CHEMICAL REACTIONS CHAPTER 2

CHEMICAL REACTIONS Chemistry: The study of matter and how matter changes

PROPERTIES OF MATTER Physical Properties > Physical changes  Characteristic of a substance that can be observed without changing the substance into another substance  Ex. Water > ice  Bending, crushing, breaking, cutting Chemical Properties > Chemical changes  Characteristic of a substance that describes its ability to change into other substances  Ex. Rusting  Color change, heat or light, change in odor, gas  Formation of new substances (precipitate) and changes in energy (endothermic/exothermic)

CHEMICAL CHANGES Properties Precipitate: a solid that forms from solution  Ex. Bread baked in the oven - dough to crunchy crust Energy Endothermic Reaction: reaction in which energy is absorbed  Ex. Fry an egg – change clear egg whites to a white solid (absorbing the heat) Exothermic Reaction: reaction that releases energy in the form of heat  Ex. Burning of airplane fuel – uses fuel and oxygen to propel and creates heat

BONDING AND CHEMICAL CHANGE Chemical changes occur when bonds break and new bonds form > new substances are produced Review: atoms form bonds when they share or transfer electrons Reaction between Magnesium and Oxygen: 1. O2 = two oxygen atoms that are sharing electrons 2. MG = one magnesium atom 3. Oxygen reacts with Magnesium (bond between 2 oxygen atoms are broken) 4. Magnesium atom transfers two of its electrons to one oxygen atom > oxygen becomes a negative ion, and magnesium because a positive ion 5. Bond forms between the magnesium and oxygen atoms > MgO

BALANCING EQUATIONS Chemical Equation: an easy way to show a chemical reaction, using symbols instead of words Chemical equations use chemical formulas and other symbols to summarize a reaction Review: chemical formulas represent the ratio of the substances within a compound

STRUCTURE OF AN EQUATION Reactant: substances you have at the beginning of a reaction Product: substances you have at the end of a reaction Arrow: shows the direction of the reaction, directing to what the reaction yields (end result) Reactants and products are separated by addition signs Reactant + Reactant  Product + Product CaCO 3  CaO + Co 2

CONSERVATION OF MASS Page 58 & 59 In a chemical reaction, matter is not created or destroyed. The total mass of the reactants must equal the total mass of the products. How does this relate to chemical equations?

BALANCING EQUATIONS Atoms are not created or destroyed during a chemical reaction. Scientists know that there must be the same number of atoms on each side of the equation. To balance the chemical equation, you must add coefficients in front of the chemical formulas in the equation. You cannot add or change subscripts!