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Causes of Change

Causes of Change. Thermochemistry. Thermodynamics. Thermodynamics is the study of the flow or exchange of energy. System Surroundings. The First Law of Thermodynamics. The law of Conservation of Energy The total amount of energy in the universe is constant

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Causes of Change

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  1. Causes of Change Thermochemistry

  2. Thermodynamics • Thermodynamics is the study of the flow or exchange of energy. • System • Surroundings

  3. The First Law of Thermodynamics • The law of Conservation of Energy • The total amount of energy in the universe is constant • The change in energy of the system + the change in energy of the surroundings equals zero • “You can’t get something for nothing”

  4. The movement of heat • Heat is always transferred from • hot to cold

  5. Enthalpy • Enthalpy (H) is the heat content of a system at a constant pressure. • H or the change in enthalpy is what we calculate • H = Hproducts - Hreactants

  6. Molar Heat Capacity • The molar heat capacity of a substance is the amount of energy needed to raise 1 mole of a pure substance by one degree Celcius or Kelvin. • C = q / mol x T • Just like specific heat, except you are using moles instead of mass

  7. Molar Enthalpy • The change in energy per mole is referred to as Molar Enthalpy • q/mol=H • So, let’s change the molar heat capacity equation . . . • If C=q/mol x T, and q/mol = H, then • C=H/T or H=CT • Remember that T= Tfinal - Tinitial

  8. What does H mean? • Chemical changes and changes in physical states either release or absorb energy. • Exothermic reactions release heat so heat is a product, let’s look at water freezing H2O(l, 292K)  H2O(s, 273K) + 459.8 J T = final – initial = 273K – 292K = -19K H=CT=(24.2J/molK)(-19K)= -459.8J/mol

  9. What does H mean? • Endothermic reactions absorb heat so it feels cold, let’s look at ice melting H2O(s, 273K) + 459.8 J  H2O(l, 292K) T = final – initial = 292K – 273K = 19K H=CT=(24.2J/molK)(19K)= 459.8J/mol

  10. Reaction Pathways • Reaction pathways are used to illustrate the amount of energy throughout the reaction • Reactants must absorb some amount of energy in order for a reaction to proceed (unless it is a spontaneous reaction) • This amount of energy is referred to as Activation Energy • Once the energy is absorbed, the reaction can proceed • These compounds are now referred to as an Activated Complex

  11. Exothermic Pathway • In an exothermic reaction, the reactants absorb the activation energy but the resulting products lose energy (H) • H= a negative number

  12. Endothermic Reaction • In an endothermic reaction, the reactants absorb the activation energy and so the resulting products have more energy (H) • H= a positive number

  13. So to review, Endo or Exo? • A positive change in enthalpy would be an endothermic reaction. • A negative change in enthalpy would be an exothermic reaction

  14. Enthalpy (cont.) • What would happen if the temperature of a sample decreased? • The kinetic energy of the sample also decreases! • If the kinetic energy of the sample decreases, the enthalpy (H) decreases

  15. Entropy • The measure of the randomness or disorder in a system is called entropy (S) and is measured in J/K • Aha! What are the variables in entropy? • You guessed it, energy and temperature! • The entropy change in a system is the measure of the entropy of the products minus the reactants • ΔS = Sproducts - Sreactants

  16. What is disorder? • Disorder is any system which is not ordered • a solid has very ordered particles • a liquid has slightly less ordered particles • a gas has very unorderly particles • your locker is a very disorderly!

  17. The Second Law of Thermodynamics • In any spontaneous change the entropy of the universe must increase • ΔSsys + ΔSsurr > 0 • ΔSreaction= ΔSproducts - ΔSreactants • “You can’t break even”

  18. Increasing order Increasing entropy Entropy Randomness of the system

  19. Trends in Entropy • What happens if . . . • Temperature increases, disorder increases • Change in phase toward gas, disorder increases • Concentration decreases, disorder increases • # of products increases, disorder increases • A solute is dissolved in a solvent, disorder increases

  20. Chemical Entropy Lower Entropy Higher Entropy

  21. Entropy Problems Will the entropy change for each of the following be positive or negative? • Sugar dissolves in tea • Air is pumped into a tire • Acetone evaporates from nail polish • CaCO3(s) CaO(s) + CO2(g) • N2(g) + 3H2(g)  2NH3(g)

  22. Gibb’s Energy • Gibb’s Energy is the measure of the available energy in a system, the energy available to do work • often referred to at “free” energy • The change in Gibb’s Energy is, • ΔG = Gproducts - Greactants

  23. Gibb’s is Not alone • Gibb’s Energy is a function of enthalpy (H), entropy (S) and temperature (K) • ΔG = ΔH - TΔS • So, what happens when the temperature goes up? • Kinetic energy increases, entropy increases and temperature increases . . . Now what!

  24. Let’s Talk Spontaneous • A reaction is spontaneous (proceeds without any help) if the change in Gibb’s Energy is negative • That would mean that the measure of the temperature times the change in entropy is a large number

  25. This will help predict

  26. 2NO2(g)  2N2(g) + O2(g) • The change in enthalpy is negative • What is the change in entropy? • Positive • Is this spontaneous or not? • Yes, it is spontaneous

  27. H2O(l)  H2O(s) • Is the reaction endothermic or exothermic? • Exothermic • Is the enthalpy positive or negative? • Negative • Does entropy increase or decrease? • Decrease • Is this spontaneous? • Only at low temperatures

  28. 2NH3(g)  N2(g) + 3H2(g) • The change in enthalpy is positive. • What is the change in entropy? • It increases. • Is this reaction spontaneous? • Yes, as long as the temperature is high

  29. 3O2(g)  2O3(g) • What would you predict the change in enthalpy would be? (endo or exo?) • Endothermic, so a positive change • What do you predict the entropy would be? • More ordered, so decreased • So . . . Spontaneous? • No, not at any temperature

  30. Hess’s Law • Well, Hess’s law states that the total amount of energy lost or gained in a given reaction is the sum of each step in the reaction. • Some reactions must occur in 2 or more steps.

  31. Apply Hess’s Law • Guess what is conserved according to Hess’s Law? • Yeah! Energy!

  32. Add Each Step • Let’s decompose water a little bit 3H2O(g)  3H2O(l) ΔH = -132 kJ 3H2O(l)  3H2(g) + (3/2)O2(g) ΔH = +858 kJ • The total change in enthalpy is 858 kJ - 132 kJ = +726 kJ

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