ENERGY AND CHANGE

• THERMODYNAMICS: Study of energy and energy transfer.
• THERMOCHEMISTRY: Study of energy involved in chemical reactions.
• LAW OF CONSERVATION OF ENERGY: total energy of the universe is constant  energy can not be created nor destroyed, only transferred.
• SYSTEM: Part of the universe that is being studied and observed (E.g. A chemical reaction.
• SURROUNDINGS: everything else in the universe.

• Heat, Q: the TRANSFER of kinetic energy.
– Expressed in the same units as energy – joules
(J).
– HEAT is transferred. Not cold.
• Temperature, T: measure of average kinetic energy of particles.
Celsius: water’s boiling point is 100⁰C and freezing is 0 o
C.
Kelvin: at 0K, substances possess no kinetic energy.

• Enthalpy, H: total internal energy of a substance at constant pressure.
• Enthalpy change, ∆H: change in enthalpy due to a chemical reaction (change in relative energy between reactants and products).
– Equal to heat change at constant pressure.

• Enthalpy changes result from chemical bonds being broken and formed.
– Bonds: sources of stored energy.
– Breaking a bond: requires energy.
– Creating a bond: releases energy.
• Example:
One mole of N≡N and one mole of O=O bonds are broken.
Two moles of N-O bonds are formed.
Reaction absorbs energy: more energy is needed to break bonds that the amount of energy released when forming new bonds.

ENDOTHERMIC: Net absorption of energy.
EXOTHERMIC: Net release of energy.

ENTHALPY OF REACTION, ∆H rxn of a chemical reaction.
: enthalpy change
Dependant on conditions such as temperature and pressure.
STANDARD ENTHALPY OF REACTION, ∆H o rxn : the enthalpy change of a chemical reaction that occurs at SATP. (Often written just as ∆H o )

REPRESENTING EXOTHERMIC REACTIONS
• THREE WAYS TO REPRESENT
– THERMOCHEMICAL EQUATION: chemical equation with the enthalpy as one of the reactants or products.
– SEPARATING ENTHALPY FROM RXN: indicates the change in enthalpy OF THE SYSTEM.
– ENTHALPY DIAGRAM:

REPRESENTING ENDOTHERMIC REACTIONS
• Also three ways:

STOICHIOMETRY AND THERMOCHEMICAL EQUATIONS
• The decomposition of TWO moles of magnesium carbonate absorbs TWICE as much energy as the decomposition of ONE mole of magnesium carbonate.
ENTHALPY OF REACTION IS LINEARLY DEPENDANT ON THE
QUANTITY OF PRODUCTS.
∆H o rxn
α #P
(if the amount of product formed doubles, the enthalpy change ___________)

• DO # 2, 3, and 4 FOR HOMEWORK.

• Heating water: heat water (add kinetic energy) until it gets to boiling point.
– At boiling point, avg. Kinetic energy does not increase: kinetic energy is used to break the intermolecular bonds between the water molecules as they change from liquid to vapour.
– Temperature remains at 100 o C until all water is vaporized.
Temperature remains steady during a phase change.

• The amount of energy absorbed by a substance in order to vaporize is equal to the amount of energy that substance condenses. released when freezing.
The same idea applies for melting and

• What would the enthalpy of freezing be for diethyl ether? ______________________
• What would the enthalpy of condensation be for methane?____________________________.

• Enthalpy of solution, ∆H soln
: enthalpy change that occurs when a solute dissolves in a solvent. Can be endothermic or exothermic.
• Cold Packs: contains water and a salt in separate compartments.
– When pack crushed, salt dissolves.
– Dissolution is endothermic (takes energy to break bonds between salt particles).
– Pack will absorb heat, feeling cold.
• Hot Packs: same idea, only the dissolution of the salt in questions is exothermic.

• Nuclear reactions involve changes in the nuclei of atoms
– Can result in the transformation of one element into a different element.
– Produce significantly more energy than physical and chemical processes.
– A significant amount of the mass of reactants is converted into energy.
•
•
 mass and energy are interconvertible.
 E = energy, m = mass, and c 2 is the square of the speed of light.
• Even a small mass is accompanied by a lot of energy!

Mass defect: the difference in mass between a nucleus and the sum of all of the nucleons.
Nuclear binding energy: energy associated with the strong force that holds a nucleus together.
Mass of Nucleus + Mass of Nuclear binding
Energy  Mass of the sum of the Nucleons.

• With the following data, calculate the nuclear binding energy for carbon-12.
Molar Mass of carbon-12 = 12.000 000 g/mol
Sum of Mass of Nucleons = 12.098 940 g/mol

• With the following data, calculate the nunclear binding energy for carbon-12.
Molar Mass of carbon-12 = 12.000 000 g/mol
Sum of Mass of Nucleons = 12.098 940 g/mol
This is the energy of the bonds that holds the nucleus together (much stronger than chemical bonds!).

• The higher the binding energy, the more stable the nucleus is.

• When a heavy nucleus splits into lighter nuclei.
– Nuclear Power Plants: controlled nuclear fission.
– Atomic bombs: uncontrolled nuclear fission.

• Two smaller nuclei fuse to form a larger nucleus.
– Source of energy produced by the Sun.

• SR, Page 233, #1-6

• SPECIFIC HEAT CAPACITY, c:
The amount of energy that is needed to raise the temperature of one gram of a substance 1 o C.
– Expressed in units J/g ∙ 0 C.
– Can be used to determine amount of energy needed to heat a given mass a certain number of degrees.
– Used to determine amount of heat released when the temperature decreases.

• All samples of the same substance have the same specific heat capacity.
• HEAT CAPACITY, C: relates the heat of a sample, object, or system to its change in temperature.
– Expressed in units of kJ/ o C.
WATER EXAMPLE
– Teacup and Bathtub: both have same specific heat capacity.
– Each have different heat capacities  bathtub has higher heat capacity.

• Page 235.
Do PPs 6-8

• Calorimeter: measures enthalpy changes for chemical and physical reactions.
– Insulates a system from its surroundings.
– Measures temperature change.
– Coffee cup calorimeter:
• Constant volume calorimeter
(open to the atmosphere)
• Measure initial and final temp.
of the solution.
We assume that the solution has the same density of water
(1 g/mL) and same specific heat capacity of water, c = 4.184 J/g∙ o C. Therefore, only well-suited to determining the enthalpy changes of reactions in dilute aqueous solutions.

• By measuring the temperature change of the system, we can determine the amount of heat that is released or absorbed by the reaction.
• THE HEAT THAT IS RELEASED BY AN
EXOTHERMIC REACTION RAISES THE
TEMPERATURE OF THE SYSTEM:

• In terms of a dilute aqueous solution, the water absorbs (or provides) the energy that is released (or absorbed) by a chemical reaction.
– (I.E: the solution itself absorbs or releases energy). m: mass of the SOLUTION.
c: specific heat capacity of water (since dilute).
REMEMBER:

DO PPs10 & 11

SR #1, 3, 4(theoretical laboratory).

• Hess’s Law:
– the enthalpy change of a physical or chemical process depends only on the beginning conditions
(reactants) and the end conditions (products).
– Independent of the pathway of the process and the number of intermediate steps in the process
• Ex// walking to school.

2 WAYS IN WHICH THIS CAN BE DONE:
1) By combining chemical reactions algebraically.
2) By using formation reactions.

• Add equations for reactions with known enthalpy changes so that the net result is the reaction we are interested in (the target equation).

• Substance is formed from elements in their standard states.
• ∆H o f
: standard molar enthalpy of formation.
– Quantity of energy that is absorbed or released when one mole of a compound is formed directly from its elements in their standard states.

Standard enthalpy of formation is usually negative. (lost energy to form compound).
Compounds are more ‘stable’ than original elements.

REMEMBER ‘HOFBrINCl’

DO #18 - 20

• We can calculate the enthalpy change of a chemical reaction by adding the heats of formation of the products and subtracting the heats of formation of the reactants. n = molar coefficient of each compound
• Need to begin with a balanced chemical equation.

For Example:

• Whether using Hess’s law or Enthalpies of
Formation, we will obtain the same enthalpy for a reaction.

DO PPs 22-23

SR
#1-5.