Chemistry - the science that describes matter properties physical and chemical changes associated energy changes

Transcription

1 Chemistry - the science that describes matter properties physical and chemical changes associated energy changes Matter - occupies space and has mass. Ex. Textbook Energy is the capacity to do work or transfer heat Kinetic energy doing work directly Rolling boulder (motion) potential energy capacity to produce kinetic energy boulder at top of mountain coal Hold textbook, drop on floor potential energy converted to kinetic energy all chemical and physical processes are accompanied by energy changes exothermic heat energy is released, ex. combustion. endothermic heat energy is consumed, ex. melting ice. Law of nature - a statement with no known exceptions. Based on experimental observation, but does not explain. 1

2 Theory - explains experimental behavior. hypothesis - explanation that is not yet supported by experimental evidence. particularly important laws of nature: The Law of conservation of matter quantity of matter remains the same during a chemical reaction or a physical change. In other words, matter is neither created nor destroyed. The law of conservation of energy energy cannot be created or destroyed in a chemical reaction or in a physical change. It can only be converted from one form to another. In other words, there s no such thing as a free lunch. If we combine these two laws, we get the law of conservation of matter and energy the combined amount of matter and energy in the universe is fixed. This can be summed up by the familiar equation E = mc 2 E energy m mass c speed of light, 3.0 x 10 8 m/s 2

3 Matter can be classified into three states: (Fig 1-2) solid rigid, definite shapes. Volume does not change much with pressure and temperature liquid flows to fit shape of container. Volume does not change much with pressure and temperature Gas occupy all parts of any vessel in which it is confined. Much less dense than liquids or solids volume changes dramatically with pressure and temperature properties - characteristics by which we distinguish different kinds of matter. Chemical properties exhibited during a chemical change (a composition change). ex. Combustion. Physical properties exhibited during physical changes (NO composition change). ex. change in state water freezes at 0ºC. another way to categorize properties: extensive properties depend on quantity present volume mass intensive properties do not depend on quantity present melting/boiling point color 3

4 the changes of matter are analogous to the properties of matter: chemical changes involve a change in the composition of matter. One (or more) substances are used up One (or more) substances are formed Energy is absorbed or released Ex. iron rusting. Iron becomes iron oxide. Ex. combustion of coal. Physical changes involve no change in the composition of matter. However, energy is still absorbed or released. Figure 1-3. ex. changes of state in H 2 O. H 2 O is still H 2 O. Figure 1-5. ex. changes of state in H 2 O. changing states requires heat. chemical composition - another way of classifying matter. Elements substances that cannot be decomposed into simpler substances by chemical reactions. simplest forms of matter. Represented by a symbol in the Periodic Table Atom the smallest particle of an element that maintains its chemical identity through all chemical and physical changes. Most elements are found in nature as atoms. 4

5 compounds substances that are composed of two or more elements in a definite ratio of mass. Can only be decomposed into elements by chemical changes. Represented by combinations of element symbols in a definite ratio. Molecule the smallest particle of two or more atoms that can have a stable, independent existence. Ex. H 2 O molecule. what if two atoms of the same element are combined to make one particle? Is it a compound? No, it s a molecule of that element. Example: O 2. substance any kind of matter which cannot be purified or further broken down by physical means. Has its own characteristic, unique properties Composition does not vary Mixture two or more substances mixed together The composition can be varied The substances retain their own characteristic properties homogeneous (solution of salt in water) heterogeneous (Mississippi river). Figure 1-8 electrolysis of H 2 O into its elements, H 2 and O 2. Law of constant composition different samples of any pure compound contain the same elements in the same proportions by mass. 5

6 scientific notation. a shorthand that scientists use when dealing with very large or very small numbers. 108g Ag =602,000,000,000,000,000,000,000 Ag atoms. 1 Ag atom = gram significant figures. Exact number - no estimation is involved. Most numbers are not exact. Accuracy describes how closely measured values agree with correct values. Precision describes how closely individual measurements agree with each other. Significant figures the digits believed to be correct by the person making the measurements. Ex. 10 ml cylinder is graduated in tenths ml how many significant figures? Ex. population of a city is 2,678,342 people. How many significant figures? 6

7 some rules for significant figures. Zeroes - leading zeroes - never significant trailing zeroes sometimes significant especially when there is a decimal point in the number zeroes in the middle of a number (4,309) are significant math involving significant figures. Addition/subtraction multiplication/division x 1.4 rounding in a multi-step calculation, carry all digits (even non-significant ones) until the end of the calculation. However, if you are reporting the results of each step of the calculation, report the correct number of significant figures. 7

8 Unit Factor method (also called dimensional analysis). 1 foot = 12 inches. This relationship can be converted into two unit factors: ex. 1 foot 12 inches 12 inches = 1 foot = yards =? millimeters? 9.32 yards 3 feet 12 inches 1 meter 1000 mm x 1 yard x 1 foot x inches x 1 meter = 8520 mm 2.61 cubic feet =? milliliters? 2.61 ft 3 (12) 3 in 3 1 m 3 (100) 3 cm 3 1 ml x 1 ft 3 x (39.37) 3 in 3 x 1 m 3 x 1 cm 3 = 7.39 x 10 4 ml 8

9 Density and Specific Gravity. density mass per unit volume. D = M/V (g/ml) 742 g / 97.3 ml = specific gravity the ratio of the density of a substance to the density of water. Density (substance) / Density (water) = specific gravity the density of water is very close to 1.00 g/ml at room temperature g Cr dropped in 5.00 ml H 2 O. volume expands to 9.32 ml. What is specific gravity of Cr? 9

10 Heat. Temperature the intensity of heat in a body. Three different scales: Fahrenheit bp H 2 O 212ºF Celsius bp H 2 O 100ºC 5 x (ºF 32)/9 Kelvin - bp H 2 O 373 K (Celsius + 273) joule (J) - the SI unit of heat and energy. calorie the amount of heat required to raise the temperature of 1 g of H 2 O from 14.5ºC to 15.5ºC. 1 cal = J. specific heat heat (joules) required to raise temp of 1g of substance 1ºC with no change in state. Heat capacity heat required to raise temp of 1 mole of ubstance 1ºC with no change in state. Specific heats of a few common substances Substance Ice Liquid water Steam specific heat 2.02 J/gºC 4.18 J/gºC 2.03 J/gºC Ex. calculate the amount of heat required to raise the temperature of 200 g H 2 O from 10.0ºC to 55.0ºC. 10