Chemical Reactions Chapter 5

5.1 The Nature of Reactions Chemical reactions change substances   Production of gas,change of color,smoke, flame are signs of chemical reactions Chemical reactions rearrange atoms

Reactant- a substance that undergoes a chemical change   Product- a substance that is the result of a chemical change Na+ + Cl-  NaCl Reactant Reactant Product

Energy and Reactions Energy must be added to break bonds   Forming bonds releases energy Energy is conserved in chemical reactions

Chemical energy- the energy stored within atoms and molecules that can be released when a substance reacts Exothermic- a reaction that transfers energy from the reactants to the surroundings usually as heat. Reactions that release energy are exothermic

Endothermic reaction a reaction in which energy is usually transferred to the reactants usually as heat from the surroundings Reactions that absorb energy are endothermic

5.2 Reaction Types A+B--------->AB Synthetic Reaction- a reaction of at least two substances that forms a new, more complex compound Synthesis reactions combine substances A+B--------->AB 2Na + Cl2  2NaCl

Decomposition Reaction- a reaction in which one compound breaks into at least two products   AB------------> A+B 2H2O  2H2 +O2 Electrolysis- the decomposition of a compound by an electric current

Combustion Reaction- a reaction in which a compound and oxygen burn. Combustion reactions use oxygen as a reactant. The products of a hydrocarbon and oxygen are carbon dioxide and water.   2CH4 + 402  2CO2 + 4H2O

Single-displacement reaction- a reaction in which atoms of one element take the place of atoms of another element in a compound   In single-displacement reactions, elements trade places XA + B  BA + X 3CuCl2 + 2Al  2AlCl3 + 3Cu

AX + BY--------->AY + BX Double-displacement Reaction- reaction in which a gas, a solid precipitate, or a molecular compound is formed from the apparent exchange of ions between two compounds   AX + BY--------->AY + BX Pb(NO3)2 + K2CrO4  PbCrO4 + 2KNO3

Reduction/oxidation Reaction- a reaction that occurs when electrons are transferred from one reactant to another   Radicals- the fragments of molecules that have at least one electron available for bonding Radicals have electrons available for bonding

5.3 Balancing Chemical Equations Describing Reactions:   Methane + Oxygen-----------> Carbon Dioxide + Water Chemical equations summarize reactions Chemical Equation- an equation that uses chemical formulas and symbols to show the reactants and products in a chemical reaction Balanced chemical equations account for the conservation of mass

Balancing Chemical Equations   Balanced equation- each side of the equation has the same number of atoms of each element. Remember the “Law of Conservation of Mass”– Same number of atoms on each side of the equation.

2H2 + O2 ---------------> 2H2O Coefficients-small whole numbers that are placed in front of the formulas in an equation in order to balance it.   2H2 + O2 ---------------> 2H2O Coefficient Coefficient To write a balanced chemical equation, first write the equation. Then use coefficients to balance the equation so that it obeys the law of conservation of mass.

Example. H2 + O2 ---------------> H2O Reactants Products 2-hydrogen atoms 2-hydrogen atoms 2-oxygen atoms 1-oxygen atom   This is not a balanced chemical equation!! To balance this equation we will use coefficients.

2H2 + O2 ---------------> 2H2O Reactants Products 4-hydrogen atoms 4-hydrogen atoms 2-oxygen atoms 2-oxygen atoms   This is a balanced equation by using the coefficient 2 in front of hydrogen giving us 4 hydrogen atoms and 2 oxygen atoms on the reactants side . The coefficient 2 on the products side gives us 4 hydrogen atoms and 2 oxygen atoms on the product side.

How to balance equations:   CH4 + O2 ------> CO2 +H2O

Information From a Balanced Equations Determining Mole Ratios   Magnesium + Oxygen-----> Magnesium Oxide 2Mg + O2----------->2MgO Balanced equations indicate particles and moles Balanced equations show the conservation of mass

The law of definite properties: A compound always contains the same elements in the same proportions, regardlessof how the compound is made or how much of the compound is formed.   Mole Ratio- the smallest relative number of moles of the substances involved in a reaction Mole ratios can be derived from the balanced equations