Chapter 7: Chemical Reactions: An Introduction. Chemical Reactions, Equations and Balancing

Transcription

1 Chapter 7: Chemical Reactions: An Introduction Chemical Reactions, Equations and Balancing

2 Learning Target

3 Chapter 7 Vocabulary 4 words 1. Chemical reaction 2. Reactant 3. Product 4. Coefficients

4 balanced equations for chemical reactions. Signs of a Chemical Reaction Chemical reaction process in which a new substance is formed. Color change A solid forms (precipitate) Bubbles form (gas is being produced) Heat and/or a flame is produced, or heat is absorbed Notice with all of these, something new is being formed

5 balanced equations for chemical reactions. Chemical Equations Chemical equations are chemist s way of writing a recipe for what is happening. There are reactants, products and a yield sign. Reactants - substances that enter a chemical reaction. Products written on the right side of the arrow in a chemical equation. The arrow ( ) indicates the direction of the reaction and is read as yields or produces in problems. Multiple reactants or products are indicated by addition symbols Reactants Products CH 4 + 2O 2 CO 2 + 2H 2 O

6 Chemical Equations Physical states of reactants and products are represented in chemical equations. The symbols and the state they represent are: (s) solid (l) liquid (g) Gas (aq) dissolved in water (in aqueous solution) Tidbits: Acids symbolized as (aq) Diatomic molecules Ammonia is NH 3 Key words: Burns in Reacts with Added to Synthesized Producing or to produce Heated or decomposed Is formed

7 Examples Write the unbalanced chemical equation for each of the following reactions. Solid mercury (II) oxide decomposes to produce liquid mercury metal and gaseous oxygen. Solid carbon reacts with gaseous oxygen to form gaseous carbon dioxide. HgO (s) Hg (l) + O 2 (g) C (s) + O 2 (g) CO 2 (g)

8 Try on your own Identify the reactants and products and write the unbalanced equation (including symbols for states) for each of the following chemical reactions. Solid magnesium metal reacts with liquid water to form solid magnesium hydroxide and hydrogen gas. Gaseous ammonia reacts with gaseous oxygen to form gaseous nitrogen monoxide and gaseous water. Mg (s) + H 2 O (l) Mg(OH) 2 (s) + H 2 (g) NH 3 (g) + O 2 (g) NO (g) + H 2 O (g)

9 Chemical Equations Atoms are neither created or destroyed in chemical reactions, only regrouped or rearranged. (Law of conservation of mass) To satisfy this, we MUST balance our equations! This means the number of atoms entering a reaction must equal the number of atoms exiting the reaction.

10 Balancing Chemical Equations You balance chemical equations with coefficients numbers used to balance chemical equations. (they must be the smallest integers possible.) Coefficients represent the number of moles or molecules of the substance it corresponds with. They should be in front and only in front of compounds or elements in the chemical equation.

11 Chemical Equations An incorrect balanced chemical equation would look like the following: H 2 (g) + O 2 (g) H 2 2O (l) A correct balanced chemical equation would look like the following: 2H 2 (g) + O 2 (g) 2H 2 O (l)

12 Balancing Chemical Equations There is no one solid way to balancing chemical equations. Some may balance by sight, some follow a strict order, and some may make a table but whatever the method, the number of atoms going into the equation should equal the number of atoms going out of the equation.

13 Balancing Chemical Equations General tips to follow: Start balancing with the most complex molecule. Balance items that exist on both sides. Make sure you consider ALL of an element on the reactant side/product side. (it may exist in more than one place) H 2 O may also be thought of as HOH. Balance polyatomic ions as groups ONLY if they exist on BOTH sides of the equation Leave hydrogens and oxygens last to balance

14 Examples Write the balanced chemical equation for each of the following reactions. Nitrogen monoxide gas reacts with hydrogen gas to form nitrogen gas and liquid water. Lithium metal reacts with chlorine gas to form solid lithium chloride. 2 NO (g) + 2 H 2 (g) N 2 (g) + 2 H 2 O (l) 2 Li (s) + Cl 2 (g) 2 LiCl (s)

15 Try on your own Write the balanced chemical equation for each of the following reactions. Iron metal reacts with oxygen gas in the air to form solid iron (III) oxide. Solid diboron trioxide reacts with liquid water to form boric acid, B(OH) 3. 4 Fe (s) + 3 O 2 (g) 2 Fe 2 O 3 (s) B 2 O 3 (s) + 3 H 2 O (l) 2 B(OH) 3 (aq)