Thermochemistry

conversion factors: 1 Cal/kcal = 1000 cal = 4.184 KJ = 4184 joules

Part 1 Thermochem

  • Two Trends in Nature:

    • Order → Disorder (Second law of thermodynamics)

    • High energy → low energy (meaning heat will transfer to cold)

  • Energy = capacity to do work or supply heat

  • Chemical Potential Energy - energy stored in chemicals

    • determined by the kinds of atoms and arrangement

  • Heat (Q/q) - energy that flows from a warmer to cooler

  • Temperature - average KE of particles, effect after heat added.

  • Law of Conservation of Energy (second law of thermodynamics): Energy cannot be created or destroye, only transferred and can change forms

  • Forms of Energy: work atoms moving or breaking/ forming bonds, stored energy within bonds, heat

Units

  • Calorie (cal) - unit of heat which brings the temperature of the water on a gram of water, 1 degree up

    • Dietery (Cal) - = 1kcal = 1000cal

  • Joule (J) = quantity of heat that raises 1 gram of water 0.239 degrees Celsius

  • 1Cal = 1kcal = 1000 cal = 4184 j = 4.184 kJ

Heat Movement

  • Endothermic -

    • heat goes into the system, making the surroundings cooler

    • (sign for heat is positive), heat on the reactant side,

  • Exothermic -

    • heat goes into surroundings, exiting the system

    • (sign for heat is negative), heat on product side.

Specific Heat - is the specific amount of heat required for a substance to be raised one degree celcius for one gram

  • Usually given in a table. If asked to calculate it. C= q/m∆t

  • Units = J/g °C or cal/g °C

  • Water has a high specific heat, meaning it can absorb a lot of heat before it changes temp, but metals have a low specific heat it can absorb fast and give fast

  • A metal spoon is better in a soup because it can conduct heat faster

  • everything has the same temperature but gives off different amount of heat at a time due to specific heat

  • lower specific heat - metals - transfer heat faster - less heat is needed to raise the temperature

Calorimetry

  • Measurement of heat changes in reaction (in an insulated container)

  • Enthalpy (H): amount of heat a substance has at a given temperature and pressure

    • q=H because we are not changing the conditions in the room

  • Q= mC∆t - Use when mass is given, specific heat, temperature change, or heat transfer is given. Use sig figs

    • use when there is a temperature change

  • mC∆t= mC∆t - use in specific situations

Thermochemical Equations

  • if heat per reaction is given, and mass is given or mole and temperature stays the same

    • ex: CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) ΔH = -890.4 kJ

    • doing the reverse changes the sign - can be used in hess law later

    • negative sign means that heat is released (exothermic), the converse is true

    • multiply the heat if there are more moles

    • physical states must be stated

Calorimetry

  • Calorimeter - an instrument that measures the gain or loss of heat

    • can be used to find the specific heat of a substance

    • can also be used to determine the heat given off or absorbed by a chemical reaction

    • It can measure heat because of change of water.

    • According to the law of conservation of energy: the heat lost by the process is gained by the water or the water loses heat and it is absorbed by the process

      • q process = -q water (4.184 J/g°C)

    • In these equations, the final temp is the same for both water and the object

    • mCAT (object) = -mCAT (water)

    • the negative sign means the substance is losing heat

Phase Changes

  • The boiling point is the temperature at which the (equilibrium) vapor pressure of a liquid is equal to the external pressure.

    • Normal is at 1 atm

    • this value is given in a table

  • Melting point = freezing point - same temp can have both solid and liquid

    • same temp, but in order to change phases, it needs to break IMF

  • Molar heat of fusion (∆Hfus = -∆Hsolidify) is the energy required to melt 1 mole of a solid substance.

    • This value is given.

    • positive is heat added in (melt), negative is heat taken away (solidify)

  • Molar Heat of Vaporization - needed heat to change the phase (positive value). The negative value would be molar heat of condensation.

  • Molar Heat of sublimation is the energy needed to sublime 1 mole (fus plus vap)

  • Heating Curve - shows when to use different equations like heat of fusion or mcat.

  • When using the phase change equation, use stoichiometry because its per mole

Stanard Enthalpy

  • enthalpy is essentially heat but is affected by pressure

  • Standard Enthaltpy of Formation- heat change when one mole of a compound is formed form its elements at 1atm

    • (stable compounds are 0, ex: o2 in period table)

    • given by table

  • Standard Enthalpy of Reaction (ΔH0 ) is the enthalpy of a reaction carried out at 1 atm.

    • Sum of Product standard Enthalpy (make sure to include moles) - the reactant sums

    • if the question then asks per mole, then you divide by the amount of mole in that reaction. (released no sign, change yes sign)

Hess Law

  • A State Function: Path independent.

    Both lines accomplished the same result, they went from start to finish.

    Net result = same.

  • When reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps.

Important Lab Info

Specific Heat

Heat of Fusion of Ice

  • Q/m

  • Heat up water, melt the ice till it goes to its melting/ freezing point. Then calculate energy

  • Caloriometru