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1 Unit 14 - CHEMICAL REACTIONS Reactants: Zn + I 2 Product: Zn I 2.

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Presentation on theme: "1 Unit 14 - CHEMICAL REACTIONS Reactants: Zn + I 2 Product: Zn I 2."— Presentation transcript:

1 1 Unit 14 - CHEMICAL REACTIONS Reactants: Zn + I 2 Product: Zn I 2

2 2 Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes.Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes. Symbols represent elements, formulas describe compounds, chemical equations describe a chemical reaction: 2 H 2 + O 2 → 2 H 2 OSymbols represent elements, formulas describe compounds, chemical equations describe a chemical reaction: 2 H 2 + O 2 → 2 H 2 O Chemical reactions occurChemical reactions occur when bonds between the outermost parts of atoms are broken and new bonds are formed. Introduction

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4 4 A chemical reaction is a process in which a substance (or substances) called reactants are changed into one or more new substances which are called products. 4 Al (s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s)

5 5 reactants products Chemical equations show the conversion of reactants (the molecules shown on the left of the arrow) into products (the molecules shown on the right of the arrow). A + sign separates molecules on the same side The arrow is read as “yields” Example C + O 2  CO 2 This reads “carbon plus oxygen react to yield carbon dioxide” Parts of a Reaction Equation

6 6 The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction, C + O 2  CO 2 contains the same information as the English sentence but has quantitative meaning as well.

7 7 Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788 Chemical Equations

8 8 A chemical reaction is written as a chemical equation using chemical formulas with symbols which represent the chemical elements and numbers written as sub-indexs, which indicate the number of atoms of each element. 4 A l (s) + 3 O 2 (g) ---> 2 A l 2 O 3 (s) The numbers in the front are called stoichiometric coefficients. The letters (s), (g) are the physical states of compounds.

9 9 Symbols Used in Equations: Chemical formulas The chemical formulas are represented by letters and numbers: l Na 2 C l O 4 The letters represent the symbols of the chemical elements present (as in the Periodic Table). l Na: sodium, C l : Chloride, O:oxygen The numbers indicate the number of atoms of each element involved. 2 atoms Na, 1 atoms C l, 4 atoms O

10 10 Symbols Used in Equations: Chemical elements

11 11 Subindexs indicate the number of atoms of each element involved. The absence of subindex means that there is 1 atom. MgBr 2 1 atom Mg, 2 atoms Br Coefficients represent the number of units of that substance. 2 MgBr 2 2 x (1 atom Mg, 2 atoms Br)  = 2 atoms Mg, 4 atoms Br Symbols Used in Equations: Subindexs and Coefficients

12 12 Symbols Used in Equations: Chemical formulas I f there is a parenthesis applies to everything inside it. Fe 2 ( S O 4 ) 3 Fe: iron S:sulphur, O:oxygen 2 atoms Fe 3 x 1 = 3 atoms S 3 x 4 = 12 atoms O

13 13 PRACTISE: Count the number of atoms in a chemical formula H 2 lKC l Na 2 O Be(OH) 2 P 2 O 5 Li 2 SO 4 Ni(BrO 3 ) 2 Ca 3 (PO 4 ) 2 2 Co 2 O 3 3 C 5 H 10

14 14 PRACTISE: Count the number of atoms in a chemical formula H 2 lKC l Na 2 O Be(OH) 2 P 2 O 5 Li 2 SO 4 Ni(BrO 3 ) 2 Ca 3 (PO 4 ) 2 2 Co 2 O 3 3 C 5 H 10 H 2 : 2 atoms H lKC l : 1 atom K, 1 atom Cl Na 2 O : 2 atoms Na, 1 atom O Be(OH) 2 : 1 atom Be, 2 atoms O, 2 atoms H P 2 O 5 : 2 atoms P, 5 atoms O Li 2 SO 4 : 2 atoms Li, 1 atom S, 4 atoms O Ni(BrO 3 ) 2 : 1 atom Ni, 2 atoms Br, 6 atoms O Ca 3 (PO 4 ) 2 : 3 atoms Ca, 2 atoms P, 8 atoms O 2 Co 2 O 3 : 4 atoms Co, 6 atoms O 3 C 5 H 10 : 15 atoms C, 30 atoms H

15 15 Solid (s) lLiquid ( l ) Gas (s) Aqueous solution (aq) Escaping gas (  ) Precipitating solid (  ) Symbols Used in Equations

16 16 notWhen balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the sub-indexs. Changing the sub-indexs changes the compound. Balancing Equations

17 17 Subindexs vs. Coefficients The sub-indexs 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.

18 18 Chemical Equations 4 Al(s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) This equation means 4 Al atoms + 3 O 2 molecules ---produces---> 2 molecules of Al 2 O 3 2 molecules of Al 2 O 3

19 19 There are four basic steps to balancing a chemical equation. 1.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! 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 in order to balance the equation. 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) Steps to Balancing Equations

20 20 Some Suggestions to Help You Take one element at a time, working left to right except for H and O. Save H for next to last, and O until last. IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element)

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22 22 Balancing Equations ___ H 2 (g) + ___ O 2 (g) ---> ___ H 2 O( l ) 22 What Happened to the Other Oxygen Atom????? This equation is not balanced! Two hydrogen atoms from a hydrogen molecule (H 2 ) combines with one of the oxygen atoms from an oxygen molecule (O 2 ) to form H 2 O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H 2 molecule) to make a second H 2 O molecule.

23 23 Balancing Equations ___ A l (s) + ___ Br 2 ( l ) ---> ___ A l 2 Br 6 (s) 23

24 24 PRACTISE: Balancing Equations ____C 3 H 8 (g) + _____ O 2 (g) → ____CO 2 (g) + _____ H 2 O(g) ____B 4 H 10 (g) + _____ O 2 (g) → ___ B 2 O 3 (g) + _____ H 2 O(g)

25 25 PRACTISE: Balancing Equations C 3 H 8 (g) + 5 O 2 (g) → 3 CO 2 (g) + 4 H 2 O(g) 2 B 4 H 10 (g) + 11 O 2 (g) → 4 B 2 O 3 (g) + 10 H 2 O(g)

26 26 Balancing Equations Mg + HI → Mgl 2 + H 2 Ca(OH) 2 + HC l → CaC l 2 + H 2 O Na 3 PO 4 + Fe 2 O 3 → Na 2 O + FePO 4 C 4 H 8 + O 2 → CO 2 + H 2 O


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