Energy Relationships in Chemical Reactions

Chapter 6

Dr. Ramy Y. Morjan

The Natural & Types of Energy

• Energy is defined as:

Capacity to Do Work

• Types of EnergyRadiant Energy Thermal EnergyChemical EnergyPotential Energy

Radiant Energy: Or solar energy, comes from the sun. It’s the earth source of energy.Thermal Energy: Results from the random motion of atoms and molecules T.E. can be calculated from temperature

measurements. More motion of atoms & molecules in a

sample gives hotter sample.

90, 30. Rem

Chemical Energy: Is stored within the structural unit of chemical substances.

The quantity of C.E. is determined by the type and arrangements of atoms.

During any chemical reaction C.E is released or stored or converted to another energy form

Potential Energy: is the stored energy of position possessed by an object.

Potential energy can be considered as form of P.E

• Two types of Potential Energy1) Gravitational Potential Energy is the energy stored in an object as

the result of its vertical position or height. The energy is stored as the result of the gravitational attraction of the Earth for the object.

2) Elastic Potential Energy is the energy stored in elastic

materials as the result of their stretching or compressing.

• All forms of energy can be converted from one form to another. That means there is no lose of energy.

• The law of conservation of energy: the total quantity of energy in the universe is assumed to be constant.

Thermal Energy & HeatAlmost all chemical reactions are

involving a change in energy (on form of heat) via either absorb or release energy.

Heat: Is the transfer of thermal energy between two bodies that are at different temperatures.

Thermochemistry is the study of heat changes in chemical reactions.

• Thermodynamics is a branch of science which deals with the energy and work of a system. In other words, thermodynamics is the study of the effects of heat, work and energy on the system.

Macroscopic properties: all the properties of a system that can be measured directly.

Intensive : A property that does not change when the amount of sample changes. Ex. density, pressure, temperature, colour, melting point, boiling point, viscosity.

Extensive:mass, volume, length, volume, mole, enthalpy, internal energy, kinetic energy.

• There are three laws of thermodynamics

• 1) Zeroth order: concerns with

thermodynamics equilibrium and

temperature.

• 2) First law: work, heat and energy.

• 3) Second law: entropy

• Important TerminologySystem: A thermodynamic system is

that part of universe which is under thermodynamic study and investigation.

30 ml HCl+

30 ml NaOH

• Surrounding: The environments that contained the system

• System and surroundings are separated from each other by a real or imaginary boundary.

• Universe = system + surroundings

• Types of System• There are three types of

thermodynamic systems• 1) Open system• 2) Closed system• 3) Isolated system

• State: T.D system is said to be in a certain state when all its properties are specified. The fundamental properties, which determine the state of a system, are:

Energy, composition, temperature, pressure and volume.

Any change in the above properties will change the state of system.

Due to this reason they are called State Functions.

State Functions: Properties that are determined by the state of the system, regardless of how that condition was achieved (bath independent)

There are two states of a system• 1) Initial state: the description of the

system before it suffers any physical or chemical change.• 2) Final state:the description of the

system after it undergoes change.

• For any change takes place in the state of a system, the value of this change depends on and can be determined from the initial and the final states.

Change in state Δ =Value final state - Value o f property in the

initial• Example:Gas at 2 atm, 300 k, 1 L (I.S)At constant Temp, P ↓ to 1 atm, V = 2L (F.S)

∆ V = Vf – Vi2-1= 1 L

Potential Energy is another example.

• The Law of Conservation of Energy

(First Law of Thermodynamics)The law of conservation of energy states

that the energy is neither created nor

destroyed but it can be changed from one form to another.

The total amount of energy remains the same regardless of what form the energy is changed into.

total amount of energy in the universe is a constant.

Forms of Energy

• Energy is found in different forms, such as light, heat, sound and motion.

Kinetic Energy KEIs motion of objects, waves, electrons, atoms, molecules and substances.

Potential Energy PEIs stored energy and the energy of position.

Internal Energy: The internal energy of a thermodynamic system E, is the total of the kinetic energy and potential energy.

The kinetic energy is due to the motion of molecules (translational, rotational, and vibrational) and the movement of electrons with in the molecules.

The potential energy is due to the attractive interactions between electrons and nuclei

and by the repulsive interactions between electrons and between nuclei in individual molecules, as well as by interaction between molecules

E = KE + PE

• Internal energy of a thermodynamic system is a state function which strictly depends upon the initial and final states of the system

• Internal Energy = K E + PE

• Calculation of the internal energy of a thermodynamic system

S(s) + O2(g) → SO2(g) + energy

• Exothermic reactions: Are reactions in which heat is released in going from reactants to product and the ΔE is negative.

• Endothermic reactions are reactions in which heat is absorbed in going from reactants to product and the ΔE is positive.

• ΔE system + Δ E surrounding = 0

• So ΔE system = -Δ E surrounding

• The First Law of Thermodynamics (another form to express)

ΔE system = Q +WQ = heat exchange between the system

and the surrounding.W = work done on or by the system.Exothermic Reactions: ΔE and Q are -Endothermic reactions: ΔE and Q are +Work done on the system by the

surrounding W is +Work done by the system on the

surrounding W is -

• Work & Heat• There is more than one type of work,

for example; mechanical work and electrical work.

• Work can be defined as the product of the force used to move an object times the

• distance the object is moved toW = F x d

• Where, F is the force and d is the distance

• An important property of any gas is its pressure, which can be defined as defined as the force per unit area

• P = F/A and A = d x d

• F = Pd2