Specific Heat of a Metal Introduction: When we wish to determine the amount of heat gained or lost during a process, we use a calorimeter (literally, a calorie counter) in which a thermometer or temperature probe measures the changes in temperature that result from the gain or loss of thermal energy. The calorimeter is an insulated container that allows as little heat as possible to be lost to or absorbed from the surroundings. We might also expect the container and the thermometer to absorb or release some heat energy; this amount of heat is the heat capacity of the calorimeter. In highly accurate calorimetric experiments the heat capacity of the calorimeter is determined first, then that value is used in calculating the heat evolved in all subsequent experiments done with that calorimeter. Knowledge of the magnitude of the temperature change that takes place in some process or reaction, the heat-absorbing capacity of the calorimeter, and the amount of one of the substances that is used in the process allows you to calculate the energy released or absorbed in terms of joules (or kilojoules) per mole of that substance. If all the heat released is used to heat water, the amount of heat involved can be found from the equation q = (mwater)(δt)(1 cal/g o C) where the last term, 1 cal/g o C, is called specific heat (or specific heat capacity) of water and is, in fact, the definition of a calorie. One calorie is the amount of heat required to heat 1 g of water 1 o C. The value for q can be changed to joules by the conversation factor, 1 calorie = 4.184 joules; thus, the equation we will use is: q = (mwater)(δt)(4.184 J/g o C) The changes in temperature can be followed in a number of ways, the most familiar being an ordinary mercury (or alcohol)-in-glass thermometer. A thermometer is an analog device in which the expansion of mercury or alcohol corresponds to changes in temperature. Your brain converts the length of the liquid column to a digital output when you read the position of the column on the thermometer scale. In a digital thermometer a tiny diode near the tip of the probe, powered by a battery, produces an output voltage that changes the temperature. This voltage, like the liquid in the thermometer, is an analog signal, but it can be easily converted to a digital signal by a computer chip. This signal is then read on the dial of the thermometer.
Pre-Lab Questions: 1. Since the specific heat of water is given in units of joules per gram degree Celsius, why do we measure the volume of water in the calorimeter instead of its mass? 2. A 22.50g piece of an unknown metal is heated to o C, then transferred quickly and without cooling into. ml of water at 20.0 o C. The final temperature reached by the system is 26.9 o C. a. Calculate the quantity of heat absorbed by the water. Show all work. b. Calculate the quantity of heat lost by the piece of metal. Show all work. c. Calculate the specific heat of the metal in J/g o C. Show all work. Materials: Balance Coffee cups with lids (2) Thermometer or temperature probe with CBL2 ml graduate cylinder Hot plate Tap water Metal sample Safety: 1. Wear goggles and apron at all times in the laboratory. 2. Handle hot metal carefully, using hot pads or tongs. 3. The hot plate and beaker are hot. Use care when working with them.
Procedure: The calorimeter used in this experiment is made of two coffee cups. Coffee cups are an excellent insulator. Why would we want our calorimeter to be an excellent insulator? Place one coffee cup right side up, cut off the top rim of the second coffee cup and nest the second coffee cups upside down inside the first right side up coffee cup. This will form a sealed chamber, why do we want a sealed chamber? You will have to insert thermometer through the top coffee cup, making a hole. Why do we want that hole as small as possible? 1. Fill a 250 ml beaker about halfway with tap water. Place it on your hot plate and begin heating the water until it reaches boiling. NO INFORMATION ABOUT THIS WATER NEEDS TO BE COPIED DOWN. It is only being used to heat the metal. In addition, place an empty test tube into the water. 2. Measure ml of tap water using the graduated cylinder and put in calorimeter. Measure and record the temperature of this water. This is the initial temperature of the water. RECORD IN DATA TABLE. 3. Obtain around 5-10 g of a metal sample. RECORD THE IDENTITY AND MASS OF YOUR METAL IN DATA TABLE. Place metal in test tube in the boiling water for at least 5 minutes. This is to ensure that the temperature of the shot is o C, the temperature of boiling water. Note: The metal is hot. Use a hot mitt OR test tube tongs 4. USE TONGS to QUICKLY transfer the metal sample at o C to the room temperature water in the calorimeter. QUICKLY place the lid containing the thermometer back on the calorimeter. 5. Record the highest temperature reached by the contents of the calorimeter. 6. Repeat the experiment with a second time with the SAME type of metal. 7. Clean up. Decant water down the drain, the metal is very expensive. DO NOT PUT METAL IN SINK. DRY OFF METAL AND PUT metal back in the container.
Type of metal used: (use the same metal for both trials DATA: TRIAL 1 TRIAL 2 Volume of water used (ml) Mass of water used (g) Initial temperature of water ( o C) Final temperature of water ( o C) Temperature difference of water ( o C) Identity and Mass of metal sample (g) Initial temperature of metal ( o C) Final temperature of metal ( o C) Temperature difference of metal ( o C) Calculated specific heat of metal (J/g o C) AVERAGE SPECIFIC HEAT OF METAL (J/g o C) Your typed lab report must include a title, claim, materials & methods, data table, calculations, and conclusion. Instead of a graph: +15 points 1. Show your work for calculating the quantity of heat gained by water for both trials and get the average. (see pre lab questions for help) 2. Assume that the quantity of heat lost by the metal is equal to the quantity of heat gained by the water. Determine the specific heat of the metal for each trial and get the average. Show your work for these calculations. For your conclusion: +5 points 1. Look up the value of the specific heat of your metal. (cite your source/website) 2. Discuss your calculated specific heat vs. the actual specific heat of your metal. Any calorimeter absorbs a certain amount of the heat released. Knowing this, is your value of the specific heat of the metal more likely to be higher or lower than the accepted value? Discuss in your conclusion. What other assumptions were made in the lab?
Sample data for Alulminum Volume of water used (ml) Mass of water used (g) Initial temperature of water ( o C) Final temperature of water ( o C) 21.0 22.36 22.0 23.40 Temperature difference of water ( o C) Identity and Mass of metal sample (g) 5.05 g of Aluminum 0.8 g Al Initial temperature of metal ( o C) C C Final temperature of metal ( o C) 22 C 23.4 Temperature difference of metal ( o C) Calculated specific heat of metal (J/g o C) AVERAGE SPECIFIC HEAT OF METAL (J/g o C)