Nature of Chemical Reactions Section 5.1 Nature of Chemical Reactions

Chemical Reactions Change Substance A chemical change can be describe by using words like grow, ripen, decay, and burn. All chemical reactions change substance.

Chemical reactions occur when substances undergo chemical change to form a new substance. Production of gas and change of color are signs of chemical reactions.

Parts of a chemical reaction: Chemical reaction rearrange atoms from reactants to products, and require energy in order to break the bonds. Reactant is the substance that undergoes a chemical change Product is the result of the chemical change.

Energy:the ability to do work Chemical energy: the energy stored within atoms and molecules that can be released when a substance reacts. Energy is conserved in chemical reactions. Forming bonds releases energy. When new bonds form, energy is released.

Exothermic Reaction (rxn) Transfer of energy from reactants to its surroundings usually as heat. Ex. Sun The temperature of the surroundings rises.

Endothermic Reaction (rxn) Transfer of energy to reactants from surroundings as heat Ex. Photosynthesis

Types of chemical reactions: Bioluminescence and respiration are exergonic reactions. Photosynthesis is an endergonic recection.

Examples: photosynthesis , polyethylene and table salt. There are 6 general reactions that are responsible for the millions different types of substances. Synthesis reaction: a reaction of at least two substances that forms a new, more complex compound. Synthesis reactions have the following general form: A + B AB For example: Na + Cl NaCl Examples: photosynthesis , polyethylene and table salt.

AB A + B For example; H2O H2 + O2 Decomposition reactions: a reaction in which one compound breaks into at least two products. The general form for decomposition reactions is as follows: AB A + B For example; H2O H2 + O2

CxHy + O2 H2O + CO2 + heat and light Combustion reaction: a reaction in which a compound and oxygen burn. The general form for combustion reactions is as follows: CxHy + O2 H2O + CO2 + heat and light Note: combustion reactions use oxygen as a reactant.

XA + B BA + X For example; CuCl2 + Al AlCl3 + Cu In single-displacement reactions, elements trade places: Single-displacement reactions are define as a reaction in which atoms of one element take the place of atoms of anther element in a compound. Single-displacement reactions have the following general form: XA + B BA + X For example; CuCl2 + Al AlCl3 + Cu Generally, in a single-displacement reaction, a more reactive element will take the place of a less reactive one.

Pb(NO3)2 + K2CrO4 PbCrO4 + KNO3 Double-displacement reaction: a reaction in which a gas, a solid precipitate, or a molecular compound is formed from the apparent exchange of ions between two compounds. Double-displacement reactions have the following general form: AX + BY AY + BX For Example: Pb(NO3)2 + K2CrO4 PbCrO4 + KNO3

Electrons and Chemical Reactions With the discovery of the electron and its role in chemical bonding, another way to classify reactions was developed. We can understand many reactions as transfers of electrons. Reduction/oxidation (redox) 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. Oil Rig: Oxidation Is Loss Of Electron. Reduction Is Gain Of

Describing Reactions: 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.

How to balance chemical equations: An equation can be balanced only by putting numbers, called coefficients, in front of the chemical formulas. Note: never add or change a subscript doing so is violating the law of conservation of mass.

Balanced equations show the conservation of mass. Information from a balanced equation is a way that chemists write about reactions to describe both the substances in the reaction and the amounts involved. Balanced equations show the conservation of mass. The law of definite proportions: A compound always contains the same elements in the same proportions, regardless of how the compound is made or h0w much of the compound is formed. Note: Mole rations can be derived from balanced equations.

Factors Affecting Reaction Rates: Most reactions go faster at higher temperature. will slow down at a lower temp. A large surface area speeds up reactions. Small surface area causes a slower reaction Concentrated solutions react faster. Dilute solutions go slower

Reactions are quicker at higher pressure. Massive, bulky molecules react slower.

Catalysts speed up the rates of chemical reactions. Enzymes are biological catalysts that speed up a specific reaction.