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Chemical Reactions. 7-1 Describing Reactions. Chemical Equations. To describe a chemical change, describe what is present before and after the change. Reactants: substances that undergo a change Products: new substances that form Using Equations to Represent Reactions Word equations:
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Chemical Equations To describe a chemical change, describe what is present before and after the change. Reactants: substances that undergo a change Products: new substances that form • Using Equations to Represent Reactions • Word equations: • Reactants → Products • Carbon + Oxygen → Carbon dioxide • Chemical formulas simplify word equations: • C + O2 → CO2 • Read “The reaction of C and O yields carbon dioxide. • Conservation of Mass Reviewed • The mass of the reactants equals the mass of the products • Number of atoms in the reactants equals the number of atoms in the products • Mass is neither created nor destroyed
Balancing Equations When hydrazine burns in the presence of oxygen, the reaction produces nitrogen, water vapor and heat. This chemical equation is at illustrated in the table to the right in an unbalanced equation and in a balanced equation. Study the table carefully.
Let’s try some! 3 3 ____ AlBr3 + ____ K ____ KBr + ____ Al Count the atoms: 1 Aluminum atom 1 Potassium atom 1 Potassium atom 1 Aluminum atom 3 Bromine atoms 1 Bromine atom Is it balanced? No! Add coefficients to balance atoms: 1 Aluminum atom 3 Potassium atoms 3 Potassium atoms 1 Aluminum atom 3 Bromine atoms 3 Bromine atoms Is it balanced? YES!
Let’s try another! ____ FeO + ____ PdF2 ____ FeF2 + ____ PdO Count the atoms: 1 Iron 1 Palladium 1 Iron 1 Palladium 1 Oxygen 2 Flourine2 Flourine 1 Oxygen Is it balanced? YES! Do we need to add coefficients? NO!
Need another? Okay! 4 6 ____ P4 + ____ Br2 ____ PBr3 Count the atoms: 4 Phosphorus 2 Bromines 1 Phosphorus 3 Bromines Is it balanced? No! Add coefficients to balance atoms: 4 Phosphorus 12 Bromines 4 Phosphorus 12 Bromines Is it balanced? YES!
Let’s try a harder one! 2 3 3 ____ CoBr3 + ____ CaSO4 ____ CaBr2 + ____ Co2(SO4)3 Count the atoms: 1 Co 1 Ca1 Ca 2 Co 3 Br 1 S 2 Br 3 S 4 O 12 O Is it balanced? No! Add coefficients to balance atoms: 2 Co 3 Ca3 Ca 2 Co 6 Br 3 S 6 Br 3 S 12 O 12 O Is it balanced? YES!
Now Try Balancing Chemical Equations on your own • More practice: 1. Na+H2O→NaOH+H2 Show your work. 2. HCl+CaCO3→CaCl2+CO2+H2O Show your work 3. Al+Cl2→AlCl3 Show your work.
Counting with Moles Counting things in terms of atoms and molecules is not always practical for chemists because chemical reactions involve large number so these particles. This is why chemists use a counting unit called a mole. 1 mole = 6.02X particles Known as Avogadro’s number Just as the mass of 12 eggs does not equal the mass of 12 oranges, a mole of carbon has a different mass than a mole of sulfur.
Moles Continued • Molar mass for an element is the same as its atomic mass expressed in grams. Carbon’s atomic mass is 12.0 amu so its molar mass is 12.0 grams. • Molar mass for a compound can be calculated by adding up the atomic masses of its atoms and expressing the result in grams.
Chemical Calculations • Chemical equations can be read as recipes for making new substances. • In chemical reactions, the mass of a reactant or product can be calculated by using a balanced chemical equations and molar masses of the reactants and products. • The chemical equation tells you how to relate amounts of reactants to amounts of products.
7-2 Types of Reactions Different reactants can produce the same product. Consider that CO2 is produced in the following multiple ways: dropping hydrochloric acid on limestone heating limestone gasoline
Classifying Reactions: 1. Synthesis & 2. Decomposition • Synthesis reaction: Two or more substances react to form a single substance A + B → AB • Sodium and Chlorine 2Na + Cl2 → 2NaCl • Hydrogen and Oxygen 2H2 + O2 → 2H2O • Decomposition reaction: The opposite of synthesis and one in which a compound breaks down (the reactant must be a compound but the products can be elements or compounds) AB → A + B • Water decomposes into hydrogen gas and oxygen gas 2H2O → 2H2 + O2 • Calcium carbonate in limestone decomposes into lime, CaO, and carbon dioxide CaCO3 → CaO + CO2
Classifying Reactions Continued: 3. Single Replacement 4. Double Replacement
Classifying Reactions Continued:5. Combustion • A combustion reaction is one in which a substance reacts rapidly with oxygen often producing heat and light. • Example: • natural gas contains methane (CH4) • combined with an unlimited supply of oxygen • the products are carbon dioxide, water, heat and light
Reactions as Electron Transfers:oxidation and reduction The discovery of subatomic particles enabled scientists to classify certain chemical reactions as transfers of electrons between atoms. A reaction where electrons are transferred from one reactant to another is called an oxidation-reduction reaction, or redox reaction.
Chemical Bonds and Energy • Chemical energy is the energy stored in the chemical bonds of a substance. • Chemical reactions involve there breaking of chemical bonds I the reactants ad the formation of chemical bonds in the products. • Propane grill = site of a chemical reaction • Reactants = propane and oxygen • Products = carbon dioxide and water • Since heat is produced (energy), heat can be included in the equation indicating heat is released in the reaction from the reactants • In the combustion of propane, the bonds in propane and oxygen molecules are broken while bonds in carbon dioxide and water molecules re formed. • Oxygen, carbon dioxide, and water molecules all have energy stored in their bonds too. Breaking Bonds Forming Bonds
Reviewing EndoThermic Reactions = Energy Absorbed • Chemical reactions occur when bonds break and new bonds form. They usually absorb heat or liberate heat(release it). • Endothermic reactions absorb energy when the bonds break • Energy is absorbed from nearby matter • Often temperature changes • Baking soda combined with vinegar absorbs heat, making it feel colder • When heat is added constantly(egg frying) • Mercury(II) oxide is heated to a temperature of about 450ºC, it breaks down into mercury and oxygen (decomposition)
Reviewing Exothermic Reactions = Energy Released • The making or breaking of chemical bonds results in a net release of energy • Example: combustion • Energy typically releases as heat Fuel and oxygen in the plane’s engine release heat causing the gases in the engine to expand. This moves the gases out of the plane which exerts a force that moves the plane forward.
Exothermic and Endothermic Reactions • In any reaction, the chemical energy reaches a peak before the reactants change into products. The peak represents the amount of energy require to break the chemical bonds of the reactants (activation energy).
Conservation of Energy Exothermic reaction = chemical energy of the reactants is converted into heat + the chemical energy of the products Endothermic reaction = heat + the chemical energy of the reactants is converted into chemical energy of the products The total amount of energy is the same before and after both types of reactions…the law of conservation of energy
Reactions over time • A reaction rate is the rate at which reactants change into products over time. • Reaction rates tell you how fast a reaction is going. • How fast the reactants are being consumed • How fast the products are being formed • How fast energy is being absorbed or released • Five Factors Affect Reaction Rates • Temperature, surface area, stirring, concentration, catalysts (inhibitors)
Factors Affecting Reaction Rates • Temperature: High = faster • Surface Area: More = faster • Stirring: Speeds up, Washing clothes • Concentration: More = faster • Catalysts: Speeds up, Enzymes • Bonus: Inhibitors: Slows reaction, preservatives
Types of Equilibria Equilibrium is a state in which the forward and reverse path of a change take place at the same rate. • Physical Equilibrium in physical changes: • When a physical change does not go to completion, a physical equilibrium is established between the forward and reverse changes. • Chemical Equilibrium in chemical changes: • In a reversible reaction, the conversion of reactants into products and the conversion of products into reactants can happen simultaneously. • When a chemical reaction does not go to completion, a chemical equilibrium is established between the forward and reverse reactions. • Synthesis of sulfur trioxide from sulfur dioxide and oxygen
Factors Affecting Chemical Equilibrium • When a change is introduced to a system in equilibrium, the equilibrium shifts in the direction that relieves the change. • Class Participation Opportunity: Research the life of Louis Le Chatelier (1850-1936) • Example: Synthesis of Ammonia (system containing nitrogen, hydrogen and ammonia in equilibrium) • Temperature: • Increase temp…system removes heat, decreasing the amount of ammonia • Pressure • Increase pressure…produces fewer gas molecules, more ammonia is produced • Concentration • Remove ammonia, system would favor the forward reaction, producing more ammonia
