Writing and Balancing Chemical Equations Quiz

PDF page 186: 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 (Figure 4.3). Likewise, these coefficients may be interpreted with regard to any amount (number)

PDF page 187: The chemical equation described in section 4.1 is balanced, meaning that equal numbers of atoms for each element involved in the reaction are represented on the reactant and product sides. This is a requirement the equation must satisfy to be consistent with the law of conservation of matter. It may be confirmed by simply summing the numbers of atoms on either side of the arrow and comparing these sums to ensure they are equal. Note that the number of atoms for a given element is calculated by multiplying the coefficient of any formula containing that element by the element’s subscript in the formula. If an element appears in more than one formula on a given side of the equation, the number of atoms represented in each must be computed and then added together. For example, both product species in the example reaction, CO2 and H2O, contain the element oxygen, and so the number of oxygen atoms on the product side of the equation is ⎛ 2 O atoms ⎞ ⎛ 1 O atom ⎞

PDF page 187: The chemical equation described in section 4.1 is balanced, meaning that equal numbers of atoms for each element involved in the reaction are represented on the reactant and product sides. This is a requirement the equation must satisfy to be consistent with the law of conservation of matter. It may be confirmed by simply summing the numbers of atoms on either side of the arrow and comparing these sums to ensure they are equal. Note that the number of atoms for a given element is calculated by multiplying the coefficient of any formula containing that element by the element’s subscript in the formula. If an element appears in more than one formula on a given side of the equation, the number of atoms represented in each must be computed and then added together. For example, both product species in the example reaction, CO2 and H2O, contain the element oxygen, and so the number of oxygen atoms on the product side of the equation is ⎛ 2 O atoms ⎞ ⎛ 1 O atom ⎞

PDF page 188: These coefficients yield equal numbers of both H and O atoms on the reactant and product sides, and the balanced equation is, therefore: 2H O ⟶ 2H + O 2 2 2

PDF page 188: The numbers of H atoms on the reactant and product sides of the equation are equal, but the numbers of O atoms are not. To achieve balance, the coefficients of the equation may be changed as needed. Keep in mind, of course, that the formula subscripts define, in part, the identity of the substance, and so these cannot be changed without altering the qualitative meaning of the equation. For example, changing the reactant formula from H2O to H2O2 would yield balance in the number of atoms, but doing so also changes the reactant’s identity (it’s now hydrogen peroxide and not water). The O atom balance may be achieved by changing the coefficient for H2O to 2. 2H O ⟶ H + O (unbalanced) 2 2 2

PDF page 188: The numbers of H atoms on the reactant and product sides of the equation are equal, but the numbers of O atoms are not. To achieve balance, the coefficients of the equation may be changed as needed. Keep in mind, of course, that the formula subscripts define, in part, the identity of the substance, and so these cannot be changed without altering the qualitative meaning of the equation. For example, changing the reactant formula from H2O to H2O2 would yield balance in the number of atoms, but doing so also changes the reactant’s identity (it’s now hydrogen peroxide and not water). The O atom balance may be achieved by changing the coefficient for H2O to 2. 2H O ⟶ H + O (unbalanced) 2 2 2

PDF page 189: The N atom balance has been upset by this change; it is restored by changing the coefficient for the reactant N2 to 2. 2N + 5O ⟶ 2N O 2 2 2 5 Element Reactants Products Balanced? N 4 = 4, yes

PDF page 187: A balanced chemical equation often may be derived from a qualitative description of some chemical reaction by a

PDF page 188: fairly simple approach known as balancing by inspection. Consider as an example the decomposition of water to yield molecular hydrogen and oxygen. This process is represented qualitatively by an unbalanced chemical equation: H O ⟶ H + O (unbalanced) 2 2 2 Comparing the number of H and O atoms on either side of this equation confirms its imbalance:

PDF page 190: C H + O ⟶ 3H O + 2CO (unbalanced) 2 2 2 2 6 This results in seven O atoms on the product side of the equation, an odd number—no integer coefficient can be used 7 with the O2 reactant to yield an odd number, so a fractional coefficient, , is used instead to yield a provisional 2 balanced equation: 7 C H + O ⟶ 3H O + 2CO 2 2 2 2 6 2 A conventional balanced equation with integer-only coefficients is derived by multiplying each coefficient by 2: 2C H + 7O ⟶ 6H O + 4CO 2 2 2 2 6 Finally with regard to balanced equations, recall that convention dictates use of the smallest whole-number coefficients. Although the equation for the reaction between molecular nitrogen and molecular hydrogen to produce ammonia is, indeed, balanced, 3N + 9H ⟶ 6NH 2 2 3 the coefficients are not the smallest possible integers representing the relative numbers of reactant and product molecules. Dividing each coefficient by the greatest common factor, 3, gives the preferred equation: N + 3H ⟶ 2NH 2 2 3

PDF page 190: C H + O ⟶ 3H O + 2CO (unbalanced) 2 2 2 2 6 This results in seven O atoms on the product side of the equation, an odd number—no integer coefficient can be used 7 with the O2 reactant to yield an odd number, so a fractional coefficient, , is used instead to yield a provisional 2 balanced equation: 7 C H + O ⟶ 3H O + 2CO 2 2 2 2 6 2 A conventional balanced equation with integer-only coefficients is derived by multiplying each coefficient by 2: 2C H + 7O ⟶ 6H O + 4CO 2 2 2 2 6 Finally with regard to balanced equations, recall that convention dictates use of the smallest whole-number coefficients. Although the equation for the reaction between molecular nitrogen and molecular hydrogen to produce ammonia is, indeed, balanced, 3N + 9H ⟶ 6NH 2 2 3 the coefficients are not the smallest possible integers representing the relative numbers of reactant and product molecules. Dividing each coefficient by the greatest common factor, 3, gives the preferred equation: N + 3H ⟶ 2NH 2 2 3

PDF page 190: The physical states of reactants and products in chemical equations very often are indicated with a parenthetical abbreviation following the formulas. Common abbreviations include s for solids, l for liquids, g for gases, and aq for substances dissolved in water (aqueous solutions, as introduced in the preceding chapter). These notations are illustrated in the example equation here: 2Na(s) + 2H O(l) ⟶ 2NaOH(aq) + H (g) 2 2 This equation represents the reaction that takes place when sodium metal is placed in water. The solid sodium reacts with liquid water to produce molecular hydrogen gas and the ionic compound sodium hydroxide (a solid in pure form, but readily dissolved in water). Special conditions necessary for a reaction are sometimes designated by writing a word or symbol above or below the equation’s arrow. For example, a reaction carried out by heating may be indicated by the uppercase Greek letter delta (Δ) over the arrow. Δ CaCO (s) ⟶ CaO(s) + CO (g) 3 2 Other examples of these special conditions will be encountered in more depth in later chapters.

PDF page 190: The physical states of reactants and products in chemical equations very often are indicated with a parenthetical abbreviation following the formulas. Common abbreviations include s for solids, l for liquids, g for gases, and aq for substances dissolved in water (aqueous solutions, as introduced in the preceding chapter). These notations are illustrated in the example equation here: 2Na(s) + 2H O(l) ⟶ 2NaOH(aq) + H (g) 2 2 This equation represents the reaction that takes place when sodium metal is placed in water. The solid sodium reacts with liquid water to produce molecular hydrogen gas and the ionic compound sodium hydroxide (a solid in pure form, but readily dissolved in water). Special conditions necessary for a reaction are sometimes designated by writing a word or symbol above or below the equation’s arrow. For example, a reaction carried out by heating may be indicated by the uppercase Greek letter delta (Δ) over the arrow. Δ CaCO (s) ⟶ CaO(s) + CO (g) 3 2 Other examples of these special conditions will be encountered in more depth in later chapters.