Ch 2 Matter and energy Energy and change Energy: the capacity to do work and involves a change in matter. Changes in matter either physical or chemical

Changes Physical change: change from one form to another without a change in chemical properties Usually a change in phase (solid, liquid, gas) Chemical change: change of one or more substances into new substance(s) with different properties Rearrangement of atoms with breaking & reforming chemical bonds.

Change involves energy change Either add or release energy Phase D s<->l melt or freeze. Phase D l<->g evaporate or condense endothermic 2 gas Temp. (K) liquid 1 exothermic solid Energy (J)

Endothermic and exothermic process Endothermic: heat is absorbed from the environment. (feels cold) Exothermic: a system that releases heat into the environment. (feels hot) Law of conservation of energy: Energy cannot be created or destroyed but can be change from one formed to another.

Energy Energy transferred: System <--> Surrounding System: components being studied. Surrounding: everything outside the system. The total energy is constant. Forms of energy (units joules) Chemical, mechanical, light, heat, electrical, and sound. Transfer of energy involves anyone of these forms

Heat Energy transferred between objects of different temperature from high energy (temp) to low energy (temp) until thermal equilibrium is reached. Exothermic: reactants --> products + energy Endothermic: energy + reactants --> products Energy from bonds breaking and bonds forming.

Heat vs. Temperature Heat: is measure of transferred energy and depends on the amount of substance. Temperature: measure of molecular motion (KE) and is independent of amount. Temperature determines the direction of energy flow between matter  temperature measure energy transferred.

Temperature scales Kelvin (K) and Celsius (oC) scales used in chemistry Kelvin scale zero pt. (absolute zero) at which all matter has minimum KE. T (K) = T (oC) + 273.15 Convert 25oC to K Convert 300K to oC T would be the same with either scale

Temperature Scales Cont.. Change Fahrenheit to Celsius use: 0F = (1.8 x C) +32 0C = 5/9 (F -32) Convert the Following 65 F to C 25 C to F 100 C to K 0K to C 50 K to F

Heat transfer and T No temperature during phase change even though energy is added. Added energy is used to break intermolecular bond rather than increasing molecular motion. 2. Temperature (oC or K) gas Heat of fusion Heat of vaporization 1. liquid solid Energy added (kJ)

Specific heat (J/gK oroC) Quantity of heat required to raise a unit mass of homogenous material 1 K or 1oC at constant P & V. (pg 60 table 1) Low SH: Little energy transferred during T High SH Large amount of energy transferred during T q = SH x m x T q = heat transfered

Studying matter and energy Scientific method: (pg46, figure 8) series of steps followed to solve problems. Strategy for drawing conclusions Collect data, formulate hypothesis, test hypothesis and state a conclusion.

Experiments Process which scientific ideas are tested. Unexpected results often give scientists as much information as expected results do. Lead to more experiments Sometime lead to important discoveries by accident (Teflon, synthetic dyes)

Scientific explanations Hypothesis: a theory or explanation that is based on observations and can be tested. (carry out experiments) Experiments: Test validity of hypothesis Identify factors that account for observations. (variables) Only one variable at a time, the rest called controls.

Experiment results Theory: an explanation for some phenomenon that is based on observations, experimentation and reasoning. Law: a summary of many experimental results and observations, how things work. Theories are explanations and cannot be completely proven (atomic theory, law of falling bodies) Models: used to explain scientific observation Used to make predictions Usually simplified representations.

Measurements and Calculations Measurements are limited by human error that can be reduced repeating and selecting proper measuring device. Accuracy: how close a measurement is to the true value of the quantity measured. Precision: exactness of a measurement. How close several measurements agree with one another. Determined by measuring device and technique. Good precision does not guarantee good accuracy.

Significant figures Related to the precision of a measured quantity. All digits know with certainty as well as the first estimated (uncertain) digit. Ruled to determine sig figs of a measured quantity. 1. Nonzero digits are significant 2. Zeros between sig figs are significant 3. Ending zeros to the right of the decimal are significant. Determine the number of sig figs in each of the following: A. 218 kPa B. 0.025 L C. 200. m D. 1.05 g

Calculations with sig figs Add or subtract: Same units and power of 10 prefix Least # of sig figs to the right of decimal. Multiply or divide Do the same math operation to the units as with the digits. Least total # of sig figs. Complete the following: A. 23.51 g + 2.1g B. (34.22m + 105m) / 2.115s

Specific heat calculations q = Cp x m x T q: heat flow, Cp: specific heat capacity of substance (pg60 table 1), m: mass of substance, T: change of temperature (Tf-Ti) Complete the following: A. A 5.00g sample of a metal was heated from 25.0oC to 40.0oC and it absorb 17.6J of energy. What is its specific heat capacity? Identity? B. Air has a heat capacity of 1.007J/goC and density 1.161 g/L. How much energy is needed to heat 2.00L of air from 293K to 298K?

Scientific notation Used to represent very large or small numbers Significant figures need to be maintained. 2 parts: Number (X) 1< X <10 Power of 10n n: positive or negative integer Determine the decimal place of the value. Form: X x 10n