General Chemistry Chapter 4 section 1

Consider as an example the decomposition of water to yield molecular hydrogen and oxygen. This process is represented qualitatively by an unbalanced chemical equation: H2O⟶H2+O2(unbalanced)

Element Reactants Products Balanced? H 1 × 2 = 2 1 × 1 = 2 2 = 2, yes O 1 × 1 = 1 1 × 2 = 2 1 ≠ 2, no 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. 2H2O⟶2H2+O2(balanced)

Plus signs (+) separate individual reactant and product formulas, and an arrow (⟶) separates

the reactant and product (left and right) sides of the equation

It is common practice to use the smallest possible ________________ in a chemical equation Realize, however, that these coefficients represent the relative numbers of reactants and products, and, therefore, they may be correctly interpreted as ratios.

whole-number coefficients

spectator ions

Ions that do not take part in a chemical reaction and are found in solution both before and after the reaction

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 molecules (H2O)

The chemical equation representing this process is provided in the upper half of Figure 4.2, with space-filling molecular models shown in the lower half of the figure.

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 (1CO2 molecule × 2 O atoms/ CO2 molecule) + (2H2O molecules × 1 O atom/ H2O molecule) = 4 O atoms

The equation for the reaction between methane and oxygen to yield carbon dioxide and water is confirmed to be balanced per this approach, as shown here: CH4+2O2⟶CO2+2H2O

Given the abundance of water on earth, it stands to reason that a great many chemical reactions take place in

aqueous media

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

The relative numbers of reactant and product species are represented by _________ (numbers placed immediately to the left of each formula). A coefficient of 1 is typically omitted.

coefficients

Explicitly representing all dissolved ions results in a

complete ionic equation

When ionic compounds dissolve in water, they may _______ into their constituent ions, which are subsequently dispersed homogenously throughout the resulting solution (a thorough discussion of this important process is provided in the chapter on solutions)

dissociate

spectator ions (net ionic equations)

ions whose presence is required to maintain charge neutrality—are neither chemically nor physically changed by the process

The substances generated by the reaction are called _________, and their formulas are placed on the right side of the equation

products

The substances undergoing reaction are called ___________, and their formulas are placed on the left side of the equation

reactants

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)

The chemical equation is balanced, meaning

that equal numbers of atoms for each element involved in the reaction are represented on the reactant and product sides.