at constant pressure)
Iron metal has a specific heat of 0.449 J/(g . ºC). How much heat is transferred to a
5.00 g piece of iron, initially at 20.0ºC, when it is placed in a pot of boiling water?
Assume that the temperature of the water is 100.0ºC and that the water remains at
this temperature, which is the final temperature of the iron.
Suppose 0.562 g of graphite is placed in a calorimeter with an excess of oxygen at
25.00ºC and 1 atm pressure. Excess O2 ensures that all the carbon burns to form
CO2. The graphite is ignited and it burns according to the equation
C(graphite) + O2(g)
CO2(g)
On reaction, the calorimeter temperature rises from 25.00ºC to 25.89ºC. The heat
capacity of the calorimeter and its contents was determined in a separate
experiment to be 20.7 kJ/ºC. What is the heat of reaction at 25.00ºC and 1 atm?
Express the answer in a thermochemical equation.
Hess's Law of Heat of Summation staes that for a chemical equation that can be
written as the sum of two or more steps, the enthalpy change for the overall
equation equals the sum of the enthalpy changes for the individual steps.
Example: Suppose you would like to find the enthalpy change for the following
reaction:
2C(graphite) + O2(g)
CO (g)
The direct determination for this change in enthalpy is very difficult. Applying
Hess's law gives us the option to use more than equation to determine this value.
Standard Enthalpy of Formation - because Hess's Law relates the enthalpy changes
of some reactions to the enthalpy changes of others, we only need to tabulate the
enthalpy changes of certain types of reactions. We also list enthalpy changes in a
standard state
Standard State refers to the standard thermodynamic conditions chosen for
substances when listing or comparing thermodynamic data: 1 atm pressure and a
specific temperature of 25ºC.
∆Hº = standard enthalpy of reaction
∆Hfº = standard enthalpy of formation
∆Hfº for diamond equals the enthalpy change from the stablest form of carbon
(graphite) to diamond
C(graphite)
C(diamond)
∆Hfº = 1.9 kJ
∆Hfº fro graphite = 0, the reference forms have zero values
In general, we can calculate the ∆Hº for a reaction by the equation
∆Hº = Σ n∆Hfº (products) - Σ m∆Hfº(reactants)
Example: Calculate the heat of vaporization, ∆Hvapº of water, using standard
enthalpies of formation (Table 6.2)
Calculate the enthalpy change for the following reaction:
3NO2(g) + H2O(l)
2HNO3(aq) + NO(g)