Chemical Reactions

Chemical Reactions

Science Enrichment 3

What is a Chemical Reaction?

• It is a chemical change in which one or more substances are destroyed and one or more new substances are created

• Before: H₂ gas and O₂ gas

• After: H₂O (liquid)

Chemical Reactions

• A chemical change takes place when chlorophyll breaks down into a new substance. This is an example of a Chemical Reaction.

• A chemical reaction is a process in which one or more substances change to make on or more new substances. The chemical and physical properties of the new substances differ from those of the original substances.

• More examples: Baking of bread (yeast), metal rusting, burning of wood

Signs of a Chemical Reaction

• Formation of a gas (bubbling or an odor) other than boiling.

• Forming of a precipitate (a new solid forms) from the reaction of two aqueous solutions.

• Evolution of light or heat

• Temperature change (increase or decrease) to the surroundings.

• Color change

Parts of a Chemical Reaction

• Reactants → Products

• Reactants: Substances that are destroyed by the chemical change (bonds break).

• Products: Substances created by the chemical change (new bonds form).

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Word Equations

• Statements that indicate the reactants and products in a chemical reaction.

• Ex. Iron (s) + chlorine (g) → iron (III) chloride (s)

• This is read as:

  “Solid iron and chlorine gas react (combine) to produce solid iron (III) chloride”

Translating Word Equations to Skeleton Equations

• A skeleton equation uses chemical formulas rather than words to identify the reactants and products of a chemical reaction.

• The word equation

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

• The skeleton equation

  Fe(s) + Cl₂(g) → FeCl₃ (s)

  
  

• A skeleton equation is not yet “balanced” by coefficients!

  
  

Conservation of Mass

During a chemical reaction, atoms are neither created nor destroyed (Conservation of Mass).

Hydrogen and oxygen gas react to form water:

H_{2 (g)} + O_{2 (g)} → H₂O _(l)

Conservation of Mass

H_{2 (g)} + O_{2 (g)} → H₂O _(l)

What is wrong with this equation above? Doesn’t it appear that one oxygen atom “went missing”?

Conservation of Mass

H_{2 (g)} + O_{2 (g)} → H₂O _(l)

What is wrong with this equation above? Doesn’t it appear that one oxygen atom “went missing”?

According to conservation of mass, the proper way to write this reaction is:

2H_{2 (g)} + 1O_{2 (g)} → 2H₂O _(l)

The red coefficients represent the # of molecules (or the # of moles) of each reactant or product.

Not All Properties are Conserved

During Chemical Reactions!

Types of Chemical Reactions

There are 5 basic types

• Synthesis (Combination)

• Decomposition

• Single Replacement

• Double Replacement

• Combustion

Synthesis Reaction (combination)

Two or more simple substances (the reactants) combine to form a more complex substance (the product). 

Ex: 2Mg + O2 → 2MgO

Decomposition

A more complex substance (the reactant) breaks down into two or more simple parts (products).

Synthesis and decomposition reactions are opposites.

Ex: 2H₂O → 2H₂ + O₂

Single Replacement

A single uncombined element replaces another element in an ionic compound.

There are two reactants and two products.

Ex: Zn + CuSO₄ → ZnSO₄ + Cu

Double Replacement

Parts of two aqueous ionic compounds switch places to form two new compounds.

There are two reactants and two products.

Example:

AgNO₃ + NaCl → AgCl + NaNO_3

Combustion

• All involve oxygen (O2) as a reactant, combining with another substance

• All combustion reactions are are exothermic

• Complete combustion of a hydrocarbon always produces CO2 and H2O

• Incomplete combustion of a hydrocarbon will produce CO and possibly C (black carbon soot) as well

Combustion (cont.)

Ex: CH₄ + 2O₂ => CO₂ + 2H₂O (complete combustion – blue flame)

Ex: CH₄ + 1.5O₂ => CO + 2H₂O (incomplete combustion – yellow flame)

Ex: CH₄ + O₂ => C + 2H₂O (incomplete combustion – yellow flame, soot)

Multiple Choice

When a new substance forms as a result of a chemical reaction, what type of change has occurred?

• Chemical Change

• Physical Change

Multiple Choice

Which is NOT an indication of a chemical change?

• Temperature Change

• Shape Change

• Formation of a precipitate

• Production of light

Multiple Choice

What is the left part of a chemical equation called?

2H_{2 +} O₂ --> 2H₂O

• Reactants

• Products

• Yields

• Chemical Equation

Multiple Choice

What is the right part of a chemical equation called...

2H_{2 +} O₂ --> 2H₂O

• Reactants

• Products

• Yields

• Chemical Equation

Multiple Choice

What does the number 6 represent in 6H₂O?

• subscript

• coefficient

• formula

• yield

Multiple Choice

What does the number ₂ represent in 6H₂O?

• subscript

• coefficient

• formula

• yield

Multiple Choice

What reaction has the following general formula:

A + B → AB

• Synthesis

• Decomposition

• Single Displacement

• Double Displacement

• Combustion

Multiple Choice

What type of reaction is the following:

2H₂+O₂ → 2H₂O

• Synthesis

• Decomposition

• Single Replacement

• Double Replacement

• Combustion

Multiple Choice

What is the general reaction scheme for a decomposition reaction?

• A + B → AB

• AB → A + B

• A + CD → C + AD

• AB + CD → CB + AD

• C_x + H_y + O₂ → CO₂ + H₂O

Multiple Choice

What reaction has the following general formula:

A + CD → C + AD?

• Synthesis

• Decomposition

• Single Displacement

• Double Displacement

• Combustion

Multiple Choice

What reaction has the following general formula:

C_x + H_y + O₂ → CO₂ + H₂O?

• Synthesis

• Decomposition

• Single Displacement

• Double Displacement

• Combustion