S ECTION 7–2: T YPES OF R EACTIONS Physical Science Coach Kelsoe Pages 199–205

O BJECTIVES Interpret chemical equations in terms of reactants, products, and conservation of mass. Balance chemical equations by manipulating coefficients. Convert between moles and mass of a substance using molar mass. Calculate amounts of reactants or products by using molar mass, mole ratios, and balanced chemical equations.

C LASSIFYING R EACTIONS There are five general chemical reactions that take place in nature: Synthesis Decomposition Single-replacement Double-replacement Combustion

S YNTHESIS R EACTIONS A synthesis reaction is a reaction in which two or more substances react to form a single substance. The general equation for a synthesis reaction is: A + B  AB 2H 2 + O 2  2H 2 O Which of the following is NOT always true about a synthesis reaction? A: There is only one reactant. An iron fence is left unpainted, and it reacts with the oxygen in the air, forming rust. The formation of rust is an oxidation-reduction reaction, but it is also an example of a synthesis reaction.

D ECOMPOSITION R EACTIONS A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances. The general equation for a decomposition reaction is AB  A + B 2H 2 O  2H 2 + O 2 CaCO 3  CaO + CO 2 A decomposition reaction is the opposite of a synthesis reaction.

S INGLE -R EPLACEMENT R EACTION A single-replacement reaction is a reaction in which one element takes the place of another element in a compound. The general equation for a single-replacement reaction is: A + BC  B +AC Cu + 2AgNO 3  2Ag + Cu(NO 3 ) 2 A cation will only replace a cation, and an anion will only replace an anion.

D OUBLE R EPLACEMENT R EACTION A double-replacement reaction is one in which two different compounds exchange positive ions and form two new compounds. The general equation for a double-replacement reaction is: AB + CD  AD + CB Pb(NO 3 ) 2 + 2KI  PbI 2 + 2KNO 3 In a double-replacement reaction, there are two reactants and two products.

C OMBUSTION R EACTION A combustion reaction is one in which a substance reacts rapidly with oxygen, often producing heat and light. Combustion reactions can also be synthesis reactions, but don’t necessarily have to be. The combustion of a hydrocarbon always produces carbon dioxide (CO 2 ) and water (H 2 O). The element oxygen is always present in a combustion reaction.

T YPES OF R EACTIONS When magnesium carbonate, MgCO 2, reacts with nitric acid, HNO 3, magnesium nitrate and carbonic acid form. Carbonic acid then breaks down into water and carbon dioxide. Which two types of reactions take place in this process? A: Double-replacement and decomposition Single-replacement reactions can take place with nonmetals. In the following equation, assume that A and C are nonmetals and B is a metal. Complete the following general equation for the replacement of a nonmetal in a compound by another nonmetal: A + BC  C + BA.

R EACTIONS AS E LECTRON T RANSFERS The discovery of subatomic particles enabled scientists to classify certain chemical reactions as transfers of electrons between atoms. A reaction in which electrons are transferred from one reactant to another is called an oxidation-reduction reaction. Which of the following takes place during a redox reaction? A: Electrons are both gained and lost.

O XIDATION Any process in which an element loses electrons during a chemical reaction is called an oxidation. Oxygen is not necessary for an element to go through oxidation. This is a broad term for losing electrons. Sodium (Na) atoms go through oxidation to become ions: Na  Na e -. Everything in groups 1-13 can go through oxidation. In a chemical reaction, an iron atom became the ion Fe 2+. What happened to the iron atom? A: It lost electrons and was oxidized.

R EDUCTION The process in which an element gains electrons during a chemical reaction is called reduction. A reactant is said to be reduced if it gains electrons. Oxidation and reduction always occur together. When one element loses electrons, another must gain them. When fluorine reacts with a metal, it forms an F - ion. The fluorine atom has gained an electron and undergone reduction.

V OCABULARY Synthesis reaction Decomposition reaction Single-replacement reaction Double-replacement reaction Combustion reaction Oxidation-reduction reaction

S TUDY G UIDE #13-16, should be completed.