Changes of Matter Physical Change- a change that alters the appearance or form of a substance but doesn’t produce a new substance. Ex: melting a sample of gold Chemical Change/Reaction- a change in matter that produces one or more new substances. Ex: Gas produced from vinegar and baking soda.
Evidence for Chemical Reactions • Gas Production • Change in Temperature/Energy • Change in Color • Change in Properties • Formation of a Precipitate
Changes in Energy • Endothermic Reactions- a reaction in which energy is absorbed. Not all endothermic reactions result in a decrease in temperature. Ex:frying an egg • Exothermic Reactions- a reaction in which energy is released. Ex: the burning of airplane fuel
Chemical Formulas • A chemical formula is a combination of symbols that represents the elements present in the formula and the ratio of elements present. Ex: MgCl2 - One atom of magnesium for every two atoms of chlorine. Ratio is 1:2 How many total atoms are present in the following: Pb(NO3)4 H2SO4 2HNO3
What Are Chemical Equations? • Using formulas and symbols instead of words to represent a chemical reaction. Ex: CaCO3 ------- CaO + CO2 Structure of an Equation *Reactants-substances you start with *Products-substances you end up with *Arrow means “yields”
Subscripts vs. Coefficients • Subscripts – A number that indicates the ratio of elements in a compound. • Ex: H2O Ratio 2:1 • Coefficients – a number in front of a chemical formula in an equation that indicates how many molecules or atoms of each reactant and product are involved in a reaction. • Ex: 3H2O Three molecules of water
Conservation of Mass • During a chemical reaction, matter is not created or destroyed. • Total mass of the reactants equals the total mass of the products. • Open System – matter can enter from or escape to the surroundings. Ex: burning wood in a fireplace • Closed System – matter is not allowed to enter or leave. Ex: a chemical reaction in a sealed bag
Balancing Equations • To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation. Ex: 2H2O + O2 2H2O2 • (4 hydrogen atoms and 4 oxygen atoms are present on both sides of the equation)
Try these!! • _Na + Cl2 _NaCl • Ca + Cl2 CaCl2 • _H2O _H2 + O2 • N2 + _H2 _NH3 • _Al2O3 _Al + _O2 • P4 + _O2 P4O6 • _Fe + _H2O Fe3O4 + _H2
Controlling Chemical ReactionsAll reactions require a certain amount of activation energy to get them started. Activation energy – the minimum amount of of energy needed to start a chemical reaction.
Factors that Influence the Rate of Reactions • Surface Area • Temperature • Concentration – amount of substance in a given volume • Use a Catalyst – a material that increases the rate of a reaction • Use an Inhibitor – a material that decreases the rate of a reaction