Chemistry Core Lab Determination of Specific Heat of Different Metals C2

Chemistry Core Lab “Determination of Specific Heat of Different Metals”

Most of my students know how to calculate the heat released or absorbed in a chemical reaction by using the equation q = mc T. However, the concept of specific heat can be tricky for them. I use this activity (lab) to reinforce this concept. I have an “equal” mass set of 5 different metals. (Zn, Cu, Al, Pb, and brass) I allow the students to handle each metal and tell me to make a prediction of each specific heat. Most will not rank them correctly. I do not allow them to see a table listing. I will have also demonstrated the specific heat of 3 liquids by taking the temperature change of each liquid on the same heater. I heat the liquids for the same amount of time. I ask them to rank the specific heat of each liquid. I do not tell them correct answers.

I set the activity as follows. I measure out 500 – 800 ml of distilled water in a 1000ml glass beaker and heat it to boiling. The students record the temperature of the water with a thermometer.(Most of the time, it is not exactly 100C). My students know that when water is boiling (undergoing a phase change), temperature does not change. All of the heat is being used to break intermolecular forces of the water molecules to allow the liquid to change phase into a gas. In a small pyrex dish or beaker, my students measure out a set amount of distilled water (usually 20 – 40 ml) and record the temperature. A Styrofoam cup can also be used. It loses less heat to the surroundings. After weighing a piece of metal, the student uses tongs to put the metal into the boiling water. I have the student leave the metal in the boiling water for a full minute. This ensures the metal will reach the same temperature as the water. The student takes the metal out of the boiling water and as quickly as possible, puts the metal into the pyrex dish or Styrofoam cup and records the change of temperature of the water in the cup. This procedure is repeated with 3 or 4 other metals. The metals can be of different masses or all can have the same mass.

When all students are done, we talk about what we need to do next to figure out the specific heat of each metal. I again ask each student to make a prediction based on just their data alone. Eventually, a student will recognize that fact that the heat lost by the metal is equal to the heat gained by the water in the dish or cup. They then set up the analysis like this.

Heat lost by metal = heat gained by water

Since q = mc t for both, then, mc T for the metal = mc T for the water. Since the specific heat of the water is known to be 4.18 J/gC, then the students can use the data from the lab to determine masses of the water and the metal and the changes in temperatures of both the water and the metal. The only unknown will be the specific heat of each metal.

You can use this example. You use 30 grams of water with an initial temperature of 22.4C. It has a specific heat of 4.18J/gC. You have a metal with a mass of 14.5 grams and an unknown specific heat. When you put the metal @ 100C into the water, the final temperature of the metal and the water is 24.6C. You calculate the change in temperature of the water to be 2.2C (24.6 – 22.4) and the change in temperature of the metal to be 75.4C. (100-24.6). You set the equation as follows

Heat gained by water = Heat lost by metal

(30g)(4.18J/gC)(2.2C) = (14.5g)(c) ( 75.4C)

The specific heat can be calculated this way for each metal. When the students are done with the lab, encourage them to look at tables that have values for specific heats and compare the answers they got to the answers on the tables.

Common mistakes:

1. I would recommend that fresh water be used each time in the dish or cup. Try to make sure the water is close or at room temperature. You will get a more accurate reading.

1. The boiling water should be either distilled or at least filtered. Ions will tend to adhere to the glass and dirty the beaker if your tap water is “hard”.

1. Have the students drop the metal into the cup and do not immerse the tongs into the water. This really skews results. Heat from the tongs also release into the water.

Materials you will need.

Hot plates. I do not like to use Bunsen burners. How plates are a more even heat. Small insulated cups or pyrex dishes, gram scale, tongs, thermometer, distilled water.