Chemical Reactions

- Observing Chemical ChangeProperties and Changes in Matter

Matter can undergo both physical change and chemical change.

Asking Questions

What are physical properties of matter?

Physical properties are characteristics that can be observed without changing one substance into another.

What is the evidence for chemical reactions?

The evidence for chemical reactions is the formation of new substances and changes in energy.

Question AnswerProperties and Changes of Matter

- Observing Chemical Change

Chemical Reactions1. Rearrange atoms

2. Involve changes in energy

Exothermic: release energy (heat, fire, fizz, etc)

Endothermic: absorb energy (drop in temp.)

Evidence of Chemical ChangesClue Example Description

Color change Bread dough bakingChanges from white to brown

Smell Eggs rotting Smellsbad

New matter created Wood burningWoodchangestoash

Gas created Baking soda added to vinegar

Createscarbondioxidegas

Heat or Light created FireworksBurningchemicalscreate light and heat

Cut along dotted lines and paste into your science notebook.

- Observing Chemical ChangeProperties and Changes in Matter

Chemical changes occur when bonds break and new bonds form.

- Describing Chemical ReactionsDescribing Chemical Reactions

Cellular phone messages make use of symbols and abbreviations to express ideas in shorter form. Similarly, chemists often use chemical equations in place of words.

- Describing Chemical ReactionsWhat Are Chemical Equations?

Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction.

The arrow symbol is read YIELDS which means “results in” or “produces”

Chemical Equations

___ CH4 + ___ O2 ___CO2 + ___ H2O

methane and oxygen yield carbon dioxide and water

Reactants Products

The molecules that combine are called REACTANTS and the molecules that result are called PRODUCTS

- Describing Chemical Reactions

Chemical FormulasThe formula of a compound identifies the elements in the compound and the ratios in which their atoms are present.

- Describing Chemical Reactions

Conservation of MassThe principle of conservation of mass states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products.

- Describing Chemical ReactionsBalancing Chemical Equations

To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation.

Balancing Chemical Equations

Magnesium metal (Mg) reacts with oxygen gas (O2), forming magnesium oxide (MgO). To write a balanced equation for this reaction, first write the equation using the formulas of the reactants and products, then count the number of atoms of each element.

- Describing Chemical Reactions

Balancing Chemical Equations

Balancing Chemical Equations:

Balance the equation for the reaction of sodium metal (Na) with oxygen gas (O2), forming sodium oxide (Na2O).

- Describing Chemical Reactions

Balancing Chemical Equations

Balancing Chemical Equations:

Balance the equation for the reaction of tin (Sn) with chlorine gas (Cl2), forming tin chloride (SnCl2).

- Describing Chemical Reactions

- Describing Chemical Reactions

Classifying Chemical ReactionsMany chemical reactions can be classified in one of three categories: synthesis, decomposition, or replacement.

Type of Reaction

Definition Equation

Synthesis

Decomposition

Single Replacement

Double Replacement

A = Red B = Blue C = Green D = Yellow

A + B → AB

AB → A + B

AB + C → AC + B

AB + CD → AC + BD

Two or more elements or compounds combine to make a more complex

substance

Compounds break down into simpler substances

Occurs when one element replaces another one in a

compound

Occurs when different atoms in two different

compounds trade places

Chemical Reactions

Identifying Chemical Reactions

____ P + O2 → P4O10 ____ Mg + O2 → MgO

____ HgO → Hg + O2 ____ Al2O3 → Al + O2

____ Cl2 + NaBr → NaCl + Br2 ____ CuCl2 + H2S → CuS + HCl

Use the code below to classify each reaction.

S = Synthesis D = Decomposition SR = Single Replacement DR = Double Replacement

 

Building Vocabulary

- Describing Chemical Reactions

Key Terms: Examples:chemical equation A chemical equation is a short, easy way to

show a chemical reaction.

reactant A substance you have at the beginning of a reaction is a reactant.

product A new substance produced in a reaction is a product.conservation of

massConservation of mass means that during a chemical reaction, matter is not created or destroyed.open system In an open system, matter can enter from or escape to the surroundings.

Key Terms: Examples:closed system

coefficient

synthesis

decomposition

In a closed system, matter is not allowed to enter or leave.

A coefficient is a number in a chemical equation telling you how many atoms or molecules of a reactant or product take part in the reaction.Synthesis means combining two or more elements or compounds to make a more complex substance.Decomposition means breaking down compounds into simpler products.

Key Terms: Examples:

replacement Replacement is the process in which one element replaces another in a compound or two elements in different compounds trade places.

End of Section:Describing Chemical Reactions