Qualitative observation descriptive observation has no numerical measurement

Transcription

1 1 Chem 047: Physical and chemical properties Chemistry 047 Properties of substances, and Physical and Chemical Changes A. Some definitions B. Properties of substances C. The Classification of Matter D. States of matter E. Phase changes of matter F. A bit about energy G. Methods to separate substances A. Some definitions: Quantitative observation can be measured numerically Qualitative observation descriptive observation has no numerical measurement Hypothesis is an unproven assumption about something which tries to explain why something is the way it is. Laws vs Theory a theory is a supposition which attempts to explain how something works (theory involves one or more hypothesis), a Law summarizes experimental evidence which points to a consistent way something behaves. Examples of Laws Law of conservation of matter states that matter cannot be created or destroyed in other words, matter does not seem to just magically appear nor disappear it might change form but it still basically exists. Law of conservation of mass = no change is observed in the total mass of substances involved in a chemical reaction. Which means: mass of reactants = mass of products Example: H2O Hydrogen + Oxygen (not a balanced eqtn) 100.0g 11.2 g 88.8 g g reactants = g of products B. Properties of substances Each substance has a unique set of properties classified as either 1) Physical or 2) Chemical 1) Physical properties characteristics determined by not altering composition i.e, color, odor, taste, state, density, melting point, boiling pt etc. 2) Chemical properties describe the ability of a substance to form new substances by reaction with other substances or by decomposition No 2 substances have identical physical and chemical properties. P a g e 1 8

2 2 Chem 047: Physical and chemical properties The physical properties of substances can also be described as 1) intensive 2) extensive 1) Extensive properties is a physical property which changes when the amount of the substance changes. For example: volume, mass, length, total charge. 2) Intensive properties is a physical property that has nothing to do with how much of it is present, such as temperature, color, hardness, melting point, boiling point, pressure, molecular weight, and density. Intensive properties are often characteristic of a particular material, so can be helpful in identifying unknown substances. Physical changes Changes in physical properties such as size, shape, density, state (solid, liquid, gas) etc. i.e., changing of ice into water or steam are physical changes NO change in composition Chemical changes New substances are formed that have different properties and composition. Shown by chemical equations: Reactants Products Starting substances produces ending substance or end product Chemical changes are often accompanied by physical changes but when classifying as physical vs chemical if any chemical change has occurred it is considered primarily a chemical change. Examples: Rusting of iron chemical change Boiling water physical Burning of S in air chem Boiling an egg chem Combustion of gas chem Digestion of food chem Sawing wood physical Burning wood chem Heating glass physical P a g e 2 8

3 3 Chem 047: Physical and chemical properties C. The Classification of Matter (= classification of substances) Matter = anything that has mass and occupies space Can be invisible All matter composed of tiny particles called atoms Matter (=substances) Homogenous substance (1 phase) Heterogeneous substances (2 or more phases) Pure substance 1 type of atom More than 1 type of atom Homogenous mixture (made of 2 or more substances) Mechanical mixtures (made of 2 or more substances) Element Compound Solution There are various ways matter is divided up remember classification of anything is scientist s attempts to make order from chaos classification of matter is usually pretty close to the above flow chart; however, some may classify all mixtures together on one side and pure substances on the other needless to say, it boils down to the same definitions for all the divisions. Hebden has chosen to divvy up matter using the above. Homogenous substances: are substances that appear to be the same throughout and has no visible phase boundaries. A homogenous substance can be a pure substance (element or compound) or a mixture. Pure Substances: are a type of matter with a definite, fixed composition pure substances are either elements or compounds. Element: one of the substances listed on the periodic table an element cannot be broken down into simpler substances and still have the same chemical and physical properties. An element only has one type of atom. Atom: is the smallest possible unit of an element which still has the same chemical and physical properties. P a g e 3 8

4 4 Chem 047: Physical and chemical properties Compound: a substance that has 2 or more different atoms chemically bonded (electric forces) together. Versus Molecule: a substance that has 2 or more atoms chemically bonded together. Homogenous mixture: a solution that is made up of two or more substances (this means physically mixed, not chemically bonded like a pure substance) that is uniform in appearance. A solution can be any of the following: Solid in solid Solid in gas Solid in liquid Liquid in liquid Liquid in gas Liquid in liquid Gas in solid Gas in gas Gas in liquid A solution is made of a solvent and a solute: A solvent: is the substance that exists in greater quantity. A solute: is the substance that exists in lesser quantity unless that substance is water in the case of water, even if water exists in lesser quantities then the other substance, water is still considered the solvent. Heterogeneous substances: also called mechanical mixture- are substances made up of two or more substances that are nonuniform in appearance. A mixture is a general term to describe both heterogeneous (mechanical) mixtures and homogenous mixtures) More definitions when classifying matter: Compound pure substance that contains 2 or more elements chemically combined in proportion by mass -fall into 2 general types: molecular (held together by covalent bonds) and ionic (held together by attraction between +ive and ive charges on the different atoms) -each compound is distinguished from other compounds by its unique chemical properties and physical characteristics P a g e 4 8

5 5 Chem 047: Physical and chemical properties Molecule smallest uncharged unit of a compound -cannot be further subdivided without destroying chemical nature of substance ie., 1 molecule of H2O - if it is broken down further it will no longer have the properties of water -or one molecule of salt has 1 atom of Na and 1 atom of Cl bonded together Ion positively or negatively charged atom or group of atoms Anion negatively charged ion Cation - positively charged ion Ionic bond attractive force between anion and cation forms an ionic compound Ionic compounds metals and nonmetals combine to form ionic compounds Covalent (molecular) compounds - Nonmetals combine with each other to form covalent compounds Particle is a miniscule fragment of matter such as an atom or molecule or subatomic particle like a proton, neutron, or electron. D. States of Matter Matter (substances) exists in 3 physical states Solid Liquid gas state shape volume particles compressibility Attractive forces Solid Definite independent of container Liquid Indefinite takes shape of container Gas Indefinite no fixed shape fills a container definite definite indefinite Cohere rigidly tightly packed crystalline (3D) or amorphous Cohere firmly thus can move freely mobile particles takes shape of container Move independently of one another particles far apart Very slight slight High and high expansion The state of matter is the form matter takes at a given temperature and pressure E. Phase changes of matter strongest weaker Very weak A phase of matter is characterized by having relatively uniform physical and chemical properties. Matter goes through phase transitions to change from one phase to the other the primary phases of matter are: solid, liquid, and gas. The difference between states of matter and phases of matter is confusing eh? Think of it this way state of matter is the physical form that matter takes at a certain phase. P a g e 5 8

6 6 Chem 047: Physical and chemical properties Some definitions look at the graph when learning the definitions. Melting temperature the temp where solid turns to liquid Freezing temperature the temp where liquid becomes solid Boiling temperature the temp where liquid becomes gas Condensation temperature the temp where gas becomes liquid When looking at this graph think of these two definitions: Heat = energy of all the total molecular motion in a substance Temperature = measure of the average energy of the molecular motion in the substance. Let me try and explain with an example temperature does not depend on how much of a substance there is - the classic example is a cup of water may have the same temperature as a bathtub full of water, but it the bathtub has more heat just because there is more water and therefore more total thermal energy. Notice that the temperature does not rise during a phase change this is because the substance needs to use all the energy into breaking the molecules apart or putting them together, this is not a change in kinetic energy (energy of motion) but a change in bonding energy so temperature will not rise until all the substance has changed from one phase to the next, then kinetic energy (energy of motion of molecules) will start changing again In other words, the average energy of molecular motion (temperature) will change again. P a g e 6 8

7 7 Chem 047: Physical and chemical properties F. A bit about Energy Energy = capacity of matter to do work Energy exists in many forms such as mechanical, chemical, electrical, heat, nuclear, radiant, light etc Potential energy (PE) = stored energy that object possesses due to its relative position. Kinetic energy (KE) = energy that matter possesses due to its motion moving bodies have KE Energy can be converted from one form to another Units for heat Unit = Joule (J) SI unit or Calorie (cal) 1 cal = J 1 cal or J = Amount of energy needed to change the temp of 1 g of water by 1 o C Because calorie and joule are very small we usually use: kilojoules (kj) or kilocalories (kcal) to express energy 1kJ = 1000J 1kcal = 1000 cal Energy in chemical changes In all chemical changes, matter either absorbs or releases energy 1. Chemical changes can produce different forms of energy Examples: the chemical change going on in a car battery produces electric energy, the chemical reaction of the combustion of fuel produce heat and light energy chemical changes in our cells produces the energy to make our body function 2. Energy can be used to produce chemical changes used to produce energy rather than a new substance - Example: electric energy is used to cause the chemical change which occurs in electroplating of metals Many chemical changes are often used for the energy they make rather than the new substances they produce - i.e. the energy the combustion of fuel creates, is more important than the substances that are formed as a result of the reaction. i.e. methane + O2 CO2 + water but it is not the CO2 or water we care about, but the energy this reaction gives off. P a g e 7 8

8 8 Chem 047: Physical and chemical properties Law of Conservation of Energy = energy can neither be created nor destroyed but it can be transformed from one form of energy to another. Energy transformation occurs whenever a chemical change occurs G. Methods to separate substances Note Hebden for a description of the following methods: 1. Hand separation 2. Filtration 3. Evaporation 4. Distillation 5. Solvent extraction 6. Recrystallization 7. Gravity separation 8. Paper, column, and thin layer chromatography P a g e 8 8