Thermochemistry is
the branch of chemistry that studies the E changes in chemical reactions
Energy is
the ability to do work (to make things move).
- its the driving force of most chem reactions
- UNIT: Joule (J), Calorie –> used in America & in food
Kinetic Energy is
E of motion/speed (of either atoms or molecules). –> cant be negative, no direction rmr
- it’s directly related to the type of motion & the speed at which the atoms or molecules r moving.
3 Types of Molecular Motions
1) Vibration–> the movement of the atoms along the bonds in a molecule (in solid, liquid, & gas)
2) Rotation–> When an atom or molecule spins on its axis (in liquids & gas)
–> rmr no empty space between atom molecules between solids & liquids
3) Translation–> When a molecule moves from 1 place to another (only in gas)
KINETIC ENERGY: Vibration < Rotation < Translation
Potential Energy
Chemical E stored in the bonds between atoms in a molecule
Temperature is
the average KE of the particles in a substance. It tells us how fast the particles in a substance, on average are moving.
- Unit: degrees Celsius, F, Kelvin
–> kelvin is the accurate one cuz Celsius is relative to water –> Ex, -10 degrees Celsius means 10 degrees Celsius below freezing of water –> rmr u can’t have negative KE
Thermal Energy is
the sum of ALL kinetic energies of the particles. The thermal E in an object depends on 2 factors
1) # of particles: the more particles there are, the greater the thermal E
2) Speed of particles: The faster the particles r moving, the greater the thermal E.
2nd law of thermodynamics
- when 2 objects r in thermal contact, heat is always transferred from the object at higher temp to the object at the lower temp until 2 objects r at the same temp
Heat is
the transfer/flow of thermal E from a HOT object to a COLD object.
- Symbol: Q
- Unit: Joules or Calorie
- when the fast-moving particles collide with the slow-moving particles some of the KE is transferred to the slower particles, so the slower ones start moving faster (gain KE) while the fast-moving particles slow down (losing KE)
- Thermal KE has been transferred as heat between the objects
Systems
A system refers to all chemical components that r involved in a chem reaction –> the reactants, products & wtv solvents they r in
Surroundings: Everything outside the system we r studying
- OPEN SYSTEM: mass transfer & heat transfer –> open can of cold drink
- CLOSED SYSTEM: heat transfer –> sealed can of cold drink
- ISOLATED SYSTEM: NO mass or heat transfer –> perfectly insulated thermos
Specific Heat Capacity is
- symbol: c
is the amount of E needed to increase the temp of 1 gram of substance by 1 degree Celsius - its a physical property of a substance
- the smaller the c value, the more quickly a substance changes in temp. –> it will get hotter & colder quickly due to not being able to resist change in E
- Ex, specific heat capacity of water=4.18 J/g degrees C
Endothermic & Exothermic reaction
Exothermic reaction = when heat leaves a system, ∆T is negative & thus so is Q
Endothermic reaction = When heat enters a system, ∆T is positive & thus so is Q
Bond Energy
- AKA bond dissociation energy
- Bond energy is the measure of bond strength
- it is experimentally determined by measuring the E required to break the atoms involved in a molecular bond into free atoms
In a chem reaction, we change both the types of molecules & the types of bonds
- PE is the E stored in chemical bonds. –> If we change the types of bonds from the reactant side to product side, the PE will change –> PE=sum of bond energy
- KE is the E of motion/speed of the molecules. –> If we change the structure (mass, formula) of the molecules, the KE will also change
–> in addition, cuz the mass of each molecule also changes the reactant side to the product side, the KE also changes - KE= 1/2mv^2
Enthalpy is
the total energy in a system
- Enthalpy= KE + PE
- during a reaction, E is absorbed to break the bonds in the reactants & released when new bonds are formed in the products
–> therefore, ∆H= change in enthalpy (energy) if a system= sum of E required to break old bonds + sum of E required to make new bonds
- if ∆H is negative –> there is a net loss of E in the reaction= EXOTHERMIC REACTION
- if ∆H is positive –> there is a net gain of energy in the reaction= ENDOTHERMIC REACTION
if Q is positive… if Q is negative…
if Q is positive, reaction is exothermic
if Q is negative, reaction is endothermic
- rmr if this if Q is calculating heat of system not surrounding
Exothermic Reactions
- Bond breaking (energy absorption) < bond forming (energy released)
- the system releases (loses) energy to the surrounding –> surrounding becomes hotter
- ∆H= negative
- in a thermochemical equation, ∆H is written on the products side
Endothermic Reactions
- Bond breaking (energy absorption) > bond forming (energy released)
- the system absorbs (gains) energy to the surrounding –> surrounding becomes colder
- ∆H= positive
- in a thermochemical equation, ∆H is written on the reactant side
Molar Enthalpy
∆Hm
- the enthalpy change of exactly 1 mole of a substance undergoing a change
- Unit: KJ/mol
Reaction Enthalpy
∆Hrxn
- the total (net) enthalpy change of a reaction, for any given amount
- Unit: KJ
Calorimetry
- A calorimeter is a device used to measure heat of a reaction
- It can be sophisticated & expensive or simple & cheap (styrofoam cup)
- the reaction is done in water inside an enclosed container (calorimeter)
- A thermometer is used to measure the temp of the solution before & after the reaction
Calorimeter: Scenario 1: the reaction is exothermic…
it releases energy (heat) into the surrounding (water)
- the temp of water increases
- the enthalpy lost by the reaction (∆Hrxn) is gained by the water (Q)
Calorimeter: Scenario 2: the reaction is endothermic…
it gains E (heat) from the surrounding (water)
- the temp of water decreases
- the enthalpy gained by the reaction (∆Hrxn) is form by the water (Q)
Hess’s law of Additivity of Reaction Enthalpies
- the enthalpy change (∆H) of a reaction depends only on the initial and final conditions of the process. ∆H of a multistep process is the sum of the enthalpy changes of its individual steps
- if a chemical equation is reversed, then the sign of ∆H changes
- if the coefficients are altered by a factor, ∆H is altered as well in the same manner.
