Chemical reactions are represented by balanced chemical equations. Section 1: Reactions and Equations K What I Know W What I Want to Find Out L What I Learned

8(D) Use the law of conservation of mass to write and balance chemical equations. Reactions and Equations Copyright © McGraw-Hill Education

Essential Questions What is evidence of chemical change? How are chemical reactions represented? Why do chemical equations need to be balanced and how is this accomplished? Reactions and Equations Copyright © McGraw-Hill Education

Review chemical change New chemical reaction reactant product chemical equation coefficient Reactions and Equations Copyright © McGraw-Hill Education Vocabulary

Chemical Reactions The process by which one or more substances are rearranged to form different substances is called a chemical reaction. Evidence that a chemical reaction may have occurred: Change in temperature Change in color Odor Gas bubbles Appearance of a solid Reactions and Equations Copyright © McGraw-Hill Education

Representing Chemical Reactions Chemists use statements called equations to represent chemical reactions. Reactants are the starting substances. Products are the substances formed in the reaction. This table summarizes the symbols used in chemical equations. Reactions and Equations Copyright © McGraw-Hill Education

Representing Chemical Reactions In word equations, aluminum(s) + bromine(l) → aluminum bromide(s) reads as “aluminum and bromine react to produce aluminum bromide”. Skeleton equations use symbols and formulas to represent the reactants and products. Al(s) + Br(l) → AlBr 3 (s) Both word and skeleton equations lack information about how many atoms are involved in the reaction. Reactions and Equations Copyright © McGraw-Hill Education

Representing Chemical Reactions A chemical equation is a statement that uses chemical formulas to show the identities and relative amounts of the substances involved in a chemical reaction. Reactions and Equations Copyright © McGraw-Hill Education

Balancing Chemical Equations This figure shows the balanced equation for the reaction between aluminum and bromine. A coefficient in a chemical equation is the number written in front of a reactant or product, describing the lowest whole-number ratio of the amounts of all the reactants and products. Reactions and Equations Copyright © McGraw-Hill Education

Reactions and Equations Copyright © McGraw-Hill Education Writing a Balanced Chemical Equation Use with Example Problem 1. Problem Write the balanced chemical equation for the reaction in which aqueous sodium hydroxide and aqueous calcium bromide react to produce solid calcium hydroxide and aqueous sodium bromide. Response ANALYZE THE PROBLEM You are given the reactants and products in a chemical reaction. Start with a skeleton equation, and use the steps given for balancing chemical equations. SOLVE FOR THE UNKNOWN Write the skeleton equation for the chemical reaction. Be sure to put the reactants on the left side of the arrow and the products on the right. Separate the substances with plus signs, and indicate their physical states. NaOH(aq) + CaBr 2 (aq) → Ca(OH) 2 (s) + NaBr(aq) Count the atoms of each element in the reactants. 1 Na, 1 O, 1 H, 1 Ca, 2 Br Count the atoms of each element in the products. 1 Na, 2 O, 2 H, 1 Ca, 1 Br

Reactions and Equations Copyright © McGraw-Hill Education EVALUATE THE ANSWER The chemical formulas for all substances are written correctly. The number of atoms of each element is equal on both sides of the equation. The coefficients are written in the lowest possible ratio. The balanced chemical equation for the reaction is 2NaOH(aq) + CaBr 2 (aq) → Ca(OH) 2 (s) + 2NaBr(aq) SOLVE FOR THE UNKNOWN Insert the coefficient 2 in front of NaOH to balance the hydroxide ions. 2NaOH + CaBr 2 → Ca(OH) 2 + NaBr Insert the coefficient 2 in front of NaBr to balance the Na and Br atoms. 2NaOH + CaBr 2 → Ca(OH) 2 + 2NaBr Write the coefficients in their lowest- possible ratio. The ratio of the coefficients is 2:1:1:2. Check to make sure that the number of atoms of each element is equal on both sides of the equation. Reactants: 2 Na, 2 OH, 1 Ca, 2 Br Products: 2 Na, 2 OH, 1 Ca, 2 Br. Writing a Balanced Chemical Equation

Balancing Chemical Equations The most fundamental law in chemistry is the law of conservation of mass. Balanced equations show this law. Reactions and Equations Copyright © McGraw-Hill Education

Reactions and Equations Copyright © McGraw-Hill Education Review Essential Questions What is evidence of chemical change? How are chemical reactions represented? Why do chemical equations need to be balanced and how is this accomplished? Vocabulary chemical reaction reactant product chemical equation coefficient