## Chapter 11Thermochemistry

### Thermodynamics --

• #### study of energy and its transformations

Work -- energy used to cause an object with mass to move
Heat -- energy used to cause the temperature of an object to increase
Kinetic Energy -- energy of motion
Potential Energy -- energy not in motion - occurs when there is attraction and repulsion between objects
Calorie (cal) -- amount of energy required to raise the temperature of 1 gram of water 1 degree Celsius
Force -- any kind of push or pull exerted on an object against the force of gravity
Energy -- the capacity to do work or to transfer heat

### First Law of Thermodynamics

• energy can be neither created nor destroyed, it just changes from one form to another
• energy is conserved - any energy lost by a system is absorbed by the surrounds - vice versa
Internal Energy - the sum of all the KE and PE of all the components of the system - represented by "E"

Relating ∆E (change in energy) to heat and work
• when a system undergoes any chemical or physical change, the change in its internal energy (∆E) is given by the heat added to or liberated from the system (q) plus the work done on or by the system (w)

∆E = q + w

(when work is done on the system by the surrounding, work is positive)

• positive ∆H = system has gained heat from the surroundings (endothermic)
• negative ∆H = system has released heat to the surroundings (exothermic)
∆H depends on the change within the system, no on how the change occurs

Enthalpies of Reactions

∆H = Hfinal - Hinitial (enthalpy of reaction = ∆Hrxn)

Guidelines for using thermochemical equations and enthalpy diagrams:

1. Enthalpy is an extensive property (depends on the quantity of sample and includes measurements of mass and volume)
• ∆H is directly proportional to the amount of reactant consumed in the process
2. The enthalpy change for a reaction is equal in magnitude but opposite in sign to ∆H for the reverse reaction
3. The enthalpy change for a reaction depends on the state of reactants and products (whether they are a gas, liquid, or solid)

Calorimetry - measure the heat flow in or out of a system
• calorimeter - device used to measure heat flow
• heat capacity - the amount of heat required to raise the temperature 1şC (the greater the heat capacity the greater the heat required to produce a rise in the temperature)
• the heat capacity of 1 mole of a substance = molar heat capacity
• the heat capacity of 1 gram of a substance = specific heat
• specific heat = measure the change in temperature that a known mass undergoes as it loses or gains a specific quantity of heat

Hess's Law
• if a reaction is carried out in a series of steps, ∆H for the reaction will be equal to the sum of the enthalpy changes for the individual steps

Enthalpies of Formation

• Enthalpy of Vaporization - ∆H for converting liquids to gases
• Enthalpy of Fusion = ∆H for melting solids
• Enthalpy of Combustion = ∆H for combusting a substance in oxygen
• Enthalpy of Formation = ∆Hf = formation of a compound from its elements (heat of formation)

1. The change depends on the state in which a substance exists - gas, liquid, or solid
2. Standard state = pure form of a substance at 1 atm and 25şC or 298 K
3. Standard Heat of Formation = ∆Hşf (kJ/mol) = 1 mol of substance is formed
4. The ∆Hf of any element is zero (0)

Heat of Reaction

∆Hşrxn = Σn∆Hşf(products) - Σm∆Hşf(reactants)

where, n and m are coefficients from the balanced equation

• Chemical Kinetics: branch of chemistry that deals with rates of reactions and mechanisms of chemical reactions.

• Thermodynamics: study of the changes in energy in chemical reactions, the influence of temperature on those changes, and the other factors that allow/cause chemical reactions to take place.

• Collision Theory: states that particles must collide in order for chemical change to take place. (these particles may be ions, atoms, or molecules)

• There are four main factors that affect the rate of a chemical reaction:
• 1. the nature of the reactants
2. the temperature of the system
3. concentration of the reactants (including the pressure of reactants in gaseous form)
4. the use of a catalyst

#### Heat Content and Enthalpy

• Every system has energy stored in it
• energy in the chemical bonds, random motion of the atoms and molecules, and potential energy (stored energy)
• The total of all these forms of energy is called the heat content or enthalpy--represented by "H".
• the exact amount of enthalpy cannot be calculated
• enthalpy will remain constant as long as no energy leaves or enters the material
• if energy leaves or enters the substance, its enthalpy will change by an amount equal to the amount of energy gained or lost
• a change in enthalpy is represented by "delta H"

• Since energy must be added to ice to change it to the liquid phase, then:

• delta H = HH2O(l) - HH2O(s)
• we can measure the amount of heat absorbed when the phase change occurs (endothermic process)
Heat of Fusion
• quantity of heat needed to change a unit mass of solid to liquid at a constant temperature
• for ice at zero degrees celsius it takes 335 joules per gram to change to liquid (endothermic process)
• for liquid at zero degrees celsius it takes -335 joules per gram to change to ice (exothermic process)
• the enthalpy for chemical reactions is different on both sides of the equation:

reactants vs. products

• this change of enthalpy is called the heat of reaction--represented by delta H
• a change in enthalpy is measured in a unit called kilojoules (kJ)

Heat of Formation

• When you have 1 mole of a compound being formed, the delta H is called the "heat of formation"--delta Hf the heat of formation is dependent on the temperature and pressure at which the reaction occurs (it also depends on the phase of the substance)
• the standard heat of formation, is the heat of formation at 25 degrees celsius and the pressure is at 1 atm
• 25 degrees celsius = 298 K
• 1 atm = 101.3 kPa
• the heat of formation of water is -286 kJ/mol ---> 1 mol of water is formed from H2 + O2 at 25 degrees celsius and 1 atm (286 kJ of heat is released)
```          Exothermic Reactions          vs.          Endothermic Reactions

*energy is a product                       *energy as a reactant
*large neg. values give                    *small neg. or pos. values--
off a lot of energy during                  unstable (explosives)
its formation--very stable
```

Calculating heat of formation:
1 mole of ethyl alcohol is -9.5 x 102 kJ or -950 kJ. How much heat is produced when 11.5 grams of ethyl alcohol is burned?
1. Change grams to moles:
```        C2H5OH    x    1 mol  =  0.25 mol
46.08 g
```
2. Change moles to kJ:
```       0.25 mol C2H5OH   x   -950 kJ  =  -237.09 kJ
1 mol
```

Hess's Law of Constant Heat Summation
• Law of Conservation of Energy-->energy can be neither created nor destroyed, but can change from one form to another
• Hess's Law of Constant Heat Summation states that when a reaction can be expressed as the algebraic sum of 2 or more other reactions, then the heat of the reaction is the algebraic sum of the heats of these other reactions
Method 2 for calculating heat of formation:
Heat of formation of the reaction = heat of formation of the products - heat of formation of the reactants
• any single element has a value of zero
• if you have more than one mole of substance (based on the balanced equation) you have to multiply the heats of formation by the coefficient of the compound involved
Entropy
• measure of disorder, randomness, or lack of organization in a system
• the more disorder, the higher the entropy value
• entropy is represented by the capital letter "S"
• therefore, delta S = Sf - Si
• where Sf = final entropy--after the change has occurred
• and Si = initial entropy--before the change has occurred
• both are positive values but the change can be either positive or negative
• an increase in entropy is positive
• a decrease in entropy is negative
• when a substance changes form the entropy will change
• when a substance is in a solid form the entropy value is low
• when a solid changes to a liquid, the entropy increases (the liquid is more "disordered" than a solid)
• when a liquid changes to a gas, the entropy again increases (the gas is more "disordered" than the liquid--gas molecules/particles tend to act independently of each other)
• when elements form a compound, a more stable condition is achieved-->entropy is low
• the decomposition of a compound into individual elements creates a more unorganized system-->the entropy increases
• (temperature can also cause a change in entropy)