5/14/14. How can you measure the amount of heat released when a match burns?

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1 CHEMISTRY & YOU Chapter 7 Thermochemistry How can you measure the amount of heat released when a match burns? 7. The Flow of Energy 7.3 Heat in Changes of State 7.4 Calculating Heats of Reaction Remember: The concept of specific heat allows you to measure heat flow in chemical and physical processes. :measurement of heat flow into or out of a system for chemical & physical processes. heat absorbed by system (+q) = heat released by surroundings (-q). heat released by system (-q) = heat absorbed by its surroundings (+q). Calorimeter: insulated device used to measure the absorption/release of heat in chemical or physical processes. Foam cups Open to environment (constant pressure) 3 4

2 Constant-Pressure Calorimeters The value of ΔH of a reaction can be determined by measuring the heat flow of the reaction at constant pressure. q = ΔH Constant-Pressure Calorimeters To calculate the heat absorbed or released by the surroundings (q surr ) : q surr = m x s x ΔT ΔT = T f -T i of water ( C) m = mass (g) s = specific heat (J/g C) 5 6 Constant-Pressure Calorimeters The heat absorbed by the surroundings is equal to, but has the opposite sign of, the heat released by the system. q surr = - q sys Constant-Pressure Calorimeters The enthalpy change for the reaction (ΔH) can be written as follows: q sys = ΔH = - q surr q = m x s x ΔT ΔH is positive for an endothermic reaction ΔH is negative for an exothermic reaction. 7 8

3 CHEMISTRY & YOU Energy Content of Food Lab What type of calorimeter would you use to measure the heat released when a match burns? Describe the experiment and how you would calculate the heat released. 9 0 Enthalpy Change in a Experiment When 5.0 ml of water containing 0.05 mol HCl at 5.0 C is added to 5.0 ml of water containing 0.05 mol NaOH at 5.0 C in a foam-cup calorimeter, a reaction occurs. Calculate the enthalpy change (in kj) during this reaction if the highest temperature observed is 3.0 C. Assume that the densities of the solutions are.00 g/ml and the volume of the final solution is equal to the sum of the volumes of the reacting solutions. Analyze List the knowns and the unknown. Use dimensional analysis to determine the mass of the water. Calculate ΔT Use ΔH = q surr = m x s x ΔT 3

4 Analyze List the knowns and the unknown. Calculate Solve for the unknown. KNOWNS UNKNOWN s water = 4.8 J/(g C) ΔH =? kj V final = V HCl + V NaOH = 5.0 ml ml = 50.0 ml First calculate the total mass of the water. m water = 50.0 ml x.00 g = 50.0 g ml T i = 5.0 C T f = 3.0 C density solution =.00 g/ml Assume that the densities of the solutions are.00 g/ml to find the total mass of the water. 3 4 Calculate Solve for the unknown. Calculate Solve for the unknown. Now calculate ΔT. ΔT = T f T i = 3.0 C 5.0 C = 7.0 C Use the values for m water, s water, and ΔT to calculate ΔH. ΔH w = q surr = m water s water ΔT = (50.0 g)(4.8 J/(g oc))(7.0 C) = 500 J =.5 kj ΔH reaction = -.5 kj 5 6 4

5 The initial temperature of the water in a constant-pressure calorimeter is 4 C. A reaction takes place in the calorimeter, and the temperature rises to 87 C. The calorimeter contains 367 g of water, which has a specific heat of 4.8 J/(g C). Calculate the enthalpy change during this reaction. The initial temperature of the water in a constantpressure calorimeter is 4 C. A reaction takes place in the calorimeter, and the temperature rises to 87 C. The calorimeter contains 367 g of water, which has a specific heat of 4.8 J/(g C). Calculate the enthalpy change during for this reaction. ΔH w = m x s x ΔT = 367 g x 4.8 J/(g C) x (87 C 4 C) = J = 97 kj ΔH reaction = -97 kj 7 8 Constant-Volume Calorimeters Measuring the temperature increase of the water, to calculate the quantity of heat released during the combustion reaction. In a chemical equation, the enthalpy change for the reaction can be written as either a reactant or a product

6 Heats of Reaction The heat of reaction is the enthalpy change for the chemical equation as it is written. Heats of reaction à ΔH. The physical state of the reactants and products must be given. Assume reaction is carried out at atm and 5 C (room temperature). Calcium oxide and water create an exothermic reaction. The enthalpy change can be considered a product. CaO(s) + H O(l) Ca(OH) (s) kj A chemical equation that includes the enthalpy change is called a thermochemical equation. CaO(s) + H O(l) Ca(OH) (s) kj Heats of Reaction mole of calcium oxide and mol of water react to form calcium hydroxide and 65. kj of heat. CaO(s) + H O(l) Ca(OH) (s) ΔH = 65. kj In exothermic processes, the chemical potential energy of the reactants is higher than the chemical potential energy of the products

7 Heats of Reaction 5 Baking soda (sodium bicarbonate) decomposes heated. This process is endothermic. NaHCO 3 (s) + 85 kj Na CO 3 (s) + H O(l) + CO (g) 6 Heats of Reaction NaHCO 3 (s) + 85 kj Na CO 3 (s) + H O(l) + CO (g) NaHCO 3 (s) Na CO 3 (s) + H O(l) + CO (g) ΔH is positive for endothermic reactions. The decomposition of mol of sodium bicarbonate requires 85 kj of heat. ΔH = 85 kj The decomposition of 4 mol would require twice as much heat, or 70 kj. 70 kj = (85 kj x ) Sample Problem 7.4 Sample Problem 7.4 Using the Heat of Reaction to Calculate Enthalpy Change NaHCO 3 (s) + 85 kj Na CO 3 (s) + H O(l) CO (g) Calculate the amount of heat (in kj) required to decompose.4 mol NaHCO 3 (s). Analyze List the knowns and the unknown. Use the thermochemical equation to write a conversion factor relating kj of heat and moles of NaHCO 3. Then use the conversion factor to determine ΔH for.4 mol NaHCO 3. KNOWNS amount of NaHCO 3 (s) that decomposes =.4 mol ΔH = 85 kj for mol NaHCO 3 UNKNOWN ΔH =? kj for.4 mol NaHCO

8 Sample Problem 7.4 Sample Problem 7.4 Calculate Solve for the unknown. Calculate Solve for the unknown. Write the conversion factor relating kj of heat and moles of NaHCO kj mol NaHCO 3 (s) Using dimensional analysis, solve for ΔH. ΔH =.4 mol NaHCO 3 (s) x = 95. kj 85 kj mol NaHCO 3 (s) The thermochemical equation indicates that 85 kj are needed to decompose mol NaHCO 3 (s) Heats of Reaction Why are the physical state of the reactants and products important? Total energy released Total energy absorbed H O(l) H (g) + O (g) ΔH = 85.8 kj H O(g) H (g) + O (g) ΔH = 4.8 kj difference = 44.0 kj 3 3 8

9 Heats of Combustion The heat of combustion is the heat of reaction for the complete burning of one mole of a substance. Exothermic Heats of Combustion Small amounts of natural gas within crude oil are burned off at oil refineries. CH 4 (g) + O (g) CO (g) + H O(l) kj Burning mol of CH 4 releases 890 kj of heat. ΔH = kj / mol CH Interpret Data Heats of Combustion at 5 C Substance Formula ΔH (kj/mol) Hydrogen H (g) 86 Carbon C(s, graphite) 394 Methane CH 4 (g) 890 Acetylene C H (g) 300 Ethanol C H 6 O(l) 368 Propane C 3 H 8 (g) 0 Glucose C 6 H O 6 (s) 808 Octane C 8 H 8 (l) 547 Sucrose C H O (s) 5645 Remember, heats of reaction/combustion are reported as the ΔH when the reactions are carried out at STP. Which of the following thermochemical equations represents an endothermic reaction? A. C graphite (s) + kj C diamond (s) B. H (g) + O (g) H O kj

10 Which of the following thermochemical equations represents an endothermic reaction? A. C graphite (s) + kj C diamond (s) B. H (g) + O (g) H O kj 37 0