Chemical Reactions

Chapter 19

Synthesis Reaction (combination reaction) -

• the combination of two or more substances to form a compound

Element or compountd + element or compound compound

General form A + B AB

Decomposition Reaction:

• is the breakdown of one substance into two or more other substances

• compound two or more elements

• AB A + B

Single Displacement Reaction:

• One element replaces another element in a compound.

element + compound element + compound

A + BC AC + B

Single Displacement

• We can predict which metal will replace another using an Activity Series, shown in the diagram, which lists metals according to how reactive they are.

• A metal will replace any less active metal.

Double Displacement Reaction:

• Two elements replace each other in compounds.

• A precipitate is an insoluble compound that comes out of solution during this type of reaction.

AB + CD AD + CB

MgCO3 + 2HCl MgCl2 + H2CO3

• An exothermic reaction in which a substance combines with oxygen forming products in which all elements are combined with oxygen. (Burning) Energy is usually released in the form of heat and light.

CxHy + O2 CO2 + H2O + heat/light

Oxidation Reduction Reactions

• Oxidation-reduction reactions- chemical changes that occur when electrons are transferred between reactants.

• Also called REDOX reactions

• Oxidation

• Modern definition - loss of electrons

•  Reduction

• Modern definition - gain of electrons

• Oxidation and reduction always occur simultaneously. One process cannot occur without the other.

Oxidation Reduction Reactions

Redox reactions

• Chemical reactions involving electron transfer of this sort often involve oxygen, which is very reactive, pulling electrons from metallic elements.

• Corrosion of metal is a visible result.

Lavoisier and the Conservation of Mass

• Chemical reactions are taking place all around you and even within you.

• A chemical reaction is a change in which one or more substances are converted into new substances.

Lavoisier and the Conservation of Mass

• The substances that react are called reactants.

• The new substances produced are called products.

• This relationship can be written as follows:

Lavoisier and the Conservation of Mass

• The French chemist Antoine Lavoisier established that the total mass of the products always equals the total mass of the reactants.

Lavoisier and the Conservation of Mass

• For example, the mass of the candles and oxygen before burning is exactly equal to the mass of the remaining candle and gaseous products.

Writing Equations

• Scientists have developed a shorthand method to describe chemical reactions.

• A chemical equation is a way to describe a chemical reaction using chemical formulas and other symbols.

Writing Equations

• Some of the symbols used in chemical equations are listed in the table.

Choosing Coefficients

• Finding out which coefficients to use to balance an equation is often a trial-and-error process.

How to Balance Equations:

1. Use Pencil!

2. Use coefficients (numbers in front of the formulas), to balance the equation. (DO NOT change the Formulas. DO NOT CHANGE THE SUBSCRIPTS!)

3. Draw a line to separate the reactants from the products.

How to Balance Equations:

4. Write down all elements that are on each side of the equation.

5. Balance polyatomic ions that appear on both sides of the equation as single units. (Ex: Count sulfate ions, not sulfur and oxygen separately)

6. Balance elements one at a time.

How to Balance Equations:

7. Balance H and O last.

8. Save the one that is in the most places for very last.

Practice:• Balance the equation:

____Mg + ____N2 3 ____Mg3N2

Mg 1

N 2

Mg 3

N 2

X 3=3

____Ca(OH)2 + H2____Ca + ____H2O 2

Ca 1

O 2

Na 1

O 2X 2=2

H 2 H 2 + 2

Coefficients are always whole numbers!

____H2 + ____O2 ____H2O

___Na + ___Cl2 ___NaCl

EXAMPLES: What kind of reactions are these?_____Na + _____Cl2

_____Fe + _____O2

_____Al + _____Cl2

_____H2O + _____CO2

____NaCl

____Fe2O3

H2CO3

____AlCl3

2

232

2

4

3 22

____H2CO3 ____H2O + ____CO2

_____KCl

_____HCl

____K + ____Cl2

____H + ____Cl22

2 2

2

EXAMPLES: What kind of reactions are these?

___Zn + ___HCl

___Cl2 + ___KI

___ZnCl2 + ___H2

___KCl + ___I2

2

22

EXAMPLES: What kind of reactions are these?____C3H8 + ____O2

____Mg + ____O2

____CO2 + ____H2O

____MgO

3 45

22

Energy Exchanges

• A dynamic explosion is an example of a rapid chemical reaction.

• Most chemical reactions proceed more slowly, but all chemical reactions release or absorb energy.

• This energy can take many forms, such as heat, light, sound, or electricity.

• Chemical bonds are the source of this energy.

• When most chemical reactions take place, some chemical bonds in the reactants are broken, which requires energy.

• In order for products to be produced, new bonds must form. Bond formation releases energy.

More Energy Out

• Chemical reactions that release energy are called exergonic reactions.

• In these reactions less energy is required to break the original bonds than is released when new bonds form.

• As a result, some form of energy, such as light or heat is given off by the reaction.

• The familiar glow from the reaction inside a glow stick is an example of an exergonic reaction, which produces visible light.

Heat Released

• When the energy given off in a reaction is primarily in the form of heat, the reaction is called an exothermic reaction.

• The burning of wood and the explosion of dynamite are exothermic reactions.

More Energy In

• Sometimes a chemical reaction requires more energy to break bonds than is released when new ones are formed.

• These reactions are called endergonic reactions.

• The energy absorbed can be in the form of light, heat or electricity.

Thermal Energy Absorbed

• When the energy needed is in the form of heat, the reaction is called an endothermic reaction.

• Some reactions are so endothermic that they can cause water to freeze.

• Certain types of ice packs are an example of this.

Endothermic or exothermic process?-evaporating alcohol-leaves burning-boiling water-water cooling-melting ice-freezing water