Chemical Reactions

Chemical reactions are taking place all around you and even within you. A chemical reaction is a change in which one or more substances are converted into new substances. Describing Chemical Reactions A chemical reaction occurs when you bake a cake.

The substances that react are called reactants. The new substances produced are called products. Describing Chemical Reactions This relationship can be written as follows:

Scientists have developed a shorthand method to describe chemical reactions. Writing Equations A chemical equation is a way to describe a chemical reaction using chemical formulas and other symbols.

Balanced Equations mercury(II) oxide reaction can be written as: Notice that the number of mercury atoms is the same on both sides of the equation but that the number of oxygen atoms is not the same.

Balanced Equations One oxygen atom appears on the reactant side of the equation and two appear on the product side. According to the law of conservation of mass, one oxygen atom cannot just become two. Nor can you simply add the subscript 2 and write HgO 2 instead of HgO.

Balanced Equations The formulas in a chemical equation must accurately represent the compounds that react. Fixing this equation requires a process called balancing. The balancing process involves changing coefficients in a reaction to achieve a balanced chemical equation, which has the same number of atoms of each element on both sides of the equation.

Choosing Coefficients Finding out which coefficients to use to balance an equation is often a trial- and-error process.

Try Your Balancing Act Step 1 Write a chemical equation for the reaction using formulas and symbols. Step 2 Count the atoms in reactants and products.

Step 3 Choose coefficients that balance the equation. Remember, never change subscripts of a correct formula to balance an equation.

Step 4 Recheck the numbers of each atom on each side of the equation and adjust coefficients again if necessary.

What do the numbers to the left of the formulas for reactants and products mean? Remember that according to the law of conservation of mass, matter is neither made or lost during a chemical reaction. Atoms are rearranged. These numbers are call Coeffiecients

Coefficients, represent the number of units of each substance taking part in a reaction. Coefficients can be thought of as unit managers.

Answer : B A change in which one or more substances are converted into new substances is a __________. A) physical change B) chemical reaction C) nuclear reaction D) chemical fusion

Answer : D New substances produced by a chemical reaction are __________. A) resultants B) reactants C) coefficients D) products

Answer : A Ten grams of mercury(II) oxide react to produce 9.3 g of mercury and oxygen. What is the mass of the oxygen produced? A) 0.7 g B) 19.3 g C) 7.0 g D) 1.7 g

Answer : A __________ is a way to describe a chemical reaction using chemical formulas and symbols. A) A chemical equation B) Synthesis C) A physical law D) The law of conservation of matter

Section 3: Types of reaction There are literally millions of chemical reactions that occur every day. With all these reactions, it would be impossible to use the information without first having some type of organization. Chemists have defined five main categories of chemical reactions.

One of the easiest reactions types to recognize is a synthesis reaction. Synthesis Reaction: Two or more substances combine to form another substance. The generalized formula for this reaction type is as follows: A + B AB

The reaction in which hydrogen burns in oxygen to form water is an exampe of a synthesis reaction. 2H 2 + O 2 2H 2 O This reaction is used to power some types of rockets

LecturePLUS Timberlake 99 Combination (Synthesis) Two or more elements or simple compounds combine to form (synthesize) one product A + B AB 2Mg + O 2 2MgO 2Na + Cl 2 2NaCl SO 3 + H 2 OH 2 SO 4

Decomposition reactions A decomposition reaction is just the reverse of a synthesis. Instead of two substances combining together to for a third, a decomposition reaction occurs when one substance breaks down, or decomposes, into two or more substances.

The general formula for this type of reaction can be expressed as follows: AB A + B Most decomposition reactions require the use of light or electricity. 2H 2 O (l) 2H 2 (g) + O 2 (g)

LecturePLUS Timberlake 99 Decomposition One substance is broken down (split) into two or more simpler substances. ABA + B 2HgO2Hg + O 2 2KClO 3 2KCl + 3 O 2

LecturePLUS Timberlake 99 Learning Check R1 Classify the following reactions as 1) synthasis or 2) decomposition: ___A. H 2 + Br 2 2HBr ___B. Al 2 (CO 3 ) 3 Al 2 O 3 + 3CO 2 ___C. 4 Al + 3C Al 4 C 3

Single Displacement When one element replaces another element in a compound, it is called a Single-Displacement Reaction. Single displacement reactions are described by the general equation A + BC AC + B

Here you can see the atom A displaces atom B to produce a new molecule, AC. Here is an example of a single displacement reaction: Cu + 2AgNO 3 Cu(NO 3 ) 2 + 2Ag

LecturePLUS Timberlake 99 Single displacement One element takes the place of an element in a reacting compound. A + BC AB + C Zn + 2HCl ZnCl 2 + H 2 Fe + CuSO 4 FeSO 4 + Cu

Sometimes single displacement reactions can cause problems. For example: if iron containing vegetables, such as spinach are cooked in aluminum pans, aluminum can displace iron form the vegetables. This causes a black deposit of iron to form on the sides of the pan. For this reason it is better to use stainless steel or enamel cookware when cooking spinach.

Double Displacement In a Double displacement reaction the positive ion of one compound replaces te positive ion of the other to form two new compounds. The generalized formula for this type of reaction is: AB + CD AD + CB Ba (NO 3 ) 2 + K 2 SO 4 BaSO KNO 3

A double displacement reaction takes place of a a precipitate, water, or a gas forms when two ionic compounds in solution are combined. A Precipitate : is an insoluble compound that comes out of solution during this type of reaction

LecturePLUS Timberlake 99 Double Displacement Two elements in reactants take the place of each other AB + CD AD + CB AgNO 3 + NaCl AgCl + NaNO 3 ZnS + 2HCl ZnCl 2 + H 2 S

LecturePLUS Timberlake 99 Classify the following reactions as 1) single displacement 2) double displacement __ A. 2Al + 3H 2 SO 4 Al 2 (SO 4 ) 3 + 3H 2 __B. Na 2 SO 4 + 2AgNO 3 Ag 2 SO 4 + 2NaNO 3 __C. 3C + Fe 2 O 3 2Fe + 3CO

Answer : A In a __________ reaction, two or more substances combine to form another substance. A) synthesis B) decomposition C) single displacement D) double displacement

Answer : B The formula AB → A + B represents what type of chemical reaction? A) synthesis B) decomposition C) single displacement D) double displacement

Answer : C Which of the following general formulas represents a single-displacement reaction? A) AB → A + B B) A + B → AB C) A + BC → AC + B D) AB + CD → AD + CB

Answer : C An insoluble compound that comes out of solution during a double displacement reaction is called a __________. A) solvent B) reactant C) precipitate D) catalyst

Answer : A What type of reaction is represented by the following equation: 2H 2 + O 2 → H 2 O? A) synthesis B) single displacement C) double displacement D) decomposition

Answer : C What type of reaction produces a precipitate? A) single displacement B) synthesis C) double displacement D) decomposition

Energy Change All reactions, whether they are physical, chemical or nuclear undergo some energy change. Reactions can be classified based on whether the change involves an absorption of energy or a release. These reactions are termed:

Exothermic Reaction in which energy is released by the system into the surrounding environment. molecules, atoms and bonds "lost" energy to the surroundings. The container in which an exothermic reaction is taking place will feel warm.

Endothermic Reaction in which energy is absorbed by the system from the surrounding environment molecules, atoms and bonds "gain" energy from their surroundings. The container in which an endothermic reaction is taking place will feel cold.