Chemical

Reactions

Chemical Reactions• A chemical reaction

is the process by which a chemical change happens

• All chemical reactions are accompanied by changes in energy

Chemical

Reactions

All chemical reactions involve the conversion of starting materials, called reactants, into new substances, called products. The products have different properties than the reactants.

For example, when solid magnesium is added to a solution of hydrochloric acid, bubbles of hydrogen gas are formed as well as aqueous magnesium chloride.

Chemical Reactions

Chemical Equations

A chemical reaction is often described by writing a chemical equation.

Chemical EquationsEvery chemical equation must have:• Reactant• Product• Arrow directing reactant to product

Chemical Equations Chemical equations can be written in different ways.

Word Equation: describes the chemical equation using the names of the reactants and products.

Ex) Solid magnesium reacts with hydrochloric acid to produce magnesium chloride and hydrogen gas.

Skeleton Equation: describes the chemical reaction using the chemical formulas of the reactants and products.

word equation:magnesium + hydrochloric acid magnesium chloride + hydrogenword equation:magnesium + hydrochloric acid magnesium chloride + hydrogen

Skeleton equation:Mg(s) + HCl(aq) MgCl2(aq) + H2(g)Skeleton equation:Mg(s) + HCl(aq) MgCl2(aq) + H2(g)

Remember: Diatomic element (HOFBrINCl)

Word vs. Skeleton Equations

Writing chemical equations is similar to math equations:

Two combines with three to make five.

Word Equation:

two + three five

Skeleton Equation:

2 + 3 5

Words with + signs between reactants and

an arrow to the products

Symbols with + signs between reactants and

an arrow to the products

Chemical EquationsThe chemical formulas in a chemical

equation will often include the state of matter of each substance• (s) = solid• (l) = liquid• (g) = gas• (aq) = aqueous (substance is dissolved in water)

The Law of Conservation of MassIn a chemical reaction, the mass of the

products always equals the mass of the reactants.

In other words, the mass is conserved.

The Law of Conservation of MassIn a reaction, no atoms are destroyed and

no new atoms are produced.

Only the chemical bonds between atoms are broken and rearranged

The Law of Conservation of MassFor example:

hydrogen + oxygen water

2H2(g) + O2(g) 2H2O(l)

4.04 g + 32.00 g 36.04 g

There are equal numbers of hydrogen atoms (4) and oxygen atoms (2) on both the reactant side and product side. The mass on each side of the reaction is also equal.

Balancing Chemical EquationsChemical reactions that

show the formulas of the reactants and products but are not balanced are called skeleton equations.

Balancing Chemical Equations

To balance a chemical equation, begin by counting the number of atoms of each element in the skeleton equation

Balancing Chemical Equations• Balance by placing coefficients in front of

the chemical formulas.• Use only whole numbers as coefficients.• Check that all of the coefficients in the

equation are the lowest common factor.• NEVER CHANGE A SUBSCRIPT in a

formula to help make atoms balance!!!

Balancing Chemical EquationsTips for Balancing:• Balance complicated looking formulas

first.• Leave hydrogen for second last and

oxygen for very last.

Balancing Chemical Equations

You may be able to treat polyatomic ions as a unit.

Example: If NO3- appears in the reactants

and products of a skeleton equation, count the number of NO3

- groups rather than the number of N and O atoms separately.

Balancing Chemical EquationsExample 1:

Balance the following chemical equation:

AlBr3(s) + Cl2(g) AlCl3(s) + Br2(g)

1. Count the number of atoms in the reactants and products:

Balancing Chemical EquationsExample 1:

2AlBr3(s) + Cl2(g) AlCl3(s) + 3Br2(g)

Balance the number of bromine atoms by adding a coefficient of 2 in front of AlBr3 and a coefficient of 3 in front of Br2. Adjust your atom count:

The number of aluminum atoms is no longer equal.

Balancing Chemical EquationsExample 1:

2AlBr3(s) + Cl2(g) 2AlCl3(s) + 3Br2(g)

Balance the number of aluminum atoms by adding a coefficient of 2 in front of AlCl3. Adjust your atom count:

The number of chlorine atoms is no longer equal.

Balancing Chemical EquationsExample 1:

2AlBr3(s) + 3Cl2(g) 2AlCl3(s) + 3Br2(g)

Balance the number of chlorine atoms by adding a coefficient of 3 in front of Cl2. Adjust your atom count:

The equation is now balanced!

Balancing Chemical EquationsExample 2:

Balance the following chemical equation:

BeCl2(aq) + AgNO3(aq) Be(NO3)2(aq) + AgCl(s)

Balancing Chemical EquationsExample 3:

Balance the following chemical equation:

Mg3N2(s) + H2O(l) MgO(aq) + NH3(g)