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Writing Chemical Equations
Chemical equations are written with the chemical formulas for each reactant on the left side of the equation and the chemical formula for the products on the right side of the equation. Each reactant and product is further specified as a liquid (l), gas (g), solid (s) or aqueous solution (aq). The reactants and products are divided by an arrow indicating the direction of the reaction -- from reactant to product. Reversible reactions are represented with a double arrow indicating that the reaction also can proceed in reverse. Further, the presence of a catalyst or specific conditions needed for a reaction to occur are written above the arrow. For example, the Greek letter delta would appear in reactions requiring heat.
Balancing Chemical Equations
The same number of each type of atom must be accounted for on both the product and reactant sides of an equation. This requires the equation to be balanced by increasing the number of moles for each reactant and product until the same number of each type of atom is on each side. For example, the reaction between hydrogen gas and oxygen gas to produce water is written as H2 (g) + O2 (g) -> H2O (l). This equation is unbalanced because there are two atoms of oxygen on the reactant side but only one atom of oxygen on the product side. Rewriting the equation as 2 H2 (g) + O2 (g) -> 2 H2O (l) balances the equation because there now are four atoms of hydrogen on each side of the equation and two atoms of oxygen on each side of the equation. This equation then can be expressed as two moles of hydrogen gas reacts with one mole of oxygen gas to produce two molecules of water.
Moles
Chemical reactions are described in terms of moles rather than individual atoms or molecules. Atoms and molecules are grouped in units known as moles. Each mole equals 6.022 x 10 to the 23rd power, also known as Avogadro's number. While this number is very large, one mole of water molecules has a mass of 18 grams and a volume of only 18 milliliters. The mole then describes the ratio between each product and reactant in the chemical equation. Because two moles of hydrogen gas and one mole of oxygen gas yields two moles of water, we can predict that four moles of hydrogen gas and two moles of oxygen gas would yield four moles of water.
Limiting and Excess Reactants
In chemical reactions, one reactant often is consumed before other reactants are consumed. A consumed reactant is termed the limiting reactant because the reaction ends when the reactant is used up. The other reactant(s) present are called excess reactants. The excess reactant(s) does not participate in the chemical reaction. Using the previous equation, if two moles of hydrogen gas are combined with two moles of oxygen gas, only two moles of water will be produced. The hydrogen is consumed and the limiting reactant. Only one mole of oxygen is consumed in the reaction and the remaining mole of oxygen remains.