Chemical Reactions & Equations

1. Chemical Reactions & Equations Chemistry Fall 2013

2. 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

3. 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.

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

5. 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 + O2 → CaO • Be sure to use the correct symbols and formulas for reactants and products.

6. 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.

7. 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.

8. 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.

9. Balancing Chemical Equations cont… • Ex: Ca + O2 → CaO • Ex: Na + Cl2 → NaCl • Ex: CH4 + O2 → CO2 + H2O

10. 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

11. 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

12. 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

13. 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

14. 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