Megan Brooks
A chemical reaction is a process where one or more substances (reactants) change into one or more different substances (products) by rearranging atoms. This involves breaking and forming chemical bonds. There are several types of chemical reactions, including combination reactions, decomposition reactions, displacement reactions, and double displacement reactions. These reactions can be exothermic, meaning they produce heat, or endothermic, meaning they absorb light. Chemical reactions are integral to many industries and customs and are constantly occurring in our surroundings, such as in the rusting of iron or the fermentation of wine.
| Characteristics | Values |
|---|---|
| Definition | A chemical reaction is a process where one or more substances (reactants) change into one or more different substances (products) by rearrangement of atoms. |
| Types | Combination reaction, Decomposition reaction, Displacement reaction, Double displacement reaction, Combustion reaction, Synthesis reaction, Neutralization reaction, Redox reaction, Precipitation, Acid-base reaction |
| Examples | Rusting of iron, Pottery, Fermentation of wine, Burning candle, Baking a cake |
| Identification | Examine the number and kind of reactants and products |
| Equation | A mathematical statement that symbolizes the product formation from reactants |
| Law of Conservation of Mass | The number of atoms of each element must be the same in both the reactants and the products |
| Molarity | A unit of concentration. Molarity = # of moles / # of liters of solution |
| Molar Volume of a Gas | At STP, one mole of most gases has a volume of 22.4 L |
| Rate of Reaction | How many reactions happen per second |
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What You'll Learn
Physical vs. chemical changes
A physical change occurs when a substance's size, shape, form, texture, or state (solid, liquid, or gas) changes without altering its chemical composition or identity. For example, ice melting into water is a physical change because the chemical composition of water (H₂O) remains the same, but its state transitions from solid to liquid. Physical changes are often reversible; the melted ice can be frozen again to return to its original state. Other examples of physical changes include cutting or bending an object, dissolving something in water, and boiling water.
On the other hand, a chemical change occurs when a substance's composition is altered through the breaking and forming of chemical bonds, resulting in the creation of a new substance. For instance, when a cake is baked, its ingredients undergo chemical changes, forming new substances such as carbon dioxide and flavour compounds. Chemical changes are typically irreversible; the cake cannot be returned to its original ingredients. Other examples of chemical changes include the rusting of iron, pottery, and the fermentation of wine.
It is important to note that the distinction between physical and chemical changes is based on whether there is a change in the composition of the substances involved. While physical changes affect the appearance, smell, or display of a substance, they do not alter its chemical makeup. In contrast, chemical changes result in the formation of entirely new substances with different identities from the original reactants.
In some cases, it can be challenging to determine whether a change is physical or chemical. For example, dissolving table salt (sodium chloride) in water involves the dissociation of the salt into positive sodium ions and negative chloride ions, which seems like a chemical change. However, when the water evaporates, the salt ions recombine to form the same salt arrangement, indicating that its composition was not fundamentally altered, and thus, this process is considered a physical change.
Understanding the difference between physical and chemical changes is essential in chemistry, as it helps us grasp the underlying principles of matter and its interactions.
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Combination reactions
Transition metals are capable of adopting multiple positive charges within their ionic compounds, allowing them to form different products in combination reactions. Iron, for instance, can react with oxygen to form both iron (II) oxide and iron (III) oxide. On the other hand, alkali metals like potassium are very reactive and must be stored under oil to prevent them from reacting with air.
In summary, combination reactions are a fundamental aspect of chemistry, involving the combination of two or more substances to form a new product. They play a significant role in various natural and industrial processes, contributing to our understanding of the properties and behaviours of matter.
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Decomposition reactions
Antoine Lavoisier, widely known as the "father of modern chemistry," was among the first to study chemical reactions in detail. Lavoisier reacted mercury with oxygen to form mercuric oxide as part of his studies on the composition of the atmosphere. He then demonstrated that the decomposition of mercuric oxide produced mercury and oxygen.
Thermal Decomposition Reaction: This type of decomposition reaction is activated by thermal energy, or heat. For example, when heated, calcium carbonate decomposes into calcium oxide and carbon dioxide:
\[ \co: 10,14> \ce{CaCO_3} \left( s \right) \rightarrow \ce{CaO} \left( s \right) + \ce{CO_2} \left( g \right) \]
Photodecomposition Reaction: In this type of decomposition reaction, the reactant absorbs energy from photons and breaks down into its constituents. An example is the decomposition of ozone into dioxygen and an oxygen radical:
\[ \co: 5> \ce{O3} + h\nu \rightarrow \ce{O2} + O \]
Electrolytic Decomposition Reaction: Electrolytic decomposition reactions are initiated by electrical energy. Passing an electric current through pure water, a binary compound composed of hydrogen and oxygen, results in its decomposition into its elements:
\[ \co: 9,15> \ce{H_2O} \left( l \right) \overset{\text{elec}}{\rightarrow} \ce{H_2} \left( g \right) + \ce{O_2} \left( g \right) \]
Double Decomposition Reaction: This type of decomposition reaction involves two constituent reactants interchanging positive and negative ions to form two new compounds. An example is the reaction between sodium and chloride (NaCl) to form sodium and chloride ions:
\[ \co: 4,6> \ce{NaCl} \rightarrow \ce{Na^+} + \ce{Cl^-} \]
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Displacement reactions
The general form of a displacement reaction can be written as:
> A + BC -> B + AC
In this equation, A is more reactive than C, resulting in the displacement of C by A.
Metal Displacement Reactions
In metal displacement reactions, a highly reactive metal replaces a less reactive metal in a compound, forming a new salt. For instance, zinc is more reactive than copper and can displace copper ions from a copper sulfate solution. The word equation for this reaction is:
> Zinc + Copper Sulfate -> Zinc Sulfate + Copper
Another example involves magnesium and copper oxide. The highly reactive magnesium metal displaces the less reactive copper from solid copper oxide, leading to the formation of magnesium oxide and copper. This particular reaction is violent and should be handled with caution.
Halogen Displacement Reaction
In halogen displacement reactions, a less reactive halide is replaced by a more reactive halogen in a compound. During this process, the more reactive halogen atoms oxidize the less reactive halide ions, causing them to lose electrons and form halogen atoms. Subsequently, these halogen atoms gain electrons to form halide ions, resulting in a reduction.
Hydrogen Displacement Reaction
In hydrogen displacement reactions, the hydrogen in an acid is replaced by an active metal. Many metals react rapidly with acids and can replace hydrogen in water. The products of this reaction are metal hydroxide and hydrogen gas.
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Double displacement reactions
A double displacement reaction, also known as a double replacement reaction, is a chemical reaction in which two reactants exchange ions to form two new compounds. The general form of a double displacement reaction is:
\[ \ce{AB} + \ce{CD} \rightarrow \ce{AD} + \ce{CB} \]
In this reaction, A and C are positively charged cations, while B and D are negatively charged anions. Double displacement reactions generally occur in aqueous solutions, and one of the products is usually a solid precipitate, a gas, or a molecular compound such as water. For example, when aqueous hydrochloric acid is reacted with aqueous sodium hydroxide, the products are aqueous sodium chloride and water:
\[ \ce{HCl} \left( aq \right) + \ce{NaOH} \left( aq \right) \rightarrow \ce{NaCl} \left( aq \right) + \ce{H_2O} \left( l \right) \]
Another example of a double displacement reaction is the reaction between silver nitrate and sodium chloride. Silver trades its nitrate ion for the sodium's chloride ion, resulting in the formation of silver chloride and sodium nitrate:
\[ \ce{AgNO3} + \ce{NaCl} \rightarrow \ce{AgCl} + \ce{NaNO3} \]
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Frequently asked questions
A chemical reaction is a process where two or more molecules interact to form a new product. This involves breaking and forming bonds within the reactant molecules to create a new substance.
Chemical reactions are all around us. For example, the rusting of iron, the fermentation of wine, and the metabolism of food in our bodies.
A physical change occurs when the size, shape, or form of a substance changes without altering its chemical identity. For example, when ice melts into water, the chemical composition remains the same (H₂O), but its state transitions from solid to liquid.
The four main types of chemical reactions are combination, decomposition, displacement, and double displacement reactions. Other types include combustion, synthesis, and acid-base reactions.
A combustion reaction occurs when a substance reacts with oxygen gas, releasing energy in the form of light and heat. These reactions are exothermic, meaning they produce heat.
