Chemical Reactions: An Introduction Chapter 6

6.1 Evidence for a Chemical Reaction

Evidence for a Chemical Reaction Some Clues That a Chemical Reaction Has Occurred: The color changes. A solid forms (precipitate). Bubbles form. Heat and/or a flame is produced, or heat is absorbed.

Chapter 6 6.2: Chemical Equations

Chemical Equations A chemical reaction is a reaction in which new products are produced or formed; it is a reorganization of atoms. A chemical reaction is represented by a chemical equation. In a chemical equation, the chemicals present before the reaction (the reactants) are shown to the left of an arrow and the chemicals formed by the reactions (the products) are show to the right of an arrow. The arrow indicates the direction of the change and is read as “yields” or “produces”.

Chemical Equations Law of Conservation of Mass: In a chemical reaction, atoms are neither created nor destroyed. All atoms present in the reaction must be accounted for among the products. The balanced chemical equation shows the actual numbers of molecules involved in a reaction. The chemical equation for a reaction provides us with two important types of information: The identities of the reactants and products. The relative numbers of each.

Physical States Besides specifying the compounds involved in the reaction, often indicated in the equation are the physical states of the reactants and products by using the following symbols: (s) = solid ( ℓ) = liquid (g) = gas (aq) = dissolved in water (aqueous)‏

Types of Chemical Equations 1. Synthesis – two or more elements/compounds combine together to form a single product A + B  C 2. Decomposition – one compound decomposes (breaks apart) to form one or more elements/compounds C  A + B 3. Single Displacement – one element displaces another AB + C  CB + A AB + C  AC + B 4. Double Displacement – two elements switch partners AB + CD  AD + CB LETS DANCE!!!!!

Chapter 6 6.3: Balancing Chemical Equations

Balancing Chemical Equations The principle that lies at the heart of balancing chemical equations is: atoms are conserved in a chemical reaction. The identities (formulas) of the compounds must never be changed in balancing a chemical equation. The accepted convention is that the “best” balanced equation is the one with the smallest integers (whole numbers). These integers are called the coefficients for the balanced equation.

How to Write and Balance Equations Step 1 Read the description of the chemical reaction. What are the reactants, the products, and their states? Write the appropriate formulas. Step 2 Write the unbalanced equation that summarizes the information from step 1. Step 3 Balance the equation by inspection, starting with the most complicated molecule. Proceed element by element to determine what coefficients are necessary so that the same number of each type of atom appears on both the reactant side and the product side. Do not change the identities (formulas) of any of the reactants or products. Step 4 Check to see that the coefficients used give the same number of each type of atom on both sides of the arrow. (Note that an “atom” may be present in an element, a compound, or an ion.) Also check to see that the coefficients used are the smallest integers that give the balanced equation. This can be done by determining whether all coefficients can be divided by the same integer to give a set of smaller integer coefficients.

Write and Balance Equations 1. Solid magnesium metal reacts with liquid water to form solid magnesium hydroxide and hydrogen gas. 2. Solid ammonium dichromate decomposes to solid chromium (III) oxide, gaseous nitrogen, and gaseous water. 3. Gaseous NH 3 reacts with gaseous oxygen to form gaseous nitrogen monoxide and gaseous water.

Write and Balance Equations 1. Mg (s) + 2H 2 O ( ℓ)  Mg(OH) 2 (s) + H 2 (g)‏ 2. (NH 4 ) 2 Cr 2 O 7 (s)  Cr 2 O 3 (s) + N 2 (g) + 4H 2 O (g) ‏ 3. 4NH 3 (g) + 5O 2 (g)  4NO (g) + 6H 2 O (g) ‏

Write and Balance Equations 4. Solid mercury (II) oxide decomposes to produce liquid mercury metal and gaseous oxygen. 5. Solid carbon reacts with gaseous oxygen to form gaseous carbon dioxide. 6. Solid zinc is added to an aqueous solution containing dissolved hydrogen chloride to produce gaseous hydrogen that bubbles out of the solution and zinc (II) chloride that remains dissolved in the water.

Write and Balance Equations 4. HgO (s)  Hg ( ℓ) + O 2 (g)‏ 5. C (s) + O 2 (g)  CO 2 (g) ‏ 6. Zn (s) + 2HCl (aq)  ZnCl 2 (aq) + H 2 (g) ‏

Write and Balance Equations 7. Solid potassium reacts with liquid water to form gaseous hydrogen and potassium hydroxide that dissolves in water. 8. Hydrofluoric acid reacts with solid silicon dioxide to form gaseous silicon tetrafluoride and liquid water.

Write and Balance Equations 7. 2K (s) + 2H 2 O ( ℓ)  H 2 (g) + 2KOH (aq) ‏ 8. 4HF (aq) + SiO 2 (s)  SiF 4 (g) + 2H 2 O ( ℓ)‏