Chemistry: Compounds and Stoichiometry Flashcards
Miolecular Weight
The sum of the atomic weights of the atoms in the molecule. The formula weight on an ionic compound is found by adding up the atomic weights according to the empirical formula of the substance.
Mole
Defined as the amount of a substance that contains the same number of particles that is found in a 12 g sample of carbon-12. This quantity, Avogadro’s number, is equal to 6.022 x 10^23. One mole of a compound has a mass in grams equal to the molecular weight of that compound in amu and contains 6.022 x 10^23 molecules of the compound.
For example, 62 g of H2CO3 represents one mole of carbonic acid and contains 6.022 x 10^23 H2CO3 molecules.
The mass of 1 mole of a compound is called its molar weight or molar mass and is usually expressed as g/mol.
Equivalent Weight
For some substances, it’s useful to define a measure of reactive capacity. This expresses the fact that some molecules are more potent than others in performing certain reactions. An example of this is the ability of different acids to donate protons (H+ ions) in solution. For instance, 1 mole of HCl can donate 1 mol of hydrogen ions, while 1 mol of H2SO4 can donate two moles of hydrogen ions. This difference is expressed using the term equivalent: 1 mole of HCl contains 1 equivalent of hydrogen ions while 1 mol of H2SO4 contains 2 equivalents of hydrogen ions.
To determine the number of equivalents a compound contains, a new measure of weight, called gram-equivalent weight (GEW), was developed.
equivalents = weight of compound/gram equivalent weight
gram equivalent weight = molar mass/n
where n is usually either the number of hydrogens used per molecule of acid in a reaction of the number of hydroxyl groups used per molecule of base in a reaction. This value is strictly dependent on reaction conditions. By using equivalents, it’s possible to say that one equivalent of acid will neutralize one equivalent of base, a statement that may not necessarily be true when dealing with moles.
Law of Constant Composition
States that any sample of a given compound with contain the same elements in the identical mass ratio.
Empirical and Molecular Formulas
Two ways to express a formula for a compound.
The empirical formula gives the simplest whole number ratio of elements in the compound.
The molecular formula gives the exact number of atoms of each element in the compound and is usually a multiple of the empirical formula.
For example, the empirical formula for benzene is CH, whereas the molecular formula is C6H6. For some compounds, the empirical and molecular formulas are the same, as in the case of H2O. An ionic compound, such as NaCl or CaCO3, will have only an empirical formula.
Percent Composition
The weight percent of the element in a specific compound.
%composition = (Mass of X in Formula/Formula Weight of Compound) x 100%
May be determined using either the empirical or molecular formula. If the percent compositions are known, the empirical formula can be derived. It’s possible to determine the molecular formula if both the percent compositions and molecular weight of the compound are known.
Combination Reactions
Reactions where two or more reactants form one product.
Ex) S(s) + O2(g) –> SO2(g)
Combination reactions can also occur when two compounds react to form a new compound.
Decomposition Reactions
One in which a compound breaks down into two or more substances, usually as a result of heating or electrolysis. When compounds are heated, for example, most of these compounds will decompose to form molecular oxygen.
Electrolysis
Electrolysis is a process that causes the decomposition of a compound by passing an electric current through the reactant.
Single Displacement Reactions
Occur when an atom (or ion) of one compound is replaced by an atom of another element.
For example, zinc metal will displace copper ions in a copper sulfate solution to form zinc sulfate.
Zn(s) + CuSO4(aq) –> Cu(s) + ZnSO4(aq)
Single displacement reactions are often further classified as redox reactions.
Double Displacement Reactions
Also called metathesis reactions, elements from two different compounds displace each other to form two new compounds. This type of reaction occurs when one of the products is removed from the solution as a precipitate or gas, or when two of the original species combine to form a weak electrolyte that remains undissociated in solution.
For example, when solutions of calcium chloride and silver nitrate are combined, insoluble silver chloride forms in a solution of calcium nitrate.
CaCl2(aq) + 2AgNO3(aq) –> Ca(NO3)2(aq) + 2AgCL(s)
Net Ionic Equations
Because reactions such as displacements often involve ions in solution, they can be written in ionic form.
When displacement reactions occur, there are usually spectator ions that do not take part in the overall reaction but simply remain in the solution throughout.. A net ionic reaction can only be written showing only the species that actually participate in the reaction. They’re important for demonstrating the actual reaction that occurs during a displacement reaction.
Neutralization Reactions
A specific type of double displacement that occurs when an acid reacts with a base to produce a solution of a salt and water.
For example, hydrochloric acid and sodium hydroxide will react to form sodium chloride and water.
HCl(aq) + NaOH(aq) –> NaCl(aq) + H2O(l)
Balancing Equations
Chemical equations express how much and what type of reactants must be used to obtain a given quantity of product. From the law of conservation of mass, the mass of the reactants in a reaction must be equal to the mass of the products. More specifically, chemical equations must be balanced so that there are the same number of atoms of each element in the products as there are in the reactants. Stoichiometric coefficients are used to indicate the number of moles of a given species involved int he reaction.
For example, the reaction for the formation of water is
2H2(g) + O2(g) –> 2H2O(g)
The coefficients indicate that two moles of H2 gas must be reacted with one mole of O2 gas to produce two moles of water. In general, stoichiometric coefficients are given as whole numbers.
Applications of Stoichiometry
Once an equation has been balanced, the ratio of moles of reactant to moles of products is known, and that info can be used to solve many types of stoichiometry problems. It’s important to use proper units when solving such problems. If and when you are faced with doing the calculations, the units should cancel out, so that the units obtained in the answer represent those asked for in the problem.
