Physical Science Chapter 6 Chemical Reactions Physical Science Chapter 6

Changes in Matter Chemistry is the study of matter and how matter changes. Matter is described in terms of either physical or chemical properties; and these are also the two changes that can occur to all types of matter.

Properties of Matter Physical properties are characteristics that can be observed without changing the substance. Ex. Color, Melting Point, Density Chemical properties describes the substance’s ability to change into a different substance. Ex. Burning, tarnishing or rusting

Changes of Matter A physical change occurs when the form or appearance of a substance is changed. Ex. Breaking, bending, cutting or melting metal. A chemical change occurs when matter is turned into one or more new substances through the formation of new chemical bonds. These can also be called chemical reactions. Ex. Burning a marshmallow

Chemical Equations A chemical equation describes a chemical reaction using symbols to represent elements and formulas to represent compounds. The reactants are written on left side of the arrow. The products are written on the right side of the arrow. The arrow symbolizes the word “yields”.

Evidence of Chemical Rxns 1.) Formation of a new substance. a. Precipitate – solid that forms when 2 liquids are mixed. b. Gas bubbles – gas that forms when 2 liquids are mixed c. Color Change – change in color of product 2.) Chemical rxns always have a change in energy. a. Endothermic rxns – energy is absorbed by reactants b. Exothermic rxns – energy is released as chem bonds reform

Conservation of Mass Conservation of mass states that mass can neither be created or destroyed. In a chemical reaction, atoms can neither be created or destroyed, only rearranged. Therefore, the atoms on the reactants side must equal the atoms on the products side of the arrow.

Balancing Chemical Equations In a balanced chemical equation, there are equal numbers of the atoms of each element on both sides of the arrow. There are basic rules to follow when balancing equations.

Rules Rules for Balancing Equations: 1.) Write a word equation and then a chemical equation. Make sure the symbols and formulas are correct. 2.) Count the number of atoms on both sides to see if they are equal. 3.) Put coefficients in front of the symbols or formulas to balance the number of atoms. 4.) Recheck the equation to see if it is still balanced.

Checking an Equation Practice Check if the following equations are balanced. We will balance if they aren’t. C + O2 > CO2 H2 + O2 > H2O Practice Nitrogen and hydrogen are combined in a reaction to produce ammonia (NH3).

Balancing Practice Balancing equations practice. 1-good lesson & practice;  2.-demo & game;   3.-practice quiz; 4.-more practice; and 5. more 

Types of Reactions Chemical reactions can be grouped into four major categories: Synthesis Reactions Decomposition Reactions: Single-Replacement Reactions: Double-replacement Reactions:

Synthesis Reactions Synthesis reactions occur when two or more reactants combine or join together to form one product. The standard formula is: A + B  C Ex. Oxidation of Metals

Decomposition Rxns In a decomposition rxn, one reactant decomposes into two or more products. Decomposition reactions are the opposite of synthesis reactions. Standard formula: C  A + B

Single-Replacement Reaction In single-replacement reactions, an element reacts with a compound, replacing one of the elements in the compound. Standard Formula: A + BX  AX + B

Double-Replacement Rxn In a double-replacement reaction, two compounds react with each other to form two new compounds. Standard Formula: AX + BY  Ay + BX Remember that positive ions can only switch with positive ion and negative ions with negative ions.

Energy of Chemical Reactions Energy is always involved in a chemical reaction. Sometimes energy is released by a chemical reaction and sometimes energy is absorbed by a chemical reaction. Chemical reactions are either classified as endothermic or exothermic.

Exothermic Reactions A chemical reaction in which energy is released is exothermic. Heat (or sound or light or kinetic-motion) is given off or released from the bonds of the reactants. In exothermic reactions the energy is higher in the reactants than in the products.

Endothermic Reactions Endothermic reactions are chemical reactions that absorb energy. Energy is taken into the reacting substance usually in the form of heat or light (or kinetic-stirring). The energy of the products is higher than the energy of the reactants.

Exogonic, Endergonic Rxns

Activation Energy Activation energy is the energy needed for the reactants to form the high energy, unstable intermediate molecules from which the products form. An energy diagram shows the activation energy of a reactions. All chemical reactions require activation energy.

Kinetics Kinetics is the study of reaction rates. Kinetics is based on collision theory, which relates particle collisions that affect reaction rates. Collision theory states the rate of the reaction is affected by four factors: concentration, surface area, temperature, and presence of catalysts.

Concentration Concentration of a substance is a measure of the amount of the substance per unit of volume. More particles = more collisions The higher the concentration of a substance, the higher its reaction rate.

Surface Area Surface area is the amount of a material that is exposed on a surface. An increase in surface area causes an increase in collisions and thus, an increase in reaction rate. An increase in surface area can be caused by breaking a substance into smaller particles or by stirring.

Temperature Temperature is a measure of the energy in the particles of a substance. If the particles are moving faster than there are more collisions. An increase in temperature results in an increase in reaction rates.

Catalysts A catalyst is a substance that speeds up a chemical reaction without changing the reaction or being used up. A catalyst works by changing one or more of the steps of a chemical reaction and thus lowering the activation energy.

Inhibitors An inhibitor is a material that decreases the rate of a reaction. Most inhibitors work by preventing reactants from coming together. Inhibitors may attach to reactants thus preventing them from attaching to the other reactant. Inhibitors are commonly used in food preservatives or are poisons (arsenic, mercury, copper, CO2)