Chemical Reactions

Chapter 2Sections 1-4Pages 26-51

Chemical Reactions• A chemical change takes place

when chlorophyll breaks down into a new substance. This is an example of a Chemical Reaction.

• A chemical reaction is a process in which one or more substances change to make one or more new substances. The chemical and physical properties of the new substances differ from those of the original substances.

• More examples: Baking bread (yeast), baking muffins (baking soda), leaves changing color, metal rusting, burning wood..

Signs of a Chemical Reaction

• Gas formation• Forming a precipitate (a

solid substance that forms in a solution)

• Color change• Light• Sound• Heat• Smell

A change in Properties

• The sure sign for a chemical reaction is the formation of a new substance.

Bonds…Chemical Bonds…

• A chemical bond is a force that holds two atoms together in a molecule.

• For a chemical reaction to take place, the original bonds must break and new bonds form.

Breaking and Making Bonds

• The chemical bonds in the starting substances break, and then new bonds form to make new substances.

New Bonds, New Substances

• New properties form as a result of a new substance.

• Don’t forget that the new properties are very different from the original.

Chemical Formulas

• A chemical formula is a shorthand way to use chemical symbols and numbers to represent a substance. It shows how many atoms of each kind are present in a molecule.

• A subscript is a number written below and to the right of a chemical symbol in a formula. If there is no subscript then only one atom of that element is present.

Formulas for Covalent Compounds

• Usually composed of two nonmetals

• Use prefixes, to represent a number.

• Prefixes tell you how many atoms of each element are in a formula.

• mono- 1, di- 2, tri- 3, tetra- 4, penta- 5, etc.

Formulas for Ionic Compounds

• Usually a metal and a nonmetal.

• Charge must be neutral or 0.

• The formula must have subscripts that cause the charges of the ions to cancel out.

Chemical Equations

• A chemical equation uses chemical symbols and formulas as a shortcut to describe a chemical reaction.

• Reactants: the starting materials

• Products: the substances formed from the reaction

• The arrow represents the equal sign.

Importance of Accuracy

• The correct symbols or formula is important • There are examples of similar symbols and

formulas that are very different:– CO₂– CO– Co

Law of Conservation of Mass

• Equations must be balanced!• Lavoisier in the 1700s found

that the total mass of the reactants are always the same total mass of the products.

• The law states that mass is neither created or destroyed in ordinary chemical and physical changes. This law means that a chemical equation must show the same numbers and kinds of atoms on both sides of the arrow.

How to Balance an Equation

1. Count the atoms of each element in the reactants and in the products.

2. Multiply the subscript of each element by its coefficient. (a coefficient is the number that is placed in front of a chemical symbol or formula)

3. Make sure the products represent the correct number of the elements using a coefficient if necessary.

4. Then add any coefficients to the reactants to represent this balance.

Balancing Equations Animation

• http://www.physics-chemistry-interactive-flash-animation.com/chemistry_interactive/basic_chemical_reaction_balance_learning.htm

Balancing Equations

• http://education.jlab.org/elementbalancing/index.html

• http://misterguch.brinkster.net/eqnbalance.html

• See extra power point on balancing equations on web site.

Types of Chemical Reactions

• Synthesis Reaction: a reaction in which two or more substances combine to form one new compound “two coming together to dance”

• Decomposition Reaction: a reaction in which a single compound breaks down into two or more simpler substances (opposite of synthesis) “the two ending the dance and separate”

Types of Chemical Reactions

• Single Displacement Reaction: a reaction in which one element or radical takes the place of another element or radical in a compound (Usually the more reactive element does the replacing) “one person cutting in on a couple dancing, leaving one person alone”

• Double Displacement Reaction: a reaction in which a gas, a solid precipitate, or a molecular compound forms from the exchange of ions between two compounds “trade partners”

Exothermic & Endothermic Reactions

• Exothermic reactions give off energy. Energy is released. Energy is a product.

2Na + Cl₂ →2NaCl + energy• Endothermic reactions take in energy. Energy

is required. Energy is written as a reactant. Example is photosynthesis 6CO₂ + 6H₂O + energy → C₆H₁₂O₆ + 6O₂Carbon Dioxide, water and sunlight → glucose and Oxygen

The Law of Conservation of Energy

• Energy is neither created or destroyed, however energy can change form

• Energy can be transferred.

Rates of Reactions

• A reaction can only take place if the particles of the reactants collide.

• Must be energy to break the bonds.

• The speed at which new particles form is called the rate of reaction.

Activation Energy

• Activation energy is the smallest amount of energy needed for a chemical reaction.

• Friction• Electric spark• Light• Looking at energy diagrams-

you can tell if it energy is given off if the reactants have a higher energy than the products (page 45)

Factors that Increase the Rates of Reaction

• Temperature increase (particles increase movement)

• High Concentration (because there are more reactants colliding)

• Surface Area increase (exposes more particles of the reactants to the other reactants)

• Catalyst can speed up reaction without itself changing (example enzymes in body or catalytic converter in car)

Factor that Decreases Rates of Reaction

• Inhibitors slow down or stops a chemical reactions.

Food preservativesAntibiotics