Chemical quantities and Chemical Reactions

There are three ways to measure the amount of something: count, mass, and volume.

a mole of a substance is 6.02 × 1023 representative particles of that substance and is the SI unit for measuring the amount of a substance. The number of representative particles in a mole, 6.02 × 1023, is called Avogadro’s number.

Avogadro’s number was named after the Italian scientist Amedeo Avogadro di Quaregna (1776–1856) who helped clarify the difference between atoms and molecules.

mass(grams) = number of moles x mass(grams) /1 mole

STP- the conditions under which the volume of a gas is usually measured; standard temperature is 0°C, and standard pressure is 101.3 kPa, or 1 atmosphere (atm) (10.2) and volume is 22.4L

Avogadro's hypothesis-Amedeo Avogadro proposed a groundbreaking explanation. Avogadro’s hypothesis states that equal volumes of gases at the same temperature and pressure contain equal numbers of particles. The particles that make up different gases are not the same size. But the particles in all gases are so far apart that a collection of relatively large particles does not require much more space than the same number of relatively small particles. Whether the particles are large or small, large expanses of space exist between individual particles of gas, as shown in Figure 10.9.

The molar volume is used to convert a known number of moles of gas to the volume of the gas at STP. The relationship 22.4 L = 1 mol at STP provides the conversion factor.

Different gases have different densities. Usually the density of a gas is measured in grams per liter (g/L) and at a specific temperature. The density of a gas at STP and the molar volume at STP (22.4 L/mol) can be used to calculate the molar mass of the gas.molar mass = density at STP × molar volume at STP

The relative amounts of the elements in a compound are expressed as the percent composition or the percent by mass of each element in the compound. The percent composition of a compound consists of a percent value for each different element in the compound.

The percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound, multiplied by 100%.

Using the individual masses of the elements and the molar mass you can calculate the percent by mass of each element in one mole of the compound. Divide the mass of each element by the molar mass and multiply the result by 100%.

The empirical formula gives the lowest whole-number ratio of the atoms of the elements in a compound.

For example, a compound may have the empirical formula CO2. The empirical formula shows the kinds and lowest relative count of atoms or moles of atoms in molecules or formula units of a compound.

The molecular formula of a compound is either the same as its experimentally determined empirical formula, or it is a simple whole-number multiple of its empirical formula.

To obtain the molecular formula of hydrogen peroxide from its empirical formula, multiply the subscripts in the empirical formula by 2. (HO) × 2 = H2O2.

A skeleton equation is a chemical equation that does not indicate the relative amounts of the reactants and products. The first step in writing a complete chemical equation is to write the skeleton equation.

How to write a balanced chemical equation: Representing a chemical reaction by a balanced chemical equation is a two-step process. To write a balanced chemical equation, first write the skeleton equation. Then use coefficients to balance the equation so that it obeys the law of conservation of mass. In every balanced equation, each side of the equation has the same number of atoms of each element

There are 5 types of chemical reactions: 1. combination reactions- a chemical change in which two or

more substances react to form a single new substance. 2. Decomposition reactions- a chemical change in which a

single compound breaks down into two or more simpler products. Decomposition reactions involve only one reactant and two or more products. The products can be any combination of elements and compounds.

3. Single replacement- a chemical change in which one element replaces a second element in a compound. You can identify a single-replacement reaction by noting that both the reactants and the products consist of an element and a compound.

4. Double displacement- is a chemical change involving an exchange of positive ions between two compounds. Double-replacement reactions are also referred to as double-displacement reactions.

5. Combustion reaction- a chemical change in which an element or a compound reacts with oxygen, often producing energy in the form of heat and light. A combustion reaction always involves oxygen as a reactant. Often the other reactant is a hydrocarbon, which is a compound composed of hydrogen and carbon.

The number of elements and/or compounds reacting is a good indicator of possible reaction type and thus possible products. For example, in a combination reaction, two or more reactants (elements or compounds) combine to form a single product. In a decomposition reaction, a single compound is the reactant; two or more substances are the products.

A net ionic equation shows only those particles involved in the reaction and is balanced with respect to both mass and charge.

Some combinations of solutions produce precipitates, while others do not. Whether or not a precipitate forms depends upon the solubility of the new compounds that form. You can predict the formation of a precipitate by using the general rules for solubility of ionic compounds.