CHEMICAL EQUATIONS AND REACTIONS CHAPTER 8 Pages

Describing Chemical Reactions Recall that a chemical reaction is when one or more substances ___________________________. Reactants are the original substances while the resulting substances are the ___________. The ___________________________ states that total mass of reactants must equal the total mass of products for a chemical reaction. How are chemical reactions described? Chemical equations represent the identities and molecular/molar amounts of reactants/products in a reaction.

Describing Chemical Reactions EXAMPLE) ammonium dichromate yields nitrogen, chromium(III) oxide and water. (NH 4 ) 2 Cr 2 O 7(s)  N 2(g) + Cr 2 O 3(s) + 4H 2 O (g) PRACTICE) Now confirm that the above chemical equation is balanced. How many atoms of nitrogen are present on the reactant side? On the product side? Do this for each element in the above chemical reaction. 2 N on both sides 8 H on both sides 2 Cr on both sides 7 O on both sides

Describing Chemical Reactions Refer to Fig 1 page 261. This demonstrates the evolution of heat and light energy. This occurs because potential energy is released as the chemical reaction takes place. Some reactions produce gas as bubbles. Refer to Fig 2a page 262. Some reactions form precipitates (solids). Refer to Fig 2b page 262. What is a precipitate? Some reactions produce a color change as indicative of a rxn. Note the diatomic molecules in Table 1 on page 263. H, N, O, F, Cl, Br, I. These are all diatomic meaning they exist in nature as a molecule of two atoms of the same element in a bond together.

Describing Chemical Reactions Law of conservation of mass (and energy) is always satisfied. Atoms are neither created nor destroyed in “normal chemical reactions”. Exceptions to this rule include nuclear chemistry. Hence, the same number of atoms of each element must appear on each side of a CORRECT chemical equation. What is a coefficient? What does a coefficient describe about a chemical reaction? PRACTICE) When methane burns in air, it combines with oxygen to produce carbon dioxide and water vapor. Methane’s formula is CH 4 while atmospheric oxygen (molecular oxygen) is O 2. What is the formula for carbon dioxide? For water vapor?

Describing Chemical Reactions PRACTICE) CH 4(g) + O 2(g)  CO 2(g) + H 2 O (g) This reaction (equation) was just described. But, is it balanced? According to the law of conservation of mass and energy, we know that the above equation is NOT balanced. The same amount of matter does not exist on the reactant side as the product side. How can we make this equation balanced? Digital Insert “Interpreting Chemical Equations” In the above equation, on the reactant side, we have 1 C, 4 H and 2 O. On the product side, we have 1 C, 2 H and 3 O. So how do we balance this chemical equation?? We need to include coefficients!

Describing Chemical Reactions CH 4(g) + 2O 2(g)  CO 2(g) + 2H 2 O (g) Now, use the above chemical equation to confirm that this is balanced. Refer to Table 2 on page 266 and MEMORIZE these symbols and what they mean in a chemical equation. You may omit the arrow that points down and the arrow that points up. These are not commonly used in chemical equations. PRACTICE) Write the word and formula equations for the chemical reaction that occurs when solid sodium oxide is added to water at room temp to form sodium hydroxide (aqueous). Include symbols for physical states in the formula equation. Finally, balance the formula equation to give a balanced chemical equation.

Describing Chemical Reactions Sodium oxide + water  sodium hydroxide (WORD EQUATION) This is the easy part! Now convert to a chemical formula equation. Remember the oxidation states (the charge of the ion) of each element involved in the reaction! Na 2 O + H 2 O  NaOH (IS THIS EQUATION BALANCED?) Don’t forget your symbols that indicate the physical states of each substance! Na 2 O (s) + H 2 O (l)  2NaOH (aq) (IS THIS EQUATION BALANCED?) Now consider: H 2(g) + Cl 2(g)  2HCl (g) -- What is the word equation for this reaction? Is it balanced?

Describing Chemical Reactions How many molecules of hydrogen are present for this reaction? Of Chlorine? Of Hydrogen Chloride (or hydrochloric acid if in aqueous solution)? Digital insert “Balancing a Chemical Equation by Inspection” PRACTICE) Balance the following reaction with a chemical equation. Zinc metal (solid) reactions with aqueous hydrochloric acid to produce a solution of zinc chloride and hydrogen gas. Don’t forget that hydrogen gas is a DIATOMIC MOLECULE. Zn(s) would represent the zinc metal. HCl (aq) would represent the hydrochloric acid. The above two substances are the ____________. (reactants or products?)

Describing Chemical Reactions The products are Zinc chloride and hydrogen gas. What are their chemical formulas? ANSWER) Zn (s) + HCl (aq)  ZnCl 2(aq) + H 2(g) (IS THIS A BALANCED EQUATION? DO THE MATH ON YOUR OWN). ANOTHER ANSWER) Zn (s) + 2HCl (aq)  ZnCl 2(aq) + H 2(g) CHALLENGE) Solid aluminum carbide, Al4C3, reacts with water to produce methane gas and solid aluminum hydroxide. Write a balanced chemical equation for this reaction. HINT) First write a word equation. Second, use your nomenclature and oxidation state skills to assign chemical formulas for the above reactants and products and then give the balanced chemical equation for this reaction.

Describing Chemical Reactions EXHAUSTED ANSWER) Aluminum carbide + water  methane gas + aluminum hydroxide. (word equation) Al 4 C 3(s) + H 2 O(l)  CH 4(g) + Al(OH) 3(s) (only thing is—is this balanced????) NOPE… So balance it... You have precisely 5 minutes. =) HINTS) We know that Aluminum always has a positive oxidation state of 3+ while Carbon, when it is an anion (-) has a 4- oxidation state. If you don’t “know” this, look at the periodic table! Notice that this formula is balanced? Now balance the entire equation so that the law of conservation of mass/energy is satisfied – meaning that, are there the same amount of atoms of each element the same on each side of the chemical equation? Remember that the arrow is the same as an equal sign.

Describing Chemical Reactions I KNOW BUT WAIT...ARE THEY EQUAL?? Are there the same amount of atoms of each elements on each side of the arrow (the yield sign)? If not, there is more work to do. This process involves using mathematic skill to satisfy the law of conservation of mass/energy. ANSWER) Al 4 C 3(s) + 12H 2 O (l)  3CH 4(g) + 4Al(OH) 3(s) CONFIRM) Do the amount of atoms of each element on the reactant side of the chemical equation the same amount of atoms of each element on the product side of the equation?

Types of Chemical Reactions Synthesis reactions – two or more substances combine to form a new compound. EXAMPLE) 2Mg (s) + O 2(g)  2MgO (s) -Is the above chemical equation balanced? -What is the word formula for the above chemical equation? A common type of synthesis reaction is the combination of an element with oxygen to produce an oxide. Most metals react with halogens in synthesis reactions to form either ionic or covalent compounds. A third type of synthesis reaction occurs between highly active metals with water to form metal hydroxides.

Types of Chemical Reactions Decomposition reactions – a single compound undergoes a reaction that produces two or more simpler substances. EXAMPLE) 2H 2 O (l)  2H 2(g) + O 2(g) (with electricity) Most of these types of reactions require energy input (heat or electricity). Decomposition of a substance by an electric current is _________. The text lists four types of decomposition. What are they? When a yield arrow has a delta on top of it, what does this mean? Digital Insert “Electrolysis”

Types of Chemical Reactions Single-displacement reactions – one element replaces a similar element in a compound. EXAMPLE) 2Al (s) + 3Pb(NO 3 ) 2(aq)  3Pb (s) + 2Al(NO 3 ) 3(aq) Many of these types of reactions take place in ___________ solutions. What does aqueous mean? What characteristic of an element determines whether or not this type of reaction occurs? Do these types of reactions require more or less energy input than synthesis or decomposition?

Types of Chemical Reactions Double-displacement reactions – the ions of two compounds exchange places in an aqueous solution to form two new compounds. EXAMPLE) 2KI (aq) + Pb(NO 3 ) 2(aq)  PbI 2(s) + 2KNO 3(aq) What is the word equation for the above formula equation? Is the formula equation balanced? Notice you have (aq) + (aq)  (s) + (aq) - What is the solid referred to as? Anytime you have this situation, the solid is called the______________. Other things that can form as a result of this reaction type are _______ & ________.

Types of Chemical Reactions Combustion reactions – a substance combines with oxygen releasing large amounts of heat and light energy. EXAMPLE) 2C 6 H 14(l) + 19O 2(g)  12 CO 2(g) + 14H 2 O (l) Is the above formula equation balanced? Can you think of an example of a combustion reaction? Digital Insert – “Combustion reactions”

Activity Series of the Elements The ability of an element to react is referred to as the element’s ____________. What is an activity series? For metals, greater activity = greater ease of ________ of electrons. This gives what charge to the ion? For nonmetals, greater activity = greater ease of _______ of electrons. This gives what charge to the ion? Review table 3 on page 286 and be able to recognize which elements are reactive and which aren’t. Digital Insert – “Activity Series”

Chapter 8 Slide Quiz 1. Name 5 indications that a chemical reaction has taken place. 2. A solid that is produced as a result of a chemical reaction in solution and that separates from the solution is called a ____________. 3. When writing chemical reactions, which law is always satisfied? 4. A coefficient in front of a chemical formula in an equation represents what? 5. What does a double arrow indicate? 6. What does aqueous mean? 7. Name 5 types of chemical reactions discussed from section Which two of the five types of chemical reactions involve energy input? 9. What is chemical activity?

Answers 1. chemical analysis, evolution of heat/light, production of gas, production of precipitate, color change 2. precipitate 3. conservation of mass 4. relative moles of a substance 5. they represent a reversible reaction 6. dissolved in water 7. synthesis, decomposition, single-replacement, double-replacement, combustion 8. synthesis & decomposition 9. the ability of an element to react