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Chapter 9

Chapter 9. Balancing Equations Part 1. Terms. A chemical reaction is the process by which one or more substances are changed into new substances. A chemical equation uses symbols to represent a chemical reaction. A reactant or reagent is a substance used to begin a chemical reaction.

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Chapter 9

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  1. Chapter 9 Balancing Equations Part 1

  2. Terms • A chemical reaction is the process by which one or more substances are changed into new substances. • A chemical equation uses symbols to represent a chemical reaction. • A reactant or reagent is a substance used to begin a chemical reaction. • A product is a substance that forms during chemical reaction.

  3. Potassium metal is added to water.

  4. K + H2O

  5. Water can also be written HOH

  6. K + HOH

  7. What are the products?

  8. What gas might be produced?

  9. What has happened to the liquid?

  10. The litmus test • Blue litmus paper turns red under acidic conditions. • Red litmus paper turns blue under basic conditions. • If both litmus tests result in no change then the solution is neutral.

  11. The resulting liquid is a base. Most common bases contain hydroxide (OHˉ).

  12. What are the products?

  13. K + H2O → KOH + H What is wrong here? → +

  14. K + H2O → KOH + H H,O,N,Cl,F,I,Br mean H should be H2 → +

  15. K + H2O → KOH + H 2 H,O,N,Cl,F,I,Br mean H should be H2 → +

  16. K + H2O → KOH + H2 Now what is wrong? → +

  17. Lavoisier, 1788 Chemical Equations Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number and type of atoms on both sides of the equation.

  18. Balancing Equations • When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts. • Changing the subscripts changes the compound.

  19. Subscripts vs. Coefficients • The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

  20. K + HOH → KOH + H2 We must balance the equation. → +

  21. Steps to Balancing Equations There are four basic steps to balancing a chemical equation. • Write the correct formula for the reactants and the products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS! • Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side. • Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element. • Check your answer to see if: • The numbers of atoms on both sides of the equation are now balanced. • The coefficients are in the lowest possible whole number ratios. (reduced)

  22. _K + _HOH → _KOH + _H2

  23. 2K + 2HOH → 2KOH + H2

  24. 2K + 2H2O → 2KOH + H2 Consider the form of these substances. → +

  25. 2K + 2H2O → 2KOH + H2 gas solid → + liquid

  26. 2K(s) + 2H2O(l) → 2KOH + H2(g) gas solid → + What about the KOH? liquid

  27. potassium hydroxide (KOH) is a solid at room temperature. • However KOH is soluble in water. Substances dissolved in water are said to be aqueous and are indicated as such by an (aq) in the equation.

  28. 2K(s) + 2H2O(l) → 2KOH + H2(g) (aq) gas solid → + aqueous solution liquid

  29. Other Symbols Used in Equations • Solid (cr) or (s) • Precipitate (↓) • “Heated” • Escaping gas () • Catalyst H2SO4 • A word may be written above an arrow to indicate something is necessary for the reaction to occur. ∆ electricity

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