Chemical Equations a chemical reaction is the process by which 1 or more substances are changed into 1 or more different substances they are represented by an equation using symbols and formulas they show what change takes place, and the relative amounts of the various elements present in the compounds involved

The starting substances are called the reactants, and the ending substances are called the products the arrow means “yields” or makes or produces Equations are like recipes they tell you what ingredients you have and how much of each ingredient

they can also show special instructions –letters in parentheses indicate the state of that substance ex’s. (g) = gas, (l) = liquid, (cr) = solid, (aq) = aqueous which means the substance is dissolved in water –they can show if heat, energy or catalysts are needed –we don’t use these all of the time, only if they are given

Writing and Balancing Equations write a word equation first, then change it to the chemical equation steps –determine the reactants and the products –assemble the parts of the equation ? + ?  ? the symbols and formulas must be correct –balance the equation

To balance an equation make sure you have equal numbers of all elements on both sides of the arrow, because mass can not be lost or gained in a reaction don’t confuse subscripts with coefficients subscripts determine the substance, coefficients tell us how much of that substance we have we don’t change the subscripts, or we change the substance, rather we change or add coefficients to adjust amounts Polyatomic ions can be balanced as a unit

Types of Reactions if we understand the basic types of reactions, we can better predict the products that will form from certain reactants we will look at the 5 basic reaction types here, there are others but we are not concerned with them The 5 types are: single displacement, double displacement, decomposition, synthesis, and combustion

displacement – an element replaces another in a compound single displacement – one element is replaced by another –a single element reacts with a compound and displaces one of the elements from the compound and takes its place in the compound –We can use an Activity Series to helps us predict which elements will replace which

Activity Series –elements are arranged in order of activity with the most active on top. –An element can displace those listed below it, but not those above it

double displacement – the positive and negative ions of 2 compounds switch –parts of 2 compounds switch places –water is sometimes written HOH

Decomposition - “break down” a substance breaks down into simpler substance only one reactant

Synthesis – “put together” 2 or more substances form together to make one new substance only one product (binary or ternary)

Combustion Usually; – a hydrocarbon + oxygen -  carbon dioxide + water can be alcohols also the substances burn in air usually used to generate energy

to determine reaction type –Look at the products and the reactants : one product = synthesis one reactant = decomposition hydrocarbon + oxygen forms carbon dioxide and water = combustion 1 element + 1 compound forming a different element and a compound = single displacement 2 compounds reacting to form 2 new compounds = double displacement