Chemical Changes J. Thousand Gainesville Middle School

PHYSICAL PROPERTIES Color Shape Odor Density Mass Luster Malleability Solubility State of Matter Bouyancy

CHEMICAL PROPERTIES Reactivity Combustibility Acid or Base (pH)

PHYSICAL CHANGE A Physical Change is a change that affects one or more of the physical properties of a substance only.

Examples of Physical Change Freezing water for ice cubes Carving a piece of wood Cutting your hair Adding sugar to tea

Can physical changes be reversed? In many cases physical changes can be reversed or put back. You can melt and refreeze water for example. In some cases though the change is not reversible, like when you carve a stick.

Chemical Changes In a chemical change, one or more substances are changed into entirely new substances with different physical and chemical properties. Chemical properties describe in what ways a substance may change, a chemical change is the actual process of change.

Examples of Chemical Change 1. Cookies after they are baked. 2. Gasoline burning in an engine 3. Milk that turned sour. 4. Iron nail rusting

Can we reverse chemical changes? Almost never! Can you un-bake a cake? Get the wood back that burned in your fire place? Chemical changes are almost never reversible.

Signs of Chemical Change A new substance has been formed A color change has occurred Heat is given off or used. Bubbles are formed. A precipitate occurs. New odors were formed.

CHEMICAL COMPOUNDS Chemical compounds are 2 or more different types of atoms put together to form a new substance. CO 2 C 6 H 12 O 6

Chemical Formulas Chemical formulas are a shorthand way of writing down the kinds and numbers of atoms in a compound. Subscript: the small number to the RIGHT of an element symbol. Coefficient: the number in front (LEFT) of a symbol or formula.

Does it add up? BaSO 4 Ba: S: O: 2 Na H 2 SO 4 (NH 4 ) 3 PO 4

Chemical Equations Chemical equations are a written expression that shows how elements and or compounds are combined to make new substances.

Conservation of Mass The law of Conservation of Mass (Matter) states that matter is neither created nor destroyed, just changed.

Balancing Equations When chemical reactions occur, the Law of Conservation of mass applies. This means that the number and kinds of reactants must also be present in the products. When you write an equation, it must be balanced with each side having the same numbers and kinds of elements.

Steps to balancing equations: 1. Label the Reactants and Products ReactantsProducts 2. Construct a T-chart and count the elements. change the coefficient of the chemical formula to balance each side. R P N=2 H=2H=3 N= Count each side and check they are balanced 2=22 6=6

Chemical Bonding The joining of atoms to form new substances that have unique chemical and physical properties from the individual atoms that make them up. The force of attraction that holds these new substances together is called a chemical bond.

Key to bonding The number of valance electrons an atom has determines how likely it is for that type of atom to create a chemical bond. Atoms that do not have full valance energy shells will either gain, lose or share electrons to stabilize this valance energy shell.

Consider this! An atom with only 1 or 2 electrons on an outer shell that holds up to 8 will probably do what? An atom with 7 electrons on an outer shell will probably do what? What about 5 valance electrons? What about an atom with a full valance shell?

What about the whole atom charge? If an atom loses two electrons what happens to it’s total charge? If an atom takes two electrons, what happens to it’s total charge? IONS: Atoms that have either a positive or negative charge.

IONIC BONDING This type of chemical bonding occurs when ions of elements are attracted to each other. Positive ions are attracted to negative ions. The new compound becomes electrically neutral. The sum of the oxidation numbers must = zero.

COVALENT BONDING Many of the compounds familiar to you are formed using covalent bonds. In covalent bonds, electrons are shared by atoms, and no ion is formed.

Every Chemical Reaction involves Energy. During chemical reactions bonds between atoms are broken apart and or remade. In order for bonds to break or form, energy is required. EXO- Go out, exit ENDO- Go in

Exothermic Reactions In an exothermic reaction energy is released. Often this energy is released in the form of heat, but not always. Light from a glow stick Electrical energy from a battery Heat from burning wood. Combustion!

Endothermic reactions In an endothermic reaction energy is absorbed into the reaction. If you were able to feel this type of reaction it would feel cold, because it is taking energy from you. Photosynthesis is an example Cold packs are examples.

How fast can it go? The rate and speed of chemical reactions can be changed. Several factors affect this: Temperature Concentration Catalysts

A catalyst is a substance that speeds up a chemical reaction, BUT is not used up in the reaction. Think of a car key. The car key starts the car but is not destroyed and can be used over and over again.

Types of Chemical Compounds Organic compounds Ionic and Covalent compounds pH: Acids, Bases, or neutral?

Organic compounds Organic: The main element is Carbon. 90% of all compounds are organic. CH 3 Cl, CH 3 OH CO 2 C 6 H 12 O 6

IONIC and COVALENT COMPOUNDS Most ionic compounds are formed when a metal bonds with a non-metal. Ex: Salt NaCl Covalent compounds are formed when two atoms share electrons. Ex: Water, sugar and Carbon Dioxide

Acids, bases and Salts One of the major chemical properties of substances is whether it is an acid, base or neutral. All substances are one of these three things to varying degrees.

pH scale The pH scale runs from 0-14 Acids: Neutral:7 Bases: to 14

ACIDS Any substance that has excess H+ are considered acids. Characteristics: A. Taste sour B. Corrosive C. Good conductors of electricity pH below 7 E. Blue litmus paper turns red

BASES A base is a compound that has extra Hydroxide ions: OH- Characteristics: A. Slippery B. PH greater than 7 C. Bitter taste D. Red Litmus turns Blue

NEUTRALIZATION If we combine an acid and a base we get water and a salt.