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Chemical reactions

2016 Massachusetts Science and Technology/Engineering Standards
HS-PS1-2. Use the periodic table model to predict and design simple reactions that result in two main classes of binary compounds, ionic and molecular. Develop an explanation based on given observational data and the electronegativity model about the relative strengths of ionic or covalent bonds.
• Simple reactions include synthesis (combination), decomposition, single displacement,
double displacement, and combustion.


Chemical reactions are written as equations

Reactants = what we start with

Products = what we end up with

reactants written on the left, products on the right.

An arrow → indicates direction of the reaction.

The →  is read aloud as “forms” or “yields”.

Many reactions can go in both directions. Here we draw a double headed arrow: ⇔


Types 0f reactions

{adapted from “Types of Chemical Reaction”, Ian Guch}

1) Synthesis: Two or more simple compounds combine to form a more complicated compound.
A + B —> AB 

Ex: iron and sulfur to form iron (II) sulfide

8 Fe + S8 —> 8 FeS


2) Decomposition: The opposite of synthesis:
A complex molecule breaks down to make simpler ones.

AB →  A + B

Ex: Electrolysis of water to make oxygen and hydrogen gas

2 H2→ 2 H2 + O2

ammonium dichromate decomposing


3) Single displacement:
One element trades places with another element in a compound.

A + BC → AC + B

Ex: Mg replaces hydrogen in water to make magnesium hydroxide and hydrogen gas:

Mg + 2 H2O → Mg(OH)2 + H2



new vocab

anion – an atom that gained an e-, becoming negative

cation – an atom that lost an e- ,     becoming positive.

anion cation

4) Double displacement:
When the anions and cations of 2 different molecules switch places, forming entirely different compounds.

AB + CD → AD + CB

Ex: Lead (II) nitrate with Potassium iodide to form Lead (II) iodide and Potassium nitrate

Pb(NO3)2 + 2 KI  PbI2 + 2 KNO3


5) Acid-Base reaction

A double displacement reaction, when an acid and base react.

H+ ion in acid reacts with OH- ion in base, creating water.

Generally, the product is some ionic salt and water:

HA + BOH → H2O + BA

Example: hydrobromic acid (HBr) with sodium hydroxide:

HBr + NaOH → NaBr + H2O

Sodium hydroxide and HCl neutralize to form water and sodium salt.

hydrochloric acid +potassium hydroxide –> water + potassium chloride

HCl(aq) + KOH(aq) → H2O(l) + KCl(aq)


Sodium hydroxide and HCl neutralizing.



6) Combustion

A type of decomposition reaction: oxygen and heat are required to occur. We study the combustion reaction here.

7. ReDox (Reduction Oxidation) reactions

In some reactions, one compounds oxidizes (loses electrons), and another is reduced (gains electrons.) These are ReDox reactions. ReDoc rxns can be single or double-replacement rxns.

Further reading http://www.chemistryland.com/CHM130S/08-Equations/TypesReactions/TypesReactions.htm

Rates of chemical reactions

There are several ways we can influence the rate of chemical reactions.

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