Describe Chemical Reactions Flashcards
(19 cards)
Valence Electron
An electron in an outer orbital that can form bonds with other atoms
reactants
In a chemical equation, the substances on the left side of the equation; the starting materials in a chemical reaction.
products
In a chemical equation, the substances on the right side of the equation are the substances that are formed in a chemical reaction.
Valence Electrons I
The electrons of the outermost shell of an atom are called valence electrons, and they have the highest energy. Readily transferred or shared during chemical reactions than those in an atom’s interior. Sharing these electrons with another atom allows the atom to achieve noble gas configuration by having eight electrons in the outermost shell and become stable. The number of valence electrons an element has is based on its group in the periodic table. Elements in the same group have the same chemical behavior and the same number of valence electrons.
Valence Electrons II
Group IA elements such as sodium and potassium have one valence electron. The elements in group IIA such as magnesium and calcium have two valence electrons. Elements on on the left side of the periodic table have relatively low ionization energies (the energy required to remove the most loosely held electron) and low electronegativity (the tendency to attract electrons). These elements are classified as metals and usually lose electrons to another atom during a chemical reaction. Group IA elements lose one valence electron, leaving the imbalance between the number of positive protons in the nucleus and electrons, resulting in positive cations with a+1 charge with an otermost shell similar to a noble gas. For example, (Na) loses the only electron in its valence shell, so now Na+ ion has the same number of electrons as neon (Ne) and therefore is more stable. Magnesium (Mg) can lose 2 valence electrons, forming Mg2+, which also has the same number and configuration as (Ne).
Valence Electrons III
The elements in the upper right side of the periodic table, such as those groups VIA and VIIA, tend to gain electrons in chemical reactions. These elements have high ionization energies and a significant ability to draw valence electrons from other atoms toward themselves. Elements in Group VIIA such as fluorine (F), chlorine (Cl), and bromine (Br) gain one electron and thus end up with more electrons than the number of protons in the nucleus, becoming negative anions with a-1 charge (F-, Cl-, and Br-). Because they end up with the same number electrons as the neighboring noble gas, they achieve stability. Group VIA elements oxygen (O) and sulfur (S) gain 2 electrons, forming O2- and S2- ions and noble gas valence electron configurations for stabilitiy.
Valence Electrons IV
Based on an elements group and placement in the periodic table, a trend can be observed to determine whether an element will gain, lose, or share valence electrons. Elements on the left side of the periodic table with low ionization energies and low electronegativity are metals, which will tend to lose electrons and become cations. The electrons lost by metals are gained by elements on the right sideof the periodic table, which have high electronegativities, that is a high affinity for them and are nonmetals. Nonmetals gain electrons to become negatively charged anions. Reactions between elements with similar tendencies to gain or lose electrons will tend to share electrons with each other. For example, when two or more nonmetals react, they become stable by sharing valence electrons.
Covalent & Ionic Bonds I
Chemical bonds occur when two or more atoms have interactions between their valence electrons. Ionic bonds can only form when the elements involved have a large difference in electronegativity, such as exists between metals and nonmetals. This difference allows for the transfer of electrons from the metal to the nonmetal. These elements then become ions or charged atoms. As mentioned earlier, metals tend to become positively charged cations and nonmetals become negatively charged anions. For example, sodium (Na) is a metal that easily transfers one electron to the nonmetal (F). Na becomes positively charged (Na+) because it has lost an electron, and F becomes negatively charged ions attract one another, forming an ionic bond and making sodium fluoride (NaF). Metals are found on the left side of the periodic table, and nonmetals are found on the right.
Covalent & Ionic Bonds II
Sometimes elements share electrons instead of transferring electrons, forming a different type of bond. Covalent bonds require the sharing of electrons and occur between two nonmetals. In covalent bonds, there is not a sufficient difference in electronegativity to gain or lose electrons. However, differenes in electronegativity with a covalently bonded molecule cause them to be polar nor nonpolar. Polar covalent compounds have a negatively charged side and a positively charged side. Water is a polar molecule; the hydrogen side of the molecule is partially positively charged, and the oxygen side is partially negatively charged. This is a result of the strong electronegativity of oxygen pulling at the shared electrons. These polar covalently bonded molecules are still neutral in overall charge.
Chemical Reactions
Chemical reactions are represented by chemical equations. Chemical equations have a basic layout going from left to right: reactants, reaction sign showing the direction of the reaction (—>), and products.
Reactants (—>) Products
Chemical Equation
Mathematical representation of a chemical reaction
chemical equation of sodium chloride
The formation of table salt is an example of a chemical equation: 2Na +Cl2 –> NaCl. In this reaction, sodium and chlorine are the reactants and the ionic compound sodium chloride is the product.
Combustion reaction
CH4+2O2 –>CO2 +2H20
Methane (CH4) and oxygen (O2) are the reactants. The products are water (H20) and carbon dioxide (C02). Looking at the two sample reactions shown above, you can see there are numbers in front of some of the formulas. These are coefficients, and they help to adjust the quantities in the reaction so that equal numbers of atoms of each element in the reactants are also in the product and the reaction is balanced.
Balancing Chemical Reactions I
During a chemical reaction there is a change in bonding between atoms that occurs, but the total mass of the elements entering and exiting the reaction does not change. Chemical equations must be shown as balanced equations, meaning there must be the same numbers of each element on both sides. Looking at the combustion reaction again, the equation could have simply shown the identities but not the quantities of the reactants and products like this: CH4+ O2—>CO2 + H2O. This reaction now suggests that two hydrogen atoms are destroyed and an oxygen atom is created going from reactants to products, and so the equation is said to be unbalanced.
Balancing Chemical Equations II
If a coefficient of 2 is placed in front of the oxygen (O2), we are now indicating that two oxygen molecules react with methane (CH4) The coefficient of 2 infront of the water (H2O) molecule in the product means the reaction forms two water molecules instead of one.
CH4 +2O2—> CO2+ 2H2O. The equation for this chemical reaction is now balanced, and the same number of atoms for each element are present on both sides.
Balancing Chemical Equations III
Take a look at another unbalanced equation. The reaction of sodium iodide (NaI) with lead nitrate NaI + Pb(NO3)2—-> NaNO3 +PbI2. Count the atoms of each element on both sides.
Reactant side: 1 Na, 1I, 1Pb, 2N, and 6O
Product side: 1 Na, 2I, 1Pb, 1N, and 3O
In order to balance the reaction, we add the smallest coefficient that will give the same number of each atom on either side of the reaction. 2NaI + Pb (NO3)2 —->2NaNO3 + PbI2. Now the reaction has 2Na, 2I, 1Pb, 2N, 6O on each side. Its balanced.
Mole
A unit of a substance that is equal to exactly 6.02214076 X10 ^23 particles of a substance.
Moles in chemical reactions I
In order to perform a chemical reaction efficiently, it is important to know the quantities of the reactants that will be mixed together to determine the amount of products formed. One way to measure amounts of a substnace is using a balance to measure out the mass of the reactants. A mole is the atomic mass (measured in grams) of a substance. For example, carbon has an atomic mass of 12 amu, so 12 g of carbon is 1 mole of carbon. One molecule of NaCl has a molecular mass of 36.5 amu, so 36.5 g of NaCl is 1 mole of table salt. This can be translated to atom or molecule numbers, because one mole of anything (carbon atoms, table salt, apples, pennies) contains exactly 6.022 x 10 ^23 particles (Avogadro’s number).
Moles in Chemical Reactions II
In the equation 2Nal + Pb(NO3)2 –>2NaNO3 + PbI2, the coefficients represent moles. This means that 2 moles of NaI react with 1 mole of Pb(NO3)2 to make 2 moles of NaNO3 and 1 mole of PbI2. To translate this to grams, the molecular mass of each compound has to be calculated using the periodic table. NaI has a molecular mass of 149.9amu (Na=23.0 amu; I =126.9 amu). Pb(NO3)2 has a molecular mass of 331.2 amu (Pb=207.2 amu; N=14.0 amu; O=16.0 amu). In the reaction, the reactants have to be mixed ina proportion that results in the appropriate molar quantities: 2 moles of NaI are equal to 299.8g, and 1 mole of Pb(NO3)2 weighs 331.2 g. You can see that mixing 2 g of NaI and 1 g of Pb(NO3)2 would not be equimolar quantities.
