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Balancing Chemical Equations

Balancing Chemical Equations. Making the seemingly impossible be easy….maybe. Balancing Chemical Equations. A chemical reaction is about to take place. H 3 PO 4 + Ca

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Balancing Chemical Equations

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  1. Balancing Chemical Equations Making the seemingly impossible be easy….maybe

  2. Balancing Chemical Equations A chemical reaction is about to take place. H3PO4 + Ca You know you should look for (1) colour changes, (2) production of gas (bubbles), (3) heat or light and/or (4) formation of a solid precipitate from two liquids Uh oh, you are asked to define the reaction chemically. That is make a prediction about type, outcome and products. Yikes…the impossible…

  3. Balancing Chemical Equations H3PO4 + Ca A little review….What does H3PO4 mean? Hydrogen (H) is bonded with the polyatomic ion phosphate (PO4). To make this compound, there are THREE hydrogen atoms for ONE phosphate polyatomic. The hydrogen-phosphate bond is an ionic bond. That is hydrogen GIVES electrons and the phosphate TAKES electrons. This makes the electron configuration in both chemicals stable…whoa….okay tough…but always recall stability is the goal PO4 is a covalent bond. Phosphorus and oxygen SHARE electrons. Recall, a polyatomic ALWAYS stays together.

  4. Balancing Chemical Equations H3PO4 + Ca What is the type of reaction? There are four types: (1) synthesis, (2) decomposition, (3) single displacement and (2) double displacement Synthesis = Two make one Decomposition = One makes two Single Displacement = One replacement Double Displacement = Two replacements Using these clues which type of reaction will we see?

  5. Balancing Chemical Equations H3PO4 + Ca This is a SINGLE DISPLACEMENT reaction. The CALCIUM ion (Ca) will replace the HYDROGEN ion (H). Why?

  6. Balancing Chemical Equations H3PO4 + Ca In an Ionic Bond, the POSITIVE ION is always written first. So in H3PO4, hydrogen is positive (H+). Using the Periodic Table, we determine calcium (Ca) is also a POSITIVE ION. Its charge is Ca2+ Since like charges repel, calcium will replace hydrogen. This is the only change…a single change. Thus, this is a SINGLE DISPLACEMENT REACTION

  7. Balancing Chemical Equations Thus, we know this… H3PO4 + Ca  CaPO4 + H The calcium replaces the hydrogen to make a new compound combining calcium and phosphate. Are we done? HINT: Recall the Law of Conservation of Mass. That is, the Mass of the Products must equal the Mass of the Reactants. Has this occurred?

  8. Balancing Chemical Equations H3PO4 + Ca  CaPO4 + H First, hydrogen will not exist in nature by itself. Thus, hydrogen bonds covalently with another hydrogen. H3PO4 + Ca  CaPO4 + H2 Are we done now? There are three H ions in the reactants and only two hydrogen in the products. The Law of Conservation of Mass is not met? We are required to BALANCE this equation…(rats)

  9. Balancing Chemical Equations H3PO4 + Ca  CaPO4 + H2 Balancing means there must be equal amounts of products and reactants. Where to start? …Let’s look at the charges of the ions. What? Okay, this is hard. Recall, elements have specific structures. Protons and neutrons reside in the centre (i.e., nucleus) of the atom, while electrons occur in various shells or orbits around the nucleus. To have a structure that is electrically stable, the outer shell of electrons (…called VALENCE SHELL) must be full. The full shells…presently…are 2,8.8. That is, Shell #1 is full when it has two electrons and Shell #2 is full with eight electrons. If the Valence Shell is not full, the element will either GIVE / TAKE electrons or SHARE electrons until the outer shell is full. This is the first part of chemistry.

  10. Balancing Chemical Equations H3PO4 + Ca  CaPO4 + H2 Calcium is element #20. That means it has 20 protons and 20 electrons (20+ and 20- respectively). The protons are in the nucleus. The number of protons CANNOT change (e.g., calcium always has 20 protons). Yet, the Valence Shell of electrons of calcium is NOT full. How can calcium get a full outer shell? It can get more electrons by taking or giving away electrons for its outer shell. Calcium has an attractive force of 20. It is hard for calcium to attract and hold enough electrons to fill the outer shell…in this case, six. So, calcium gives away the two electrons in its outer shell. The calcium ion has 20+ and 18- for a charge of 2+ H3PO4 + Ca2+ CaPO4 + H2

  11. Balancing Chemical Equations H3PO4 + Ca2+ CaPO4 + H2 Does the charge for calcium also occur in the products? Yes, it must. Why? H3PO4 + Ca2+ Ca2+PO4 + H2 Looking at the Periodic Table, hydrogen has a charge of 1+ Thus…. (H3)1+PO4 + Ca2+ Ca2+PO4 + (H2)1+

  12. Balancing Chemical Equations (H3)1+PO4 + Ca2+ Ca2+PO4 + (H2)1+ Finally, the PO4 is a polyatomic. It STAYS TOGETHER in a chemical reaction. This is the hard one. How many PO4 occur in this reaction? There is only one PO4. The subscript one is NOT written; rather, it is implied. (H3)1+(PO4)1 + Ca2+ Ca2++ (PO4)1 + (H2)1+ What is the ionic charge of the phosphate polyatomic ion? Recall, the CRISSCROSS RULE. In this case, the rule is applied backwards (i.e., cross upwards). Since it is H3, the PO4 must have a charge of 3- (i.e., three hydrogen giving one electron each to bond with one PO4 that needs three electrons). (H3)1+(PO4)13- + Ca2+ Ca2++ (PO4)13- + (H2)1+

  13. Balancing Chemical Equations Now, apply the Crisscross Rule to make sure the right number of atoms are used to make the new compounds (H3)1+(PO4)13- + Ca2+ Ca32+(PO4)23- + (H2)1+ Finally, eliminate all the charges (…superscripts) H3PO4 + Ca  Ca3(PO4)2 + H2 Holy crap, Batman…we did it. All the chemicals are present AND in their right proportions. Wait…look again…it is not chemically balanced. For example, there are more hydrogen reactants than hydrogen products. Can this be? No. Why?

  14. Balancing Chemical Equations Let’s balance… Pick any chemical that is not balanced and start. So, hydrogen….three reactants and two products. What value makes the same number of both sides? SIX. Yet, how do you make six hydrogen on both sides? If I have 2(H3PO4), I have six hydrogen in the reactants (2 X 3). As well, I have 3H2 which is 3 X 2 or six hydrogen in the products. Hydrogen is balanced. 2(H3PO4) + Ca  Ca3(PO4)2 + 3H2 If I look closely, I also have two PO4 on both sides: 2PO4 and (PO4)2

  15. Balancing Chemical Equations Almost done…look closely…look at calcium and decide what to do 2(H3PO4) + 3Ca  Ca3(PO4)2 + 3H2 To check, add it all up – Reactants = 6H, 2PO4 and 3Ca Products = 6H, 2PO4 and 3Ca The correct, balanced chemical formula for this reaction is 2(H3PO4) + 3Ca  Ca3(PO4)2 + 3H2 In other words, I must mix 2 parts of H3PO4 and three parts Ca to get one Ca3(PO4)2 and three H2 Hooray…you are starting to understand…learning is fun

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