CHEM10007 Lecture Notes - Lecture 17: Calorimetry, Lithium Chloride, High Energy Stereoscopic System
12 Jun 2018
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LECTURE 17 - CALORIMETRY & HESS‘ LAW
CHAPTER 8 - CHEMICAL THERMODYNAMICS
8.2 THE FIRST LAW OF THERMODYNAMICS
•The energy transferred as heat during chemical reactinos comes from an object’s internal energy.
Internal energy is the sum of energies for all of the individual particles in a sample of matter, and
it is given the symbol U.
•In studying chemical and physical changes, we are interested in the change in internal energy
(ΔU) that accompanies a particular process.
ΔU = U(final) - U(initial)
•For a chemical reaction, U(final) corresponds to the internal energy of the products, so it can be
written as U(products). Similarly, U(reactants) = U(initial).
•If the change in a thermodynamic quantity is for a chemical reaction, we add an ‘r’ to the symbol.
ΔrU = U(products) - U(reactants)
•When we dissolve certain substances (Eg. NaCl) in water, the system absorbs energy from its
surroundings, and the final energy is therefore greater than the initial energy.
•Therefore ΔrU = U(products) - U(reactants) > 0.
•When other certain substances are dissolved in water (Eg. LiCl), the system gives out energy to its
surroundings, and its final energy is therefore less than the initial energy.
•Therefore ΔrU = U(products) - U(reactants) < 0.
•Energy can also be exchanged between the system and surroundings if the system does work on
the surroundings or the surroundings do work on the system. This work (in a chemical reaction)
is usually the compression or expansion of gas produced in the reaction.
•The magnitude of the work done during a volume change, ΔV, against an opposing pressure p, is
given by the equation;
w = -pΔV
•If a gas expands, V(final) > V(initial), and so ΔV is positive. The work is therefore negative for
this process at constant temperature.
•If a gas is compressed, V(final) < V(initial) and so ΔV is negative, and the work is therefore
positive for this process.
•Heat and work are the only ways by which a closed chemical system can exchange energy with
its surroundings, so the change in internal energy of a chemical system during a chemical or
physical change must be equal to the sum of the heat absorbed or emitted by the system and the
work done on or by the system. This is the first law of thermodynamics.
ΔU = q + w
•Where q = heat and w = work. This law means that energy can be transferred between a system
and its surroundings as either heat or work, but it can never be created or destroyed.
•The energy of an isolated system is constant.
•If there is no change in the volume then ΔV = 0 and no work is done, such as a reaction
performed in a sealed container whose volume cannot change. In this case, ΔrU = q. The internal
energy change ΔrU is the heat change at constant volume, qv:
ΔrU = qv
HEAT CAPACITY
•If we take two objects initially at different
temperatures and bring them into contact,
they will eventually both come to the same temperature, through the transfer of heat from the
hotter object to the colder object. We can say that heat is a transfer of energy due to a
temperature difference.
•There is a linear relationship between heat and temperature change;
Q = CΔT
•Where q is heat and C is the heat capacity, the quantity of heat needed to raise the temperature of
an object by 1K, of the object in question.
•Heat capacity is measured in the units J/K.
•Heat capacity depends on the size of the sample. Any property with a value that depends on the
size of the sample is called an extensive property, a property of an object that is described by a
physical quantity that is proportional to the size or amount of that object.
•Eg. Volume or mass of a system.
•Any property with a value that is the same regardless of the size of the sample is called an
intensive property, a property of an object that is described by a physical quantity (such as density
and temperature) that is independent of the size of the sample.
•Eg. temperature of a system.
•Heat capacity, an extensive property, can be turned into an intensive property called specific heat
capacity, by dividing the mass of the sample.
•Specific heat capacity is the quantity of heat that will raise the temperature of 1g of a substance
by 1K, usually in units of J/gK.
•It is a measure of how effectively a substance stores energy, is different for different substances
and is used to calculate the energy change that occurs when a mass of a compound changes temp.
•It has the symbol c and is defined as;
c = C
M
•Giving c the units of J g-1 K-1.
•Using the intensive property specific heat allows us to easily compare values for the same mass of
different substances by inspection.
•The heat required to raise the temperature of 1 mole of a substance by 1K is called the molar heat
capacity, and has units of J mol-1 K-1.
•We can calculate the heat absorbed or emitted by an object given its mass (m), temperature
change (ΔT) and specific heat (c) by;
Q = cmΔT
Quickpoll Quiz 1:
•The temperature of a 50 g block of
copper increases from 20C to 50C
after absorbing 585 J of energy. What is the
specific heat of copper?
•A "2.6 J g-1 oC-1
•B "975 J g-1 oC-1
•C "351 J g-1 oC-1
•D "0.39 J g-1 oC-1
•Quickpoll Quiz 1 Answer:
•Use the formula: q = CmΔT
•C = q/mΔT!
= 585 / (50 × 30) = 0.39 J g-1 o C-1
THE DETERMINATION OF HEAT
•The relationship between temperature change and heat can be used to determine the heat lost or
gained in a chemical reaction or physical process.
•Calorimetry is the science of measuring heat. It is based on the observation of temperature
changes when a body absorbs or releases heat.
•A calorimeter measures temperature changes and relates this to the heat associated with a
chemical reaction.
Document Summary
8. 2 the first law of thermodynamics: the energy transferred as heat during chemical reactinos comes from an object"s internal energy. U = u(final) - u(initial: for a chemical reaction, u(final) corresponds to the internal energy of the products, so it can be written as u(products). Similarly, u(reactants) = u(initial): if the change in a thermodynamic quantity is for a chemical reaction, we add an r" to the symbol. Ru = u(products) - u(reactants: when we dissolve certain substances (eg. Nacl) in water, the system absorbs energy from its surroundings, and the final energy is therefore greater than the initial energy: therefore ru = u(products) - u(reactants) > 0, when other certain substances are dissolved in water (eg. U = q + w: where q = heat and w = work. The internal energy change ru is the heat change at constant volume, qv:
