posted by Paul .
Ok i have done 2 experiments (See below) the question i need help with is i need to explain the difference for the two combustions and i just don't know, please help me and thankyou for your help.
The Combustion Of Marshmallows
To investigate how much energy is transferred from a burning marshmallow to water and to calculate the efficiency of the energy transfer process.
Test tube clamp, Measuring cylinder, Bunsen burner, Mounting needle, Thermometer, Test tube, Marshmallow, Beaker of water.
20cm³ (20g) of water was measured in the measuring cylinder, this was then poured into the boiling tube. The boiling tube was then clamped to the stand and the thermometer inserted into the tube. After allowing the water to achieve room temperature an initial temperature reading was taken. The marshmallow was then weighed on the balance and the mass recorded. The marshmallow was then fixed to the mounting needle. The Bunsen burner was then lit and adjusted to have the hole fully open in order to produce a roaring flame into which the mounted marshmallow was placed. When the marshmallow began to burn it was moved from the flame to a point under the boiling tube to heat the water. This continued until the marshmallow stopped burning whilst ensuring that the water was stirred to distribute the heat evenly. Once the marshmallow stopped burning a final water temperature reading was taken. All measurements were taken to one decimal place.
MASS OF MARSHMALLOW 1.11g
TEMPERATURE OF COLD WATER 20.0°C
TEMPERATURE OF WATER AFTER HEATING 22.0°C
RISE IN TEMPERATURE 2.0°C
Therefore in order to calculate the energy produced by the marshmallow the following equation will be used:
Mass of water x rise in temperature x 4.2
(Where 4.2 is the figure we know as the amount of energy needed to raise the temperature of 1cm³ of water by 1°C)
Therefore energy produced by the marshmallow = 20.0 x 2.0 x 4.2
= 151351 Joules
As the energy unit used in describing the energy value of food is the kilo joule:
Energy produced by the marshmallow is 3.7kJ
The experiment showed that the energy transfer was very inefficient with a large difference between the official figure for the energy content of mini marshmallows and the results that were achieved in the laboratory.
Suggested reasons for this inefficiency and discrepancy would be:
1. Light produced during burning
2. Heating of air around the experiment area
3. Heating of the test tube
4. Heating through conduction of the mounting needle
5. Conduction at the clamp
6. Energy loss during transfer from Bunsen burner to boiling tube.
In order for this experiment to work more accurately and be more efficient it would be necessary to conduct it in a more controlled environment with many less variables to affect it.
The Combustion Of Pasta In The Calorimeter
To determine the energy content of one pasta twirl and witness the energy conversion.
Pasta twirl (1g), Calorimeter, 600cm3 water, Thermometer, Scales.
Fill the calorimeter with 600cm³ of water. Then the temperature of the water must be recorded before the experiment is conducted. Weigh the pasta on scales and record the weight of it. Put the pasta twirl in the crucible inside of the calorimeter. The next step is to ignite (heat) the pasta until it stops burning and the flame is extinguished. As the pasta is burning it is imperative to stir the water with the stirrer to ensure the heat is spread equally around the whole amount of water. When the burning finally stops, swiftly measure the temperature of the water once again and record the final temperature.
How A Calorimeter Works
A calorimeter is a container with two chambers. The first chamber holds the reaction you want to measure. The second chamber has a measured volume of water. These two chambers are separated by a metal wall that conducts the heat from the reaction to the water without letting the water mix in. They are both insulated so the heat stays inside the calorimeter as much as possible. A thermometer measures the temperature of the water. The calorimeter's sealed around the thermometer to prevent heat and water from escaping.
To use the calorimeter, a scientist will put a precisely known amount of pure water into the water chamber. The amount will vary. The temperature of the water is then recorded. Then measure out a precise amount of chemicals etc that is to be studied, put them in the reaction chamber, and close the lid. Watch the thermometer very closely for changes in the temperature. As the chemical reaction progresses, the temperature will rise or drop. If it goes up, it will reach a peak value, then decline. The opposites true if the temperature goes down. It's important to note the maximum or minimum temperature.
Once this temperature is discovered, the scientist can determine the reactions energy by minusing the first temperature from the final reading, then multiplying by the amount of water used. If the temperature went up, it is an exothermic reaction, giving off heat. If the temperature goes down, it is called an endothermic reaction, absorbing heat.
Role of Water
Water is the key to making the calorimeter work. The definition for the calorie was set down as the amount of energy that raises the temperature of 1 mL of water one degree Celsius. That's also called water's specific heat capacity. When water freezes to ice or boils to steam, the specific heat capacity changes. But as long as we're dealing with water, we can count on the relation.
INITIAL TEMPRATURE OF THE WATER
600g of water ÷ 0.6 Kg
4.1 x 4.2 x 0.6 = 10.3Kj Efficiency Output Energy
I have a couple of comments:
Are you sure of the energy content of the marshmellow? You just stated it as a fact. I would have given students a value of about 4.2.
Now, you caculations. On the marshmellow,
heat= 20g*2C*4.2J/goC= 151J Where did you get 151 thousand?
You need to use UNITS in your calculations, it stops errors such as this. Same on the pasta.