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 MgCO 3 + 2HCl  MgCl 2 + H 2 CO 3

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. C x H y + O 2  CO 2 + H 2 O + 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 + ____N 2  3 ____Mg 3 N 2 Mg 1 N 2 Mg 3 N 2 X 3=3

____Ca(OH) 2 + H 2 ____Ca + ____H 2 O  2 Ca 1 O 2 Na 1 O 2 X 2=2 H 2 H 2 + 2

Coefficients are always whole numbers!

____H 2 + ____O 2  ____H 2 O ___Na + ___Cl 2  ___NaCl

EXAMPLES: What kind of reactions are these? _____Na + _____Cl 2  _____Fe + _____O 2  _____Al + _____Cl 2  _____H 2 O + _____CO 2  ____NaCl ____Fe 2 O 3 H 2 CO 3 ____AlCl

____H 2 CO 3  ____H 2 O + ____CO 2 _____KCl  _____HCl  ____K + ____Cl 2 ____H + ____Cl EXAMPLES: What kind of reactions are these?

___Zn + ___HCl  ___Cl 2 + ___KI  ___ZnCl 2 + ___H 2 ___KCl + ___I

EXAMPLES: What kind of reactions are these? ____C 3 H 8 + ____O 2  ____Mg + ____O 2  ____CO 2 + ____H 2 O ____MgO

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