CHAPTER Chemical Reactions

Writing Chemical Equations A chemical reaction occurs when matter changes from one composition to another.

Writing Chemical Equations Chemical equations are made up of reactants and products  Reactant: a substance present at the start of the reaction  Product: a substance produced in a chemical reaction An arrow separates reactants and products You read the arrow as yields, gives, or produces

Writing Chemical Equations Word equations: Names of the reactants to the left of the arrow and names of the products to the right of the arrow.  Hydrogen + Oxygen  Dihydrogen Monoxide Chemical equation is a representation of a reaction using chemical formulas to represent reactants and products.  H 2 + O 2  H 2 O

Balancing Chemical Equations To write a balanced chemical equation, first write the chemical equation and then use coefficients to balance the equation so that it obeys the law of conservation of mass The amount of atoms of the reactants must equal the amount of atoms of the products

States of matter States of matter are written after the compound (s) – solid (l) – liquid (g) – gas (aq) – aqueous (dissolved in water)

Types of Chemical Reactions The five general types of reaction are combination (synthesis), decomposition, single-replacement, double-replacement, and combustion

Combination (Synthesis) A combination reaction is a chemical change in which two or more substances react to form a single new substance.  Generic Equation: A + B  AB  Example: 2Mg (s) + O 2 (g)  2MgO (s)

Decomposition A decomposition reaction is a chemical change in which a single compound breaks down into two or more simpler products  Generic Equation: AB  A + B  Examples: 2HgO (s)  2Hg (l) + O 2 (g)

Single Replacement A single-replacement reaction is a chemical change in which one element replaces a second element in a compound.  Generic Equation: AB + X  XB + A  Example: Cu(NO 3 ) 2 (aq) + Zn (s)  Cu (s) + Zn(NO 3 ) 2

Double Replacement Reactions A double replacement reaction is a chemical change involving an exchange of positive ions between two compounds  Generic Equation: AB + XY  XB + AY  Example: Na 2 S (aq) + Cd(NO 3 ) 2 (aq)  CdS (s) + 2NaNO 3 (aq)

Combustion Reactions A combustion reaction is a chemical change in which an element or a compound reacts with oxygen, often producing energy in the form of light and heat. When the reactant is a hydrocarbon, the products will always be water and carbon dioxide.

Predicting the Products of a Chemical Reaction The number of elements/compounds in the reactants is a good indicator of possible reaction type and thus possible products. Use the types of reactions to help predict the products of the reaction.

Diatomic Molecules When predicting products, do not forget about diatomic molecules.  BrINClHOF When these seven elements are alone, they must be written with a subscripted 2

Predicting Products When predicting products of single and double replacement reactions, the like charges of the two reactants must switch places  Examples  MgCl 2 + Na 2 O  NaCl + MgO  Magnesium and Sodium are both positively charged and therefore switch places during the reaction.  DON’T FORGET TO CRISS CROSS

The Mole The mole: A measurement of matter You often measure the amount of something by one of three different methods- by count, by mass, and by volume.

The Mole A mole of a substance is 6.02 x representative particles of that substance. The number of representative particles in a mole (6.02 x ), is called Avogadro's number.  The term representative particle refers to atoms, molecules, or formula units.

Mass of a Mole The atomic mass of an element expressed in grams is the mass of 1 mole of the element. To calculate the molar mass (formula mass) of a compound, find the number of grams of each element in one mole of the compound. Then add the masses of the elements together. The molar mass of a compound is the mass of a mole of the compound

Mass to Mass Conversion In order to convert between masses of one element or compound to another element or compound, quantities should be converted to moles first.  To convert from mass to moles  Divide the mass by the molar mass  To convert from moles to mass  Multiply the moles by the molar mass