January 11, 2015

Unit 4: Chemical Reactions

What is a chemical reaction?• the process by which the atoms of one or more

substances are rearranged to form different substances

• chemical change

What chemical reactions are important in your life?

Physical change: substance does not change identity

Chemical change: new substances are formed

January 11, 2015

Evidence of Chemical Reaction (or chemical change)

1. Temperature change2. Light3. Color change4. Production of gasses5. Production of solids (precipitate)6. Odor change

What evidence of a chemical reaction do you see in a fire?

What evidence of a chemical reaction do you see when you roast a marshmallow?

January 11, 2015

Representing Chemical Reactions

• As chemists, we can describe chemical reactions in different ways.> In a sentence> Word equations> Skeleton equation> Chemical equations

January 11, 2015

This arrow separates the reactants from the products (kind of like the equal sign in math)

When you have more than one reactant or product, use a plus sign

Representing Chemical Reactions

• Reactants are the starting substances of a reaction.• Products are the substances formed during the

reaction.

reactant 1 + reactant 2 product 1 + product 2

January 11, 2015

2. Word Equation

iron(s) + chlorine(g) iron (III) chloride(s)

1. In a sentence

"Solid iron and chlorine gas react to produce solid iron (III) chloride."

The letters in parentheses shows you the state of matter:

(s) = solid state

(l) = liquid state

(g) = gas state

(aq) = water solution

3. Skeleton Equation: uses chemical formulas

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

January 11, 2015

Example 1:

Write skeleton equations for these word equations.

1. hydrogen(g) + bromine(g) hydrogen bromide(g)

2. carbon monoxide(g) + oxygen(g) carbon dioxide(g)

3. potassium chlorate(s) potassium chloride(s) + oxygen(g)

HON17: Always diatomic

January 11, 2015

4. Chemical equation: Balanced equation that shows the reactants and products, and the relative amounts involved in the reaction.

> Conservation of mass (mass of reactants = mass of products)

> Need to have same # of atoms of each element in the reactants and products.

January 11, 2015

How to balance chemical equations

1. Write the unbalanced (skeleton) equation.2. Count the atoms of the elements in the reactants

and products.3. Use coefficients to make the # of atoms of each

element equal on both sides of the equation.> Coefficients:numbers written in front of a

reactant or product. Tells us the number of particles of the substance involved in the reaction.

4. Make sure coefficients are in their lowest possible ratio.

5. Check your work!

unbalanced equation: Fe(s) + Cl2(g) FeCl3(s)

chemical equation: Fe(s) + Cl2(g) FeCl3(s)

January 11, 2015

Example 2: Write the balanced chemical equation for each.1. Hydrogen gas and chlorine gas combine to form hydrochloric acid (HCl)

2. __CS2(l) + __O2(g) __CO2(g) + __SO2(g)

January 11, 2015

Example 2 (cont): Balance the following

H2(g) + Cl2(g) HCl(g)

CS2(l) + O2(g) CO2(g) + SO2(g)

January 11, 2015

Example 2 (cont): Balance the following

CaO(s) + H2O(l) Ca(OH)2(s)

NaOH(aq) + CaBr2(aq) Ca(OH)2(s) + NaBr(aq)

*When balancing equations with Polyatomic Ions that appear in the reactant and product, keep it together as one "unit" and balance.

January 11, 2015