CHEMICAL REACTIONS AND ENERGY

Energy Capacity to do work

work is done when a force is exerted over a distance. For chemists  work is energy change resulting from a process.

Types of energies: Kinetic Potential Radiant Thermal Chemical

Kinetic Energy Energy of motion (chemists’ particular ineterest)

Potential energy Available by virtue (effective force) of an object’s position relative to other objects.

Electrostatic energy is one of the most important forms of PE. It arises from the interactions between charged particles. It is directly proportional w/ the charges of 2 interacting particles & inversely proportional to the distance separating the 2 particles.

Radiant energy Comes from the sun. Earth’s primary source of energy.

Thermal energy Energy associated w/ the random motion of atoms & molecules. Thermal energy≠ temperature, but it’s dependent on the temp. The more vigorous the motion of the atoms&molecules in a sample of matter, the hotter the sample is & the greater its thermal energy.

Chemical energy is the potential energy stored in the arrangements of the atoms of the substances. When a substance participates in a chemical rxn, the chemical energy is gained, lost, or converted into another type of energy.

SI(systems integration) unit of Energy(E) Joule (J) 1 kJ=1000J 1cal= 4.186 J

1st LAW OF THERMODYNAMICS: THE LAW OF CONSERVATION OF ENERGY Energy is conserved!!! Energy can neither be created nor destroyed. When one form of energy disappears, another form of energy w/ equal magnitude appears.

All forms of energy are interconvertible!!!

ENERGY CHANGES IN CHEMICAL REACTIONS Almost all chemical reactions absorb or release energy, generally in the form of HEAT

Heat (Q) Is the transfer of thermal energy between two bodies that are at different temperatures. Heat flow From hot object ---- cold object

Thermochemistry Is the study of heat changes in chemical rxns.

system The specific part of the universe that’s of interest to us. Surroundings: Includes everything else in the universe.

The chemicals usually constitute the system &the container and everything beyond them (including us) are considered surroundings.

Types of systems Open Closed Isolated Isothermal Isochoric isobaric

Types of systems 1) open system

Types of systems 2) closed system

Types of systems 3) isolated system

Types of systems 4) isochoric (isovolumetric)system Volume of the closed system remains constant (ΔV= 0)

Types of systems 5) isobaric system Pressure stays constant (ΔP=0)

Types of systems 6) isothermal system Temperature remains constant (ΔT=0)

Energy in the form of heat can be transferred from the system to the surroundings, or from the surroundings to the system.

energy More specifically is, The capacity to do work or to transfer heat.

Internal energy (E or U) Internal energy of the system is the sum of all kinetic and potential energies of all the “particles” in the system.

Internal energy (E or U) For the system of frozen ice or still water, internal energy includes motion of H2O molecules, their rotations and vibrations, energies of the nuclei and electrons.

Internal energy (E or U)

Internal energy (E or U) We can’t calculate the actual numerical value of U. Instead, we can calculate ΔE or ΔU (change in internal energy).

Internal energy (E or U) ΔU= Ufinal – Uinitial Uinitial : Internal energy of the system at the beginning( refers to the reactants in a chemical rxn) Ufinal : Internal energy of the system after the change( refers to the products in a chemical rxn)

Internal energy (E or U) (+) Ufinal > Uinitial The system has gained energy from its surroundings (-) Ufinal < Uinitial The system has lost energy to its surroundings

Internal energy (U) ΔUsys + ΔU surr =0 (because the total energy of the universe is constant)

Heat and work are 2 equivalent ways of changing the internal energy of a system

RELATING ΔU TO HEAT AND WORK

U = Q (heat) + W (work) + = Change in internal energy Energy supplied to system as heat Energy supplied to system as work U = Q (heat) + W (work)

q w U U like reserves of a bank: bank accepts deposits or withdrawals in two currencies (q & w) but stores them as common fund, U.

Signs (+/-) will tell you if energy is entering or leaving a system + indicates energy enters a system - indicates energy leaves a system

When q is (+) & w is (+) ΔU>0 (energy of the system increases) q(+): heat is transferred from the surroundings to the system. q (-): heat is transferred from the system to the surroundings W (+): work is done by the surroundings on the system W (-): work is done by the system on the surroundings When q is (+) & w is (+) ΔU>0 (energy of the system increases) When q is (-) & w is (-) ΔU <0 (energy of the system decreases)

Excercise Calculate ΔU for a system undergoing an endothermic process in which 15.6 kJ of heat flows & where 1.4 kJ of work is done on the system.

solution Q= +15.6kJ W= +1.4kJ ∆U= (+15.6kJ) + (+1.4kJ)= 17.0kJ

Exercise During a reaction, the decrease in the internal energy of the system is 125 J and the system gains 55 J of heat. The system does the work. Volume of the system increases. W= - 70 J. Which of the statements are correct? (Esen yayinlari,2010)

answer: I and II

W = - PΔV 1) when pressure (P) of the system of a gas is constant : a) ΔV might increase because of expansion of the gas. - w (-), since work is done by the system through expansion ΔU= qp-w qp: heat change at constant pressure

W = - PΔV 1) when pressure (P) of the system of a gas is constant: b) ΔV might decrease because of compression of the gas. - w (+), since work is done on the system through compression ΔU= qp+w qp: heat change at constant pressure

W = - PΔV 2) when volume (V) of the system of a gas is constant: ΔV = o, w = o ΔU = qv + w ΔU = qv (the subscript “v” indicates that volume is constant) qp≠qv

Exercise A system loses 21 kJ of its internal energy when it releases 125 kJ of heat. A) calculate the work associated w/ this process? B) is the work done on or by the system?

solution A) ∆U= - 21kJ q= -125 kJ ∆U= q+W -21=(-125)+w w= +104kJ B) on the system.

References www.erlanger.k12.ky.us/teachers/Gels.../chemical%20reactions.ppt library.tedankara.k12.tr/IB/mustafa/.../Internal%20Energy%2010A.ppt http://www.goalfinder.com/product.asp?productid=102 Esen yayinlari, 11. sinif konu ozetli soru cozumleri, 2010.