Determining the Heat of ReactionExperiment 3 Determining Heat of Reaction:Ice Calorimeter Pre-Lab Readings:. Introduction In this experiment,you will determine the amount of heat evolved per mole ofmagnesium metal when it reacts with an acid according to equationE1 below: E1: Mg(s) + 2 H+ (aq) â†’ Mg2+(aq) + H2(g) Apply! Why mighta chemist want to know the amount of heat given off by a reaction?______________________________ _ ______________________________ _One method to accomplish this is with
Determining the Heat of Reaction Experiment 3 Determining Heat of Reaction: Ice Calorimeter Pre-Lab Readings: . Introduction In this experiment, you will determine the amount of heat evolved per mole of magnesium metal when it reacts with an acid according to equation E1 below: E1: Mg(s) + 2 H+ (aq) â†’ Mg2+(aq) + H2(g) Apply! Why might a chemist want to know the amount of heat given off by a reaction? ______________________________ _ ______________________________ _ One method to accomplish this is with the use of an ice calorimeter. The simple setup you will use is shown in Figure 1. The reaction takes place in a small test tube suspended in a sealed beaker filled with ice and water. As heat given off by an exothermic reaction is absorbed by the surroundings, the ice melts. Since ice and liquid water have very different densities, melting results in a change in volume of the ice-water mixture. The heat required to melt a given mass of ice is shown in equation E2: E2: where q is heat absorbed by ice , w is mass of ice melted (in grams), and Hf is the latent heat of fusion of ice (333.5 J/g) This means that for every gram of ice melted, 333.5 J of heat is absorbed by the ice and, therefore, this same amount of heat must be given off by the reaction. Since it would be very difficult to directly measure the weight of ice that gets melted, another approach is in order. You can measure the change in volume (caused by the melting ice): E3: where Vi is the volume of ice, and Vw is the volume of liquid water (after melting w grams of ice) Remember that you can relate mass and volume by density and derive equation E4: E4: where Di is the density of ice at 0Â°C (0.917 g/mL), and Dw is the density of water at 0Â°C (1.000 g/mL) Then rearrange E4 to solve for w (which is the unknown term in E2 that you need): E5: q ï€½ w ï‚´ H f ï„V ï€½ Vi ï€Vw i Dw w D w ï„V ï€½ ï€ ( ) w i i w D D D D w V ï€Substitute E5 into E2 to get: E6: This equation means that you can calculate the heat change of a reaction easily by measuring the change in volume of the icewater mixture in your calorimeter. Conceptualize! Since a sample of ice occupies (more / less) space than the same weight of liquid water, the final volume measured will be (more / less) than the initial volume and Î”V will be (positive / negative). This will also result in a (positive / negative) value for q, which is expected since heat is (released / absorbed) by an (exothermic / endothermic) reaction. The heat per mole (Î”H) of Mg reacted is: E7: where y is the number of moles of Mg reacted in the experiment The accuracy of any calorimeter depends on being able to minimize heat exchange between all other sources except for the reaction in question. To do this, you will insulate your ice calorimeter as much as possible from the heat of the room with paper towels, water, and ice. After your setup has been assembled (without any air bubbles inside the beaker), and acid has been added to the test tube, the entire apparatus must come to equilibrium and sit for 10 minutes before you start to take readings. Method (to be done in pairs) The ice calorimeter setup is shown in Figure 1. A: 1.000 mL pipette B: Tygon tubing with screwpinch clamp C: loose stopper on test tube D: stopper assembly E: 200 mL beaker with ice/water F: 1000 mL beaker with ice/water and paper towel Figure 1: Ice calorimeter setup Crumple some paper towels and pack them in the bottom of a 1000 mL beaker. Thoroughly dry the inside of the test tube that is a part of the stopper assembly using a Kim Wipe wrapped around a pen. Add 5mL of 2.0 M HCl and stopper the test tube tightly. Fill the 200 mL beaker 3/4 full of crushed ice, then add cold water until it overflows. Gently stir the 200 mL beaker with a glass rod to allow as many air bubbles to escape as possible. Slowly push the stopper assembly into the 200 mL beaker so that the ice is pushed down and water spills out onto the counter top. f w i i w H D D D D q V ï‚´ ï€ ï€½ ï„ ï‚´ ( ) J mol y q ï„H ï€½ / A B C D E F 27 Do not trap any air bubbles under the stopper! With the clamp open, push the stopper assembly firmly into the 200 mL beaker until water rises into the tubing and remains at a fairly constant level. If there are large air bubbles trapped under the stopper, or the test tube is not completely surrounded by crushed ice, remove the stopper assembly, add more crushed ice, refill the beaker with cold water, and repeat the above procedure until the water level in the tube remains relatively constant. If the water level remains relatively constant, place the 200 mL beaker inside the 1000 mL beaker so that the top of the stopper assembly is below the lip of the large beaker and pack as many ice cubes around the sides as you can fit. Add water until the top of the stopper assembly is submerged and covered with ice, but the mouth of the test tube remains above water. Allow the system to come to equilibrium for about 5 minutes. Use a distilled water bottle to force water (and no air bubbles!) into the system until the pipette is filled. Have your partner clamp the tubing closed. Wait 5 more minutes for the water level to settle. Accurately weigh a 0.06-0.08 g sample of magnesium metal turnings. Avoid touching the metal with your fingers! Loosen the small test tube stopper so that you can open and close it easily. Adjust the meniscus so that it is near the top of the pipette but not past the calibration marks. Take all pipette readings to three significant figures! The first pipette reading should be the largest. Notice! What is the total volume of the pipette? _________________________ Take a pipette reading at , and every 30 seconds thereafter for seven minutes, to measure the base rate of melting due to the room temperature environment. Despite your best efforts to insulate the ice calorimeter, you will observe this slight decrease in volume before ever starting an exothermic reaction, but the base rate should remain constant throughout the experiment and not interfere with your results. At the seven minute mark, take the pipette reading and then drop in the magnesium metal. Loosely replace the small stopper on the test tube. Hypothesize! Why should you not seal the test tube tightly with the small stopper? ______________________________ _ ______________________________ _ Continue to take pipette readings every 30 seconds. Four minutes after the magnesium is added, check to make sure that all of the metal made it into the acid. If there are any magnesium pieces stuck on the sides of the test tube, use a pen to push them down. Continue to take pipette readings until the rate of volume change returns to the initial base rate you observed (or for about 18 minutes of total timing). t ï€½ 0 28 Rinse the test tube with lots of water and repeat the experiment using another sample of magnesium and 5 mL of 1.0 M H2SO4 . Details of the Experiment Your graph should end up looking roughly like the one shown in Figure 2 below: Figure 2: Sample graph of pipette volume (mL) vs. time (mins) The first 14 volume measurements you took were to determine the background melting rate of the ice, caused by a hopefully small but constant absorption of heat from the room. Since the temperature of the room is not fluctuating wildly, the changes in volume are expected to be constant and a line of best fit should be straight. This background melting rate should continue at the same rate even while the reaction is taking place. When the magnesium metal turnings are dropped into the acid, the heat produced is greatest at first when there is the most available metal to react. The rate of heat production then continues to decrease as the magnesium is used up. As the reaction proceeds and the reaction mixture is heated up, the total rate of melting is not constant and the line of best fit will be a smooth curve through the graphed points rather than a straight or zig-zag line. After the reaction is finished and the test tube has cooled back to the initial (nearly 0Â°C) conditions, the absorption of room heat again becomes the only reason the ice in the calorimeter is melting. If the rate of room heat melting has not changed during the experiment, a straight line of best fit through the last few points should have exactly the same slope as the first 14 points. Thus, any vertical distance between these two extrapolated parallel lines is the change in volume (Î”V) due to the reaction, as shown in Figure 2. This graphing method takes any effects from the room temperature environment out of the calculation for Î”V. If the initial and final melt rates are not the same (i.e. the straight lines are not parallel with each other) calculate the Î”V at the t = 12 minute mark. Thermodynamic Supplement Why do we use two symbols (Î”H and Î”E) to designate the heat of a reaction? Many chemical reactions produce a different amount of heat depending on whether they are done under constant pressure or constant volume conditions! Î”E is the heat change per mole of product produced under constant volume conditions, and is referred to as the internal energy change. Î”H is the heat change per mole of product produced under constant pressure conditions, and is referred to as the enthalpy change. The difference in magnitude between Î”E and Î”H depends on whether work (w = PÎ”V) is done during the reaction. This kind of work can only be done if the reaction volume changes, which is determined by examining only the gases in a balanced equation. 1) Question is Name 2 sources of errors (not personal errors) in this experiment.