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Chemical Reactions Part 1: Physical and Chemical Change Types of Change There are two ways that a substance can change: Physical Change Changes that may alter the appearance but do NOT produce new substances Chemical Change Changes that produce one or more new substances Types of Change There are two ways that a substance can change: Physical Change Bending, ripping, mixing and changing physical states Chemical Change Burning, rusting, baking Types of Change Ripping Burning Physical Change Chemical Change No new substances: it’s still paper! New substances: smoke and ash Types of Change Mixing the dough Rolling the dough Baking the dough Physical Change Physical Change Chemical Change 4 Signs of a Chemical Change A change in color A change in energy Formation of a gas Formation of a solid Chemical Reactions A chemical change occurs when a substance changes into one or more new substances. This change is also called a chemical reaction. During a chemical reaction, atoms rearrange themselves to form new substances. Chemical Reactions Part 2: Chemical Equations Chemical Reactions Reactions can be written as chemical equations. A chemical equation uses chemical formulas and symbols to describe a chemical reaction. Let’s look at one chemical reaction: magnesium (Mg) burns brightly in oxygen (O2) Chemical Reactions + Magnesium Oxygen Magnesium Oxide Reactants Products Substances that react at the start of a reaction Substances formed at the end of a reaction Chemical Equations A chemical equation shows how the atoms rearrange during a reaction in a simple way. + Magnesium 2 Mg Oxygen + O2 Magnesium Oxide à 2 MgO Chemical Equations Reactants 2 Mg + produces O2 à 2 MgO Products Chemical Equations These cannot be changed! chemical formulas 22 Mg Mg + O2 à MgO 2 MgO Chemical Equations These cannot be changed! chemical formulas 22 Mg Mg + O2 à coefficients These can be changed! MgO 2 MgO Law of Conservation of Mass Chemical equations MUST be balanced. This is because they must follow the Law of Conservation of Mass. “In a chemical reaction, mass is not created or destroyed, it is simply rearranged.” Law of Conservation of Mass The numbers of reactants on one side MUST equal the number of products on the other side. 2 Mg + O2 à 2 MgO 2 Mg 2 O 2 MgO 4 atoms 4 atoms How to Balance Equations To balance equations, use the MeNOH method! Metals – balance all metals first Nonmetals – balance nonmetals (not O,H) Oxygen – balance any oxygen atoms Hydrogen – balance any hydrogen atoms Balancing Equations 1. SiCl4 à Si + Cl2 1. SiCl4 à Si + 2 Cl2 Balancing Equations 2. Na + O2 à Na2O 2. 4 Na + O2 à 2 Na2O Balancing Equations 3. H2 + N2 à NH3 3. 3 H2 + N2 à 2 NH3 Balancing Equations 4. Mg + HCl à H2 + MgCl2 4. Mg + 2 HCl à H2 + MgCl2 Balancing Equations 5. H2O2 à H 2O + O2 5. 2 H2O2 à 2 H 2O + O2 Chemical Reactions Part 3: Types of Chemical Reactions Types of Reactions There are 5 major types of chemical reactions: 1. Synthesis 2. Decomposition 3. Combustion 4. Single Displacement 5. Double Displacement 1. Synthesis A synthesis reaction is when 2+ substances combine to form a single substance. Example: synthesis of water: + Hydrogen (H2) Oxygen (O2) Water (H2O) Decomposition A decomposition reaction is when 1 substance breaks down into 2+ simpler substances. Example: decomposition of water: + Water (H2O) Hydrogen (H2) Oxygen (O2) Combustion A combustion reaction is when a hydrocarbon (CXHX) reacts with oxygen to produce CO2 and H2O and heat and light energy. Example: combustion of methane gas: + Methane (CH4 ) + Oxygen (O2 ) Carbon Dioxide (CO2) Water (H2O) Single/Double Displacement In displacement reactions, elements replace other elements in a compound. Example: displacement of Fe with CuSO4 + Iron (Fe ) + Copper Sulfate (CuSO4 ) Iron Sulfate (FeSO4) Copper (Cu) Summary SYNTHESIS A DECOMPOSITION A COMBUSTION H H C + B A B H H A + O2 + 1 DISPLACEMENT A 2 DISPLACEMENT A D B + B C C + B B CO2 + H2O + A C A C + B B D Summary SYNTHESIS DECOMPOSITION COMBUSTION Mg + O2 MgO NaHCO3 Na2CO3 + H2O CH4 + O2 CO2 + H2O 1 DISPLACEMENT Cu + AgNO3 CuNO3 + Ag 2 DISPLACEMENT KI + PbNO3 KNO3 + PbI Classify These Reactions! 1. 2 HgO 2 Hg + O2 2. 4 Sb + 3 O2 Sb4O6 3. Fe2O3 + 3 Mg 4. C3H8 + 5 O2 2 Fe + 3 MgO 3 CO2 + 4 H2O 5. NaCl + CuSO4 NaSO4 + CuCl Classify These Reactions! 1. 2 HgO DECOMPOSITION 2 Hg + O2 2. 4 Sb + 3 OSYNTHESIS 2 Sb4O6 3. Fe2O3 + 13 DISPLACEMENT Mg 2 Fe + 3 MgO 4. C3H8 + 5 COMBUSTION O2 3 CO2 + 4 H2O 2 DISPLACEMENT 5. NaCl + CuSO 4 NaSO4 + CuCl Chemical Reactions Part 4: Rates of Chemical Reactions Introduction Some reactions are quick, others are slow. Rusting (slow!) Combustion (fast!) The area of chemistry that studies the speed of reactions is called kinetics. Rate of Reactions The speed of a reaction is called its rate. Specifically, the rate of a reaction measures how fast the reactants are changed to products in a certain time. To understand reaction rates, we must first learn about Collision Theory… Collision Theory Collision Theory states that for substances to react, they must physically collide. Reaction However, not ALL collisions cause a reaction! 1. Reactions Need Energy Firstly, for two reactants to collide with success, they must have enough energy. No Reaction To give the reactants more energy, you must increase the temperature. 1. Reactions Need Energy Firstly, for two reactants to collide with success, they must have enough energy. Reaction! To give the reactants more energy, you must increase the temperature. 2. Reactions Need Orientation Secondly, for two reactants to collide with success, they must be lined up properly. No Reaction If the reactants are orientated correctly, the collision will be successful. 2. Reactions Need Orientation Secondly, for two reactants to collide with success, they must be lined up properly. Reaction! Note: Orientation is by random chance only! It cannot truly be controlled. Increasing the Rate of Reaction The rate of reaction can be increased. Collision Theory states that as the number of collisions (per second) increases, so will the rate of reaction. The number of collisions can be increased in 4 ways: h the temperature h the concentration h the surface area Use of a catalyst Effect of Temperature Increasing the temperature increases the rate. As temperature increases, the reactants gain more kinetic energy. This increases the number of successful collisions. Low Temperature High Temperature Effect of Concentration Increasing the concentration increases the rate. Increasing the number of molecules of one or more reactants makes the collisions more likely and more frequent. Low Concentration High Concentration Effect of Surface Area Increasing surface area increases the rate. Breaking a reactant into smaller pieces (or a powder) makes more of it exposed to collide with another reactant. Low Surface Area High Surface Area Catalysts Using a catalyst increases the rate. A catalyst is a substance that speeds up the reaction without being consumed. It helps make the reactants collide easier. Catalysts work by lowering the energy the reactants need to react. Reaction! Any Questions?