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Warmup: (5 min)

Warmup: (5 min). Please get your new packet if you don’t have it already! Molecules of WHICH phase of matter have the most energy? What is the difference between temperature and heat? Do you think heat flows from cold to hot or from hot to cold? Give an example. Energy.

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Warmup: (5 min)

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  1. Warmup: (5 min) Please get your new packet if you don’t have it already! • Molecules of WHICH phase of matter have the most energy? • What is the difference between temperature and heat? • Do you think heat flows from cold to hot or from hot to cold? Give an example.

  2. Energy

  3. Energy (E): ability to do work or produce heat In chemistry, we talk about work and energy in terms of rearranging atoms and molecules by forming and breaking bonds. Heat (q): thermal energy “COOL” IS NOT A THING. IT IS AN ABSENCE OF HEAT

  4. The Law of Conservation of Energy states that energy: can NOT be created or destroyed can be converted from one type (potential, kinetic, thermal) to the other can be exchanged between the system and the surroundings

  5. Heat flows from HOT to COLD

  6. (3 min) Suppose you take a sample of chromium metal and put it into a boiling pot of water. After 10 minutes, you take the chromium piece out and place it into a cup of room temperature water. a) Draw and label the system and surroundings for each of the two situations. b) Draw an arrow to indicate the direction of heat flow for each of the two situations.. c) WHY this is the direction of heat flow for each of the two situations?

  7. Temperature: -measures the amount of thermal energy (heat) in a system -depends on how fast the molecules are moving (KE) TK = T°C + 273 average KE of a sample of molecules

  8. Energy Units • the Joule(J) • the calorie (cal) (amount of energy needed to raise the temperature of 1 gram of water by 1 °C) This is a Calorie (kcal) NOT a calorie!

  9. One serving of yogurt contains 6.01 x 104 calories. Convert this to kiloJoules of energy. 1 kJ = 1000 J 1 cal = 4.184 J 1000 cal = 1 Cal 6.01 x 104 cal (4.184 J) (1 kJ) 0.251 kJ = (1 calorie) (1000 J)

  10. How many Joules is 26000 Calories? 1 kJ = 1000 J 1 cal = 4.184 J 1000 cal = 1 Cal 26000 Cal (4.184 J) (1000 cal) 1.1 x 108 J = (1 Cal) (1 cal)

  11. Cold packs (read only) In cold packs, the chemical ammonium nitrate is often used because it absorbs a lot of heat when it dissolves in water.  Ammonium nitrate dissolves in water “endothermically.” Water and ammonium nitrate are kept in separate compartments in the pack until the pack is needed.  Then the chambers are broken and the ammonium nitrate dissolves in the water, absorbing heat and making the pack as cold as 0°C. 19.2 kJ + NH4NO3(s) → NH4+(aq) + NO3-(aq) reactant

  12. Hot packs (read only)CaCl2(s) → Ca2+(aq) + 2Cl-(aq) +13.9kJ product In hot packs, calcium chloride or magnesium sulfate frequently are used because these chemicals dissolve in water exothermically.  In other words, they release a lot of heat when they are dissolved in water.  Hot packs can reach temperatures around 90°C.

  13. Enthalpy (∆H) • Potential energy is stored in bonds • In chemical reactions, reactant bonds are broken (energy absorbed) and product bonds form (energy released) • ∆H: net amount of heat released or absorbed; depends on the strength of bonds made/broken

  14. +251 kJ Xe(g) + 2F2(g) + 251 kJ --> XeF4(s) endo or exo? ∆H = XeF4(s) --> Xe(g) + 2F2(g) + 251 kJ endo or exo? ∆H = - 251 kJ When a reaction is reversed, the sign of ∆H is reversed too xenon tetrafluoride

  15. Identify each as an exothermic or endothermic reaction:*write just the answer • When solid KBr is dissolved in water, the liquid gets colder • 2KClO3(s) + 84.9 kJ  2KCl(s) + 3O2(g) • When concentrated sulfuric acid is added to water, the solution gets very hot • The net enthalpy change for a reaction is negative 768kJ (ΔH = - 768 kJ)

  16. exothermic reaction pathway: shows ∆H of reaction and compares the total energy stored by the reactants vs the products endothermic

  17. What is the value of ∆H? ∆H = Hproducts – Hreactants= 500 – 150 = +350 kJ Endothermic! More energy stored by products than was stored by reactants (kJ) products ΔH reactants

  18. Enthalpy and Stoichiometry Ex. 2C2H4O(l)+ 2H2O(l)2C2H6O(l) +O2(g) ∆H = 915.7 kJ • Considering the reaction above, how much energy is required to produce 0.467 moles of oxygen gas? 0.467 moles O2 b. If I want to completely burn 3.4 moles liquid C2H4O, how much energy must be present? 3.4 moles C2H4O (915.7 kJ) (1 mole O2 ) = 428 kJ (915.7 kJ) (2 mole C2H4O) = 1600 kJ

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