Chapter 11
Thermochemistry
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:
- 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
- The enthalpy change for a reaction is equal in magnitude but opposite in sign to ∆H for the reverse reaction
- 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 1ºC (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)
- The change depends on the state in which a substance exists - gas, liquid, or solid
- Standard state = pure form of a substance at 1 atm and 25ºC or 298 K
- Standard Heat of Formation = ∆Hºf (kJ/mol) = 1 mol of substance is formed
- The ∆Hf of any element is zero (0)
Heat of Reaction
∆Hºrxn = Σn∆Hºf(products) - Σm∆Hºf(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)
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
- 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)
- the minus sign indicates that heat is being given off
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
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)