The enthalpy of reaction \u0394 Hrxn is an extensive property of a system that can

The enthalpy of reaction δ hrxn is an extensive

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2)The enthalpy of reaction(ΔHrxn) is an extensive property of a system that can beused to obtain the heat absorbed/evolved in a reaction at a given temperature andpressure. a.If at constant pressure, the qpcan be determined from the followingformula:i.ΔHrxn= qp. Introduction to Hess’s Law : 1) Hess’s law of heat summationstates that for a chemical equation that can be written as the sum of two or more steps, the enthalpy change for the overall equation is the sum of the enthalpy changes for the individual steps. 2) An example calculation for the decomposition of 2 moles of CaCO 3 . a. Unknown equation: 2 CaCO 3 ( s ) barb2right 2 CaO( s ) + 2 CO 2 ( g ) Δ H = ? b. Known equations: i. Ca( s ) + C( s ) + 3/2 O 2 ( g ) barb2right CaCO 3 ( s ) ΔH o 1 = –1208 kJ ii. C( s ) + O 2 ( g ) barb2right CO 2 ( g ) ΔH o 2 = – 393.5 kJ iii. Ca( s ) + ½ O 2 ( g ) barb2right CaO( s ) ΔH o 3 = – 635.0 kJ c. Step 1: Flip Equation #1 (and the sign of ΔH o 1 ) i. CaCO 3 ( s ) barb2right Ca( s ) + C( s ) + 3/2 O 2 ( g ) ΔH o 1 = –(–1208 kJ) d. Step 2: Cancel Like Terms and Add ΔH o i. CaCO 3 ( s ) barb2right Ca( s ) + C( s ) + 3/2 O 2 ( g ) ΔH o 1 = +1208 kJ ii. C( s ) + O 2 ( g ) barb2right CO 2 ( g ) ΔH o 2 = –393.5 kJ iii. Ca( s ) + ½ O 2 ( g ) barb2right CaO( s ) ΔH o 3 = –635.0 kJ iv. CaCO 3 ( s ) barb2right CaO( s ) + CO 2 ( g ) ΔH o rxn = +179.5 kJ e. Step 3: Multiply by Entire Equation by 2 &ΔH o rxn i. 2(CaCO 3 ( s ) barb2right CaO( s ) + CO 2 ( g )) ΔH o rxn = 2(+179.5 kJ) f. Final Answer: i. 2 CaCO 3 ( s ) barb2right 2 CaO( s ) + 2 CO 2 ( g ) ΔH o rxn = +359 kJ Introduction to Specific Heat : 1) Specific heat is the quantity of heat required to raise the temperature of one gram of a substance, one degree Celsius (or Kelvin) at constant pressure. a. Specific heats are characteristic for that substance at 25 o C and constant pressure. Experiment #8B Barnett
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49 b. Example specific heats of pure materials are listed below: Element/Compound Specific Heat (J/(g o C)) Aluminum 0.901 Iron 0.450 Lead 0.128 Copper 0.387 Water ( l ) 4.184 Procedure for the Specific Heat of an Unknown Metal: 1) Place approximately 400. mL of tap water into a 600 mL beaker and place onto a hotplate. a. Heat the water to the boiling point. 2) While waiting for the water to boil, obtain an unknown metal cylinder and determine the mass on an ANALYTICAL balance. a. Record this mass in the data table attached. b. Record the unknown number/letter in data table. c. The unknown metals are aluminum, iron, lead, and copper. 3) Carefully, place the metal cylinder into a dry test tube and clamp the tube into the boiling such that the metal cylinder is below surface. a. The metal should NOT be in direct contact with the water. 4) Obtain the mass of two clean and dried empty Styrofoam cups with a cap on an ANALYTICAL balance. a. These two cups should be stacked on top of one another and the cap should have a hole in it to facilitate the use of your thermometer. b. This set of Styrofoam cups will function as your calorimeter. c. Record mass in data table.
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  • Spring '19
  • Chemistry, Chemical reaction, Timothy Barnett

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