Predict : Rendement and Product of a Reaction

Rendement is actually a term in the field of chemistry studies. The results show the inaccuracy of the reaction result, which results from the mathematical calculation. For example, in a chemical reaction, it is best to produce a 80g gram, mathematically, but the result is only 30 grams.

The percentage of results (or fractional or relative yield), which serves to measure the effectiveness of synthetic procedures, is calculated by dividing the desired amount of product obtained by the theoretical results (units of measure for both must be equal):

 

The theoretical result is the amount estimated by the stoichiometric calculation based on the number of moles of all the reactants present. This calculation assumes that only one reaction occurs and that the limiting reactant reacts completely. But the actual results are very often smaller (yield percentage less than 100%) for several reasons:

1. Many reactions are incomplete and the reactants are not fully converted to the product. If a backlash occurs, the final state contains reactants and products in a chemical equilibrium state.

2. Two or more reactions may occur together, so some of the reactants are converted into unwanted byproducts.

3. Losses occur in the separation and purification of the desired product from the reaction mixture.

4. There are no rejects

          The ideal or theoretical result of a chemical reaction is 100%. According to Vogel Textbook of Practical Organic Chemistry, the results of about 100% are called quantitative, yield above 90% is called very good, yield above 80% is excellent, yield above 70% is good, yield above 50% is fair, and Yields under 40% are called poor. These names are arbitrary and not universally accepted, and depending on the nature of the reaction in question, these expectations may be unrealistic. The yield may appear above 100% when the product is not pure, as the weight of the measured product will include any waste weight.
          The purification step always reduces the yield, through the losses incurred during the delivery of the material between the reaction vessel and the purifying device or the improper product separation of the impurities, which may require the removal of the fraction considered impure.

          The product yield measured after purification (typically up to 95% purity of spectroscopy, or with sufficient purity to bypass the combustion analysis) is called the isolated reaction product. The results can also be calculated by measuring the amount of product formed (usually in crude and unrefined products) compared to a number of internal standards added, using techniques such as gas chromatography / liquid, or NMR spectroscopy. The results determined using this approach are known as the result of internal standards. The yield is usually obtained in this way to accurately determine the amount of product produced by the reaction, regardless of the potential isolation problem. In addition, they can be useful when product isolation is challenging or boring, or when a quick determination of the expected outcome is desired. Unless otherwise stated, results reported in synthetic organic and chemical literature refers to the isolated results, which further reflect the amount of pure product which may be obtained under the conditions reported, after repeated experimental procedures.
When more than one reactant participates in the reaction, the result is usually calculated on the basis of a limiting amount of reagent, which is less than the stoichiometric equivalent (or equivalent) to the sum of all other reactants. Other reactants present in an amount greater than those required to react with all current limiting reagents are considered excessive. Consequently, results should not be automatically taken as a measure for reaction efficiency.

Example:

The yield percentage of 31.06% is not a result too bad. Chemical chemists and chemical engineers, however, prefer to produce more than 90%. One industry that uses the Haber Process has a yield percentage greater than 99%.



Predicting Chemical Reactions

Burning
Every time we light a match, burn a candle, make a fire, or light a grill, we will see a burning reaction. Combustion combines energetic molecules with oxygen to produce carbon dioxide and water.
For example, the propane combustion reaction, found in gas grills and some fireplaces, are:

C3H8 + 5O2 → 4H2O + 3CO2 + energy

Chemical mixing

If we merely combine vinegar and baking soda to make chemical volcanoes or milk with baking powder in the recipe, we experience multiple displacements or metathesis reactions (plus a few other things). Recombines to produce carbon dioxide and water gases. Carbon dioxide forms bubbles in volcanoes and can help improve roasting. This reaction seems simple in practice, but it often consists of several stages. This is the overall chemical equation of the reaction between baking soda and vinegar:

CH3COOH (aq) + NaHCO3 (aq) → NaC2H3O2 (aq) + H2O (s) + CO2 (g)

Rust
Rust is iron oxide, usually a red oxide formed by redox reactions of iron and oxygen in the presence of water or air humidity. Rust is a general term for corrosion of iron and its alloys, such as steel. Many other metals have the same corrosion, but the resulting oxide is not commonly called rust.
Here is the chemical rust chemical equation:

Fe + O2 + H2O → Fe2O3. XH2O



Reference:
Andy. 2009. Pre-College Chemistry.
Chang, Raymond. 2007. Ninth Edition Chemistry. New York: Mc Graw Hill.
Moore, John T. 2003. Chemistry For Dummies. Indonesia: great experts