Chemical Reactions & Equations

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 reactionProducts- 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 equationEx: calcium + oxygen → calcium oxide

Arrow indicates the direction of the reaction

Formula equationsEx: Ca + O2 → 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 + O2 → CaO

Ex: Na + Cl2 → NaCl

Ex: CH4 + O2 → CO2 + H2O

Classifying Chemical Reactions Some reactions combine elements to form

compoundsSome reactions break down compounds into

elementsIn some reactions, one element replaces

another in a compound. There are 4 general types of chemical

reactions:Direct combinationDecompositionSingle-replacementDouble-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