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Reactions and Equations Learning Objectives • Accurately represent chemical reactions with equations. • Balance chemical equations. Review • What is a chemical change? • How do you know that a chemical change has taken place? Review • Remember that in a chemical change, a substance undergoes a change in chemical composition. • A chemical change always involves the conversion of a material into one or more new substances. Review • Signs of a chemical change – Temperature change – Release of energy in the form of light or sound – Gas formation – Precipitate formation – Color change – Odor change Review Write formulas for the following: 1. Barium nitrate 2. Sulfurous acid 3. Copper (II) sulfide 4. Nitrogon trioxide Chemical Reactions • A chemical reaction is a process by which the atoms of one or more substances are rearranged to form different substances. Chemical Equations • 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. Chemical Reaction • Chemical equations are used to represent chemical reactions. – Reactants are the starting substances – Products are the substances that are formed Chemical Reactions • A subscripted number tells you how many atoms of an element there are directly to the left of the number. – Ex: there are two atoms of hydrogen in a water molecule Chemical Reactions • A coefficient is the number written in front of a reactant or product. – Whole numbers – Not written if one • A coefficient tells you how many of that atom or molecule there is. The coefficient applies to all atoms in the molecule. – Ex: There are two water molecules – Ex: There are 4 hydrogen atoms and 2 oxygen atoms 3 Chemical Reactions • In equations, symbols are used to show the physical states of the reactants and products. – – – – Solid (s) Liquid (l) Gas (g) Aqueous (aq) • Aqueous means dissolved in water. Learning Check Identify each of the following: • A • B • C • D • E • F Learning Check • How many sodium oxide molecules are there? • How many sodium atoms are there on the left side of the equation? • How many oxygen atoms are there in the water molecules on the left side of the equation? Learning Check • Write a chemical equation for the reaction in which hydrogen gas reacts with oxygen to yield water (remember that both hydrogen and oxygen are diatomic molecules). Balancing Chemical Equations • The law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction—it is conserved. • In other words, the mass of the reactants equals the mass of the products. Chemical Equations • To accurately represent a chemical reaction by an equation, the equation must show equal numbers of atoms of each reactant and each product on both sides of the arrow. • Such an equation is called a balanced chemical equation. Balancing Chemical Equations • To balance an equation, you must find the correct coefficient for the chemical formula. Step 1 Write the skeletal equation for the reaction. Step 2 Count the atoms of the elements in the reactants. Step 3 Count the atoms of the elements in the products. Step 4 1. Change the coefficients to make the number of atoms of each element equal on both sides. DO NOT CHANGE COEFFICIENTS! Write coefficients in lowest possible ratio. Step 5 Check your work. Diatomic Molecules When writing out chemical equations, it is important to know which elements form diatomic molecules. There are seven elements that form diatomic molecules. The seven diatomic elements are: Hydrogen (H2) Nitrogen (N2) Oxygen (O2) Fluorine (F2) Bromine (Br2) Iodine (I2) Chlorine (Cl2) How to Remember the Diatomic Elements • The elements ending with "-gen" including halogens form diatomic molecules. An easy-to-remember mnemonic for the diatomic elements is: Have No Fear Of Ice Cold Beverages Example Write a balanced chemical equation for the reaction of carbon monoxide gas and oxygen gas that produces carbon dioxide gas. Example 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. Practice Write the balanced chemical equation for the reaction in which, in water, iron (III) chloride reacts with sodium hydroxide, producing solid iron (III) hydroxide and sodium chloride. Practice Write the balanced chemical equation for the reaction in which liquid carbon disulfide reacts with oxygen gas, producing carbon dioxide gas and sulfur dioxide gas.