Chemistry Fall 2013

Chemical Reactions A chemical reaction is a process in which one or more substances are converted into new substances with different physical and chemical properties. All chemical reactions involve 2 types of substances: Reactants- substances present before the chemical reaction Products- the substance or substances produced by the chemical reaction

Chemical Reactions cont… Some substances do not under go chemical reactions because they have a full set of valence electrons Chemical reactions allow substances to obtain a complete set of valence electrons.

Chemical Equations Chemical reactions are represented by sentences known as chemical equations. Chemical equations describe what happens in a chemical reaction.

Chemical Equations cont… Word equations Simplest type of chemical equation Ex: calcium + oxygen → calcium oxide Arrow indicates the direction of the reaction Formula equations Ex: Ca + O 2 → CaO Be sure to use the correct symbols and formulas for reactants and products.

Balancing Chemical Equations Remember the law of conservation of mass? Mass can be neither created or destroyed Mass is neither created or destroyed during a chemical reactions Mass of total products must equal mass of the total reactants For mass to remain constant before and after a chemical reaction, the number of atoms of each element must be the same before and after a chemical reaction.

Balancing Chemical Equations cont… When balancing a chemical equation DO NOT ever change the subscripts, as this changes the compound being used to produce the reaction or the product of the reaction. To balance a chemical equation you change the coefficients (whole numbers written before the formulas) for the reactants and products.

Balancing Chemical Equations cont… Steps to balance a chemical equation 1. Write the formula equation with correct symbols & formulas. 2. Count the number of atoms of each element on each side of the arrow. 3. Balance atoms using coefficients. 4. Check your work by counting atoms of each element.

Balancing Chemical Equations cont… Ex: Ca + O 2 → CaO Ex: Na + Cl 2 → NaCl Ex: CH 4 + O 2 → CO 2 + H 2 O

Classifying Chemical Reactions Some reactions combine elements to form compounds Some reactions break down compounds into elements In some reactions, one element replaces another in a compound. There are 4 general types of chemical reactions: Direct combination Decomposition Single-replacement Double-replacement

Classifying Chemical Reactions Direct combination reactions Two or more reactants come together to form a single product Also known as synthesis reactions A + B → AB This is the only type of reaction in which there is a single product 2 or more simple reactants form a single, more complex product

Classifying Chemical Reactions Decomposition Reactions a single compound is broken down into two or more smaller compounds or elements Reverse of a direct combination reactions There is only one reactant AB → A + B

Classifying Chemical Reactions Single- Replacement Reactions An uncombined element displaces an element that is part of a compound Reactants are always one element and one compound A + BX → AX + B BX & AX are generally ionic compounds Not all elements can replace other elements Depends on chemical reactivity Usually metals replace nonmetals or hydrogen and nonmetals replace metals

Classifying Chemical Reactions Double-replacement Reactions Atoms or ions from two different compounds replace each other Two compounds as reactants and two compounds as products AX + BY → AY + BX