Introduction to Chemical Equations. Introduction to Chemical Equations. How do you write a skeleton equation?

Transcription

1 Introduction to Chemical Equations Introduction to Chemical Equations How do you write a skeleton equation?

2 Introduction to Chemical Equations All chemical reactions involve changing substances. In a chemical reaction, one or more reactants change into one or more products.

3 Introduction to Chemical Equations Word Equations How do you describe what happens in a chemical reaction? The reactants are written on the left and the products on the right. An arrow separates them. You read the arrow as yields, gives, or reacts to produce. Reactants products

4 Introduction to Chemical Equations Word Equations How could you describe the rusting of iron? You could say, Iron reacts with oxygen to produce iron(iii) oxide (rust). It is quicker to identify the reactants and product by means of a word equation. Iron + oxygen iron(iii) oxide

5 Introduction to Chemical Equations Word Equations In a word equation, write the names of the reactants to the left of the arrow, separated by plus signs. Write the names of the products to the right of the arrow, also separated by plus signs. Notice that no plus sign is needed on the product side of the equation on the previous slide because iron(iii) oxide is the only product. Iron + oxygen iron(iii) oxide

6 Introduction to Chemical Equations Chemical Equations A chemical equation is a representation of a chemical reaction; the formulas of the reactants (on the left) are connected by an arrow with the formulas of the products (on the right). Here is a chemical equation for rusting: Fe + O 2 Fe 2 O 3

7 Introduction to Chemical Equations Chemical Equations Equations that show just the formulas of the reactants and products are called skeleton equations. A skeleton equation is a chemical equation that does not indicate the relative amounts of the reactants and products. The first step in writing a complete chemical equation is to write the skeleton equation. Fe + O 2 Fe 2 O 3

8 Introduction to Chemical Equations Chemical Equations To write a skeleton equation, write the chemical formulas for the reactants to the left of the yields sign (arrow) and the formulas for the products to the right. Fe + O 2 Fe 2 O 3

9 Introduction to Chemical Equations Chemical Equations To add more information to the equation, you can indicate the physical states of substances by putting a symbol after each formula. Use (s) for a solid, (l) for a liquid, (g) for a gas, and (aq) for a substance in an aqueous solution (a substance dissolved in water). Fe(s) + O 2 (g) Fe 2 O 3 (s)

10 Introduction to Chemical Equations Chemical Equations In many chemical reactions, a catalyst is added to the reaction mixture. A catalyst is a substance that speeds up the reaction but is not used up in the reaction. A catalyst is neither a reactant nor a product, so its formula is written above the arrow in a chemical equation. H 2 O 2 (aq) MnO 2 H 2 O(l) + O 2 (g)

11 Interpret Data Symbols Used in Chemical Equations Symbol Explanation + Separates two reactants or two products Yields, separates reactants from products Use in place of for reversible reactions (s), (l), (g) (aq) Δ heat Pt Designates a reactant or product in the solid state, liquid state, or gaseous state; placed after the formula Designates an aqueous solution; the substance is dissolved in water; placed after the formula Indicates that heat is supplied to the reaction A formula written above or below the yields sign indicates its use as a catalyst (in this example, platinum).

12 Sample Problem 11.1 Writing a Skeleton Equation Hydrochloric acid reacts with solid sodium hydrogen carbonate. The products formed are aqueous sodium chloride, water, and carbon dioxide gas. Write a skeleton equation for this chemical reaction.

13 Sample Problem Solve Apply concepts to this situation. Start with the names of reactants and products. Include their physical states. Reactants sodium hydrogen carbonate (solid) hydrochloric acid (aqueous) Products sodium chloride (aqueous) water (liquid) carbon dioxide (gas)

14 Sample Problem Solve Apply concepts to this situation. Write the correct formula for each reactant and product. Reactants NaHCO 3 (s) HCl(aq) Products NaCl(aq) H 2 O(l) CO 2 (g)

15 Sample Problem Solve Apply concepts to this situation. Separate the reactants from the products with an arrow. Use plus signs to separate the reactants and the products. NaHCO 3 (s) + HCl(aq) NaCl(aq) + H 2 O(l) + CO 2 (g)

16 Balancing Chemical Equations Balancing Chemical Equations What are the steps for writing and balancing a chemical equation?

17 Balancing Chemical Equations To write a balanced chemical equation, first write the skeleton equation. Then use coefficients to balance the equation so that it obeys the law of conservation of mass. In every balanced equation, each side of the equation has the same number of atoms of each element.

18 Balancing Chemical Equations Carbon burns in the presence of oxygen to produce carbon dioxide. C(s) Carbon + O 2 (g) Oxygen Reactants 1 carbon atom, 2 oxygen atoms CO 2 (g) Carbon dioxide Product 1 carbon atom, 2 oxygen atoms This equation is balanced. You do not need to change the coefficients. They are all understood to be 1.

19 Balancing Chemical Equations When hydrogen and oxygen are mixed, the product is water. The formulas for all the reactants and the product are correct, but this equation is not balanced. As written, the equation does not obey the law of conservation of mass.

20 Balancing Chemical Equations When hydrogen and oxygen are mixed, the product is water. If you put the coefficient 2 in front of H 2 O, oxygen will be balanced. Now twice as many hydrogen atoms are in the product as are in the reactants.

21 Balancing Chemical Equations When hydrogen and oxygen are mixed, the product is water. To correct this equation, put the coefficient 2 in front of H 2. The equation is now balanced.

22 Interpret Data Rules for Writing and Balancing Equations 1. Determine the correct formulas for all the reactants and products. 2. Write the skeleton equation by placing the formulas for the reactants on the left and the formulas for the products on the right with a yields sign ( ) in between. If two or more reactants or products are involved, separate their formulas with plus signs. 3. Determine the number of atoms of each element in the reactants and products. Count a polyatomic ion as a single unit if it appears unchanged on both sides of the equation. 4. Balance the elements one at a time by using coefficients. When no coefficient is written, it is assumed to be 1. Begin by balancing elements that appear only once on each side of the equation. Never balance an equation by changing the subscripts in a chemical formula. Each substance only has one correct formula. 5. Check each atom or polyatomic ion to be sure that the number is equal on both sides of the equation. 6. Make sure all the coefficients are in the lowest possible ratio.

23 Sample Problem 11.2 Balancing a Chemical Equation Students suspended copper wire in an aqueous solution of silver nitrate. They noticed a deposit of silver crystals on the copper wire when the copper reacted with the silver nitrate. They recorded the equation for this reaction but didn t balance it. Balance their equation: AgNO 3 (aq) + Cu(s) Cu(NO 3 ) 2 (aq) + Ag(s)

24 Sample Problem Solve Apply concepts to this situation. Balance the nitrate ion. Put a coefficient 2 in front of AgNO 3 (aq). 2AgNO 3 (aq) + Cu(s) Cu(NO 3 ) 2 (aq) + Ag(s) Remember that a coefficient must always go in front of a compound s formula, not in the middle of it.

25 Sample Problem Solve Apply concepts to this situation. Balance the silver. Put a coefficient 2 in front of Ag(s). 2AgNO 3 (aq) + Cu(s) Cu(NO 3 ) 2 (aq) + 2Ag(s)

26 Sample Problem 11.3 Balancing a Chemical Equation Aluminum is a good choice for outdoor furniture because it reacts with oxygen in the air to form a thin protective coat of aluminum oxide. Balance the equation for this reaction. Al(s) + O 2 (s) Al 2 O 3 (s)

27 Sample Problem Analyze Identify the relevant concepts. Apply the rules for balancing equations. Notice the odd number of oxygen atoms in the product. Al(s) + O 2 (s) Al 2 O 3 (s)

28 Sample Problem Solve Apply concepts to this situation. First balance the aluminum by placing the coefficient 2 in front of Al(s). 2Al(s) + O 2 (s) Al 2 O 3 (s)

29 Sample Problem Solve Apply concepts to this situation. Multiply the formula with the odd number of oxygen atoms (on the right) by 2 to get an even number of oxygen atoms on the right. 2Al(s) + O 2 (s) 2Al 2 O 3 (s) Any whole-number coefficient placed in front of O 2 will always give an even number of oxygen atoms on the left.

30 Sample Problem Solve Apply concepts to this situation. Balance the oxygens on the left by placing a 3 in front of O 2. 2Al(s) + 3O 2 (s) 2Al 2 O 3 (s)

31 Sample Problem Solve Apply concepts to this situation. Then rebalance the aluminum by changing the coefficient of Al(s) from 2 to 4. 4Al(s) + 3O 2 (s) 2Al 2 O 3 (s)

32 Balance the following equation. C 3 H 8 (g) + O 2 (g) CO 2 (g) + H 2 O(l) C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(l)

33 Key Concepts To write a skeleton equation, write the formulas for the reactants to the left of the yields sign and the formulas for the products to the right. After writing the skeleton equation, use coefficients to balance the equation so that it obeys the law of conservation of mass.

34 Glossary Terms chemical equation: an expression representing a chemical reaction; the formulas of the reactants (on the left) are connected by an arrow with the formulas for the products (on the right) skeleton equation: a chemical equation that does not indicate the relative amounts of reactants and products

35 Glossary Terms catalyst: a substance that increases the rate of reaction by lowering the activation-energy barrier; the catalyst is not used up in the reaction coefficient: a small whole number that appears in front of a formula in a balanced chemical equation balanced equation: a chemical equation in which mass is conserved; each side of the equation has the same number of atoms of each element

36 BIG IDEA Reactions The law of conservation of mass states that mass is neither created nor destroyed. In order to show that mass is conserved during a reaction, a chemical equation must be balanced.