Notes: Matter & Change (text Ch. 1 &10)

Transcription

1 Name Per. Notes: Matter & Change (text Ch. 1 &10) NOTE: This set of class notes is not complete. We will be filling in information in class. If you are absent, it is your responsibility to get missing information from a fellow classmate or the chemistry website: I. Classification of Matter Matter: anything that has mass and takes up space Examples of matter: Non-examples of matter: 3 states of matter and their properties: Solid Structure of particles Compressibility Motion of particles Shape Volume Liquid Gas Viscosity is a term related to fluids (liquids and gases). Viscosity is defined as resistance to flow. What are 2 examples of highly viscous liquids?, Matter can be classified according to the following diagram: Matter Pure Substances definite composition (one chemical formula) Mixtures 2 or more pure substances physically combined, can be separated physically, indefinite composition Elements Compounds Homogeneous Heterogeneous One type of atom; 2 or more elements Also known as solutions; Not uniform (can see on Periodic Table chemically bonded look uniform (same) throughout different particles) Ex: Fe, Cl2 Ex: H2O Ex: Gatorade, salt water, alloys Ex: Italian salad dressing, oil & water 1

2 II. Properties of Matter Physical Property: a characteristic that is observed without changing the chemical makeup of the substance. Physical properties can be extensive or intensive. Extensive properties depend on the amount of matter present; think extent. (examples: mass, volume) Intensive properties are not influenced by the amount of matter present; they are constant. (examples: color, melting point, density, specific heat). Chemical Property: characteristic that is observed when the substance undergoes a chemical change (reaction); these properties show how a substance reacts with other substances. Examples: III. Density Density is an intensive property and can be found by dividing a substance s mass by its volume: Density = mass volume D = m v mass: measure of the quantity of matter an object contains volume: the amount of space an object takes up ***How could you rearrange the equation to solve for mass? For volume?*** Density can be thought of as a measure of how tightly packed a substance s molecules are. A substance with a high density probably has massive particles that are closely packed. A substance with a low density is likely to have less massive particles that are more loosely packed. What state of matter is the least dense? The most dense (usually)? What substance is an important exception to this (the solid form is less dense than the liquid)? What evidence can you think of to support this statement? The density of a particular substance does not change with the amount of that substance! Example: a cube of wood has a mass of 80.0 g and a volume of 100. cm 3. What is the density of the block? If you cut the block in half, what is the density of each half? Prove it with math. 2

3 IV. Changes in Matter A. Physical change: a change that does not produce a new substance (no chemical rxn) Examples of physical changes: Phase changes are changes in the state of matter of a substance. They are always physical changes because the substance s chemical identity remains the same. Example: Water is still H2O, whether it s ice, liquid, or steam. Phase changes can be represented on a heating curve. The heating curve below is specific for water only. Label the phase changes and the direction of the arrows as endo- or exothermic. Assume standard pressure (1 atm). Exothermic- heat is exiting or being released Endothermic- heat is entering or being absorbed Name of phase change States of matter involved? Dir. of Heat flow (heat absorbed or removed? Exothermic or endothermic? Melting Solid liquid Absorbed Endothermic Freezing Boiling Condensing Sublimation* Deposition** 3

4 Sublimation: solid to gas phase change without passing through the liquid phase (Examples: dry ice, solid air fresheners, mothballs, shrinking ice cubes) Deposition: gas to solid phase change without passing through the liquid phase (Example: frost on a windshield--water vapor in the air crystallizes on the cold glass) B. Chemical change (reaction): a change that does produce a new substance Examples of chemical changes: One rule of thumb that can be useful in distinguishing physical from chemical changes: Ask yourself, Can the change be reversed? If yes: probably physical. If no: probably chemical. Example: Milk goes sour. Can the change be reversed? NO. You cannot get the milk to taste good again, so it is a CHEMICAL change. A new substance (lactic acid) has been produced. This acid is formed when bacteria break down the lactose in the milk. Example: An ice cube melts. Can the change be reversed? YES. Just put the water back in the freezer. No new substance is produced. Remember, phase changes (changes between states of matter solid, liquid, gas) are PHYSICAL changes! ***This rule is not perfect; it is just meant as a guide. For ex: consider the change that takes place when paper is cut into pieces. Can you get the whole sheet of paper back again? What kind of change is it? Consider each case carefully. Don t just apply the rule without thinking. Besides the general indicator of a chemical reaction (new substances are formed), other specific signs of a chemical reaction are: 1. a gas is produced 2. light is produced 3. a solid (precipitate) is formed from 2 liquids 4. temperature change [temp goes up (exothermic) or down (endothermic)] 5. permanent color change Exothermic reactions are those in which heat flows FROM the system of interest TO the surroundings. Example of an exothermic chemical reaction: burning a log in a fireplace Diagram this example below to show which way the heat energy flows for an exothermic process: (system = burning log) Which way does the arrow go? SURROUNDINGS (atmosphere) SYSTEM (fire) 4

5 Endothermic reactions are those in which heat flows FROM the surroundings TO the system of interest. Example of an endothermic chemical reaction: using a cold pack/ice for an injury Diagram this example below to show which way the heat energy flows for an endothermic process: (system = cold pack/ice) Which way does the arrow go? SURROUNDINGS (injury) SYSTEM (cold pack) V. Energy Energy is the capacity to do work. Energy is used to make things happen. It is best understood by descriptions and not by definition. Some categories of energy are described below: Kinetic: energy of motion (moving car) Potential: energy of position (boulder on hill) Chemical: energy stored in chemical bonds (like bonds in food or fuel) Thermal: heat energy Electrical: flowing electrons (current) Radiant: light energy Nuclear: stored in nucleus of atoms The Law of Conservation of Energy states that the amount of energy in the universe is constant; in other words, energy can be converted from one form to another, but is not created nor destroyed in the process. Can you trace the energy conversions that occur starting with the sun s light and ending with a rabbit hopping? (answers may vary) What about the energy conversions that occur starting with the sun s light and ending with a car driving down the road? (answers may vary) 5

6 Thermal energy (HEAT) can be transported via 3 possible pathways: 1. Convection: the transfer of heat energy in a gas or liquid (not in solids) by movement of currents. The heat moves with the fluid. Example: At the beach, hot air rises, cooler air from the ocean comes in to replace it, and then the cool breeze cools your body. 2. Conduction: the transfer of heat energy through matter from particle to particle. The materials are actually touching during the transfer! Conduction is most effective in solids, but can occur in fluids. Example: A chemical reaction occurring inside a test tube causes the test tube to heat up. 3. Radiation: the transfer of heat energy by electromagnetic waves. Radiation is the only form of heat transfer that can occur in empty space, without the aid of any solids, liquids or gases. Sunlight is a type of electromagnetic wave. It travels through space via radiation. Example: A lamp spreads light through a room. How is thermal energy transported in the following examples? 1. A beaker on a hot plate becomes warmer. 2. The pilot light in your car illuminates a map. 3. Hot soup transfers heat to the air above it. VI. Chemical Reactions and the Law of Conservation of Mass The starting materials are called reactants. The ending materials are called products. They are separated by an arrow pointing toward the products. Reactants go on the left of the arrow, products on the right. The reaction reads left to right, just like a sentence. REACTANTS PRODUCTS Example: photosynthesis 6 CO2 + 6 H2O C6H12O6 + 6 O2 The Law of Conservation of Mass states that during a chemical reaction, matter cannot be created or destroyed. This means that the original atoms can move around and find new partners, but all of the original atoms that were in the reactants WILL BE present in the products as well. How many atoms of C are on the reactants side of the reaction above? products side? How many atoms of H are on the reactants side of the reaction above? products side? How many atoms of O are on the reactants side of the reaction above? products side? ***This balanced equation illustrates that the Law of Conservation of Mass is obeyed! 6

7 Another way to illustrate the Law of Conservation of Mass: 6 CO2 + 6 H2O C6H12O6 + 6 O2 264 g g = 180 g +??? g The mass of the O2 is ***The mass of the reactants must equal the mass of the products!!!** Now work through this problem: Mg + 2 HCl MgCl2 + H2 How many atoms of Mg are on the reactants side of the reaction above? products side? How many atoms of H are on the reactants side of the reaction above? products side? How many atoms of Cl are on the reactants side of the reaction above? products side? Provided the following masses, solve for the mass of the Mg. Mg + 2 HCl MgCl2 + H2?? g 73 g 95 g 2 g The mass of the Mg is 7