Chemical Reactions

Chemical Reactions

What do you notice about the two pictures?

A. Chemical reactions are changes of one or more

substances into one or more new substances.

Reactants Products(produce)

1. Reactants are the substances that combine or change.

2. Products are the result of the reaction.

B. The law of conservation of mass – in a chemical reaction, matter is

not created or destroyed.The mass of the

candles and oxygen before burning . . .

Is exactly equal to

The mass of the remaining candle and gaseous products.

According to the law of conservation of mass, what is

known about mass in the reaction?

In the reaction above, how much zinc was present in the zinc carbonate? _

64g 200g 160g ?= 264g

=264g – 160g =

104g

Practice conservation Problems130 g NaN3 + 88.0g N2 + ?g Na

223.4 g Fe + 128.0 g O2 ?g Fe2O3

?g NH4NO3 18.0 g H2O + 22.0 g N2O

44.0 g C3H8 + 112.0 g O2 132.0 g CO2 + ?g H2O

42g

351.4g

40.0g

24.0g

C. Chemical equations use chemical formulas and symbols

to describe a chemical reaction.

1. Chemical Symbols represent the elements.

2. Chemical formulas express the ratio of elements in compounds.

3. Subscripts – numbers representing the atoms of an element in a compound.

4. Parenthesis – used to represent polyatomic ions within a compound.

Table 1 Symbols Used In Chemical Equations

produces, yields, or forms.

+ plus

(s) Solid

(l) Liquid

(g) Gas

(aq) Aqueous – dissolved in water

Analyze a Chemical Equation

Let’s take close look at a chemical equation.

2Na + Cl2 2NaCl

First, you should notice that the equation is broken into two parts with an arrow.

The arrow represents the chemical reaction.

The “ingredients” on the left side of the equation are called the

Reactants

Reactants

The “results” on the right side of the equation are called the

Products

Products

Analyze a Chemical Equation

Identify the reactants and the products in the following reactions.

N2 + 3H2 2NH3

BaCO3 BaO + CO2

MnO2 + 4HCl MnCl2 + Cl2 + 2H2O

Reactants Products

Coefficients – numbers in equations which represent the number of units of each substance in a reaction.

Subscripts – number that represent the ratio of atoms in the formula of that substance.

2Na + Cl2 2NaCl

You probably noticed the numbers associated with chemical formulas for the

reactants and the products.

The large numbers in front of the formulas are called Coefficients.

(Coefficients of one are not written out.)

Coefficients

Coefficients tell the number of atoms or molecules that are required for the reaction.

Coefficients are the amounts of the reactants and products.

The coefficients in this equation tell us…Two sodium atoms will react with One chlorine molecule to produce Two molecules of sodium chloride.

The small numbers following the chemical symbols are

called Subscripts.(Subscripts of one are not written out.)

Subscripts

Subscripts tell the number of atoms that are bonded together to makea single molecule of a substance.

They determine the substance itself.

The subscripts in this equation tell us… Na is in elemental form.

2 Cl atoms are bonded to form a molecule of chlorine gas.1 Na is bonded to 1 Cl to make a molecule of sodium chloride.

Analyze a Chemical Reaction with it’s Equation

NiCl2(aq) + 2NaOH(aq) Ni(OH)2(s) + 2NaCl(aq)

• A white precipitate of nickel(II) hydroxide forms when sodium hydroxide is added to a green solution of nickel(II) chloride.

• Sodium chloride, the other product formed is in the solution.

NiCl2 + 2NaOH Ni(OH)2 + 2NaCl

First, you should notice that the equation is broken into two parts with an arrow.

The arrow represents the chemical reaction.

The “ingredients” on the left side of the equation are called the

Reactants

Reactants

The “results” on the right side of the equation are called the

Products

Products

Let’s take another look at that chemical reaction.

NiCl2(aq) + 2NaOH(aq) Ni(OH)2(s) + 2NaCl(aq)

You probably noticed the numbers associated with chemical formulas for the

reactants and the products.

The large numbers in front of the formulas are called Coefficients.

(Coefficients of one are not written out.)

Coefficients

Coefficients tell the number of atoms or molecules that are required for the reaction.

Coefficients are the amounts of the reactants and products.

The coefficients in this equation tell us… One molecule of nickle chloride react with

Two molecules of sodium hydroxide to produce

One molecule of nickle hydroxide and Two molecules of sodium chloride.

The small numbers following the chemical symbols are

called Subscripts.

(Subscripts of one are not written out.)

Subscripts

Subscripts tell the number of atoms that are bonded together to make one unit of the

substance. In this equation 2 chloride Ions in NiCl2

and 2 OH- Ions in Ni(OH)2

Let’s take another look at that chemical reaction.

Back

Analyze a Chemical Equation

What do the chemical equations below tell you?

N2 (g) + 3H2 (g) 2NH3 (NH3 is Ammonia)

2MgO + Si 2Mg + SiO2

1 molecule of nitrogen reacts with 3 molecules of hydrogen to form 2 molecules of ammonia. Nitrogen and hydrogen molecules are each formed from 2 atoms bonded together. Ammonia is formed from 1 N atom bonded to 3 H atoms.

2 molecules of Magnesium Oxide (each with 1Mg bonded to 1 O) react with 1 atom of silicon to form 2 atoms of magnesium and 1 molecule of silicon dioxide (having 1 Si bonded to 2 O)