Chemical equations and reactions
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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

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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


Chemical equations and reactions

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.


Chemical equations and reactions

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!


Chemical equations and reactions

Easy, Right?

Not that easy?

Let’s try...


Visualizing to balance an equation

VISUALIZING to Balance an Equation…


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.


Chemical equations and reactions

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…


Chemical equations and reactions

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


Chemical equations and reactions

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!


Chemical equations and reactions

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


Chemical equations and reactions

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.


Chemical equations and reactions

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?


Chemical equations and reactions

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

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

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

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

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

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”?


Let s try another1

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)


Chemical equations and reactions

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)


Chemical equations and reactions

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)


Chemical equations and reactions

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

4 Types of Chemical Reactions

Synthesis

Decomposition

Single Displacement

Double Displacement


Synthesis

Synthesis

A + B AB

2H2 + O2 2H2O

Two or more substances combine to form a new one.


Decomposition

Decomposition

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

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

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

  • 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

Ea = Activation Energy

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


Activation energy1

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 energy2

Activation Energy

How do you usually light a match?

Friction

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


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