Notes: Matter and Change

Transcription

1 Name Chemistry-PAP Notes: Matter and Change Period: I. What is Chemistry? is the study of composition, structure, and properties of matter and energy associated with the changes it undergoes. is defined as anything that has and. is defined as the amount of matter in something. (Note: mass is not the same as. Weight is mass times the gravitational acceleration. will not change if you go to the moon.) II. States Of Matter There are five main states of matter: - definite shape and volume - indefinite shape and definite volume, takes shape of container but does not expand or contract detectably - indefinite shape and volume; it can expand/contract to fill container; easily compressible - created by heating a gas to high temperatures, ionizing some of the particles; most common state of matter in universe (intergalactic plasma) (BEC or Superfluid)- a dilute gas of bosons cooled to ultra-low temperatures very near absolute zero III. Matter Classification of Matter All matter can be classified into 2 types:

2 1. Pure Substances Can be either (ex. Al or H 2 ) or (ex. H 2 O, CO 2 ) Pure substances cannot be separated. are the simplest forms of matter and consist of of all one type. An is the smallest part of an element that retains all the properties of the element. are formed when elements chemically combine (bond) and consist of (Ex. H 2 O) or formula units (Ex. NaCl). A or is the smallest part of a compound that retains all the properties of the compound. Compounds have a composition. 2. Mixtures A is a physical blend of substances. The composition of mixtures can. Mixtures can be separated. How could a mixture of salt and water be separated? What about sand and water? Sugar, iron filings, and water? Two types of mixtures: uniform composition (looks the same throughout) not uniform composition Practice Determine whether the following are pure substances or mixtures. a) concrete d) ammonia, NH 3 b) oxygen e) potassium fluoride, KF c) saline solution 2

3 Properties of Matter A is a quality or condition of a substance that can be observed or measured without changing the substance s chemical identity. Physical properties can be intensive or extensive. Intensive physical properties do not change with amount of substance. Extensive physical properties do change with amount. The density of a substance is a physical property. Recall: D = m v mass: measure of the quantity of matter an object contains volume: the amount of space an object takes up 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. Something to think about: 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. The density of a particular substance does not change with the amount of that substance! That s why we can use density to help us identify an unknown substance. Considering these facts, is density an intensive or extensive property? A refers to a substance s ability to undergo a chemical reaction and form new substances. In order to observe a chemical property, you must change the substance by breaking old chemical bonds / forming new ones (in other words, a chemical reaction takes place). 3

4 Practice Determine whether the following are physical or chemical properties: a) Sulfur is yellow. b) Sulfur will combine with iron to form iron sulfide. c) Alcohol is flammable. d) Copper is a good conductor of electricity. e) A car bumper corrodes in a salty, humid environment. Changes in Matter Physical vs. Chemical Change A change alters a substance without changing its composition. No new substance is produced and no chemical reaction occurs. 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 H 2 O, whether it s ice, liquid, or steam. phase changes involve an absorption of heat from the surroundings; phase changes involve a release of heat to the surroundings. 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** *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) 4

5 Label the phase changes on the arrows in the diagram below: A change occurs when a substance (or substances) change(s) into new substances. (also called a chemical reaction) Practice Classify the following as a physical change or chemical change: a) a match burns b) meat freezes c) sugar dissolves in tea d) ice cream melts e) a burger is grilled IV. Chemical Reactions A chemical occurs when a new substance has been formed. This is shown typically by a chemical 5

6 Besides the general indicator of a chemical reaction (new substance(s) is/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 chemical 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 **Endothermic chemical reactions are those in which heat flows FROM the surroundings TO the system of interest. Example of an endothermic chemical reaction: using a chemical cold pack for an injury Law of Conservation of Mass: States that mass is neither created nor destroyed during ordinary chemical reactions or physical processes. This means in any chemical reaction, the total mass of the must equal the total mass of the. Reactants Products 6 CO H 2 O C 6 H 12 O O g g = 180 g g The starting materials are called reactants. The ending materials are called products. How many atoms of C are on the left hand side of the reaction? on the right? How many atoms of H are on the left hand side of the reaction? on the right? How many atoms of O are on the left hand side of the reaction? on the right? Notice how the number of atoms of each type is the same on each side of the equation. This also illustrates the Law of Conservation of Mass. Provided the following masses, solve for the mass of the Mg. Mg + 2 HCl MgCl 2 + H 2?? g 73 g 95 g 2 g The mass of the Mg is 6

7 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 important for chemistry are described below: Kinetic: energy of motion (moving car) Potential: energy of position (rock on hill) Chemical: energy stored in chemical bonds Thermal: heat energy Radiant: light energy Nuclear: energy stored in the 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. Trace the path of energy from sunlight to a rabbit hopping. Thermal energy (HEAT) can be transported via 3 possible pathways: 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. 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 it can happen in fluids. Example: A chemical reaction occurring inside a test tube causes the test tube to heat up. 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. 7

8 VI. SPECIFIC HEAT The specific heat capacity (C), or simply the specific heat, of a substance is the amount of heat it takes to raise the temperature of 1 gram of the substance 1 o C. C = q, where q = heat energy in Joules, m = mass, T = change in temperature m T T = T final - T initial Therefore the units for specific heat are J/(g o C) or cal/(g o C). To solve for heat energy, rearrange the equation for q: q = Some substances, such as metals, have low specific heats. This means it doesn t take a lot of energy to cause a temperature change. Other substances, such as water, have high specific heats. It takes more energy to cause a temperature change. On a summer day, why does the concrete deck around a swimming pool become hot, while the water stays much cooler? 8