Chemical Reactions: The Basics

Chemical Reactions describe chemical changes.  Chemical Equations are shorthand descriptions of chemical reactions that use coefficients, symbols and subscripts to describe the ratios of a reaction.  We call the substances before a reaction the reactants and we call the substances that are formed the products.

Antoine-Laurent de Lavoisier (1743-1794) is considered by many to be the father of chemistry. He was the first to clearly state the Law of Conservation of Mass.  This states that matter is neither created nor destroyed in a chemical change.  We can now add to that the idea that the atoms or building blocks of matter are simply rearranged in a chemical reaction.

John Dalton (1766-1844) took Lavoisier’s ideas further by developing the first basic atomic theory.  He stated that an atom is the smallest unit of an element that can exist either alone or in combination with other atoms of the same or different elements.

His supporting evidence:

1. All matter is made up of very small particles called atoms.

2. Atoms of the same element are all chemically alike: atoms of different elements are chemically different.

3. Individual atoms of the same element may not have the same exact mass (isotopes), but for all practical purposes, they all have a definite average mass.

4. The atoms of different elements have different average masses.

5. Atoms are not subdivided in chemical reactions, they unite in simple ratios to form compounds.

He also developed the Law of Multiple Proportions.  If two elements combine to form more than one compound, they will combine in distinct whole number ratios.

Matter: Part 2 Mixtures

Pure substances are uniform throughout with a definite composition and properties, while mixtures are physical combinations of two or more pure substances. The properties of the substances in a mixture retain their own properties.

We can further divide pure substance in chemistry into elements and compounds.  An element is basically the name of a type of atom defined by the number of protons in the nucleus.  A compound in a chemical combination of two or more elements.  A compound is the name of a type of molecule (chemically bonded atoms).

Note:  Physical combination means that substances are just dispersed or close together. A chemical combination means that the atoms are chemical attached creating new molecules with new properties.  Chemical bonds cannot be separated by physical means.

Mixtures can be divided into homogeneous mixtures that appear to be a single substance and heterogeneous mixtures that are obviously two or more substances.

All mixtures are composed of a solute and a solvent. The solute is the substance that is dissolved or dispersed, while the solvent is the substance that does the dissolving.  The solvent separates and keeps the solute particles apart.

Mixture can be divided into three categories:

Solutions are homogeneous and clear.  The particles are so small they cannot be seen and do not reflect light.

     Ex:  windex, tap water, air

Colloids are homogeneous but appear cloudy.  Some of the particles are large enough to reflect light even though they can’t be seen with the naked eye.

     Ex:  milk, fog, mayonnaise

Suspensions are heterogeneous.  Given time gravity will separate a suspension with the most dense particles on the bottom, and the least dense (lightest) particles rising to the top.

     Ex:  Italian salad dressing, oil and water

Matter: Part 1 Describing Matter

In lab we make both qualitative and quantitative measurements.  We can further describe matter in terms of extensive and intensive properties.  Extensive properties are quantity specific, such as mass and volume. The mass of a sample of water depends on how much water you have in a sample.  Intensive properties are dependent only on the type of matter, not the quantity.  The density of water is 1.0 g/mL whether you have a drop or a swimming pool full.

We can further divide observations into physical and chemical properties.  Physical properties describe matter without changing its composition.  Chemical properties describe how matter interacts or changes with other matter.

Physical Properties                                Chemical Properties

Color                                                                Rusting

Odor                                                                 Burning

Density                                                            Tarnishing

Boiling Point

Malleability

The state of matter, and its boiling and melting points are all physical properties.  The state of matter is determined by the arrangement of its particles.

The change of state of a substance is a physical change.

                                    Boiling                                                Evaporating

                                    Melting                                               Condensing

                                    Freezing                                              Sublimation

                                    Solidifying

If the identity of a substance is changed, or a new substance is formed, it is a chemical change.

Density with UA

You must use unit analysis to solve density problems in chemistry.  Remember you are learning the process  of unit analysis not simply finding the correct answer!

Use the technique you have learned in problem solving:  what are you looking for, what are you given, put the units together to cancel and solve.

Example #1

A sample of a known metal is dropped into a graduated cylinder with 25.0 mL of water. The level rises to 31.5 mL. If the given density of the metal is 2.56 kg/L, what is the mass of the sample in grams?

Example #2

What is the volume of a sample of aluminum in L that weighs 56.7 g?  The density of aluminum is 2.71 g/mL.

Possible Questions

Why flip the density?  To make sure the units of volume will be in the numerator.

I got a different answer, why? Remember that 2.71 is in the denominator.  You must divide not multiply. You can either punch in 56.7 first or start with 1/2.71.

Problem Solving with UA

Unit Analysis can make solving complex word problems much easier.  First, DON’T BE INTIMIDATED!  The problem is not going to jump off the page and bite you if you get it wrong!  Just TRY!  Follow these basic steps to simplify problem solving.

1. What are your looking for?  Read through the problem and determine the exact units requested.

          WRITE THAT DOWN!

2. What are you given?  Sometimes, there is so much information given it is a good idea to write it all down or underline it in the problem.  It also helps if you will label what type of given information it is.  For instance:  mass, distance, …

3. Is there any other information you need?  Conversions, molar mass, reactions, …

4. Put the units together in such as way that you cancel out the units you don’t want and end up with only the units requested.  If the units are reversed, just flip your calculation.

Example #1

A farmer has 2 cows and he decides to change to chickens.  He can barter 4 emu for each cow, 3 emu for 5 pigs, 8 pigs for 3 llama,  a llama for 20 rabbits and 3 rabbits for 2 chickens.  How many chickens can he get for both his cows?

1. What are you looking for?  chickens

2. What are you given?  Many ridiculous ratios with animals.

3. Is there any other information needed?  Not for this problem, just watch out where you step.

4. Use UA to determine the units requested.

Example #2

A machine produces 4.5 x 10³ m of spaghetti noodles each minute. A package of noodles contains 128 noodles that are each 12.5 inches long.  The company sells the noodles in cartons containing 20 packages for $75.50.  If the machine runs 12.0 hr a day, 5 days a week, 50 weeks a year, how much money can the company make each year from that one machine?

Unit Analysis: Both Numerator & Denominator

Changing both the unit in the numerator and the denominator is just like changing only one set of units.  Just remember that all the units except the ones needed must cancel.

Change 3.40 m/sec to km/yr.

Unit Analysis: The Basics

Unit analysis or dimensional analysis is a method used to calculate values based on the units of each measurement.  We will start by using this method to simply convert one measurement in one unit to another unit. This technique may seem more complicated than necessary at this point, but remember you are learning how to use the units.  Later in the semester you will see that unit analysis will make problem solving so much easier!

Unit analysis is based on two very fundamental mathematical principles.

1. any number multiplied by one is equal to itself
2. a fraction equals one if the value of the numerator equals the value of the denominator

These two properties allow us to let the measurements determine how to do the calculation.  The final answer must have the units desired and all other units must be canceled.

Measurement, Part 2: Measurement & Uncertainty

There are two types of numbers in science: exact (counting) numbers and inexact (measurements and calculated quantities).  Exact or counting numbers represent objects.  For instance, a dozen eggs has exactly 12 eggs.  You can’t have 12.01 eggs.  Measurements and numbers based on calculations will always have some uncertainty. Significant digits are used to represent that uncertainty or the amount of confidence you have in a measurement.

Uncertainty occurs because we use equipment to make measurements.  You can only measure a length as exact as the increments on the ruler you are using.  Significant digits are the numbers we know with certainty plus one more that is estimated.

Basic Rules:

1. All non-zero digits are significant
2. All zeros between non-zero digits are significant
3. Zeros to the right of the decimal and to the right of a non-zero digit are significant
4. Zeros to the right of the decimal, but to the left of all non-zero digits are not significant
5. If there is no decimal, zeros to the right of the last non-zero digit are not significant

Rules for Calculations:

1. In addition and subtraction, use the LEAST number of DECIMALS.
2. In multiplication and division, use the LEAST number of SIGNIFICANT DIGITS.
3. Apply each rule using the order of operations.

Measurement, Part 3: Accuracy and Precision

Accuracy and precision are terms used to explain the sources of error in a data set.  Accuracy describes how close a measurement is to the correct answer. Precision describes the spread of the data or how close the measurements are to each other.

To determine the accuracy of a measurement, the correct or accepted value must be known.  The most common calculation associated with accuracy is percent error.

percent error = |(accepted value - experimental value)|   x   100

                                   experimental value

The precision of a data set can be determined in a number of ways, including range, standard deviation and percent deviation. Range is determined by subtracting the smallest value from the largest value in a data set.

Deviation literally means difference, so we can calculate it using subtraction.  By finding the difference between an individual measurement and the average of all the measurements in a data set, we can find how "off" that single measurements is from all the others. A very basic way of looking at standard deviation is to think of it as the average of all the deviations of the individual measurements from the average of the data set.  

The problem with simply using standard deviation to determine precision is magnitude (the size of the numbers.) A standard deviation of 1.00 may sound large or small without some idea of the magnitude of the measurements in the data set.  If your measurements range from 1.20 to 3.56, it is huge!  But if the range of the data is 1000.0 to 1002.0, it would be much more acceptable.

Percent compares the part to the whole, so it takes away the uncertainty of magnitude.  Percent deviation allows us to compare the standard deviation to the average of the data set.  The lower the percentage that each individual measurement differs from the average of the data set, the better the precision.

percent deviation = standard deviation  x  100

                                average of the data set

Measurement, Part 1: Introduction

All science is based on analyzing data. There are two types of data in chemistry. Qualitative data is based on descriptions such as color, state and luster. Quantitative data is based on numerical measurements.

Chemistry represents its quantitative data using the metric system.  Mass is measured in grams, volume in liters, length in meters, and temperature in Celsius or the Kelvin scale. EVERY QUANTITATIVE MEASUREMENT MUST HAVE BOTH A QUANTITY AND A UNIT.

Numbers mean nothing without a unit!