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Learn about the signs of chemical changes, including the release of heat and/or light, the production of gas or bubbles, the formation of a precipitate, and color changes. Explore the symbols used to represent chemical reactions and factors that affect reaction rates.
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Signs of a Chemical Change 1. Heat and/or light is given off 2. A gas is produced (often seen as bubbles) • A precipitate is formed (when 2 solutions are mixed, a solid sometimes forms and separates from the mixture) 4. A color change occurs
ENERGY • Exothermic Reactions – when energy is released during a reaction it is called an exothermic reaction. • Endothermic Reactions – when energy is absorbed or taken in during a chemical reaction.
Chemical Reaction Symbols • + Separates 2 or more reactants or products • “Yield” Separates reactants from products • Used in place of to indicate a reversible reaction • Designates a reactant or product in the solid state • (l) Indicates a liquid reactant or product • (aq) Indicates an aqueous solution (where some solute has been dissolved in water) • (g) Indicates a gaseous reactant or product • Indicates that heat is supplied to a reaction • A formula written above sign indicates that a catalyst is used (something that speeds up the reaction)
Factors that Affect Reactions:NOTE: A reaction between two molecules can only occur when a collision occurs between the two molecules. • 1. Temperature: Raising the temperature increases the speed at which the molecules move. When molecules move faster, more collisions occur thus making the reaction proceed faster. The opposite is true for decreasing the temperature.
Factors that Affect Reactions: • 2. Concentration: When the concentration is increased, the number of molecules are increased. When there are more molecules, more collisions occur thus making the reaction proceed faster. The opposite is true for decreasing the concentration.
Factors that Affect Reactions • 3. Particle Size: When the size of the particles are smaller the surface area is increased. This provides access to a greater number of molecules and increases the speed of the reaction. The opposite is true for using larger particle sizes.
Factors that Affect Reactions • 4. Catalysts: Even though a catalyst is never used up in a reaction, their presence can speed up a reaction by lowering the energy it takes for the collisions to occur.
Don’t forget… • Chemical reactions must follow the Law of Conservation of Mass, which states that mass is neither created nor destroyed. The form of matter may change, but the TOTAL amount of mass stays constant. This law also means that the number of atoms must stay the same too!
