Chapter 11

Goals Describe chemical reactions using word equations. Know the correct symbols to use in order to write skeleton equations for chemical reactions. Use coefficients to balance chemical equations so that the law of conservation of mass is followed. Review: Define and give general equations for combination, decomposition, single-replacement, and double- replacement reactions. Classify a reaction as combination, decomposition, single- replacement, double-replacement, or combustion.

Chemistry is about change... When chemical reactions take place, stuff changes. Old substances are broken down. New substances are formed. The chemical composition of the new is different from the old, but it is still made of the same types of matter. Matter changes in the form of how atoms are combined chemically. Matter is ALWAYS conserved. The changes in arrangement of matter can be represented by chemical equations.

*Reminder* Evidence for a Chemical Reaction   There are 4 visual signals that indicate a chemical reaction has occurred. 1. The color changes. 2. A solid forms in a liquid solution. 3. Bubbles form. 4. Heat produced or absorbed.

Chemical Equations Once it is known that a chemical change is taking place, it is useful to represent the chemical reaction that takes place by writing a chemical equation. Chemical Equations contain the following: Molecular formulae of all chemicals involved before and after the reaction takes place. The states of matter of all chemicals involved. Numeric prefixes that represent the proportions in which the chemicals are involved in the reaction.

Reactants  Products In a chemical reaction, reactants become products, and a chemical equations shows this. REACTANTS The chemicals present before a reaction The sum of the reactants are ALWAYS on the left side of the arrow in a chemical equation. PRODUCTS The chemicals formed by the reaction The sum of the products are ALWAYS on the right of the arrow in a chemical reaction. The ARROW indicates the direction of the change and means “yields” or “produces” or “becomes” but not “equals”.

An Example of a Chemical Equation When methane, (CH 4 ) or natural gas, combines with oxygen (O 2 ) in the air and burns, carbon dioxide (CO 2 ) and water (H 2 O) are formed. CH 4 + O 2  CO 2 + H 2 O Methane Oxygen Carbon Dioxide Water Reactants Products

Physical States in Reactions In addition to specifying the compounds involved in the reaction, the physical states of the reactants and products are also represented by using the following symbols: Solid (s) Liquid (l) Gas (g) Dissolved in an aqueous solution (aq) Our example equation becomes: CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O(l)

Practice: Writing Unbalanced Equations Identify the reactants and products and write an unbalanced chemical equation for the following:  Solid magnesium metal reacts with liquid water to form solid magnesium hydroxide and hydrogen gas. Mg(s) + H 2 O(l)  Mg(OH) 2 (s) + H 2 (g)

Practice: Writing Unbalanced Equations Identify the reactants and products and write an unbalanced chemical equation for the following:  Solid ammonium dichromate decomposes to solid chromium (III) oxide, gaseous nitrogen, and gaseous water. (NH 4 ) 2 Cr 2 O 7 (s)  Cr 2 O 3 (s) + N 2 (g) + H 2 O(g)

Practice: Writing Unbalanced Equations Identify the reactants and products and write an unbalanced chemical equation for the following:  Gaseous ammonia reacts with gaseous oxygen to form gaseous nitrogen monoxide and gaseous water. NH 3 (g) + O 2 (g)  NO(g) + H 2 O(g)

CH 4 (g) + O 2 (g)  CO 2 (g) +H 2 O(l) Notice in this equation that the products contain the same atoms as the reactants but that the atoms are associated in different ways. A chemical reaction involves changing the ways the atoms are grouped. It is important to recognize that in a chemical reaction, atoms are neither created nor destroyed. All atoms present in the reactants must be accounted for among the products. In other words, matter must be conserved, there must be the same number of each type of atom on the product side as on the reactant side of the arrow. Making sure that the equation for a reaction obeys this rule is called balancing the chemical equation for a reaction.

Our Example Balanced… CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(l)  The chemical equation is complete. 1. The chemicals involved are represented by molecular formulae. 2. The states of the chemicals are indicated. 3. Conservation of matter is observed because there are 1 carbon, 4 oxygens, and 4 hydrogens on each side of the arrow.

Rules for Balancing Chemical Equations 1. The identities (formulas) of the compounds must never be changed in balancing a chemical equation. You cannot change the subscripts in chemical formulas. You cannot add or subtract atoms to chemical formulas. 2. The “best” balanced equation is the one with the smallest possible whole number integers as the coefficients for the chemicals in the reaction. 3. Each individual atom must be present in equal numbers in the reactants and products. Atoms can exist as elements, in compounds, and as ions. Remember some elemental atoms occur in pairs, (i.e FOHNCl) 4. The total charge on each side of the equation must also be balanced.

A Method for Balancing There isn’t one…  Inspection Guess & Check Trial & Error Keep trying until you find the numbers of reactants and products that give the same number of each type of atom on both sides of the arrow.  Hint: When one molecule in an equation is more complicated (contains more elements) than the others, it is best to start with that molecule. How to get better? Practice

Steps to Writing a Balanced Chemical Equation. 1. Read the description of the chemical reaction. Think about the chemical identities involved. What are the reactants, the products, and their states? 2. Write the appropriate formulas in an unbalanced equation that summarizes the information from step 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. 4. Check to see that the coefficients used give the same number of each type of atom on both sides of the arrow. 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.

Balancing 1. Write the chemical equations using the correct compounds or atoms for the reactants and products including states of matter. Remember, you can ONLY change the coefficients of reactants and products when balancing, NOT the subscripts or superscripts. A coefficient is the number in FRONT of an atom or molecule 2. Then…

PīMē-NōCHO!

Balancing for Dummies. 2. Balance the atoms in the equation in the following order: Pī.Mē.Nō.CHO P olyatomic ī ons Mē tals Nōnmetals C arbon H ydrogen O xygen Balancing in this order usually makes it easy ;)

Balancing for Dummies Once you have all the atoms balance, make sure the coefficients (the numbers in front of the molecules) are in a lowest whole number ratio. For example, the following equation has a ratio that can be divided by 3: 6H 2 O(l)  6H 2 (g) + 3O 2 (g) When each coefficient is divided by three, the equation is correctly balanced: 2H 2 O(l)  2H 2 (g) + O 2 (g)  The ONLY way to get better at this is to practice.

Practice: Writing Balanced Equations Identify the reactants and products and write an balanced chemical equation for the following:  When solid ammonium nitrite is heated, it produces nitrogen gas and water vapor. NH 4 NO2(s)  N 2 (g) + 2H 2 O(g)

Practice: Writing Balanced Equations Identify the reactants and products and write an balanced chemical equation for the following:  Gaseous nitrogen monoxide decomposes to produce dinitrogen monoxide gas and nitrogen dioxide gas. 3 NO(g)  N 2 O(g) + NO 2 (g)

Practice: Writing Balanced Equations Identify the reactants and products and write an balanced chemical equation for the following:  Liquid nitric acid decomposes to reddish-brown nitrogen dioxide gas, liquid water, and oxygen gas. 4 HNO 3 (l)  4NO 2 (g) + 2H 2 O(l) + O 2 (g)