1 Calorimetry (4-2)

Calorimetry Calorimetry - the accurate and precise measurement of heat change for chemical and physical processes. The device used to measure the absorption or release of heat in chemical or physical processes is called a Calorimeter

Calorimetry Foam cups are excellent heat insulators, and are commonly used as simple calorimeters

A Cheap Calorimeter

For systems at constant pressure, the heat content is the same as a property called Enthalpy (H) of the system

Calorimetry Changes in enthalpy =  H q =  H These terms will be used interchangeably in this textbook Thus, q =  H = m x C x  T  H is negative for an exothermic reaction  H is positive for an endothermic reaction

Calorimetry Calorimetry experiments can be performed at a constant volume using a device called a “bomb calorimeter” - a closed system

9 In terms of bonds C O O C O O Breaking this bond will require energy. C O O O O C Making these bonds gives you energy. In this case making the bonds gives you more energy than breaking them.

10 Exothermic The products are lower in energy than the reactants Releases energy 2Al (s) + 3Cl 2 (g) --> 2 AlCl 3 (s) kJ ∆H= kJ

11 C + O 2  CO 2 Energy ReactantsProducts  C + O 2 C O 2 ΔH = -395kJ kJ

12 Endothermic The products are higher in energy than the reactants Absorbs energy 2 H 2 O kJ > 2 H O 2 (g) ∆H = kJ

13 CaCO 3  CaO + CO 2 Energy ReactantsProducts  CaCO 3 CaO + CO 2 ΔH = 176 kJ CaCO kJ  CaO + CO 2

14 Chemistry Happens in MOLES An equation that includes energy is called a thermochemical equation CH 4 + 2O 2  CO 2 + 2H 2 O kJ 1 mole of CH 4 releases kJ of energy. When you make kJ you also make 2 moles of water

What is the molar enthalpy of CO2 (g) in the reaction for the burning of butane below? 2 C 4 H O 2  8 CO H 2 O ∆H=-5315 kJ Answer: Molar enthalpy is the enthalpy change in equation divided by the balance of CO2 Molar enthalpy, ∆H substance = 5315 kJ ÷ 8 mol = 664 kJ / mol.

For each of the following rewrite the equation in " H " notation, for one mole of the underlined substance. Fe 2 O 3 (s)+3CO(g)→3CO 2 (g)+2Fe(s)+25kJ Answer: 1/3 Fe2O3 (s)+CO(g)CO2(g)+2/3 Fe(s) ∆H = KJ 4 NH 3 (g)+5O 2 (g)→4 NO(g)+6H 2 O(l)+1170kJ

2 HCl (g)+96 KJ → H 2 (g)+Cl 2 (g) N 2 (g)+3 H 2 (g) → 2 NH 3 (g)+92 KJ 2 CO 2 (g)+566 KJ →2 CO (g)+ O 2 (g) 4 Al (s) +3 O 2 (g) →2 Al 2 O 3 (s)+3360 KJ

18 Thermochemical Equations A heat of reaction is the heat change for the equation, exactly as written –The physical state of reactants and products must also be given. –Standard conditions for the reaction is kPa (1 atm.) and 25 o C

19 CH O 2  CO H 2 O kJ If grams of CH 4 are burned completely, how much heat will be produced? g CH g CH 4 1 mol CH kJ = 514 kJ

20 CH O 2  CO H 2 O kJ How many liters of O 2 at STP would be required to produce 23 kJ of heat? How many grams of water would be produced with 506 kJ of heat?

How much heat will be released if 65 grams of butane is burned in a lighter according the equation: 2 C 4 H O 2  8 CO H 2 O ∆H=-5315 kJ = kJ = 3.0 MJ

Calculate the heat released when 120 grams of Iron (III) oxide is formed by the following equation 2 Fe 2 O 3 (s) → 4 Fe(s)+3 O 2 (g) ∆H=1625 kJ = kJ = 610 kJ

Q = n ∆H (substance) Where n = # of moles

What mass of carbon dioxide must form to create 1200 kJ of heat when the following reaction occurs? C 6 H 12 O 6 (s)+6O 2 (g)→6CO 2 (g)+6H 2 O(l) ∆H=- 2808kJ Answer: 110 grams

3) What mass of oxygen is needed to completely react and release 550 kJ of heat in the following reaction? 4Fe (s)+3O 2 (g) → 2 Fe 2 O 3 (s) ∆H= kJ Answer: 32 grams

26 Summary, so far...

27 Enthalpy The heat content a substance has at a given temperature and pressure Can’t be measured directly because there is no set starting point The reactants start with a heat content The products end up with a heat content So we can measure how much enthalpy changes

28 Enthalpy Symbol is H Change in enthalpy is  H (delta H) If heat is released, the heat content of the products is lower  H is negative (exothermic) If heat is absorbed, the heat content of the products is higher  H is positive (endothermic)

29 Energy ReactantsProducts  Change is down  H is <0

30 Energy ReactantsProducts  Change is up  H is > 0

31 Heat of Reaction The heat that is released or absorbed in a chemical reaction Equivalent to  H C + O 2 (g)  CO 2 (g) kJ C + O 2 (g)  CO 2 (g)  H = kJ In thermochemical equation, it is important to indicate the physical state H 2 (g) + 1/2O 2 (g)  H 2 O(g)  H = kJ H 2 (g) + 1/2O 2 (g)  H 2 O(l)  H = kJ

32 Heat of Combustion The heat from the reaction that completely burns 1 mole of a substance