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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.

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Chemical equations and reactions

Chemical Equations and Reactions

What are they?


The two sides of a chemical reaction

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
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
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...



Equations must be balanced
Equations Must Be Balanced

H2O

H2 + O2

Law of Conservation of Mass


Equations must be balanced1
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


Here is what it means

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 equation balance…

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
Four Steps to Balance Equations: equation balance…

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
1. Set up your equation. equation balance…

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
2. Count and write the number of atoms you have of each on both sides.

H2 + O2 H2O

1

2

O

2

H

2


3 balance by changing the coefficients and recounting

H both sides.2 + 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
Let’s try another: both sides.

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”?


Let s try another1

Na + HCl both sides.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 + both sides.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 + both sides.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. both sides.

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

4 Types of Chemical Reactions both sides.

Synthesis

Decomposition

Single Displacement

Double Displacement


Synthesis
Synthesis both sides.

A + B AB

2H2 + O2 2H2O

Two or more substances combine to form a new one.


Decomposition
Decomposition both sides.

AB  A + B

2MgO Mg+O2

Decomposition the opposite of synthesis; a substance breaks down into 2 or more simpler substances.


Single displacement
Single Displacement both sides.

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
Double Displacement both sides.

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
Combustion Reaction both sides.

  • 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
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
Activation Energy fire?

Ea = Activation Energy

It is the minimum amount of energy required to initiate a reaction.


Activation energy1
Activation Energy fire?

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 energy2
Activation Energy fire?

How do you usually light a match?

Friction

Mechanical EnergyFrictionHeat Energy until heat energy = Activation energy


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