Warmup: (3 min) 1. What is the difference between temperature and heat? 2. Do you think heat flows from cold to hot or from hot to cold? Give an example.

Energy and Enthalpy

The First Law of Thermodynamics 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

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

HOTtoCOLD Heat flows from

Temperature: -measures the amount of thermal energy (heat) in a system -depends on how fast the molecules are moving (KE) T K = T °C T °C = (T F – 32)(5) 9 average KE of a sample of molecules are different.

Energy Units 1. the Joule(J) or kJ 2. 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!

One serving of yogurt contains 6.01 x 10 4 calories. Convert this to kiloJoules of energy. 1 kJ = 1000 J1 cal = J 1000 cal = 1 Cal 6.01 x 10 4 cal (1 calorie) (4.184 J) = 251 kJ (1000 J) (1 kJ)

How many Joules is Calories? 1 kJ = 1000 J1 cal = J 1000 cal = 1 Cal Cal (1 Cal) (1000 cal) = 1.1 x 10 8 J (4.184 J) (1 cal)

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

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. 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 kJ + NH 4 NO 3 (s) → NH 4 + (aq) + NO 3 - (aq) reactant

Hot packs (read only) CaCl 2 (s) → Ca 2+ (aq) + 2Cl - (aq) +13.9kJ In hot packs, calcium chloride or magnesium sulfate frequently are used because these chemicals dissolve in water exothermically. 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. product

Exothermic Rxns Endothermic Rxns Heat flows from: system  surroundings surroundings  system ΔHΔH-+ Surroundings feel: HOTCOLD because:Absorbed energy released by reaction System stole energy to break bonds

Xe(g) + 2F 2 (g) kJ --> XeF 4 (s) endo or exo? ∆H = XeF 4 (s) --> Xe(g) + 2F 2 (g) kJ endo or exo? ∆H = kJ When a reaction is reversed, the sign of ∆H is reversed too xenon tetrafluoride +251 kJ

Identify each as an exothermic or endothermic reaction: *write just the answer 1) When solid KBr is dissolved in water, the liquid gets colder 2) 2KClO 3 (s) kJ  2KCl(s) + 3O 2 (g) 3) When concentrated sulfuric acid is added to water, the solution gets very hot 4) The net enthalpy change for a reaction is negative 768kJ (ΔH = kJ)

Enthalpy and Stoichiometry Ex. 2C 2 H 4 O(l)+ 2H 2 O(l)  2C 2 H 6 O(l) +O 2 (g) ∆H = kJ a. Considering the reaction above, how much energy is required to produce moles of oxygen gas? moles O 2 b. If I want to completely burn 3.4 moles liquid C 2 H 4 O, how much energy must be present? 3.4 moles C 2 H 4 O (915.7 kJ) (1 mole O 2 ) = 428 kJ (915.7 kJ) (915.7 kJ) (2 mole C 2 H 4 O) = 1600 kJ

Energy must be added or absorbed to break bonds and that energy is released when bonds are formed. You can calculate the total enthalpy of the reaction using the following formula: ΔH = bonds broken - bonds formed Calculate the change in energy that accompanies the reaction: H 2(g) + F 2(g)  2HF (g) Bond type Bond energy (kJ/mol ) H—H 432 F—F 154 H—F 565 ΔH = {breaking H-H + F-F} – (forming 2 H-F bonds) = [1(432) + 1(154)] - [2(565)] = -544 kJ ΔH = -544 kJ More energy is released by forming the new bonds than was required to break the old bonds kJ

2H 2 O  2H 2 + 1O 2 2H 2 O  2H 2 + 1O 2 brokenformed 2(two O-H bonds) – {2(one H-H bond) and 1(one O=O bond)} 2(2 x 464kJ) - {2(436kJ)+ 1(498kJ)} 1856 kJ kJ ∆H = 1856 kJ kJ = kJ Bond Bond Energy (kJ/mol) Bond Bond Energy (kJ/mol) Bond Bond Energy (kJ/mol) Bond Bond Energy (kJ/mol) C and H414O and O142Ca and Cl398 H and H436 H and Cl 431O = O498Ca and O452 Cl and Cl243 H and N389O and C360Na and Cl310 N and Cl425 H and O464O = C736K and Cl437 C and C kJ