Chapter 5-- Section 2 Writing Chemical Equations

What you should learn b What does a chemical equation tell you? b How does mass change during a chemical reaction? b What are the three categories of chemical reactions? b Vocabulary: Chemical equation, subscripts, products, reactants.

What is a chemical equation? b A CHEMICAL EQUATION is a shorter, easier way to show chemical reactions, using symbols instead of words. Ex. “Hydrogen molecules react with oxygen molecules to form water molecules” would be written 2H 2 + O 2 --> 2H 2 O.Ex. “Hydrogen molecules react with oxygen molecules to form water molecules” would be written 2H 2 + O 2 --> 2H 2 O. b Symbols express a concept in a shorter form.

Writing Chemical Formulas b A formula (like H 2 0) identifies the elements in the compound and shows the ratios of different atoms that make up that substance. b Notice that the numbers in a formula are written smaller and lower that the letter symbols. These numbers are subscripts. SUBSCRIPTS show the ratio of the atoms of different elements in a formula. If a letter symbol is a chemical formula doesn’t have a subscript, the number 1 is understood.If a letter symbol is a chemical formula doesn’t have a subscript, the number 1 is understood. Ex. H 2 0 means 2 hydrogen and 1 oxygenEx. H 2 0 means 2 hydrogen and 1 oxygen

Structure of an Equation b Chemical equation summarizes a reaction. b The materials you have at the beginning are called REACTANTS. b When the reaction is done, you have different materials, called the PRODUCTS. b A chemical equation uses symbols to show the reactants and the products of a reaction. Ex. CaCO 3 --> CaO + CO 2.Ex. CaCO 3 --> CaO + CO 2. Reactant Product Product Reactant Product Product

Conservation of Mass b No matter how many reactants and products are involved, all of the atoms at the start of the reaction are present at the end of the reaction. b The amount of matter involved in a chemical reaction does not change. The total mass of the reactants must equal the total mass of the products. This principle, called the CONSERVATION OF MASS, means that during a chemical reaction, matter is not created or destroyed.This principle, called the CONSERVATION OF MASS, means that during a chemical reaction, matter is not created or destroyed.

Balancing Chemical Equations b Steps for balancing equations: Write the word equation.Write the word equation. Write the chemical equation.Write the chemical equation. Count the number of atoms or each element on each side of the equation.Count the number of atoms or each element on each side of the equation. Chose coefficients to balance the equation.Chose coefficients to balance the equation. Think about it.Think about it.

Now try a few. b Example Problems b H 2 + O 2 --> H 2 O b Mg + O 2 --> MgO b C + Cl 2 --> CCl 4 b Al 2 O 3 --> Al + O 2 b Answers b H 2 + O 2 --> H 2 O 2H 2 + O 2 --> 2H 2 O b Mg + O 2 --> MgO 2Mg + O 2 --> 2MgO b C + Cl 2 --> CCl 4 4C + 2Cl 2 --> CCl 4 b Al 2 O 3 --> Al + O 2 2Al 2 O 3 --> 4Al + 3O 2

Classifying Chemical Reactions b Many Chemical reactions can be classified in one of three Categories: synthesis, decomposition, or replacement. When two or more substance combine to form a more complex substance is a process called synthesis.When two or more substance combine to form a more complex substance is a process called synthesis. –Ex. 2 Mg + O 2 --> 2 MgO Decomposition breaks down compounds into simpler products.Decomposition breaks down compounds into simpler products. –Ex. 2 Al 2 O 3 --> 4 Al + 3 O 2 Replacement is when one element replaces another in a compound or when two element in different compounds trade places.Replacement is when one element replaces another in a compound or when two element in different compounds trade places. –Ex. 2 CuO + C --> 2 Cu + Co 2

Answer these Questions. b What does a chemical equation tell you? b How does mass change during a chemical reaction? b What are the three categories of chemical reactions?