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Chemical Reactions

Chemical Reactions. Writing and balancing chemical equations. Reactions. Writing an equation to represent a chemical reaction is like writing a sentence. Sentences Subject and a verb with punctuation. Equations Reactants and products with an arrow in between. Reactants and Products.

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Chemical Reactions

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  1. Chemical Reactions Writing and balancing chemical equations

  2. Reactions • Writing an equation to represent a chemical reaction is like writing a sentence. • Sentences • Subject and a verb with punctuation. • Equations • Reactants and products with an arrow in between.

  3. Reactants and Products • In a chemical reaction, the ways in which atoms are joined together are changed. • Reactants are converted to products when bonds that hold atoms together are broken and new bonds form. (conservation of mass) • Chemical reactions can be described in different ways. • Word equations • Reactants are written on the left, products written on the right.

  4. Symbols of Chemical Reactions +Used to separate two reactants or two products →"yields," separates reactants from products ↔Used in place of an arrow for reversible reactions (s)solid (l)liquid (aq)substance dissolved in water (g)gas

  5. Writing Chemical Equations • Important to remember: • Arrows separate the reactants (on the left) and products (on the right) • Example: • Fe + O2Fe2O3 • Skeleton equations do not tell the relative amounts of the reactants and products. • A catalyst speeds up chemical reactions with being used up itself. These are written above the arrow. • Example: H2O2(aq) MnO2 H2O(l) + O2(g)

  6. Example • Write a skeleton equation for this chemical reaction: When calcium carbonate is heated, calcium oxide and carbon dioxide are produced. • Answer: (go through in class)

  7. Writing Word Equations • NaHCO3(s) + HCl(aq) NaCl(aq) + H2O + CO2(g) • Answer: (go through in class)

  8. Balancing Everyday Equations • Analogy: Football Offensive Team • 1 center, 2 guards, 2 tackles, 1 tight end, 2 wide receivers, 2 running backs and 1 quarterback • C + 2G + 2T + Te + 2Wr + 2Rb + QbCG2T2TeWr2Rb2Qb

  9. Balancing Chemical Equations • Important: • Each side of the chemical equation must have the same number of atoms of each element.

  10. Rules • Rules for Balancing Chemical Equations • Determine the correct formulas for all reactants and products. • Write the formulas for the reactants on the left and the products on the right with a “yields” sign in between. If two or more reactants are involved use a “plus” sign to separate their formulas. • Count the number of atoms of each element in the reactants and products. • Balance the elements one at a time by using coefficients(= a small whole number that appears in front of a chemical formula in an equation.) When no coefficient is written, it is assumed to be 1. • Check each atom or polyatomic ion to be sure that the equation is balanced. • Make sure that all the coefficients are in the lowest possible ratio.

  11. Example • H2 (g) + O2(g) H2O (l) • H2 (g) + O2(g) 2H2O (l) • 2H2 (g) + O2(g) 2H2O (l)

  12. Clarification • What is the difference between 2O and O2? • 2O = 2 single oxygen atoms • O2 = 1 oxygen molecule (O – O) • Analogy: disposable razors • A single razor = R • A package of two razors = R2 • Two single razors = 2R • Two packages of razors = 2R2

  13. Boxing • AgNO3 (aq) + Cu(s) Cu(NO3)2 (aq)+ Ag(s)

  14. Types of Reactions • Combination • Two or more substances react to form a single substance • Decomposition • A single compound is broken down into two or more products • Single-Replacement • Atoms of an element replace the atoms of a second element in a compound • Double-Replacement • Exchange of the positive ions between two compounds • Combustion • An element or a compound reacts with oxygen and produces heat and light

  15. Combination Reactions • In a combination reaction, two or more substances react to form a single substance. • Example: S + O2 SO2

  16. Decomposition Reactions • A single compound is broken down into two or more products. • Example: CaCO3 CaO + CO2

  17. Single-Replacement Reactions • Atoms of an element replace the atoms of a second element. • Example: Mg + Zn(NO3)2 Mg(NO3)2 + Zn Mg + LiNO3 no reaction*** ***Refer to Table 7.2 “Activity Series of Metals” A reactive metal will replace any metal below it in the activity series.

  18. Double-Replacement Reactions • Involve an exchange of positive ions between two compounds. • Example: Na2S + Cd(NO3)2CdS + 2NaNO3

  19. Combustion Reactions • An element or a compound reacts with oxygen. • Example: CH4 + 2O2 CO2 + 2H2O + heat and light A combustion reaction between an element and oxygen is also an example of a combination reaction.

  20. Predicting Products of a Chemical Reaction • Table 7.3 “Keys to Indentifying Types of Chemical Reactions”

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