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Mass and Volume Relationships Balancing Chemical Equations

Mass and Volume Relationships Balancing Chemical Equations. Overview : Description of the Chemical Equation Coefficients vs. Subscripts The four basic steps to balancing a chemical equation 4. Law of Combined Volumes. Chemical Sentences: Equations.

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Mass and Volume Relationships Balancing Chemical Equations

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  1. Mass and Volume Relationships Balancing Chemical Equations

  2. Overview: • Description of the Chemical Equation • Coefficients vs. Subscripts • The four basic steps to balancing a chemical equation • 4. Law of Combined Volumes

  3. Chemical Sentences: Equations Chemical equations represent the sentencesin the language of chemistry. P4(s) + 6Br2(l) 4PBr3(g) They are the means of communicating a chemical change using the symbols and formulas to represent the elements and compounds involved in a chemical reaction.

  4. Chemical Sentences: Equations Reactantsare the species present beforethe reaction. Productsare the species present afterthe reaction: Reactants→Products The arrow (→) means “yield(s)” or “react(s) to produce”.

  5. Chemical Sentences: Equations The following are used to denote the states of matter of a species in an equation: (s) = solid (l) = liquid (g) = gas (aq) = aqueous solution

  6. Chemical Sentences: Equations Coefficientsare numbers used to balance a chemical equation. Never change the subscripts!!!

  7. Chemical Equations are Balanced • In a balanced • chemical reaction • no atoms are lost or gained. • the number of reacting atoms is equal to the number of product atoms.

  8. A Balanced Chemical Equation In a balanced chemical equation, • the number of each type of atom on the reactant side is equal to the number of each type of atom on the product side. • numbers called coefficientsare used in front of one or more formulas to balance the number of atoms. Al + S Al2S3 Not Balanced 1 Al  2 Al Unequal number of Al atoms 1 S  3S Unequal number of S atoms 2Al + 3S Al2S3Balanced using coefficients 2 Al = 2 Al Equal number of Al atoms 3 S = 3S Equal number of S atoms

  9. Steps to Balancing Equations There are four basic steps to balancing a chemical equation. 1a. Write the correct formula for the reactants and the products. (Write a skeleton equation) b. Indicate the state of the matter 2. 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. 3. 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 4. 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)

  10. Example: N2H4 + N2O4 N2 + H2O Steps to Balancing Equations Step 1 a&b : Write a skeleton equation; Indicate the state of the matter: N2H4(l)+ N2O4 (l)  N2(g) + H2O (l) Step 2: Find the number of atoms on theleft side and compare to right 4N, 4H, 4O 2N, 2H, 1O N2H4(l)+ N2O4 (l)  N2(g) + H2O (l) Step 3: Determine where to place coefficients Note that the product side needs 4O: Multiply the H2O by 4

  11. Steps to Balancing Equations Writing a Balanced Equation 2N, 8H, 4O 4N, 4H, 4O N2H4(l)+ N2O4 (l)  N2(g) + 4 H2O(l) Theproductside now has 8 Hcompared to the 4H on the reactantside. What reactant can we multiply to get 8reactantH? Try multiplying N2H4by 2 6N, 8H, 4O 2N, 8H, 4O recount 2N2H4(l)+ N2O4 (l)  N2(g) + 4 H2O (l) *Now use the N2to balance the N: multiple N2 by 3 2 N2H4(l)+ N2O4 (l)  3N2(g) + 4 H2O (l) Step 4: Check your answer:Both sides: 6N, 8H, 4O

  12. A Balanced Chemical Equation State the number of atoms of each element on the reactantside and the productside for each of the following balanced equation. P4(s) + Br2(l) PBr3(g) P4(s) + 6Br2(l) 4PBr3(g) 4 P atoms = 4 P atoms 12 Br atoms = 12 Br atoms

  13. A Balanced Chemical Equation State the number of atoms of each element on the reactantside and the productside for each of the following balanced equation. Al(s) + Fe2O3(s) Fe(s) + Al2O3(s) 2Al(s) + Fe2O3(s) 2Fe(s) + Al2O3(s) 2 Al atoms = 2 Al atoms 2 Fe atoms = 2 Fe atoms 3 O atoms = 3 O atoms

  14. Volume Relationships in Chemical Equations Law of Combined Volumes: When all measurements are made at the same temperatureand pressure, the volumesof gaseous reactants and products are in a small whole-number ratio. 3H2 + N2  2 NH3

  15. Summary • Chemical equations represent the sentencesin the language of chemistry • 2. Reactantsare the species present beforethe reaction. • 3. Productsare the species present afterthe reaction: • 4. Coefficientsare numbers used to balance a chemical equation. Never change the subscripts!!! • 5. Four basic steps to balancing a chemical equation. • Write a skeleton equation • Find the number of atoms for each element on the left side. Compare them to those on the right side. • Determinewhere to place coefficients in front of formulas so that each side has the same number of atoms • Check your answer • Law of Combined Volumes: When all measurements are made at the same temperature and pressure, the volumes of gaseous reactants and products are in a small whole-number ratio.

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