The Properties of Chemical Reactions

Introduction

Chemical reactions are a part of our daily lives, from rusting metal to making bread to leaves changing colour in the fall. A chemical reaction is the process that occurs when two or more substances combine to produce a chemical change. When a chemical reaction takes place, the change is indicated by one or more qualitative properties. The colour or odor could change, gas could be produced, a precipitate – a solid substance in a solution - could be formed, or energy could be absorbed or released.

The substances initially involved in a chemical reaction are called reactants. When chemical reactions occur, the end result is called a product. Products usually have different properties than reactants; bonds between atoms will be changed during the reaction, switch the atoms’ arrangement in different compounds. A compound is a chemical substance that consists of two or more different chemically bonded elements. In this experiment, several different compounds are being worked with – potassium iodide, lead (II) nitrate, acetic acid and sodium bicarbonate.

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The purpose of this experiment is to determine whether or not a reaction has taken place using qualitative and quantitative analysis.

Procedure

Part 1 - Qualitative observations of both potassium iodide and lead (II) nitrate were recorded in a table. Then, the equipment and chemicals needed were gathered – potassium iodide and lead (II) nitrate, a graduated cylinder, Erlenmeyer flask, small test tube, rubber stopper, and small scale. Using the graduated cylinder, 10 mL of potassium iodide solution was measured and poured into the Erlenmeyer flask.

Next, the lead (II) nitrate solution was poured into the small test tube until it was approximately half full. The test tube was placed inside the Erlenmeyer flask, and sealed with a rubber stopper – the solutions were not mixed. The mass of flask, stopper and contents were determined by being placed and weighed on a small scale.

The mass determined was then recorded in another table. The Erlenmeyer flask was tipped so the previously separate solutions were allowed to mix, and the new mixture was again weighed and recorded in the second table. Changes in appearance from the original solutions were recorded in the first table. Lastly, leftover materials were disposed of.

Part 2 – Qualitative observations of both acetic acid and sodium bicarbonate were recorded in a third table. Equipment and chemicals were gathered – acetic acid and sodium bicarbonate, a small scale, two plastic “weigh boats”, scoopula, and graduated cylinder. The first weigh boat was placed on the scale and the scale was “zeroed”. One scoop of sodium bicarbonate was poured into the weigh boat with the scoopula, and the results were recorded in a fourth table.

Next, the second weigh boat was placed on the scale and zeroed. Using the graduated cylinder, 15 mL of acetic acid was measured and poured into the weigh boat on the scale. The mass of the acetic acid was recorded in the fourth table. The sodium bicarbonate, measured previously, was poured into the weigh boat of acetic acid, and qualitative observations of that were recorded in the third table. When the reaction was complete, the mass of the products shown on the scale was recorded in the fourth table. Lastly, leftover materials were disposed of.

Results

Qualitative Observations Part 1 – Before the reaction took place, both the potassium iodide and lead (II) nitrate had similar characteristics – they were both in a liquid state, as well as being clear and colorless. After the reaction, the qualitative observations of the new substance showed several differences. It was yellow in color, and a grainy precipitate had formed. The substance remained in a liquid state.

Qualitative Observations Part 2 – Before the reaction took place, the acetic acid and sodium bicarbonate each had different qualitative characteristics. The acetic acid was clear, colorless and in a liquid state. The sodium bicarbonate, however, was a white powder, in a solid state. The reaction caused the new substance to bubble and foam, producing a gas.

Analysis

Part 1: The chemical reaction that took place was evidenced by several qualitative observations. The colour of the two original substances was clear, and when combined the colour changed to yellow – a change of colour is clear evidence of a chemical reaction. The word equation for this reaction would be - potassium iodide + lead (II) nitrate Potassium nitrate and lead (II) iodide. In the form of a balanced equation, this reaction would be - 2KI + Pb(NO3)2 2KNO3 + PbI2.

Weighing both the reactants and the product showed that the mass of the reactants, with the apparatus, was 140. 26 grams. The mass of the product, with the apparatus, was also 140. 6 grams – the product of this reaction had the same mass as the reactants. These results were expected, based on the Law of Conservation of Mass.

The Law of Conservation of Mass is neither created nor destroyed in a chemical reaction. The application of the Law of Conservation of Mass means that the mass of products in a chemical reaction will equal the mass of the reactants, and this is consistent with the results of the first experiment.

Part 2: Several qualitative observations determined that a chemical reaction took place – the product bubbled, and a gas was produced.

The chemical equation for this reaction is CH3COOH + NaHCO3 NaCH3COO + H2O + CO2. Put into a word equation, the equation would be acetic acid + sodium bicarbonate Sodium acetate + water + carbon dioxide, carbon dioxide being a gas. Because a gas is present on the product side of the equation but not on the reactant side, the gas has been produced, which is evidence of a chemical reaction. Weighing the reactants and products showed that the combined mass of the reactants was 15. 30 grams.

The combined mass of the products was 14. 8 grams, weighing slightly less than the reactants. This result was expected, because it is consistent with the Law of Conservation of Mass. Although the product weighed on the scale showed a slightly lower mass, this was because the carbon dioxide was a gas, and was not accounted for on the scale. To achieve better results and gain an accurate measurement of the products’ combined mass, it would be necessary to capture the gas and weigh it as well.

Part 3: Extension For the chemical reaction AgNO3 + NaCl NaNO3 + AgCl, the total molecular mass of the reactants would be 228. grams – silver nitrate (AgNO3) would have a mass of 169. 9 grams, and sodium chloride (NaCl) would have a mass of 58. 5 grams. Based on the experiments performed in the lab, and the Law of Conservation of Mass, it is expected that the mass of the products would be 228. 4 grams as well. This prediction can be proved by finding the molecular mass of the reactions products - multiply the mass number of each atom by the number of said atom, and add the amounts to determine the total mass of sodium nitrate and silver chloride, the products.

Conclusion

Using qualitative and quantitative analysis, it was determined that reactions took place in each portion of the lab – the first between potassium iodide and lead (II) nitrate; the second between sodium bicarbonate and acetic acid. In each section, the reactions were evidenced by several qualitative observations. When potassium iodide and lead (II) nitrate were combined, a change of colour occurred in the reactant and a precipitate was formed. This is evidence of a chemical change. When acetic acid and sodium bicarbonate were combined, a gas was produced; also evidence of a chemical reaction.

In the experiments, the Law of Conservation of Mass was discovered. The quantitative evidence in each section proved this law. In the first experiment, the mass of the products was equal to the mass of the reaction, indicating that matter was not created or destroyed in the reaction. The results of the second reaction showed a decrease in the mass of the product – however, this was only because the gas formed in the reaction could not be weighed. Application Much of the ease of our current lifestyle is due to the availability of fresh air, pure water and access to resources – all of which are almost entirely taken from the environment.

When fossil fuels like coal and oil are mined and utilize, the result is a huge negative impact on the many aspects of the environment, and as a direct consequence, our lifestyles. Coal and oil have to be extracted from deep under the earth, transported, and burned. At each stage of this process, greenhouse gases - i. e. , carbon dioxide – are produced. Burning is especially bad for the environment; when burned, fossil fuels like coal and oil release mass amounts of carbon, which then combines with oxygen to form carbon dioxide.

References

1. University of Washington. Chemical Reactions – an Introduction. http://depts. washington. edu/chemcrs/bulkdisk/chem110A_aut01/notes_Week_5. pdf (accessed Oct. 30, 2012).
2. Science Daily. Science Reference – Chemical Compound. http://www. sciencedaily. com/articles/c/chemical_compound. htm (accessed Oct. 30, 2012).
3. American University. Oil Production and Environmental Damage. http://www1. american. edu/ted/projects/tedcross/xoilpr15. htm (accessed Oct. 30, 2012).
4. Chem Professor. Reactants and Products. http://www. chemprofessor. com/outline7b. htm (accessed Oct. 30, 2012).

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