Chemistry For Engineers - Thermochem

CHEMISTRY FOR ENGINEERS kinetic energy of 15.75 J?
The density of iron is

7.87 g/cm3.
Chapter 1: Thermochemistry
2. What is the potential energy of an object whise
Defining Energy mass is 5 kg that is placed 10 m above ground
Kinetic Energy - energy in motion level?
1
KE  mv 2 3. The total kinetic energy of a sample’s molecules
2
KE - kinetic energy is 100 J. The total potential energy is 50 J. What is
m - mass of the object its internal energy?
v - velocity of the object
4. How many kJ of work is done if a cylinder’s
Potential Energy - energy by virtue of an object’s volume decreases by 3.27 L against a constant
position/ elevation pressure of 1 atm?
PE  mgh
PE - potential energy of the body
g - gravitational acceleration (9.81 m/s2)
h - the object’s height/ elevation
Internal energy - combination of the potential and

kinetic energies of the atoms and molecules of a
substance
Chemical energy - energy released in the formation Units of Energy

of bonds in a chemical reaction
calorie - amount of energy required to heat one
Radiant energy - energy from light/ gram of water by 1°C
electromagnetic radiation
Joule (J) - SI unit of energy defined as 1 kg-m2/s2
Mechanical energy - energy from the movement of
macroscopic objects Calorie (Cal) - energy unit in food, defined as 1000
calories
Thermal energy - energy that arises from the
temperature of an object Btu (British thermal unit) - amount of energy
required to heat one pound of water by 1°F
Electrical energy - energy that arises from moving
charges Conversion: 1 Btu = 252 calories
Nuclear energy - energy that comes from nuclear Energy Transformation and Conservation
fission and fusion processes
System - part of the Universe being considered
Heat (q) - flow of energy between two objects due
to a thermal difference Surroundings - remainder of the Universe being
considered
Work (w) - transfer of energy accomplished by a
force moving an object a certain distance against Boundary - separation between the system and the
resistance surroundings
Pressure - volume (PV) work - most common type
of work encountered in chemistry First Law of Thermodynamics - energy can neither
be created nor destroyed
Sample Problems E  q  w
1. How fast (in meters per second) must an iron ball
with a mass of 56.6 g be traveling in order to have a E  E final  Einitial
Path properties - properties that depend on the path chemical reaction that occurs at constant pressure
that the system takes to get there. Example: q and w
Exothermic - heat is evolved from the system
State properties - properties that are independent of
the path taken to get there. Example: temperature, Endothermic - heat is absorbed from the system
volume and enthalpy
Fusion - also known as melting, change from the
State properties solid phase to the liquid phase
Sample Problems
If a machine does 4.8 × 103 kJ of work after an Freezing - change from the liquid phase to the solid
input of 7.31 × 104 kJ of heat, what is the change in phase
internal energy for the machine?
Vaporization - change from the liquid phase to the
Heat Capacity and Calorimetry vapor phase
Calorimetry - observation of heat flow in and out of Condensation - change from the vapor phase to the
the system using various techniques liquid phase
Specific heat capacity - measures the heat required Sublimation - change from the vapor phase to the
to heat one gram of a material by 1°C solid phase
Molar heat capacity - measures the heat required to Deposition - change from the solid phase to the
heat one mole of a material by 1°C vapor phase
Sample Problem:
1. Heating a 24.0-g aluminum can raises its
temperature by 15.0°C. Find the value of
q for the can (Cp= 0.9 J/g-°C).
2. The molar heat capacity of liquid water is 75.3

J/mol K. If 37.5 g of water is cooled from 42.0 to
7.0°C, what is q for the water? For water:
Specific heat of ice = 2.108 kJ/kg-K
3. A glass contains 250.0 g of warm water at Specific heat of water = 4.187 kJ/kg-K
78.0°C. A piece of gold at 2.30°C is placed in the Specific heat of steam = 1.996 kJ/kg-K
water. The final temperature reached by this system Latent heat of fusion = 333.55 kJ/kg
is 76.9°C. What was the mass of gold? The specific Latent heat of vaporization = 2257 kJ/kg
heat of water is 4.184 J/g °C, and that of gold is
0.129 J/g °C. The total enthalpy change is equal to the sum of
the individual enthalpy changes that the sytem
4. A calorimeter is to be used to compare the energy undergoes.
content of some fuels. In the calibration of the
calorimeter, an electrical resistance heater supplies
100.0 J of heat and a temperature increase of Sample Problems:
0.850°C is observed. Then 0.245 g of a particular 1. The heat of fusion of pure silicon is 43.4 kJ/mol.
fuel is burned in this same calorimeter, and the How much energy would be needed to melt a 5.24-
temperature increases by 5.23°C. Calculate the g sample of silicon at its melting point of 1693 K?
energy density of this fuel, which is the amount of
energy liberated per gram of fuel burned. 2. ΔHvap = 31.3 kJ/mol for acetone. If 1.40 kg of
water were vaporized to steam in a boiler, how
much acetone (in kg) would need to be vaporized to
use the same amount of heat?
3. Calculate the energy required to convert 1.70 g of

Enthalpy - heat absorbed/released as a result of a ice originally at –12.0°C into steam at 105°C.
oxide, what quantity of thermal energy (in kJ) is
Hess’ Law and Heats of Reaction absorbed or evolved?
Thermochemical equation - summary of the over- Energy and Stochiometry

all energetics of a chemical reaction
Sample Problem
Standard Enthalpy Change for Heat of Formation
(Δ°Hf ) - enthalpy change when one mole of a Nitroglycerine, C3H5(NO3)3(l), is an explosive most
substance is formed from its elements at standard often used in mine or quarry blasting. It is a
state powerful explosive because four gases (N2, O2,
CO2, and steam) are formed when nitroglycerine is
Standard state - Substances in their natural phase at detonated. In addition, 6.26 kJ of heat is given off
physical conditions of 298.15 K and 1 atm per gram of nitroglycerine detonated.
Values for Δ°Hf can be looked up in tables like (a) Write a balanced thermochemical equation for
this one: the reaction.
(b) What is ΔH when 4.65 mol of products is

formed?
Cumulative Problems
1. A runner generates 418 kJ of energy per

kilometer from the cellular oxidation of food. The
runner’s body must dissipate this heat or the body
will overheat. Suppose that sweat evaporation is the
only important cooling mechanism. If you estimate
the enthalpy of evaporation of water as 44 kJ/mol
and assume that sweat can be treated as
Hess’ Law - the enthalpy change for any process is water,describe how you would estimate the volume
independent of the particular way the process is of sweat that would have to be evaporated if the
carried out. Enthalpy is a state property. runner runs a 10-km race.
Rules:
1. The reaction can be reversed. This will change
the sign of ΔHf.
2. The reaction can be multiplied by a constant. The
value of ΔHf must be multiplied by the same
constant.
3. Any combination of the first two rules may be
used.
Sample Problem:
Using these reactions, find the standard enthalpy

change for the formation of 1 mol PbO(s) from lead
metal and oxygen gas.
PbO(s) + C(graphite) → Pb(s) + CO(g) ΔH° =

106.8 kJ
2C(graphite) + O2(g) → 2CO(g) ΔH°= –221.0 kJ
If 250. g of lead reacts with oxygen to form lead(II)

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CHEMISTRY FOR ENGINEERS kinetic energy of 15.75 J?

The density of iron is

Internal energy - combination of the potential and

Chemical energy - energy released in the formation Units of Energy

2. The molar heat capacity of liquid water is 75.3

3. Calculate the energy required to convert 1.70 g of

Thermochemical equation - summary of the over- Energy and Stochiometry

(b) What is ΔH when 4.65 mol of products is

1. A runner generates 418 kJ of energy per

Using these reactions, find the standard enthalpy

PbO(s) + C(graphite) → Pb(s) + CO(g) ΔH° =

2C(graphite) + O2(g) → 2CO(g) ΔH°= –221.0 kJ

If 250. g of lead reacts with oxygen to form lead(II)

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