moles/stoichiometry.....aim 3

what does a chemical equation represent?

.....aim 2

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A chemical equation represents a qualitative and quantitative representation of a chemical reaction.

Example

Chemical equation

2 C2H6 (g) + 7 O2(g) --> 4 CO2 (g) + 6 H2O (l)

                   reactants --> products

 

Word chemical equation

2 moles of ethane gas react with 7 moles of oxygen gas to produce 4 moles of carbon dioxide gas and 6 moles of hydrogen oxide liquid.

 

MU #1 A balanced chemical equation represents conservation of atoms. In all chemical reactions there is a conservation of mass, energy, and charge.

mass, energy, and charge of reactants = mass, energy, and charge of products

 

The method to balance equations is trial and error where atoms are checked one at a time starting with the most complex compound.

Example

unbalanced: H2 + O2 --> H2O  

balanced: 2 H2 + O2 --> 2 H2O

 

Problems

Balance and write a word chemical equation for the following chemical equations.

  1. Na + Cl2 --> NaCl   

  2. Al + O2 --> Al2O3

  3. CuS + O2 --> CuO + SO2

  4. Cu + O2 --> CuO

  5. Mg + O2 --> MgO

  6. PbO + C --> Pb + CO2

  7. CuCO3 --> CuO + CO2

  8. Mg + H2O --> Mg(OH)2 + H2

  9. Zn + Pb(NO3)2 --> Pb + Zn(NO3)2

  10. Fe2O3 + C --> CO2 + Fe

  11. C3H8 + O2 --> CO2 + H2O

  12. Zn + HCl --> ZnCl2 + H2

  13. Na + H2O --> NaOH + H2

  14. KClO3 --> KCl + O2

  15. HgO --> Hg + O2

  16. C + O2 --> CO2

  17. Cu + AgNO3 --> Ag + CuNO3

  18. Na2SO4 + BaCl2 --> NaCl + BaSO4

  19. Ba(NO3)2 + Na3PO4 --> Ba3(PO4)2 + NaNO3

  20. Fe + HCl --> H2 + FeCl2

  21. BaCl2 + Na3PO4 --> Ba3(PO4)2 + NaCl

  22. H2CO3 + Ca(OH)2 --> HOH + CaCO3

  23. NaHCO3 --> Na2CO3 + H2O + CO2

  24. CO2 + H2O --> C6H12O6 +  O2  

  25. C2H6 + O2 --> CO2 + H2O

MU #2 The coefficients in a balanced chemical equation can be used to determine mole ratios in the reaction.

Example

Consider the balance equation 2 C2H6 + 7 O2(g) --> 4 CO2 + 6 H2O

A set of relationships can be written since each compound  is linked to all the others.

2 moles of C2H6 to 7 moles of O2

4 moles of CO2 to 2 moles of C2H6      

6 moles of H2O to 2 moles of C2H6  

etc.....

 

Mole ratios are used to solve simple mole to mole problems.

Example

How many moles of H2 are needed to completely react with 2 moles of N2 in the reaction  N2 + 3 H2 --> 2 NH3?

Solution

Step 1: Write the quantity given in the question.

            2 moles of N2

Step 2: Write what you are looking for to the right of the given.

            2 moles of N2                           = ? moles of H2

Step 3: Find the relationship in the reaction that links the given and what you are looking for.

            1 mole of N2 to 3 moles of H2

Step 4: Place the relationship as a fraction next to the given so that the correct units cancel.

            (2 moles of N2 ) (3 moles of H2 ) / (1 mole of N2 ) = 6 moles of H2

 

Problems

1. How many moles of oxygen gas are produced by the decomposition of six moles of potassium chlorate in the reaction

    2 KClO3 --> 2 KCl + 3 O2

2. How many moles of hydrogen gas are produced from the reaction of three moles of zinc with an excess of hydrochloric acid in the reaction

    Zn + 2 HCl --> ZnCl2 + H2

3. How many moles of oxygen gas are necessary to react completely with four moles of propane in the reaction

    C3H8 + 5 O2 --> 3 CO2+ 4 H2O

4. How many moles of potassium nitrate are produced when two moles of potassium phosphate are used in the reaction

    K3PO4  + Al(NO3)3 --> 3 KNO3 + AlPO4

5. One of the iron minerals used to produce iron is siderite (iron (II) carbonate).  Siderite is first heated to produce iron II oxide and carbon dioxide then the oxide is made to react with oxygen gas to yield iron (III) oxide and last the new oxide reacts with carbon solid to produce iron  and carbon monoxide. Then the purification by reduction continues by reacting the iron (IV) oxide with carbon monoxide to produce iron (II) oxide and carbon dioxide gas. The last reaction in the process of purification produces iron solid and carbon dioxide gas when iron (II) oxide reacts again with carbon monoxide gas.

                a) Write all the above word chemical reactions into formula reactions and balance them.

 

TEST YOUR UNDERSTANDING

1/07

10 Given the balanced equation representing the reaction between propane and oxygen: C3H8 + 5 O2 ---> 3 CO2 + 4 H2O

According to this equation, which ratio of oxygen to propane is correct?

36 Given the balanced equation representing a reaction:

What is the minimum number of moles of O2 that are needed to completely react with 16 moles of NH3?    (1) 16 mol     (2) 20. mol    (3) 64 mol    (4) 80. mol

6/07

59 The unbalanced equation below represents the decomposition of potassium chlorate.

Balance the equation using the smallest whole-number coefficients.

77 Determine the total number of moles of CH3Br in 19 grams of CH3Br (gram-formula mass = 95 grams/mol).

8/07

10 Given the balanced equation representing a reaction:    2CO(g) + O2(g) ---> 2CO2(g)

    What is the mole ratio of CO(g) to CO2(g) in this reaction?    (1) 1:1 (2) 1:2 (3) 2:1 (4) 3:2

 11 Given the balanced equation representing a reaction:

In this reaction there is conservation of    (1) mass, only (2) mass and charge, only (3) mass and energy, only (4) mass, charge, and energy

6/02

Given the reaction: 6 CO2 + 6 H2O --> C6H12O6 + 6 O2 What is the total number of moles of water needed to make 2.5 moles of C6H12O6?  (1) 2.5 (2) 6.0  (3) 12  (4) 15

8/02

1. If an equation is balanced properly, both sides of the equation must have the same number of

            (1) atoms (2) coefficients (3) molecules (4) moles of molecules

2. Given the unbalanced equation: ___ Al  + ___ CuSO4 --> ___ Al2(SO4)3 + ___ Cu

When the equation is balanced using the smallest whole-number coefficients, what is the coefficient of Al?  (1) 1 (2) 2 (3) 3  (4) 4

1/03

1. Given the equation: 2 C2H2(g) + 5 O2(g) à 4 CO2(g) + 2 H2O(g) How many moles of oxygen are required to react completely with 1.0 mole of C2H2? (1) 2.5     (2) 2.0     (3) 5.0     (4) 10

2. Given the unbalanced equation: ___ Fe2O3 + ___ CO à ___ Fe + ___ CO2

When the equation is correctly balanced using the smallest whole-number coefficients, what is the coefficient of CO? (1) 1  (2) 2  (3) 3  (4) 4

6/03

20. Given the reaction:  PbCl2(aq) + Na2CrO4(aq) -->  PbCrO4(S) + 2 NaCl(aq)

What is the total number of moles of NaCl formed when 2 moles of Na2CrO4 react completely? (1) 1mole  (2) 2 moles  (3) 3 moles (4) 4 moles

8/03

10 What is conserved during a chemical reaction? (1) mass, only (2) charge, only (3) both mass and charge (4) neither mass nor charge

39 Given the balanced equation: 2 C4H10 (g) + 13 O2 (g) --> 8 CO2 (g) + 10 H2O(g) What is the total number of moles of O2 (g) that must react completely with 5.00 moles of C4H10 (g)? (1) 10.0 (3) 26.5 (2) 20.0 (4) 32.5

Base your answers to questions 54 and 55 on the information below.
Given the unbalanced equation:
                            enzyme

____ C6H12O6 -----------> ____ C2H5OH + ____ CO2
54 Balance the equation using the lowest whole-number coefficients.

55 Identify the type of reaction represented.

1/04

Base your answers to questions 51 and 52 on the unbalanced equation provided below.
__________C5H12(g) + ___________ O2(g) ---> ___________ CO2(g) + ___________ H2O(g)

51 Balance the equation using the smallest whole-number coefficients. 

52

    a Using your balanced equation, show a correct numerical setup for calculating the total number of moles of H2 O(g) produced when 5.0 moles of O2(g) are completely consumed.

    b Record your answer. ____________ mol H2O 

6/04

7 Given the reaction: N2(g) + 3H2(g) 2NH3(g)   What is the mole-to-mole ratio between nitrogen gas and hydrogen gas?

        (1) 1: 2 (3) 2: 2 (2) 1: 3 (4) 2: 3

8/04

8 All chemical reactions have a conservation of

                (1) mass, only (2) mass and charge, only (3) charge and energy, only (4) mass, charge, and energy

52 Given the equation: 2 H2(g) + O2(g) → 2 H2O(g) If 8.0 moles of O2 are completely consumed, what is the total number of moles of H2O produced?

62 show a correct numerical setup for calculating the number of moles of CO2 (gram-formula mass = 44 g/mol) present in 11 grams of CO2.

1/05

35 Which equation shows a conservation of mass?

                (1) Na + Cl2 → NaCl (2) Al + Br2 → AlBr3 (3) H2O → H2 + O2 (4) PCl5 → PCl3 + Cl2

6/05

54 Given the balanced equation: 4Al(s) + 3O2(g) ---> 2Al2O3(s) 
What is the total number of moles of O2(g) that must react completely with 8.0 moles of Al(s) in order to form Al2O3(s)?

1/06

10 Which chemical equation is correctly balanced?

    (1) H2(g) + O2(g) -> H2O(g)     (2) N2(g) + H2(g) -> NH3(g)    (3) 2NaCl(s) -> Na(s) + Cl2(g)    (4) 2KCl(s) -> 2K(s) + Cl2(g)

6/06

Base your answers to questions 69 on the information below.

Air bags are an important safety feature in modern automobiles. An air bag is inflated in milliseconds by the explosive decomposition of NaN3(s). The decomposition reaction produces N2(g), as well as Na(s), according to the unbalanced equation below.

NaN3(s) -> Na(s) + N2(g)

69 Balance the equation for the decomposition of NaN3, using the smallest whole-number coefficients.

8/06

38 Given the balanced equation: CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g) What is the total number of moles of CO2 formed when 20. moles of HCl is completely consumed?    (1) 5.0 mol     (2) 10. mol     (3) 20. mol    (4) 40. mol

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