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Chemical Reactions What’s the MATTER? • Matter is anything that has MASS and takes up SPACE Physical Changes • Changes in the physical properties of a substance: including changes in size, shape, or its state of matter (whether it’s solid, liquid, or gas). • How are these pictures physical changes?? Physical Changes • Examples: • Tearing paper • Freezing water • Water boiling and becoming water vapor • Cutting your hair • Cutting food • Flipping a light switch • Mixing Kool-Aid and water Chemical Changes • Changes that produce new substances with chemical properties different from the original substances. • Can you name some chemical changes? Chemical Changes • Examples: • Process of rusting • Burning paper • Silver tarnishing • Light bulb coming on • Milk spoiling • Baking soda and vinegar reacting Chemical Reactions • A process that produces a chemical change is a chemical reaction. • Signs that a chemical reaction has occurred include: Color change Bubbles released Heat released Light given off Temperature change New substance formed Substances CANNOT be changed back to original substances by physical means NEXT 4 SLIDES are for Pre-AP CLASSES **Bonus for General Ed Classes Energy in Chemical Reactions • Exothermic reactions RELEASE heat energy resulting in the raising of temperature. • Endothermic reactions ABSORB heat energy resulting in the lowering of temperature Exothermic Reactions • The extra energy is released in various forms—light, sound, and heat. • Example: burning is an exothermic chemical reaction in which a substance combines with oxygen to produce heat along with light, carbon dioxide, and water. Endothermic Reactions • Example: instant cold pack • The heavy plastic cold pack holds ammonium nitrate and water. • The two substances are separated by a plastic divider. • When you squeeze the bag, you break the divider so that the ammonium nitrate dissolves in the water. • The dissolving process absorbs heat energy. • The reaction requires energy which is absorbed, used in the reaction, and thus the temperature DROPS and the pack gets cold! Endothermic OR Exothermic? Chemical Formulas • Chemical formulas are combinations of chemical symbols and numbers that indicates how many atoms of each element are in a compound • Subscripts, located to the right of a symbol, tell you the number of atoms of that element only (O3) • If an atom has no subscript, it means that only one atom of that element is in the compound. NaHCO3 (baking soda) Chemical Equations • To describe a chemical reaction, you must know which substances react and which substances are formed in the reaction. • A chemical equation tells chemists at a glance the reactants, products, and the amounts of each substance present in the reaction. Reactants & Products • Reactants are the substances that exist before the reaction begins (LEFT side of arrow) • Products are the substances that form as a result of the reaction (RIGHT side of arrow) • The arrow in the equations is known as the yield arrow and it means: “produces” The Law of Conservation of Mass • States that mass/matter is neither created nor destroyed, but only changes from one form to another. The Law of Conservation of Mass • Chemical equations are much like math equations: the left and right sides of the equation are numerically EQUAL • The number and type of atoms must be the same on each side of the equation (the mass • This means the equation is BALANCED and the of the reactants must law of conservation of be the same as the mass/matter is observed. mass of the products) Practice #1 Na + Cl • Reactants: – # of atoms of Na = 1 – # of atoms of Cl = 1 NaCl • Products: – # of atoms of Na = 1 – # of atoms of Cl = 1 Does this example follow the law of conservation of mass? YES! Practice #2 H2 + O2 • Reactants: – # of atoms of H = – # of atoms of O = H2O • Products: 2 2 – # of atoms of H = – # of atoms of O = Does this example follow the law of conservation of mass? 2 1 NO! Adjusting Coefficients • When balancing chemical equations, the numbers placed BEFORE the formulas are coefficients. • They are the ONLY numbers changed when balancing equations (unlike when building compounds) • Example: 2H2O H = 4 O = 2 • The coefficient numbers are distributed across entire compound Practice with Coefficients 1. 2KF K=2 F=2 2. 4CO2 C=4 O=8 3. 5NaCl Na = 5 Cl = 5 4. 3AlCl3 Al = 3 Cl = 9 5. 6CaCO2 Ca = 6 C = 6 O = 12 Practice #2, Continued… 2 H2O 2 H2 + O2 • Reactants: – # of atoms of H = – # of atoms of O = • Products: 24 2 – # of atoms of H = – # of atoms of O = 24 12 Is it now BALANCED and follow the law of conservation of mass? YES!