HESI A2 Exam: Chemical Reactions
Intro
Elements are substances that cannot be broken down into other substances.
Compounds are substances made up of molecules containing atoms of different elements.
Inorganic substances do not contain carbon. Organic substances do.
Bonds
You can recognize ionic compounds because they consist of a metal bonded to a nonmetal. Ionic bonds form between two atoms that have different electronegativity values. Because the ability to attract electrons is so different between the atoms, it's like one atom donates its electron to the other atom in the chemical bond.
Covalent bonds are formed between non-metal atoms. Each of the atoms involved in bonding contribute one, two, three or more electrons to form the shared pair. HCl is an example of a covalent bond.
A water molecule, abbreviated as H2O, is an example of a polar covalent bond. The electrons are unequally shared, with the oxygen atom spending more time with electrons than the hydrogen atoms. Since electrons spend more time with the oxygen atom, it carries a partial negative charge.
A non-polar bond, on the other hand, involves electrons being shared equally with a molecule.
Chemical Reactions
A chemical reaction involves the rearranging of atoms of the same or different elements to form new substances. It is represented by a chemical equation in which the reactants (substances that are broken apart) are written on the left and the products (new substances formed) are written on the right. If more than one reactant or product is needed, they are separated with a + sign. An arrow is used to separate the reactant side of the equation from the product side.
Reactants are the inputs in a chemical equation.
Products are the resulting outputs.
Subscripts represent how many atoms or ions are in one molecule. For example, the subscript in NO3 indicates that there are 3 oxygen atoms in each nitrate ion.
Coefficients are what go in front of the substance (for example, 3NaCl). This can be adjusted and changed. Subscripts, however, cannot be changed for any reactant or product.
A synthesis reaction involves multiple chemicals being combined to create a more complex product.
A redox reaction involves electrons being transferred from one atom to another. This changes the oxidation numbers of the atoms.
Single displacement reaction is a chemical reaction where one reactant is exchanged for one ion of a second reactant. Single displacement reactions take the form A + BC = B + AC. The reaction between zinc metal and hydrochloric acid to produce zinc chloride and hydrogen gas is an example of a single-displacement reaction
Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g)
Double displacement reaction is a type of reaction in which two reactants exchange ions to form two new compounds. Double displacement reactions take the form
AB + CD → AD + CB. The reaction between silver nitrate and sodium chloride is a double displacement reaction. The silver trades its nitrite ion for the sodium's chloride ion, causing the sodium to pick up the nitrate anion.
AgNO3 + NaCl → AgCl + NaNO3
A decomposition reaction occurs when a reactant molecule is broken down into its component parts. Decomposition reactions take the form AB → A + B. The electrolysis of water into oxygen and hydrogen gas is an example of a decomposition reaction:
2 H2O → 2 H2 + O2
Direct combination reaction has two or more chemical species combine to form a more complex product: A + B → AB. The combination of iron and sulfur to form iron sulfide is a direct combination reaction.
8 Fe + S8 → 8 FeS
Combustion reactions, in the most general sense, involve a reaction between any combustible material and an oxidizer to form an oxidized product. It usually occurs when a hydrocarbon reacts with oxygen to produce carbon dioxide and water. For example, the combustion of methane is represented by
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)
An endothermic reaction is any chemical reaction that absorbs heat from its environment. Baking bread or an egg solidifying are common examples.
An exothermic reaction involves the energy usually being transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. A burning candle would be a good example of an exothermic reaction.
Activation energy is the amount of energy needed to get a reaction going. Starting a fire would be an example of activation energy.
Law of Conservation of Mass
The law of conservation of mass states that matter is neither created nor destroyed during a chemical reaction, and therefore the mass of the products always equals the mass of the reactants. This means that chemical equations must balance.
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