Thermochemistry
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Thermochemistry!. AP Chapter 5. Temperature vs. Heat. Temperature is the average kinetic energy of the particles in a substance. Heat is the energy that is transferred from one object to another. Heat always flows from the hotter object to the colder object. Energy!.

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Thermochemistry

Thermochemistry!

AP Chapter 5


Temperature vs heat
Temperature vs. Heat

  • Temperature is the average kinetic energy of the particles in a substance.

  • Heat is the energy that is transferred from one object to another.

  • Heat always flows from the hotter object to the colder object.


Energy
Energy!

  • Energy is the ability to do work.

  • Kinetic Energy- the energy of motion

  • Potential Energy – the energy that an object has as a result of its composition or its position with respect to another object.


Units of energy
Units of Energy

  • 1 Joule = 1 kg m2/s2 (1 kJ is 1000J)

    • Used to calculate the energies associated with chemical reactions.

  • Calorie – Amount of energy required to raise the temperature of 1 gram of substance 1 °C. (This is specific heat!)

    1 calorie will raise the temperature of 1 g of H2O from 14.5 °C to 15.5 °C.

    1 calorie is equal to 4.184 Joules (exactly!)


  • Systems and surroundings
    Systems and Surroundings

    • System – the portion used in a study.

      • It can be an open system or a closed system.

    • Opensystem – matter and energy can interact with the surroundings.

    • Closedsystem – the matter cannot interact with the surroundings.


    First law of thermodynamics
    First Law of Thermodynamics

    • Energy Is Conserved!


    Internal energy
    Internal Energy

    • Internal Energy is the sum of all the kinetic and potential energies of all its components.

      ΔE = Efinal - Einitial


    Thermochemistry
    ΔE

    • A positive value for ΔE is when Efinal > Einitial

    • If energy has been absorbed from its surroundings, it is endothermic.

    • If energy is given off to the surroundings, it is exothermic.



    Thermochemistry

    Endothermic reaction refers to the products.

    Exothermic reaction


    Thermochemistry

    A system composed of H refers to the products.2 (g) and O2 (g) has greater internal energy than a system composed of H2O (l).

    Gases have greater kinetic energy and must lose some of that energy to change states back to the liquid state.



    Thermochemistry


    Enthalpy
    Enthalpy environment only as heat, no work is done.

    • The change in enthalpy for a reaction (∆H) is the overall measure of energy that is absorbed to break bonds and the energy that is released when new bonds form.

    • A reaction is said to be spontaneous if it occurs without being driven by an outside force. (driving forces are enthalpy & entropy)

    • ∆H = ΣH(products) - ΣH(reactants)


    Thermochemistry

    In an endothermic system where it absorbs heat, environment only as heat, no work is done.∆H will be positive (∆H > 0).

    In an exothermic system, where heat is given off, ∆H will be negative (∆H < 0).


    Enthalpy diagrams
    Enthalpy Diagrams environment only as heat, no work is done.

    • Enthalpy is an extensive property – it depends on how much you have. If 1mol of CH4 and 2 mol O2 yield -890 kJ, then 2 mol CH4 and 4 mol O2 would yield double that.

    • The enthalpy change for a reaction is equal in magnitude, but opposite sign, for a reverse reaction.


    Calorimetry
    Calorimetry environment only as heat, no work is done.

    • This is a measure of the amount of energy that is needed or lost when a certain mass of a substance changes temperature.

    • q = mC∆T

    • q is the amount of energy (J)

    • m is the mass of the substance (g)

    • C is the specific heat capacity of the substance

    • ∆T is the change in temperature


    Calorimeters
    Calorimeters environment only as heat, no work is done.

    • Calorimeters are devices that measure the transfer of heat from one object to another.


    Heat of formation h f
    Heat of Formation ( environment only as heat, no work is done.∆H°f)

    • The heat change that occurs when one mole of a compound is formed from its elements at 1 atm pressure.

    • Generally, the standard enthalpy of formation for any element in its most stable form is 0. (i.e. O2 gas would have a standard enthalpy of 0.)

    • Remember Appendix C!


    Standard enthalpy changes
    Standard Enthalpy Changes environment only as heat, no work is done.

    • The standard enthalpy change can be calculated from the standard enthalpies of formation of the reactants and products in the reaction (see Appendix C for values.)

    • ∆H°rxn = Σn∆H°f(products) - Σm ∆H°f(reactants)

    • The n and m refer to the molar coefficients in the chemical equation.


    Thermochemistry

    Also refer to Appendix C! environment only as heat, no work is done.


    Hess s law
    Hess’s Law environment only as heat, no work is done.

    • If you can break a chemical reaction into several steps, add up all of the ∆H’s for each step to get the overall ∆H for the reaction.


    Entropy
    Entropy environment only as heat, no work is done.

    • Entropy is a measure of randomness or disorder of a system. The greater the disorder, the greater the entropy.

    • In terms of entropy, gases>liquids>solids.

    • When pure substance dissolves in a liquid, its entropy increases.

    • When gas molecules escape a solvent, entropy increases.

    • Entropy increases with molecular complexity.

    • Reactions that increase the number of moles of particles often increase the entropy of the system.


    Predict
    Predict! environment only as heat, no work is done.

    • Na+(aq) + Cl-(aq) → NaCl (s) ∆S is negative

    • NH4Cl (s)→ NH3(g) + HCl (g) ∆S is positive