Chemical Reactions 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 substance will change. Signs of a chemical reaction: -Precipitate – solid substance that is formed in a solution. -Light -Heat change -Electricity -Color change -Gas formation

Chemical Formulas The names of many covalent compounds use prefixes to express the number of atoms of each element. mono-1Hexa-6 di-2hepta-7 tri-3octa-8 tetra-4nona-9 penta-5deca-10

Chemical Formulas 2 In an ionic compound the name of metallic element is written first and –ide is added to the nonmetal.

Chemical Equations A chemical equation is a representation of a chemical reaction in which the reactants and products are expressed as formulas. Reactant + Reactant → Product Reactants are the substances that undergo the change. Products are the new substances formed. C + O 2 → CO 2 Reactants → Product

2 + 3 = 5 Antoine Lavoiser determined that the mass of the products is always equal to the mass of the reactants. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. In order to show mass is conserved an chemical equation must be balanced.

Balanced Equations Equations need to be balanced. –Matter can not be created nor destroyed. This means there needs to be the same number of atoms on each side of the equation. –The equation below is balanced S O 2 →8 SO 3 –Subscript – The small numbers to the lower right of chemical symbols. Subscripts represent the number of atoms of each element in the molecule –Coefficients - The large numbers in front of chemical formulas. Coefficients represent the number of molecules of the substance in the reaction

How Many Atoms? Using coefficients and subscripts to count atoms in equations: –number of atoms = coefficient X subscript Example: How many of each type of atom are represented by: 2Al 2 (SO 4 ) 3 # of Al atoms = 2 X 2 = 4 # of S atoms = 2 X 1 X 3 = 6 # of O atoms = 2 X 4 X 3 = 24

Types of Reactions A synthesis reaction is a reaction in which two or more substances form a single substance. (Simple → Complex) A + B → AB A decomposition reaction is when a compound breaks down into two or more simpler substances. (Complex → Simple) AB → A + B

Types of Reactions II Single Displacement reaction in which one element takes the place of another element in a compound. (One person breaks up a couple and goes out with one of them. A + BC→ AC + B Double Displacement reaction is one in which two different compounds exchange positive ions and form two new compounds. AB + CD → AD + CB

Energy Changes Chemical energy is the energy stored in the chemical bonds of a substance. Breaking a chemical bond requires energy. During a chemical reaction, energy is either gained or released.

Conservation of Energy The law of conservation of energy states neither mass or energy can be created or destroyed in a chemical reaction. Energy can be transferred from one object to another.

In/Out Exothermic Reaction – the energy released as the products form is greater than the energy required to break the bonds. –Exo = Exit Examples: –Fire –Digestion –Hand warmers Endothermic Reaction – More energy is required to break the bonds in the reactants than is released by the formation of the products. –Endo = Enters Examples: –Alka-seltzer –Ice packets –Camera

EndothermicExothermic

Reaction Rate Reaction rates tell you how fast a reaction is going. Temperature – Increasing the temperature generally increases the reaction rate. Stirring – Increases the exposure of the reactant increases the reaction rate. Surface Area – Increasing surface area increases the reaction rate. Concentration – Increasing concentration increases the reaction rate. Catalyst – A catalyst is a substance that affects the reaction rate with out being used up in the reaction.

Get Started Many reactions occur at room temperature, but some need a “little help” to get started. You may need to add heat (Lighting a candle) or pressure (Diesel engine) to start the reaction. Activation Energy – Energy needed to start a reaction. This is represented by an activation curve on a graph.

Reaction Rate & Catalyst You can speed up a reaction (rxn) rate by: –Increasing Temperature –Increase Surface Area –Increase Concentration –Using a catalyst A catalyst is a substance that speeds up a reaction without being part of the reaction. A catalyst works by decreasing the amount of activation energy required for the reaction to occur. Remember, reactions require activation energy, a catalyst simply lowers the amount of energy.

Catalyst Continued A catalyst is neither a reactant or a product in the reaction. When you finish the reaction all the catalyst is still there – none was used or changed in the reaction. It is shown above the yield sign.

Inhibit An inhibitor is a substance that slows down or stopping a reaction may sometimes be useful. Examples: –Penicillin –Preservatives –Fungicide