12 Experiment 6: Conservation of Energy

https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Principles_of_Modern_Chemistry_(Oxtoby_et_al.)/Unit_4%3A_Equilibrium_in_Chemical_Reactions/12%3A_Thermodynamic_Processes_and_Thermochemistry/12.3%3A_Heat_Capacity_Enthalpy_and_Calorimetry

https://physics.ucf.edu/~saul/02_FALL_1121C/HWs/Heat4UCF-P1121.pdf

https://chemdemos.uoregon.edu/demos/Comparing-Specific-Heats-of-Metals#

https://www.middleschoolchemistry.com/lessonplans/chapter2/lesson1

ACS/Ron’s project 7

conservation of energy

objective: To study the principles of energy transfer and conservation

learning points:

1. be able to calculate energy as calories
2. track energy transfer of heated object

The law of conservation of energy states that the total energy of an isolated system cannot change and is conserved over time. Energy cannot be either created or destroyed, but energy can change form, for example chemical energy can be converted to kinetic energy in the explosion of a stick of dynamite.  An example of the consequence of the law of conservation of energy is that a perpetual motion machine of the first kind cannot exist. Stated differently, no system without an external energy supply can deliver an unlimited amount of energy to its surroundings.

The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes: The change of internal energy of a system is equal to the heat added to the system minus the work done by the same system.  In equation form:  ΔU = q – W or Change in internal energy = (heat added to the system) – (work done by the system).

Typically for chemistry you will see the first law written as:  ΔU = q + W or Change in internal energy = (heat added to system) + (work done on the system).  It is the same law, the thermodynamic expression of the conservation of energy principle. It is just that work is defined as the work done on the system instead of work done by the system. In context of chemical reactions and processes, it may be more common to deal with situations where work is done on the system rather than by it.  The law of conservation of energy states that energy cannot be created or destroyed, but only changed from one form into another or transferred from one object to another.  For example in a closed system, meaning a system that is isolated from its surroundings, the total energy of the system is conserved.

Some forms of energy: 1.Kinetic energy – energy of motion;. 2. Potential energy – energy of location with respect to some reference point; 3.Chemical energy – energy stored in chemical bonds, which can be released in reactions; and 4.Electrical energy – energy created by separating charges (energy stored in a battery); 5.Thermal energy – energy given off as heat, such as friction.

The specific heat is the amount of heat per unit mass of material required to raise the temperature by one degree Celsius. The relationship between heat and temperature change is usually expressed in the form shown below where cp is the specific heat. The relationship does not apply if a phase change is encountered, because the heat added or removed during a phase change does not change the temperature.

Q =  cpmΔT

Where,

Q = heat

m = mass

ΔT = change in temperature of object (Tfinal – Tinitial)

cp = the specific heat

Some Metal Specific Heat Capacy

The heat generated into the water (if boiling water) is:

q  =  4.183 J/goC    x   mass of water    x    ΔT of water                  [1]

Expressed differently to include specific heat capacity of a metal becomes:

-q  =  cp     x     mass of metal    x     ΔT of metal                             [2]

You will determine if the heat associated with the water is equal to the heat associated with the metal.

SAFETY

This project involves the dangers related to working with glassware and heating with a burner.  Avoid touching anything heated until it has cooled to a comfortable temperature.  Dispose of matches into the trash after dosing with water.

Hazards: boiling water, flame, hot metal.

PROCEDURE

1. Obtain the following supplies: tripod, asbestos pad, burner, 400 mL beaker, glass stir rod, piece of metal, string, 2 styrofoam cups, thermometer, and matches.
2. Place the asbestos pad atop the tripod and place on that a 400 mL beaker that is ¾ full with distilled water.
3. Place the burner below the beaker for heating. Take the metal piece to the balance and record the weight of the metal piece to three decimal places. Tie a 9 inch string to the metal piece and tie the other end to the middle portion of the glass stir rod.
4. Now suspend the metal in the beaker water and make sure it is NOT sitting on the bottom of the beaker.
5. Your apparatus should look as follows:

6. Light the burner and adjust the air flow (by the barrel of the burner) so that you get a blue flame. (The gas is adjusted by a pin wheel at the bottom of the burner).
7. When the water is boiling then take the thermometer and holding it in the water (do NOT allow it to touch the bottom of the beaker), record the temperature of the boiling water (read the thermometer to the tenths of degree).
8. Weigh the empty dry styrofoam cups (one within another) on the balance and record the mass. Place enough distilled water into the styrofoam cup to submerge the metal piece but not to fill the cup with water.  Take it to the balance and record the weight of the cup with water.  Obtain and record the temperature of this water in the cup (read thermometer to the tenths of degree).  Leave the thermometer inside the cup.
9. After the metal piece has been suspended in the boiling water for about 5 minutes then carefully transfer the metal piece to the styrofoam cup water (the string can be present but do not place the glass stir rod into the cup water).
10. One student holds the styrofoam cup and reads the thermometer while the second student monitors the clock and records temperature every 30 seconds.
11. You will notice the temperature of the cup water rises quickly at first but flattens out or even decreases. When you have four temperature readings at the maximum or below the maximum then you can stop.  If you visualize the data, then your data would look like the following:

CALCULATIONS

You will determine the heat absorbed by the metal in the boiling water and compare that to the heat released by the metal piece into the cup water.

Heat flow into the water of the cup (units are Joules or J:

Mass of water = weight of cups with water – weight of empty dry cups

ΔT  =  maximum temperature of water in cup – temperature of water before metal added

=  Tfinal – Tinitial

Heat flow into water = q = 4.183 J/goC    x   mass of water    x    ΔT of water

Heat flow out of the metal piece (units are Joules or J):

Mass of metal = mass obtained directly from balance

ΔT of metal  =  maximum temperature of water in cup – boiling temperature of water

Cp is obtained for your metal from the table given above.

Heat flow out of metal =  -q  =  cp     x     mass of metal    x     ΔT of metal

Heat flow into the water of the cup =   Heat flow out of the metal piece

ANSWER THE FOLLOWING QUESTIONS:

1. Why did we keep the metal piece off the bottom of the beaker of boiling water?
2. Why did you keep the thermometer off the bottom of the beaker of boiling water?
3. What was your metal and specific heat capacity of the metal?
4. a. What was your heat flow into the water of the cup?
5. What was your heat flow out of the metal piece?
6. Did these two values equal to each other?
7. Subtract the two values and report the absolute value of the difference.
8. Calculate the average value of heat by adding 4a. and 4b., then divide by two.
9. Determine the percent difference by dividing the absolute value of the difference in step 4d. by the average value of heat in step 4e., then multiply by 100.

alternative procedure for conservation of energy

PROCEDURE

1. Obtain a 100 mL graduated cylinder and weigh it dry before beginning, record mass=________.

2. Obtain one cup of cold water and one cup of very hot water (use water heater for source).

3. Combining hot and cold water and using the 100 mL graduated cylinder, fill the graduated cylinder near to the 75.00 mL mark. RECORD THE EXACT VOLUME TO 2 DECIMAL PLACES (ie. 75.00 mL)

4. Obtain the mass to three decimal places (ie. 45.124 g) and record the exact mass with the exact volume in the table.

***REPEAT WITH DIFFERENT COMBINATIONS OF HOT AND COLD WATER.

5. Record these masses and volumes for as many times as instructed.

6. Subtract the empty mass of the 100 mL graduated cylinder from the masses of “water + cylinder”. Now you can calculate densities of the water at the different temperatures

Example:density = mass of water/volume of water = 87.786 g/89.10 mL   = 0.9853 g/mL

7. Go to the plot of specific gravity versus temperature for water provided to you.

Determine the temperature of the water off the X-axis based on your densities you calculated.

When completed then plot YOUR resulting temperature and densities as described above.

Report Preparation:

Title of Project

Objective of Study

Safety (minimal: wear goggles, no sandals)

Procedure

Data (Place the data in a table similar to example below)

Table with four columns:

Graph (plot Temperature (oC) on X-axis versus your Density value (g/mL) on Y-axis)

Answer the questions below

Conclusion

Questions:

1. How does density of the water vary with temperature of water?

2. Does your graph appear as a straight line? Describe your plot.

3. What happens to water at low temperatures that make this approach ineffective?

4. The density of sea water at the ARCTIC and ANTARCTIC is great or lesser than at the equator? How would that likely to affect sea life at the equator compared to the POLES?