3.10
Chemical Equations
Chemical equations represent the identities and relative quantities of substances involved in a chemical reaction. The substances undergoing reaction…
Chemical reactions involve chemical transformations of one or more substances into different materials. A reaction is conventionally represented using a chemical equation, which utilizes chemical formulas to represent atoms, molecules, or compounds involved.
The substances present at the start of a reaction and consumed during the process are called reactants. Reactants are specified on the left side of the equation. The substances formed during the reaction are called products and are indicated on the right side of the equation.
An arrow separating the reactants and products shows the direction of the reaction. The different states of matter are depicted using parenthesized abbreviations.
A chemical reaction observes the law of conservation of mass, where the total masses of all participating elements are conserved. Therefore, the cumulative number of atoms of each element will match in both the reactants and products.
Chemical equations are balanced using numerical coefficients to reflect the actual number of atoms involved in the reaction. It is important to only change the numerical coefficients and not numerical subscripts that define the identity of a compound while balancing any chemical equation.
General guidelines help illustrate and adjust a chemical equation efficiently.
For example, the combustion of liquid benzoic acid, forming gaseous carbon dioxide and water is represented initially using a skeletal chemical equation. The atoms are balanced next by first adjusting those within the compounds, followed by the atoms of free elements.
The inclusion of coefficients seven and three before the formulas for carbon dioxide and water balances the carbon and hydrogen atoms. A fractional coefficient of 15/2 adjusts the oxygen atoms.
Fractional coefficients are converted to whole numbers by multiplying the complete equation by the denominator. The collective number of atoms of each element equals on both sides in a balanced equation.
In chemical equations representing reactions involving ionic compounds, the cations are balanced first, followed by the anions. Polyatomic ions are always balanced as a unit.
For example, aqueous aluminum sulfate reacts with aqueous calcium hydroxide forming solid aluminum hydroxide and solid calcium sulfate. Using a skeletal equation, the cations of aluminum and calcium are balanced, followed by the anions of polyatomic hydroxide and sulfate ions. Lastly, the coefficients are adjusted to get an overall balanced equation.
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Q1: What are reactants and products in a chemical equation?
Reactants are substances consumed at the start of a chemical reaction and appear on the left side of the equation. Products are substances formed during the reaction and appear on the right side. An arrow separates reactants from products, showing the reaction direction. Chemical formulas represent the atoms, molecules, or compounds involved in the transformation.
Q2: Why do chemical equations need to be balanced?
Chemical equations must be balanced to satisfy the law of conservation of mass, which states that the total mass of all participating elements is conserved. Balancing ensures the cumulative number of atoms of each element matches on both the reactant and product sides, accurately representing how atoms rearrange during the reaction.
Q3: How do you balance a chemical equation?
Balance equations by adjusting numerical coefficients placed before formulas to equalize atom counts for each element on both sides. Change only coefficients, never subscripts within formulas. Start with atoms in compounds, then balance free elements. Fractional coefficients can be used initially, then multiplied by a whole number to convert all coefficients to integers.
Q4: What do the parenthetical abbreviations in chemical equations represent?
Parenthetical abbreviations indicate the physical states of reactants and products. Standard abbreviations include 's' for solids, 'l' for liquids, 'g' for gases, and 'aq' for substances dissolved in water. These notations provide complete information about the reaction conditions and the form of each substance involved.
Q5: How are polyatomic ions handled when balancing ionic compound equations?
Polyatomic ions are always balanced as complete units rather than individual atoms. When balancing equations with ionic compounds, cations are adjusted first, followed by anions. The polyatomic ion maintains its identity and charge throughout the balancing process, ensuring the equation accurately represents the chemical transformation.
Q6: What do numerical coefficients in a balanced equation represent?
Numerical coefficients represent the relative ratios of reactants and products in a chemical reaction. They indicate the smallest whole-number proportions of substances involved and can be interpreted as ratios for any unit, including individual molecules or moles. A coefficient of 1 is typically not shown in the equation.
Q7: Why should you never change subscripts when balancing chemical equations?
Subscripts define the chemical identity of a compound and cannot be altered during balancing. Changing subscripts would create different substances entirely, misrepresenting the actual reaction. Only numerical coefficients should be adjusted to balance atom counts while preserving the identity and composition of each reactant and product.