Intro to Chemical Reactions Physical vs. Chemical Changes Chemical Reactions Identifying a chemical Reaction Symbolizing a Chemical Reaction Balancing Chemical Equations Energy in Chemical Reactions Chemical Reactions in Life and Industry

Physical vs. Chemical Changes Physical Changes Changes the appearance of the substance but substance remains the same. EXAMPLE- Tearing paper or wetting dirt Chemical Changes Changes substance into a new substance EXAMPLE – Frying an egg or baking a cake.

Chemical Reactions A chemical reaction is the process where the atoms in a substance are rearranged and bonded into a new substance. Chemical reactions form new substances by breaking and making chemical bonds. Chemical changes occur as a result of chemical reactions.

Identifying a Chemical Reaction How can we tell if a chemical reaction is taking place? Release (evolution) of heat Absorption of heat Emission of light Formation of a solid (precipitate) Formation of gas bubbles Color change

Symbolizing Chemical Reactions We can use chemical equations to represent a chemical reaction. An equation has four parts: Reactants: starting materials Products: materials produced from the reaction Coefficients: numbers placed in front of a chemical formula Arrow: Indicates the direction of the reaction. Always points to the product.

Parts of a Chemical Equation Chemical Equation: CuCl2 + H2S  CuS + 2 HCl Chemical Formulas: CuCl2, H2S, CuS, HCl Reactants: CuCl2 + H2S Products: CuS + 2 HCl Arrow read as “yields”:  Coefficients: numbers used to indicate number of molecules of a compound like the 2 in front of the HCl.

Balancing a Chemical Equation When writing a chemical equation you must obey the law of conservation of mass. This means that each side of the chemical equation (reactant and product) must have the same number and type of elements. For example CuCl2 + H2S  CuS + 2 HCl Reactants: 1Cu, 2Cl, 2H and 1S Products: 1Cu, 1S, 2H, and 2Cl.

Balancing… (cont.) To balance a chemical equation: Count the number of atoms of each element on both sides of the arrow. Put in a coefficient to get the same number of atoms of an element on both sides of the arrow. Repeat until all elements have the same number of atoms on both sides of the arrow.

Balance these Equations NaHCO3  Na2CO3 + H2O + CO2 Cu + AgNO3  Cu(NO3)2 + Ag NH3 + H2S  (NH4)2S NH4Cl + Ca(OH)2  CaCl2 + NH4(OH) NH4(OH)  NH3 + H2O CH4 + O2  CO2 + H2O

Classifying Chemical Reactions Chemical Reactions can be classified for easier understanding. Synthesis: A + B  AB Decomposition: AB  A + B Combustion: A + O2  varies Replacement: AX + B  A + BX Double Displacement: AX + BY  AY + BX

Classify these Equations NaHCO3  Na2CO3 + H2O + CO2 Cu + AgNO3  Cu(NO3)2 + Ag NH3 + H2S  (NH4)2S NH4Cl + Ca(OH)2  CaCl2 + NH4(OH) NH4(OH)  NH3 + H2O CH4 + O2  CO2 + H2O

Energy in Chemical Reactions Energy for chemical reactions are stored in the bonds of the chemical in the form of bond energy. Chemical reactions involve breaking bonds and forming new bonds. Breaking bonds require/absorb energy Forming bonds release energy. If more energy is absorbed than released, the reaction is endothermic. If more energy is released than absorbed, the reactions is exothermic.

Endothermic vs. Exothermic Exothermic Reaction Net release of energy Products have more bond energy than reactants releases heat or light. Causes an increase in temperature of surroundings (gets hotter). Endothermic Reaction More energy absorbed Reactants have more bond energy than products. Light or heat absorbed Causes a decrease in temperature of surrounding (gets colder).

Rates of Chemical Reactions Vary Concentration: An increase in concentration of reactants increases the number of particles that can interact and speeds up a reaction. Surface Area: An increase in surface area is made by decreasing the particle sizes allowing more interactions and speeding up a reaction. Temperature: Adding energy raises the temperature and makes particles move faster and collide more frequently, speeding up a reaction. Catalysts: chemicals added to a chemical reaction that speeds up a chemical reaction but are not changed in the reaction.

Chemical Reactions in Life and Industry Living things require chemical reactions for energy: photosynthesis and respiration. Chemical reactions are used in technology: energy in a battery, gasoline combustion, and catalytic converters. Chemical reactions are used in industry to make useful products: silicon production for use in electronic devices.