Chemical Equations and Reactions

Chemical Equations and Reactions What are they?

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

Symbol for Chemical Equations Pb(NO3)4+2KI  PbI2+2KNO3 “Pb(NO3)4” is a formula. Means produces (or yields) + Means added together Coefficients – Number of molecules Subscripts - Number of a particular atom in a molecule

Father of Modern Chemistry 1743 - 1794 Antoine Lavoisier First Described the “Law of Conservation of Mass”

Conservation of Mass Antoine Lavoisier found that the mass of the reactants and the products are equal, even when the states of matter change. 0.7g Oxygen became a gas HgO Hg + O2 He started with: 10g of Mercury Oxide (HgO) He ended up with:and 9.3g Mercury… ...But what happened to the O2? 10 g. = 0.7 + 9.3 g. This is the Law of the Conservation of Mass –Matter is neither created nor destroyed.

Lavoisier’s Chemical Equation: 2 2 HgO  Hg + O2 but, oxygen will need to bond with another oxygen to make O2 Hg (mercury) can exist by itself... To balance the atoms we need to: Put the coefficient of 2 in front of reactant HgO. Put the coefficient of 2 in front the product Hg. This balances the equation!

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

VISUALIZING to Balance an Equation…

Equations Must Be Balanced H2O H2 + O2 Law of Conservation of Mass

Equations Must Be Balanced 2 H2O 2 H2 + O2 Matter is neither created nor destroyed.

N H H H N N H H 11 Is this reaction possible? H2 N2 NH3 Why or Why Not? Let’s Count the Atoms: There are 2 nitrogen atoms There are 2 hydrogen atoms 1 nitrogen and 3 hydrogen…

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

Atoms can only bond in certain ways.. N N N N N N N N N N N N H H H H H H H H H H H H H2 N2 H2 N2 H2 H2 That’s why we can’t change the subscripts. N2 H2 N2 N2

Subscripts - Small #’s below an element. Coefficients - Large #’s in front of the formulas. H H H H H H Here is what it means... H2 2H2 I can’t live without you! When balancing equations,we can only change the coefficients!

N H H H N N H H 15 Now, back to the balancing... H2 1 1 N2 NH3 1 We can only change coefficients before the symbols. 1 molecule of nitrogen 1 molecule of hydrogen 1 molecule of ammonia

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

A little common sense math goes a long way in making this equation balance… N N H H H H H H N N H H H H H H There are 2 nitrogen atoms. We will get 2 more molecules for a total of 6 hydrogen atoms. That will make 2 NH3. Count the atoms. Is it balanced?

N N H H H H H H N N H H H H H H N2+3H22NH3 Do both sides have the same amount of atoms? Then it is a balanced equation.

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.

1. Set up your equation. Write the elements in this order: Metals, Nonmetals, Oxygen, and Hydrogen H2 + O2 H2O O H

2. Count and write the number of atoms you have of each on both sides. H2 + O2 H2O 1 2 O 2 H 2

H2 + O2 H2O 1 2 O 2 H 2 3. Balance by changing the coefficients and recounting. 2 2 (2) Need to have at least 2 “O” (4) (4) But it changes the number of “H” How are you going to make “H” add up to 4? Is this balanced? Yes!

Let’s try another: 2 Mg + HCl  H2 + MgCl2 1 Mg 1 2 (2) Cl 1 Need to have at least 2 “Cl” Is this balanced? Yes! 2 H 1 (2) Changing the Cl changes the “H”?

Na + HCl NaCl + H2 1 Na 1 Cl 1 1 H 2 1 Let’s try another: 2 In this case, we will start with hydrogen since it is the only one unbalanced. (2)

Na + HCl NaCl + H2 1 Na 1 Cl 1 1 H 2 1 But, changing the hydrogen in HCl affects the number of chlorine atoms. 2 2 (2) (2) (2)

Na + 2HCl NaCl + H2 2 1 Na 1 Cl (2) 1 (2) 1 H (2) 2 1 2 Changing the chlorine on the product side affects the sodium (Na) on the reactants side. So we must now change sodium as well. (2) (2)

Click on the web sites below and balance the equations. http://www.wfu.edu/~ylwong/balanceeq/balanceq.html http://www.chem.vt.edu/RVGS/ACT/notes/scripts/bal_eq1.html

4 Types of Chemical Reactions Synthesis Decomposition Single Displacement Double Displacement

Synthesis A + B AB 2H2 + O2 2H2O Two or more substances combine to form a new one.

Decomposition AB  A + B 2MgO Mg+O2 Decomposition the opposite of synthesis; a substance breaks down into 2 or more simpler substances.

Single Displacement A+ BC  AC + B Zn + Cu(NO3)2 Zn(NO3)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).

Double Displacement AB + CD AD + CB HCl + NaOH HOH+NaCl (H2O) This occurs when two elements “switch” with one another to form two new compounds.

Combustion Reaction • Oxygen is a reactant in every combustion reaction. • A reaction in which a compound and oxygen burn. • 2CH4 +4O2 - 2CO2 + 4H2O

Why can firewood be stored in open air and not catch on fire? Do you have to have fire to start a fire?

Activation Energy Ea = Activation Energy It is the minimum amount of energy required to initiate a reaction.

Activation Energy What has happened to the paper and the match heads? The material was exposed to enough energy( heat energy not flame) in order to start a chemical reaction.

Activation Energy How do you usually light a match? Friction Mechanical EnergyFrictionHeat Energy until heat energy = Activation energy

Chemical Equations and Reactions