CHEMICAL REACTIONS

Chapters 7 and 8

What is a chemical reaction A chemical reaction (a.k.a, chemical

change) is a process in which one or more substances change into new substances.

This happens when atoms or molecules of the starting substances collide, bonds break, and new bonds are formed. ATOMS ARE NOT CREATED NOR DESTROYED. They are merely rearranged.

Evidence of a chemical reaction

Not every chemical reaction is accompanied by observable evidence, however many reactions are. Some signs that a chemical reaction has taken place are:

temperature changeproduction of heat and lightcolor changeproduction of an odorFormation of a precipitate (solid forming)Formation of bubbles

Representing chemical reactions

Chemical reactions are represented with “equations”. All equations have reactants on the left and products on the right, with an arrow separating the two. This arrow says “reacts to produce” or “yields”.

Ex: reactant 1 + reactant 2 product 1 + product 2

Word equations – use names of reactants and products only

Ex: iron + chlorine iron (III) chloride

Skeleton Equations – use correct chemical formulas for the reactants and products

Ex: Fe + Cl2 FeCl3

Balanced Chemical Equations – use correct chemical formulas for the reactants and products AND coefficients to reflect the law of conservation of mass

Ex: 2Fe + 3Cl2 2FeCl3

Other symbols often found in chemical equations (s) = solid (l) = liquid (g) = gas (aq) = aqueous (dissolved in water) Δ (over the arrow) = reactants must be

heated ↑(after a product) = produced as a gas ↓(after a product) = produced as a solid

precipitate = reactants and products are in equilibrium

Some examples Solid iron reacts with chlorine gas to produce

solid iron (III) chloride.

2Fe(s) + 3Cl2(g) 2FeCl3(s)

Aqueous solutions of sodium carbonate and calcium chloride react to produce aqueous sodium chloride and solid calcium carbonate.

Na2CO3(aq) + CaCl2(aq) 2NaCl(aq) + CaCO3(s)