1 Chemical Equations and Reactions What are they?

2 The Two Sides of a Chemical Reaction Definition: In a chemical reaction one or more substances are changed into new substances. Na + Cl  NaCl Reactant – About to be changed Product – The new substance

3 Symbol for Chemical Equations “Pb(NO 3 ) 4 ” is a formula.  Means produces (or yields) + Means added together Coefficients – Number of molecules Subscripts - Number of a particular atom in a molecule Pb(NO 3 ) 4 + 2KI  PbI 2 + 2KNO 3

4 First Described the “Law of Conservation of Mass” Father of Modern Chemistry

5 Conservation of Mass Antoine Lavoisier found that the mass of the reactants and the products are equal, even when the states of matter change. This is the Law of the Conservation of Mass – Matter is neither created nor destroyed. Hg + O 2 HgO He started with: 10g of Mercury Oxide (HgO) He ended up with: and 9.3g Mercury… 0.7g Oxygen became a gas 10 g. = g....But what happened to the O 2 ?

6 Lavoisier’s Chemical Equation: Hg (mercury) can exist by itself... but, oxygen will need to bond with another oxygen to make O 2 HgO  Hg +O2O2 2 2 To balance the atoms we need to: This balances the equation! Put the coefficient of 2 in front the product Hg. Put the coefficient of 2 in front of reactant HgO.

7 Not that easy? Let’s try... Easy, Right?

8 VISUALIZING to Balance an Equation…

9 Equations Must Be Balanced H 2 + O 2 H2OH2OH2OH2O Law of Conservation of Mass

10 Equations Must Be Balanced 2 H 2 + O 2 2 H 2 O Matter is neither created nor destroyed.

Is this reaction possible? N N N N H H H H N N H H H H H H N2N2 NH 3 Why or Why Not? Let’s Count the Atoms: There are 2 nitrogen atoms There are 2 hydrogen atoms 1 nitrogen and 3 hydrogen… H2H2 11

12 Before we begin balancing, we will look at a couple of rules.

13 Atoms can only bond in certain ways.. subscripts. That’s why we can’t change the subscripts. H H H H N N N N N N N N N N N N N N N N H H H H H H H H H H H H H H H H N N N N N N N N H H H H H2H2 N2N2 N2N2 N2N2 N2N2 N2N2 H2H2 H2H2 H2H2 H2H2

14 Here is what it means... Subscripts - Small #’s below an element. Coefficients - Large #’s in front of the formulas. 2H22H2 H2H2 When balancing equations, we can only change the coefficients! H H H H H H H H H H H H I can’t live without you!

Now, back to the balancing... N N N N H H H H N N H H H H H H N2N2 NH 3 We can only change coefficients before the symbols. 1 molecule of nitrogen 1 molecule of hydrogen 1 molecule of ammonia H2H

16 N N N N H H H H N N H H H H H H After counting the atoms we know... There are too many nitrogen atoms and not enough hydrogen atoms.

17 N N N N H H H H N N H H H H H H A little common sense math goes a long way in making this equation balance… Count the atoms. Is it balanced? There are 2 nitrogen atoms. We will get 2 more molecules for a total of 6 hydrogen atoms. That will make 2 NH 3. H H H H H H H H N N H H H H H H

18 N H 2 2NH 3 N N N N H H H H N N H H H H H H H H H H H H H H N N H H H H H H Then it is a balanced equation. Do both sides have the same amount of atoms?

19 Four Steps to Balance Equations: 1. Set up your equation. Write the metals, nonmetals, oxygen, and hydrogen. 2. Count and then write the number of atoms you have on both sides. 3. Balance by changing the coefficients and recounting. 4. Start the process again if it still does not balance.

20 1. Set up your equation. H 2 + O 2  H 2 O Write the elements in this order: Metals, Nonmetals, Oxygen, and Hydrogen O H

21 2. Count and write the number of atoms you have of each on both sides. H 2 + O 2  H 2 O O H

22 H 2 + O 2  H 2 O O H (2) 2 (4) 2 (4) 3. Balance by changing the coefficients and recounting. Need to have at least 2 “O” But it changes the number of “H” How are you going to make “H” add up to 4? Is this balanced? Yes!

23 Mg + HCl  H 2 + MgCl 2 Mg Cl (2) 2 (2) Let’s try another: Need to have at least 2 “Cl” Changing the Cl changes the “H”? Is this balanced? Yes! H 2 1

24 Na + HCl  NaCl + H 2 Na Cl H (2) 2 In this case, we will start with hydrogen since it is the only one unbalanced. Let’s try another:

25 Na + HCl  NaCl + H 2 Na Cl H (2) 2 (2) (2) 2 But, changing the hydrogen in HCl affects the number of chlorine atoms.

26 Changing the chlorine on the product side affects the sodium (Na) on the reactants side. So we must now change sodium as well. Na + 2HCl  NaCl + H 2 Na Cl H (2) (2) (2) 2 2 (2) (2)

Click on the web sites below and balance the equations.

28 4 Types of Chemical Reactions Synthesis Decomposition Single Displacement Double Displacement

29 Synthesis A + B  AB 2H 2 + O 2  2H 2 O Two or more substances combine to form a new one.

30 Decomposition AB  A + B 2MgO  Mg + O 2 Decomposition the opposite of synthesis; a substance breaks down into 2 or more simpler substances.

31 Single Displacement A + BC  AC + B Zn + Cu(NO 3 ) 2  Zn(NO 3 ) 2 + Cu This occurs when one element replaces another in a compound; a zinc strip is put into a copper nitrate solution and the copper precipitates out (it turns solid).

32 Double Displacement AB + CD  AD + CB HCl + NaOH  HOH + NaCl (H 2 O) This occurs when two elements “switch” with one another to form two new compounds.