Sec. 2.1 Energy Objectives: 1. Explain that physical and chemical changes in matter involve transfers of energy 2. Apply the law of conservation of energy to analyze changes in matter 3. Distinguish between heat and temperature 4. Convert between Celsius and Kelvin temperature Vocabulary energy physical change chemical change evaporation Energy and Change endothermic exothermic law of conservation of energy heat kinetic energy temperature specific heat Energy is the capacity to do some kind of work such as moving and object, forming a new compound, or generating light. Energy is involved whenever there is a change in matter. Physical Change does not change matter at the molecular level Chemical Change changes matter at the molecular level and creates different substances. Every physical or chemical change in matter involves a change in energy. All material that has a temperature has energy. The amount of energy a substance has determines the movement of the particles that make up the substance. Molecules in solids are held in a crystal lattice, but they still vibrate. Melting (fusion) occurs when molecules have enough energy to slide past one another. Evaporation occurs when molecules have enough energy to leave a liquid s surface. 1
Law of Conservation of Energy states that energy cannot be created or destroyed but can be changed from one form to another. When energy appears to be created or destroyed, energy is actually being transformed from one form to another. Forms of energy include: heat mechanical light electrical chemical sounds Photosynthesis transfers light from the sun into chemical energy. Chemical energy the energy that is released or absorbed when a substance undergoes a chemical reaction. Endothermic occurs when energy is absorbed from the surroundings during a chemical reaction. Exothermic occurs when energy is released to the surroundings during a chemical reaction. Heat energy transferred between objects that are at different temperatures. Note: energy is always transferred from the high temperature object to the lowertemperature object until thermal equilibrium is established (both objects are the same temperature). Change of State heat transfer does not always cause a temperature change. Temperature does not change during a change of state (solid to liquid, liquid to solid, liquid to gas, gas to liquid, solid to gas, gas to solid) because all energy is used to transfer molecules between states. Temperature a measure of the hotness or coldness of an object. Temperature indicates the average movement or kinetic energy of the particles in an object. Kinetic energy is the energy of an object that is due to the object s motion. The English system unit for temperature uses Fahrenheit ( o F). The metric system uses Celsius and Kelvin. t ( o C) = T (K) - 273K T(K) = t ( o C) + 273K Specific heat the amount of heat needed to raise one unit mass of material 1 K or 1 o C. The SI unit for energy is the joule (J). Specific heat can be expressed as joules per gram per change in kelvin (J/g oc). Metals have low specific heats. Water has an extremely high specific heat. Example: how much heat is needed to raise the temperature of 5 g of water from 20 o C to 100 o C given that the specific heat of water is 4.2 J/g oc? (5 g)( 100 o C - 20 o C)(4. 2 J/g o C) = 1680 J 2
Section 2.2 Studying Matter and Energy Objectives: 1. Describe how chemists use the scientific method. 2. Explain the purpose of controlling the conditions of an experiment. 3. Explain the difference between a hypothesis, a theory, and a law. Vocabulary: scientific method hypothesis theory law law of conservation of mass The Scientific Method is a strategy for determining sound, defendable conclusions on how nature works. A general sequence of steps used in the scientific method is shown below. Aspects of Scientific Research: Conclusions are based on experimental results Experimental results often do not turn out as expected Unexpected results are as important as expected results Some important discoveries are due to serendipity, e.g., Teflon, synthetic dyes Ultimately, the success of the scientific method depends on publishing research so that others can repeat experiments to verify results. Scientific Explanations Scientists look for patterns that suggest an explanation for observations of the natural world. The proposed explanation is called a hypothesis. A scientific hypothesis must be a reasonable and testable explanation for observations. Scientists design controlled experiments to test the validity of their hypotheses. A good experiment identifies all factors that could account for observations and results. A factor that could affect results is called a variable. The types of variables in a controlled experiment include: 3
Independent variable: the variable that is changed. To ensure a fair test, a scientific experiment has only one independent variable. As the scientist changes the independent variable, he or she observes what happens. Dependent Variable(s): the variable that is observed. The scientist observes how the dependent variable responds to the changes made to the independent variable. The new value of the dependent variable is caused by and depends on the value of the independent variable. There can be more than one dependent variable. Control variables: Controlled variables are quantities that a scientist wants to remain constant, and they must be observed as carefully as the dependent variables. Good variables must be able to be measureable. Mass is an example of a variable that is easy to measure. However, imagine trying to do an experiment where one of the variables is love. There is no such thing as a "love-meter." This creates a situation where scientific progress depends on the technology of the measuring device. Scientific Theory A hypothesis that is supported by repeated testing may become part of a theory. In science, a theory is a well-tested explanation of observations. Because theories are explanations, not facts, they can be disproved but can never be completely proven. Note that in common usage the word theory means a guess. Do not get confused. Scientific Law A scientific law is a statement or mathematical expression that reliably describes a behavior of the natural world. A theory attempts to explain the cause of events in the natural world, a scientific law describes the event. For example, the Law of Conservation of Mass states that the products of a chemical reaction have the same mass as the reactants. Or, matter can be neither created nor destroyed. This law does not attempt to explain why, it simply describes the behavior. Models Models play a major role in science. A model represents an object, a system, a process, or an idea. A reliable model may become a scientific law. In chemistry, models are used to describe objects, particles, and systems that are too small to see. 4
Sec. 2.3 Measurement and Calculations in Chemistry Objectives: 1. Distinguish between accuracy and precision in measurements. 2. Determine the number of significant figures in a measurement and apply rules for significant figures in calculations. 3. Calculate changes in energy using the equation for specific heat, and round the results to the correct number of significant figures. 4. Write very large and very small numbers in scientific notation. Vocabulary accuracy precision significant figure scientific notation Accuracy and Precision No value collected in an experiment is exact because all measurements are subject to limits and errors. The main types of experimental errors are: human errors method (procedural) errors equipment errors Equipment Errors The first step in reducing experimental is to choose the correct piece of equipment to make measurements. The accuracy of a piece of equipment is based on the smallest division or interval used to read a value. For example, you would not use a 400 ml beaker to measure 8.5 ml of fluid because there is 50 ml (accuracy ±25 ml) between the smallest intervals on the beaker. To measure 8.5 ml, it is necessary to use a 10 ml graduated cylinder, whose accuracy is ±0.05 ml. Accuracy and Precision: These terms have very different meanings: An accurate measurement is one in which the results of the experiment are in agreement with the accepted value. * A precise measurement is one that we can make to a large number of decimal places. *This only applies to experiments where accuracy is the goal measuring the speed of light, for example. 5
Significant Figures Scientists always report values using significant figures. The significant figures of a measurement or a calculation consist of all the digits known with certainty as well as one estimated, or uncertain digit. Notice that the term significant does not mean certain. The last digit or significant figure reported after a measurement is uncertain or estimated. Significant figure rules are used so that everyone understands the precision of the reported experimental data. 1. If there is a decimal point anywhere in the number: Start with the first non-zero number and count all digits until the end. 2. If there is not a decimal point anywhere in the number: Start with the first nonzero number and count until the last non-zero number. When using a calculator to find a result, you must pay special attention to significant figures to make sure that your result is meaningful. Adding & Subtracting: 1. Perform the calculation 2. Determine the least # of decimal places in problem 3. Round answer to that # of decimal places Multiplying & Dividing: 1. Perform the calculation 2. Determine the least # of sig figures in problem 3. Round answer to that # of sig figures Exact Values Note that exact values have unlimited significant figures. This is because there is no uncertainty with exact values. Another value that can have an unlimited number of significant figures is a conversion factor. There is no uncertainty in a conversion factor such as 1 m = 1000 mm, because the millimeter is exactly one-thousandth of a meter. Measurements & Uncertainty When measuring volumes, always read from the bottom of the meniscus. When recording measurements, record 1 more decimal place than the lines give you. o Record a 0 at the end if it is on the line o Record a 5 at the end if it is in-between lines Zero s at the end of a reading are important they indicate that it was on the line Scientific Notation Scientific Notation is a short hand method of writing numbers using powers of 10. The rules for writing scientific notation follow: 1. The decimal point is always moved to after the 1st nonzero number. 2. Count the number of times the decimal point is moved and use this as the power of 10. 6
3. Big numbers (>1) have positive exponents. Small numbers (<1) have negative exponents. Reading scientific notation: 1. Power of 10 = number of times to move decimal point 2. Positive powers = make the number Big (>1). 3. Negative exponents = make the number Small (<1) Specific Heat The specific heat of a substance is the quantity of energy that must be transferred as heat to raise or lower the temperature of 1 g of a substance by 1 K. The quantity of energy transferred as heat during a temperature change depends on the nature of the material that is changing temperature, the mass of the material and the size of the temperature change. Specific heat is usually measured under constant pressure conditions, indicated by the subscript pi in the symbol for specific heat, cp. c p = q m x ΔT Where cp is the specific heat at a given pressure, q is the energy transferred as heat, m is the mass of the substance, and ΔT is the difference between the initial and final temperatures. Element Specific Heat (j/g K) Element Specific Heat (j/g K) Aluminum 0.897 Lead 0.129 Cadmium 0.232 Neon 1.030 Calcium 0.647 Nickel 0.444 Carbon (graphite) 0.709 Platinum 0.133 Copper 0.385 Silver 0.235 Gold 0.129 Water 4.18 Iron 0.449 Zinc 0.388 7