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Section 5.1

Section 5.1. Nature of Chemical Reactions. Chapter 5 Chemical Reactions. Chapter Preview: 5.1 The Nature of Chemical Reactions Chemical Reactions Change Substances Energy and Reactions 5.2 Reaction Types Classifying Reactions Electrons and Chemical Reactions

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Section 5.1

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  1. Section 5.1 Nature of Chemical Reactions

  2. Chapter 5Chemical Reactions • Chapter Preview: • 5.1 The Nature of Chemical Reactions • Chemical Reactions Change Substances • Energy and Reactions • 5.2 Reaction Types • Classifying Reactions • Electrons and Chemical Reactions • 5.3 Balancing Chemical Equations • Describing Reaction • Determining Mole Ratios • 5.4 Rates of Change • Factors Affecting Reaction Rates • Equilibrium Systems

  3. Section 5.1The Nature of Chemical Reactions • Objectives: • Recognize some signs that a chemical reaction is taking place. • Explain chemical changes in terms of the structure and motion of atoms and molecules. • Describe the differences between endothermic and exothermic reactions. • Identify situations involving chemical energy.

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

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

  6. A closer microscopic look at chemical reactions: • Chemical reaction rearrange atoms which are the reactants and 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. • Parts of a chemical reactions:

  7. Energy is conserved in chemical reactions • Chemical energy: the energy stored within atoms and molecules that can be released when a substance reacts.

  8. Energy: the ability to do work • Forming bonds releases energy. • When new bonds form, energy is released.

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

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

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

  12. Section 5.1 Summary • During a chemical reaction, atoms are rearranged. • Signs of a chemical reaction include any of the following: a substance that has different properties than the reactants have; a color change; the formation of a gas or a solid precipitate; or the transfer to energy. • Mass and energy are conserved in chemical reactions. • Chemical energy can be given off or taken in. • Energy must be added to the reactants for bonds between atoms to be broken.

  13. Section 5.2Reaction Types • Objectives • Distinguish among five general types of chemical reactions. • Predict the products of some reactions based on the reaction type. • Describe reactions that transfer or share electrons between molecules, atoms, or ions.

  14. Classifying reaction: even though there are millions of unique substances and many millions of possible reactions, there are only a few general types of reactions. • 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 + BAB • For example: • Na + Cl NaCl • Examples: photosynthesis , polyethylene and table salt.

  15. Decomposition reactions: a reaction in which one compound breaks into at least two products. • The general form for decomposition reactions is as follows: • ABA + B • For example; • H2O H2 + O2 • Note: This type of reaction can be reversed by a process known as electrolysis • Electrolysis: the decomposition of a compound by an electric current.

  16. 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.

  17. 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 + BBA + X • For example; • CuCl2 + AlAlCl3 + Cu • Generally, in a single-displacement reaction, a more reactive element will take the place of a less reactive one.

  18. 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 + BYAY + BX • For Example: • Pb(NO3)2 + K2CrO4PbCrO4 + KNO3

  19. 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 Electron.

  20. Section 5.3Balancing Chemical Equations • Objectives • Demonstrate how to balance chemical equations. • Interpret chemical equations to determine the relative number of moles of reactants needed and moles of product is formed. • Explain how the law of definite proportions allows for predictions about reaction amounts. • Identify mole ratios in a balanced chemical equations. • Calculate the relative masses of reactants and products from a chemical equation.

  21. 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.

  22. 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.

  23. 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 indicate particles and moles. • 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.

  24. Chapter 5 Summary Report • A chemical equation shows the reactants that combine and the products that results from the reaction. • Balanced chemical equations show the proportions of reactants and products needed for the mass to be conserved. • A compound always contains the same elements in the same proportions, regardless of how the compound is made or how much of the compounds is formed. • A mole ratio relates the amounts of any two or more substances involved in a chemical reactions.

  25. Section 5.4Rates of Change • Objectives: • Describe the factors affecting reaction rates. • Explain the effect a catalyst has on a chemical reaction. • Explain chemical equilibrium in terms of equal forward and reverse reaction rates. • Apply Le chatelier’s principle to predict the effect of changes in concentration, temperature, and pressure in an equilibrium process.

  26. Factors Affecting Reaction Rates: • Most reactions go faster at higher temperature. • A large surface area speeds up reactions. • Concentrated solutions react faster.

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

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

  29. Substrate: the specific substance affected by an enzyme. For example:

  30. Section 5.4 Summary Report • Increasing the temperature, surface area, concentration, or pressure of reactants may speed up chemical reactions. • Catalysts alter the rate of chemical reactions. Most catalysts speed up chemical reactions. Others, called inhibitors, slow reactions down. • In a chemical reaction, chemical equilibrium is achieved when reactants change to products and products change to reactants at the same time. • At chemical equilibrium, no changes are apparent even though individual particles are reacting. • Le Chatelier’s principle states that for any change made to a system in equilibrium, the equilibrium will shift to minimize the effects of the change.

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