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# Stoichiometry

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INTRODUCTION :

stoichiometry is the study of the quantitative relationships in substances and their reactions.

Solid–fuel are a central feature in the world’s space exploration programs, including the new space launch system being developed by the national aeronautics and space administration (NASA) to replace to retired space shuttle feet. The engines of these rockets rely on carefully prepared solid mixtures of chemicals combined in precisely measured amounts. Igniting the mixture initiaties a vigorous chemical reaction that rapidly generates large amounts of gaseous products. These gases are ejected from the rocket engine through it’s nozzle, providing the thrust needed to propel heavy payloads into space. Both the nature of this chemical reaction and the relationships between the amount of the substances being consumed and produced by the reaction are critically important considerations that determine the success of the technology. This chapter will describe how to symbolize chemical reactions using chemical equations, how to classify some common chemical reactions by identifying patterns of reactivity , and how to determine the quantitative relations  between the amounts of substances involved in chemical reaction — that is , the reaction stiochemistry .

WRITING AND BALANCING CHEMICAL EQUATIONS :

The preceding chapter introduced the use of element symbols to represent individual atoms.when atoms gain or lose electrons to yield ions, or combine with other atoms to form molecules, their symbols are modified or combined to generate chemical formulas that appropriately represent these species extending this symbolism to represent both the identities and the relative quantities of substances undergoing a chemical (or physical) change involves writing and balancing a Chemical equation. Consider as an example the reaction between one methane molecule (CH4) and two diatomic oxygen molecules (O2) to produce one carbon dioxide molecule (CO2) and two water molecule (H2O). The chemical equation representing this process is provided in the upper half of with space– filling.  Molecular models shown in lower half of the figure.

The example illustrates the fundamental aspects of any chemical equation :

1. The substances unndergoing reaction are called Reactants, and their formulas are placed on the left side of the equation.
2. The substances generated by the reaction are called Products, and their formula are placed on the right side of the equation.
3. Plus signs (+) separate individual reactant and product formulas, and an arrow (—> ) separates the reactant and product ( left and right ) sides of the equation.
4. The relative numbers of reactant and product species are represented by Coefficient ( number placed immediately to the left of each formula ) A coefficient of 1 is typically omitted.

It is common practice to use the smallest possible whole number coefficients in a chemical equation, as is done in this example. Realize,however, that these coefficients represent the relative numbers of reactants and products, and, therefore, they may be correctly interpreted  as ratios. Methane and oxygen react to yield carbon dioxide and water in a 1:2:1:2 ratio. This ratio is satisfied if the numbers of these molecules are , respectively, 1–2–1–2, or 2–4–2–4  or 3–6–3–6 and so on. Likewise, these coefficients may be interpreted with regard to any amount (number) unit , and so this equation may be correctly read in many ways, including :

• One methane molecule and two oxygens molecules react to yield one carbon dioxide molecule and two water molecules.
• One dozen methane molecules and two dozen oxygen molecules react to yield one dozen carbon dioxide molecules and two dozen water molecules.
• One mole of methane molecules and 2 moles of oxygen molecules react to yield 1 mole of carbon dioxide molecules and 2 miles of water molecules.

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