(BCR) Types of Chemical Reactions Lesson Cont'd

Types of Chemical Reactions 

Single Replacement Reactions

A single replacement reaction occurs when one ion takes the place of another in a single compound. This type of reaction has the general equation:

A+ BC → AC + BC

Do you see how A has replaced B in the compound? The compound BC has become the compound AC.

An example of a single replacement reaction occurs when zinc (Zn) reacts with hydrochloric acid (HCl). Hydrogen gas (H₂) is released and zinc chloride remains. The equation for the reaction is:

Zn + 2HCl → H₂ + ZnCl₂

Double Replacement Reactions

A double replacement reaction occurs when two compounds exchange ions. This produces two new compounds. A double replacement reaction can be represented by the general equation:

AB + CD → AD + CB

Do you see how B and D have changed places? Both reactant compounds have changed.

An example of a double replacement reaction is a colorless solution lead (II) nitrate (Pb(NO₃)₂) reacting with a colorless solution of potassium iodide (KI). In this reaction, a yellow solid, lead (II) iodide (PbI₂) is instantly produced as seen in the picture below.  This reaction is represented by the equation:

Pb(No₃)₂ + 2 Kl → PbI₂ + 2 KNO₃

Another example of a very important double replacement reaction is when an acid and a base combine.  When an acid and a base react, the reaction is called a neutralization reaction.   That's because the reaction produces neutral products. Water is always one product, and a salt is also produced.      

Let's see how a neutralization reaction produces both water and a salt, using as an example the reaction between solutions of sodium hydroxide and hydrochloric acid. The overall equation for this reaction is:

NaOH + HCl → H₂O + NaCl

Remember from a previous module that acids start with Hydrogen and bases typically end in Hydroxide (OH). Now let's break this reaction down into two parts to see how each product forms during double replacement.

• Positive hydrogen ions from HCl and negative hydroxide ions from NaOH combine to form water. This part of the reaction can be represented by the equation:

H⁺ + OH^- → HOH which is H₂O

• Positive sodium ions from NaOH and negative chloride ions from HCl combine to form the salt sodium chloride (NaCl), commonly called table salt. This part of the reaction can be represented by the equation:

Na⁺ + Cl^- → NaCl

Neutralization reactions not only occur in chemistry class with chemicals but also any time you experience indigestion. Antacid tablets contain the base calcium carbonate (CaCO₃). The base reacts with hydrochloric acid (HCl) in the stomach. The reaction neutralizes the acid to relieve acid indigestion. The products of this reaction are water and the salt calcium chloride (CaCl₂). Carbon dioxide (CO₂) is also produced. The reaction is represented by the following chemical equation:

CACO₃ + 2HCl → H₂O + CaCl₂ + CO₂

Summary

To review the 4 types of reactions discussed, look at the table below.

Sometimes analogies and pictures help information make sense. View the presentation below for an easy way to remember the 4 types of reactions.

Energy In and Energy Out

All chemical reactions involve energy. Energy is needed to break bonds in reactants. These bonds may be very strong. Energy is released when new bonds form in the products. That's because the atoms now have a more stable arrangement of electrons. Which energy is greater: that needed for breaking bonds in reactants or that released by bonds forming in products? It depends on the type of reaction. When it comes to energy, chemical reactions may be endothermic or exothermic.

In an endothermic reaction, it takes more energy to break bonds in the reactants than is released when new bonds form in the products. The word "endothermic" literally means "taking in heat." A constant input of energy, often in the form of heat, is needed in an endothermic reaction. Not enough energy is released when products form to break more bonds in the reactants. Additional energy is needed to keep the reaction going. The general equation for an endothermic reaction is:

Reactants + Energy → Product

In an exothermic reaction, it takes less energy to break bonds in the reactants than is released when new bonds form in the products. The word "exothermic" literally means "turning out heat." Energy, often in the form of heat, is released as an exothermic reaction occurs. The general equation for an exothermic reaction is:

Reactants → Product + Energy

If the energy is released as heat, an exothermic reaction results in a rise in temperature.

Conservation of Energy

Whether a reaction absorbs energy or releases energy, there is no overall change in the amount of energy. Energy cannot be created or destroyed. This is the law of conservation of energy. Energy can change form — for example, from electricity to light — but the same amount of energy always remains.

If energy cannot be destroyed, what happens to the energy that is absorbed in an endothermic reaction? The energy is stored in the chemical bonds of the products. This form of energy is called chemical energy. In an endothermic reaction, the products have more stored chemical energy than the reactants. In an exothermic reaction, the opposite is true. The products have less stored chemical energy than the reactants. The excess energy in the reactants is released to the surroundings when the reaction occurs. The graphs below show the chemical energy of reactants and products in each type of reaction.

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