Percent Yeild

In doing stoichiometry calculations, we assume things do not go wrong (there is no error). In real terms this assumption is unrealistic. No reaction is going to proceed perfectly. When an equation is used to calculate the amount of product that will form in a reaction, then the value obtained is called the theoretical yield.

By contrast the amount of product that forms when a reaction is carried out in a laboratory is called the actual yield. The percent yield therefore is a ratio of actual yield to the percent yield. The percent yield is therefore a ratio of the actual yield to the percent yield.

Percent yield = (Actual yield/Theoretical yield) x 100

Limiting Reagents

As the name implies, limiting agents limits or determines the amount of product that can be formed in a reaction. The reaction proceeds until the limiting agent is used up. Then the reaction stops. The opposite of the limiting reagent is the excess reagent. The quality of an excess reagent, is more than enough to react with a limiting reagent.

Moles

In chemistry we measure quantities in many ways. We measure mass in Kg, volume in litres or cm3, for chemical quantities we also use the mole. A mole represents a number of particles. This is a very large number. This number is 6.02E23, and known as Avogadro's number. So a mole in chemistry represents Avogadro's number particles of any given substance.

The mole is one of the seven SI units, and it is represented by the symbol 'n' when used in a mathematical formula. We understand that matter is made of different kinds of particles, so the term representative particles refers to either atoms, ions or molecules.

Molecular mass

Mole = Mass/Molecular Mass

Measured in g/Mol
E.g. H2O 1+1+16 = 18g/Mol

Volume of a Mole of gas

Mole = Particles/Avogadro's number

The volume of a mole of gas is more predictable than the volume of a mole of a liquid or a solid. The volume of a mole of gas is usually measured at Standard Temperature & Pressure (STP).
Standard temperature is: 0° (273K) and
Standard pressure is: 1 Atmosphere (atm), 760mmHg or 103.8Kpa
At STP, one mole of a gas occupies a volume of 22.4 litres. This quantity of 22.4 litres is also know as the molar volume of gas.

Mole = Volume (litres)/22.4 litres

Balancing Equations

In every balanced chemical equation, each side of the equation (reactants and products) has the same number of atoms of each element on each side.

Rules of Balancing
1. Determine the correct formula for all the reactants and products involved.
2. Write the reactants on the left, and the products on the right.
3. Count the number of atoms of each element in the reactants and products.
4. Balance elements one at a time by adding coefficients to the front of the formulas.
*when there is no coefficient, it is assumed there is an invisible 1.
5. Check each atom to be sure the equation is balanced.
6. Make sure all coefficients are the smallest whole number ratio.

Acids

Acids are compounds that give off hydrogen ions when dissolved in water.

E.g. Hydrochloric Acid:

H+ + Cl- Water HCl

The formula for writing acids usually includes: Hx, where 'x' is a mono-atomic or polyatomic anion and H is Hydrogen.

Common acids are:

Acids

Acids are compounds that give off hydrogen ions when dissolved in water.

E.g. Hydrochloric Acid:

H+ + Cl- Water HCl

The formula for writing acids usually includes: Hx, where 'x' is a mono-atomic or polyatomic anion and H is Hydrogen.

Common acids are:

Covalent compounds aka Molecular compounds, are formed from non-metals that share electrons. Because there are many sharing between two non-metals, the formula cannot be guessed unless we have a naming system that reveals the atoms involved.

For this we use a set of prefixes:

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The only time we drop a prefix is if the mono is to appear at the beginning of the name.

E.g. CO = Carbon Monoxide

CO2 = Carbon Dioxide *Note we don't say 'mono carbon'.

This doesn't work for metals, only non-metals. There last element changes to an 'ide'.

Polyatomic ions

Polyatomic ions are ions that are formed from the combination of multiple atoms. When a polyatomic ion forms, the whole group of atoms act as a unit, including the charge e.g. Sulphate = SO4-2

It contains one sulphur atom and four oxygen atoms all bound together as one entire unit. The whole unit carries a charge of -2.

Nitrate = NO4-

Contains one nitrogen atom and three oxygen atoms. The whole unit has a charge of -1.

*Most polyatomic ions carry a negative charge.

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Writing chemical formulas, the charges of the ions must be equal. Compounds are always neutral.

E.g. Sodium(s) + Hydroxide(l) react Sodium Hydroxide.

Na+ + OH- --> NaOH

Ions

In order to be able to write chemical formulas, we need to know the ions that elements tend to form. We can get this information directly from the periodic table for the most common elements we deal with.

Across the top of the table is the symbols 1A through to 8A (or 0). We can determine the ions these elements have by directly counting along their columns.

Compounds and Molecules

Compounds are pure substances that differ from elements, in that they contain more than one kind of atom. Compounds are formed when atoms join chemically. Compounds always obey the law of definite proportions. In any compound, the elements are always combined in the same proportions by mass.
E.g. H2O = Water
= 2g of Hydrogen, for every 16g of Oxygen
In many compounds the atoms are bound together in molecules. A molecule is a neutral group of atoms that act as a single unit. All molecules of an given compound are identical. Compounds composed of molecules are called molecular compounds. They tend to have low melting and boiling points.

Not all compounds are molecular compounds. Ionic compounds are composed of positive and negative ions. The ions are arranged in an orderly 3-D pattern. A formula unit is the lowest whole number ratio of ions in an ionic compound.

A chemical formula shows the numbers of atoms in the smallest representative unit of a compound, there are two types f chemical formulas.

A molecular formula shows the number and kinds of atoms present in a molecule or compound.
E.g. C6H12O6 (Sugar)

A formula unit shows the number of kinds of ions/atoms present in the smallest unit of an ionic compound e.g. MgSO4 (Magnesium Sulphate)

Atoms and Ions

Atoms are neutrally charged particles, as the positively charged particles (protons) equal the number of negatively charged particles (electrons). When atoms lose or gain electrons they become ions.

When the atom loses an electron its overall charge becomes positive, as there are more protons (positively charged) than electrons (negatively charged). A positive ion is called a cation.

When the atom gains an electron its overall charge becomes negative, as there more electrons (negatively charged) than protons (positively charged). A negative ion is called an anion.

Atoms have electrons orbiting around them in defined energy levels. The first level can hold two electrons, and the second level can hold eight, for simplicity, we will say that the third, fourth and fifth can hold eight as well. The need for an ion to have a complete outer layer is why ions form.

Octet Rule

Atoms react by gaining or losing electrons so they can acquire the stable electron configuration of a noble gas (they have complete energy levels). These outer energy levels, electrons are also know as valence electrons.

Common ions:

Isotopes

Isotopes are atoms of the same element with the same number of atoms but a different number of neutrons. Atoms of a same element have the same number of protons, but they can have different numbers of neutrons in the nucleus. This causes a change in the mass of an atom, but doesn't produce a new element.

Hydrogen Isotopes:











The heavier hydrogen atoms are radioactive because they are unstable an tend to break down in the nucleus.

Atomic Mass

The total number of protons and neutrons in the nucleus is the atomic mass number, the electrons are not included, because their weight is negligible.

The mass of individual elements is useful information. But it is unpractical to work with. They should use an isotope of carbon 12 as a basis and then compare the other atoms to the carbon atoms. Carbon has 12.00 AMU - (Atomic - Mass - Units).

E.g. He(4) would have 1/3 the mass of carbon - 4 AMU

Ni (60) would have 5/1 the mass of carbon - 60 AMU

Most of the atoms have an atomic mass, which is approximately a whole number, but atoms like chlorine (Cl) has an atomic mass of 35.453 AMU.

Units; Mass, Length, Volume, Density, Pressure and Specific Gravity

Mass

The amount of matter in an object determines its own mass. Weight is the force due to gravity and can change depending on the force acting on the object. Mass stays the same. Kg is the weight of an object.

Length

The metre (m) is the basic unit for measuring distance or linear measurement.

Volume

The shape occupied by any sample of matter is called its volumes.

Density

Density is the ratio of the mass of an object, to its volume. Density is a physical property of a substance.

Density = Mass/Volume

The standard units for density are grams per cubic centimetres (g/cm3) though grams per litre (g/l) may be used.

Pressure

Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of air above that surface at any given point in the Earth's atmosphere.

Atmospheric Pressure Variation:

Specific Gravity

Specific gravity is comparison of density of a substance, to the density of a reference substance usually at the same temperature e.g. water at 4c° = 1g/cm2

A hydrometer is an instrument that measures the specific gravity of a liquid.

SI Units

Every measurement depends on a reference standard. The metric system of measurement is a simple international system, because all units are based on 10 or more multiples of 10 so it is easy to convert one unit to another.

The International System of Units (SI), is a revised version of the metric system. There are seven SI based units. From these base units, other SI units of measurement can be derived, such as; volume, density and pressure.
SI Units:

Significant Figures

When calculating quantities you apply the rules of significant figures in calculations to round off numbers correctly. In general an answer cannot be more precise than the least precise measurement from which it was calculated.

e.g. Area = L x W = 7.7 x 5.4 = 41.58m2

The smallest number of significant figures in the measurements is two significant figures, therefore the answer must be rounded to two significant figures.

Addition and Subtraction

When adding or subtracting numbers in calculations, the answer should be rounded off to have the same number of decimal places as the measurement with the least number of decimal places.

e.g. 38.45 + 3.68 + 124.5 = 166.6

Multiplication and Division

When a calculation involves multiplication or division, the answer must be rounded off to the least number of significant numbers as the least precise term in the calculation.

e.g. 48.354 x 1.6 = 77.3

Scientific Measurement

Measurement is fundamental to the experimental science. The references we use are called the metric system, and a base 10 system.

There are two types of measurement:
Quantitative; give results in a definite form, usually as numbers.
E.g. The temperature is 31.7°

Qualitative; give results in a non-descriptive, non-numeric form.
E.g. The temperature is very warm

Accuracy & Precision
Accuracy is how close a single measurement comes to the actual dimension or true value of whatever is measured. Precision is how close several measurements are to the same value.

Scientific Notation
In scientific notation, a number is written as the product of two numbers as a coefficient and the power of 10 or 'E' E.g. 0.00000327 = 3.27E-6

Simple math with scientific notation:
- When multiplying notation; multiply the coefficients, and add the indices
e.g. 6.0E5 x 4.0E6 = 24E11 = 2.4E12
- When dividing notation; divide the coefficients and subtract the indices
e.g. 6.0xE4 / 4.0E8 = 1.5E-4

Scientific Notation Prefixes

Energy

Energy is the capacity for doing work. Nearly all physical and chemical changes in nature involve the absorption or emission of energy.

There are two general types of energy, potential and kinetic:

Potential Energy: is the energy of position or can be considered as stored energy ready for use.

Kinetic Energy: is the energy of motion or movement.

Energy is moved between or from potential to kinetic to potential. A common form of energy is heat. Heat is energy that is transferred from one body to another because of a temperature difference.

Chemical Equations

Arrows mean 'Change into'
E.g. Hydrogen + Oxygen --> Water (Word Equation)
2H2 + O2 --> 2H2O (Chemical Equation)

Chemical symbols
Writing word equations is often not practical. To avoid this, scientists assign a symbol to each element.

Writing chemical equations
Word equations adequately describe chemical equations, but are cumbersome. As a result, we use chemical equations. In a chemical equation, an arrow separates the formulas of the reactants (on the left) from the formulas products (on the right).

Equations that show only show the formulas of the reactants and the products are called skeletal equations. A skeleton equation does not indicate the relative amounts of reactants and products or the state of them. The state of the products and reactants is indicated by putting the symbol into the formula, three states of matter (g), (s), (l). A fourth state is used to indicate if a product or a reactant is in an aqueous (aq) (water).

*A catalyst is written above the arrow, only if it is involved in a reaction.

Chemical Reactions

In a chemical reaction, one or more substances change into new substances. The change in composition of the compound results from a chemical reaction. In a chemical reaction, the starting substances are called reactants and the new substances are called the products.

Types of chemical reactions
When dealing with chemical reactions, the type of chemical reactions can be split into five different categories. The five different categories are:
- Combination reactions:
In this type of reaction two or more substance react to make one substance.
E.g. S + O2 = SO2

- Decomposition reactions:
In this type of reaction, ne substance reacts (breaks down) to form two or more simpler substances.
E.g. CaCO3 = CaO + CO2

- Single replacement reactions:
In this type of reaction, atoms of an element replace atoms of a second element in a compound.
E.g. Fe + CuSO4 = FeSO4 + Cu

- Double replacement reactions:
In this type of reaction there is an exchange of positive ions between two compounds.
E.g. 3H2SO4 + 2Al(OH)3 = 6H2O + Al2(SO4)3

- Combustion reactions:
This is when oxygen reacts with another substance, often producing energy in the form of light and heat.
E.g. CH4 + 2O2 = 2H2O + CO2

Mixture, Pure substances, Compounds and Elements

Mixture

A mixture is a combination of two or more substances which retain to their distinctive properties

e.g. Gravel, Milk.

Pure Substances

Pure substances are substances that have distinct properties and have a constant composition no matter where they are found or how they are made e.g. water, salt, gold, oxygen, petrol.

Elements

Are pure substances that cannot be broken down into simpler substances. These are building blocks for all other substances. All matter is made up of elements and there are 92 natural occuring elements in nature e.g. Oxygen, gold, iron.

Compounds

Are pure substances that can only be broken down by chemical reactions. Compounds can be synthesised (made up) or decomposed (broken down) e.g. water, sugar, acids.

Homogeneous

The same thing throughout (e.g. Distilled Water).

Heterogeneous

Not the same thing throughout (e.g. Milk).

States of Matter

There are three common states of matter. They are gaseous, liquid and solid state. At present there are five know states of matter, but the other two are not usually dealt with at this level (Plasma, Bose Condensate).



Gas vs. Vapor
A gas is a substance that is in a gaseous state at room temperature otherwise it is a vapor.

Laws of Chemistry

Law of conservation of mass

Law of conservation of mass states that in any physical or chemical reaction mass is neither created nor destroyed, but it is always conserved.

Law of conservation of energy

Law of conservation of energy states that in any chemical or physical process, energy is neither created nor destroyed.