Chemical Reactions

Chemical ReactionsChapter 10

• When a substance undergoes a chemical change, it takes part in a chemical reaction.

Recognizing Chemical Reactions

Chemical Reactions and Equations: Basic Concepts

• After it reacts, it no longer has the same chemical identity.

Topic 8

Writing Chemical Equations

• A substance that undergoes a reaction is called a reactant.

• When reactants undergo a chemical change, each new substance formed is called a product.

Chemical Reactions and Equations: Basic ConceptsTopic 8

Word Equations

• The simplest way to represent a reaction is by using words to describe all the reactants and products, with an arrow placed between them to represent change.

• Reactants are placed to the left of the arrow, and products are placed to the right.

• Plus signs are used to separate reactants and also to separate products.


Chemical Equations

• Word equations describe reactants and products, but they are long and awkward and do not adequately identify the substances involved.

• Word equations can be converted into chemical equations by substituting chemical formulas for the names of compounds and elements.


Chemical Equations

• The equation for the reaction of vinegar and baking soda can be written using the chemical formulas of the reactants and products.

• By examining a chemical equation, you can determine exactly what elements make up the substances that react and form.


Chemical Equations

• It may also be important to know the physical state of each reactant and product.

• How can we indicate the bubbles we see during this reaction are CO2?

• Symbols in the parentheses are put after formulas to indicate the state of the substance.

• Solids, liquids, gases, and water (aqueous) solutions are indicated by the symbols (s), (l), (g), and (aq).


Chemical Equations

• The following equation shows these symbols added to the equation for the reaction of vinegar and baking soda.


Balancing Chemical Equations

• The mass of the products is always the same as the mass of the reactants that react to form them.

• The law of conservation of mass summarizes these findings.

• Matter is neither created nor destroyed during a chemical reaction.



• Remember that atoms don’t change in a chemical reaction; they just rearrange.

• The number and kinds of atoms present in the reactants of a chemical reaction are the same as those present in the products.

• When stated this way, it becomes the law of conservation of atoms.



• For a chemical equation to accurately represent a reaction, the same number of each kind of atom must be on the left side of the arrow as are on the right side.

• If an equation follows the law of conservation of atoms, it is said to be balanced.


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• The easiest way to count atoms is to practice—first with a simple reaction and then with some that are more complex.

• For example, consider the equation that represents breaking down carbonic acid into water and carbon dioxide.



• Because a subscript after the symbol for an element represents how many atoms of that element are found in a compound, you can see that there are two hydrogen, one carbon, and three oxygen.

• All of the atoms in the reactants are the same as those found in the products.


Balancing an Equation

• The balanced equation tells us that when sodium hydroxide and carbon dioxide react, two units of sodium hydroxide react with each molecule of carbon dioxide to form one unit of sodium carbonate and one molecule of water.


Major Classes of Reactions

• In one type of reaction, two substances—either elements or compounds—combine to form a compound.

• Whenever two or more substances combine to form a single product, the reaction is called a synthesis reaction.


A Synthesis Reaction

• When iron rusts, iron metal and oxygen gas combine to form one new substance, iron(III) oxide.

• The balanced equation for this synthesis reaction shows that there is more than one reactant but only one product.


A Synthesis Reaction



• In a decomposition reaction, a compound breaks down into two or more simpler substances.

• The compound may break down into individual elements, such as when mercury(II) oxide decomposes into mercury and oxygen.


A Decomposition Reaction

• When ammonium nitrate is heated to a high temperature, it explosively breaks down into dinitrogen monoxide and water.

• The decomposition reaction taking place is represented by a balanced equation that shows one reactant and more than one product.


A Decomposition Reaction



• In a single-displacement reaction, one element takes the place of another in a compound.

• The element can replace the first part of a compound, or it can replace the last part of a compound.


Single Displacement

• If an iron nail is placed into an aqueous solution of copper(II) sulfate, the iron displaces the copper ions in solution, and copper metal forms on the nail.


Single Displacement



• In double-displacement reactions, the positive portions of two ionic compounds are interchanged.

• For a double-displacement reaction to take place, at least one of the products must be a precipitate or water.


Double Displacement

• When clear aqueous solutions of lead(II) nitrate and potassium iodine are mixed, a double-displacement reaction takes place and a yellow solid appears in the mixture.

• This solid is lead(II) iodine, and it precipitates out because it is insoluble in water, unlike the two reactants and the other product.


Double Displacement


Combustion

• When welding is done with an acetylene torch, acetylene combines with oxygen to form carbon dioxide and water.

• This combustion reaction is exothermic, and enough energy is released to melt metal.


Combustion


Basic Assessment Questions

decomposition; already balanced

Answer 3b

Question 3b Topic 8

Basic Assessment Questions

synthesis;

Answer 3a

Question 3a Topic 8

Chemical Reactions

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