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UT CH 204 - Experiment and Thermochemistry

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1 Experiment 8 Thermochemistry CH 204 Fall 2006 Dr. Brian Anderson What is Thermochemistry? The study of heat in chemical reactions. Forming chemical bonds releases energy. Breaking chemical bonds requires energy. How much energy? Depends on the bond. Add up all the energies to get the heat of reaction, ∆H rxn . This week in lab We will measure the amount of heat given off by 50 ml of hot water, by some chunks of hot metal, and by two chemical reactions Mg + 2H + → Mg 2+ + H 2 + heat MgO + 2H + → Mg 2+ + H 2 O + heat We’ll do all these reactions in a coffee cup calorimeter.2 The basic operation of calorimetry - Start with a known volume of a solution in the calorimeter. - Drop in something hot, or start a reaction that generates heat. - Close the calorimeter and measure the increase in temperature as heat is generated. - Keep measuring the temperature until it finally levels out. Part One: Add hot water to cold 50 mL of cold water (5ºC). Add 50 mL of hot water (75ºC). Final temp should be (75 + 5) ÷ 2 = 40ºC But the final temp will actually be lower than that because the cup itself will absorb a little bit of the heat. Heat Capacity We will use the data in part 1 to calculate the heat capacity of the cup, in units of J/K. This will tell us how many Joules of heat the cup absorbs for every K (or degree C) the water heats up. We know how many Joules of heat we added with the hot water, and we can calculate how many Joules were absorbed by the cold water as it warmed up. The “unused” Joules were absorbed by the cup. C cal = Joules/∆T3 Part 2 Unknown Metal We will determine the identity of an unknown metal by calculating its specific heat capacity. Specific heat capacity is how much heat (in Joules) it takes to heat up one gram of something by one Kelvin. A word on heat capacities SPECIFIC HEAT CAPACITY is an intensive property. Specific heat capacity tells how much heat (in Joules) is required to raise the temperature of one gram of the substance by one degree Kelvin. HEAT CAPACITY is an extensive property. It takes into account how much mass you have. Part 2 again We know the mass of the water, T C and T M , and we know how much energy is required to heat 1 gram of water by 1 Kelvin (c s = 4.184 J/gK). With this information (and the heat capacity of the calorimeter calculated from Part 1), we can calculate how much heat energy the hot metal supplied to the water.4 Every equation is a sentence The total heat given up by the metal is equal to the amount of heat absorbed by the water plus the amount of heat absorbed by the calorimeter: q total = q water + q calorimeter The amount of heat absorbed by the water is equal to the mass of the water times its specific heat capacity times the change in temperature: q water = m w × c w × ∆T The amount of heat absorbed by the calorimeter is equal to its heat capacity times the change in temperature: q cal = C cal × ∆T Identifying the unknown metal Since we know the mass of the metal, its initial and final temperatures, and how much heat it gave up, we can calculate its specific heat capacity. c m = Joules/(mass metal × ∆T) q total Parts 3 and 4 The reactions of magnesium and magnesium oxide with HCl. Mix these continuously, expecially the MgO. IMPORTANT: Use 2.0M HCl to react with the Mg metal (Part 3). Use 6.0M HCl to react with the MgO (Part 4). HCl in the hood is 6.0 M.5 Making graphs in Excel Draw lines on the graphs yourself or have Excel do it for you. Get equation for the line, plug in time of mixing to get T M and T C . Get it right the first time 1) Start recording temps before starting the reaction 2) Cover and swirl immediately! 3) Continue recording temps on the same timeline throughout the experiment. 4) Keep taking temperature readings until the temp is constant or declining Measuring temperature The tip of the probe should be IN THE SOLUTION, not resting on the bottom or stuck through the bottom.6 Four Things to Remember 1) The Alamo 2) Return digital thermometers to the beaker of water in the hood as soon as you finish with them. 3) Put both lab partners’ names on the unknown request slip. 4) Return your metal unknown to the stockroom before you leave. Some Fatherly Advice Start the report early Don’t wait until all the TA’s office hours have passed before you start on this. The calculations are not hard, but students have more questions on this lab report than on any other.7 Post-Lab Typo Problem 4: CO 2 (g) + 2 H 2 O(l) CH 4 (g) + 2 O 2 (g) Next Week’s Quiz Hess’s Law Calculate a heat capacity given some reaction

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