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Writing Chemical Equations

Building Science Champions. Writing Chemical Equations. Objectives. Describe the information conveyed in a chemical equation. Apply the principle of conservation of mass to chemical reactions. Identify and describe three classes of chemical reactions. Key Terms. Chemical equation Subscript

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Writing Chemical Equations

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  1. Building Science Champions Writing Chemical Equations

  2. Objectives • Describe the information conveyed in a chemical equation. • Apply the principle of conservation of mass to chemical reactions. • Identify and describe three classes of chemical reactions.

  3. Key Terms • Chemical equation • Subscript • Reactant • Product • Conservation of mass • Coefficient • Synthesis • Decomposition • Replacement

  4. Chemical Equations • Chemical Equations are short, easy ways to show a chemical reaction by using elements’ symbols instead of words. • Symbols are easy to understand.

  5. Writing Chemical Equations • The formula will show the ratio of elements in a compound. • C12H22O11 is the chemical formula for sugar. • The letter corresponds to a particular element on the periodic table. • The subscript corresponds to the element that precedes the subscript. • If there is no subscript after a letter then there is one (1) of that particular element. • CO2

  6. Structure of an Equation • A chemical equation summarizes a chemical reaction. • The material you begin with is the REACTANT. • The completed reaction is the PRODUCT. Reactant + Reactant Product + Product = YIELDS

  7. Structure of an Equation • When there are two or more reactants they are separated by a plus sign. • When there are two or more products they are separated by a plus sign. • The ratio of products to reactant can change. Reactant + Reactant Product Reactant Product + Product Reactant + Reactant + Reactant Product

  8. Conservation of Mass • Regardless of the total number of reactants and products involved, all the atoms at the start of the reaction have to be present at the end of the reaction. • The amount of matter involved in a chemical reaction does not change. The total mass of the reactants must equal the total mass of the product.

  9. The LAW of CONSERVATION • The law of conservation states: • “Matter cannot be created nor destroyed during a chemical reaction.”

  10. Review • Chemical equations are short hand for chemical reactions. • Subscripts go to the element before them. • To the left of the yield signs are reactants and to the right are products. • There can be multiple products and reactants. • Matter cannot be created nor destroyed.

  11. Checkpoint • How do masses of the atoms in the reactants of a chemical reaction compare with the atoms in the products? • The number of atoms remain equal and the total of mass is unchanged.

  12. Balancing Chemical Equations • A chemical reaction must show the same number of each type of atom on both sides of the equation. • When this happens the equation is BALANCED.

  13. Balancing Equation • H2 + O2 H2O • Does the left side equal the right side? • 2 Hydrogen and 2 Oxygen on the left • 2 Hydrogen and 1 Oxygen on the right • Not equal. So what can be done to balance the equation?

  14. H2 + O2 H2O • Add a coefficient of 2 in front of the H2O • Coefficient is the number placed in front of chemical formulas in an equation. • A coefficient tells how many atoms or molecules there are. • The coefficient is factored out until the next sign. • H2+ O2 2 H2O • Left side – 2 Hydrogen and 2 Oxygen • Right side – 4 hydrogen and 2 Oxygen • How can we balance them out?

  15. H2 + O22 H2O • Place a 2 in front of the H2 in the reactant. • 2H2 + O2 2H2O • Left side – 4 Hydrogen and 2 Oxygen • Right side – 4 Hydrogen and 2 Oxygen • We are balanced!!!!!!!!!!!!!!!!!

  16. Mg + O2MgO • Left side – 1 Magnesium and 2 Oxygen • Right Side – 1 Magnesium and 1 Oxygen • Is the equation balanced? • Now what? • Add 2 in front of Magnesium in the reactant and Magnesium in the product. 2Mg + O2 2MgO • Are we balanced?

  17. You Try • C + Cl2 CCl4 • C + 2 Cl2 CCl4 • Al2O3 Al + O2 • 2 Al2O3 4 Al + 3 O2 • ***You can only change coefficients you CANNOT change subscripts. ***

  18. Classifying Chemical Reactions • There are 3 major classifications of chemical reactions: • Synthesis • Decomposition • Replacement

  19. Synthesis • When two or more substances combine to make a more complex substance the process is called synthesis. • Put things together. • Simple + Simple Complex • More reactants than products 2 SO2 + O2 + 2 H2O 2 H2SO4 Sulfur dioxide + Oxygen + water yields sulfuric acid

  20. Decomposition • Decomposition breaks down reactants into smaller products. • Complex Simple + Simple • More products than reactants 2 H2O2 2 H2O + O2 Hydrogen peroxide yields water and oxygen gas

  21. Replacement • When one element replaces another in a compound, or when 2 elements in different compound trade places, this is replacement. • Reactant + Reactant Product + Product CaCO3 + 2 HCl CaCl2 + H2O3 2 CuO + C 2 Cu + CO2

  22. Review • Chemical equations show chemical reactions • Matter cannot be created nor destroyed • Three types of chemical reactions • Synthesis • Decomposition • Replacement

  23. References • Anderson, M. et all (2012) Physical Science. McGraw-Hill: Columbus • Frank, D.V et al (2001). Physical Science. Prentice Hall: New Jersey

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