Unit 3 - Stoichiometry Flashcards
Molecular formula
The molecular formula of a compound is the number and type of different atoms in one molecule.
Writing molecular formula
Subscript numbers represent the number of atoms of that element within the compound.
Empirical formula
The empirical formula of a compound is the simplest whole number ratio of the different atoms or ions. Useful when very large molecules that contain hundreds, if not thousands of atoms.
Ions
An ion is an electrically charged particle formed when an atom (or group of atoms) loses or gains electrons. When an atom becomes an ion it seeks to gain the very stable noble gas electron structure. The charge of the ion shows how many electrons it has gained or lost.
Charge of ionic formula
The overall charge of the positive ions and the negative ions must be neutral.
Valency
The valency of an element, also known as its combining power, is the number of electrons gained, lost or shared by an atom of the element when it forms a compound. The valency of an element is given by the number of electrons in the outer shell (known as valence electrons) and can be used to determine the charge on an ion.
Group 1
All metals in Group 1 have one electron in their outer shell. They can lose this one electron to form ions with a charge of 1+.
Group 2
All metals in Group 2 have two valence electrons, so have a valency of 2. They can lose 2 electrons to form ions with a charge of 2+.
Group 3
The valency of Group 3 is 3 so in this group three electrons can be lost to form a 3+ charge.
Group 4
Group 4 elements have a valency of 4. They do not usually form ions, instead elements tend to achieve a full outer shell by sharing electrons.
Group 5
They have a valency of 3 and five outer electrons. These elements can achieve a full outer shell by gaining three electrons to form ions with a 3- charge.
Group 6
They have six outer shell electrons and a valency of 2. They can gain 2 electrons to form ions with a 2- charge.
Group 7
They have seven outer shell electrons and a valency of 1. They can achieve a full outer shell by gaining one electron.
Group 8
They are the noble gases and already have a outer shell so they don’t need to gain or lose electrons.
Chemical reactions
It is a process when one set of substances, the reactants, are converted into another set of substances, the products. Any chemical reaction can be represented by a chemical equation.
Chemical equation
- A word equation in which the chemical names of all reactants and products are displayed.
- A symbol equation in which the names of the substances are replaced by their elemental symbols.
Polyatomic ions
Some ions are composed of more than one element.
Balancing equations
When creating a symbol equation the number of atoms present on the left and right for each element must be the same. The total mass of the reactants must equal the total mass of the products.
Balancing complex equations
With more complicated equations it is useful to attempt to balance one atom at a time.
State symbols
State symbols add extra information that allow the reader to know which physical state each substance is during the reaction.
Aqueous solution
The is used to represent a substance fully dissolved in water. This is called the aqueous state and is represented by: (aq)
State symbols (representation)
- Solid = (s)
- Liquid = (l)
- Gas = (g)
Molecular ions (polyatomic ions)
Compounds comprised of atoms that have been covalently bonded together but contain unequal numbers of protons and electrons.
If there is more than one molecular ion in a compound, then brackets are required to make sure the reader understands which ions are involved. Also put brackets around it to show how many of them there are.
Ionic compounds
Formed between two or more ionic molecules. The formula of an ionic compound can be involving a molecular ion can be worked out in the same way as of an ionic compound formed of a metal and non metal. Charges must balance each other an result in a overall charge of zero.
Relative atomic mass
The standard unit comes from another atom, carbon-12. Carbon was chosen because it has a mass of exactly 12. The mass of every other atom is give relative to it. It is defined as the number of times heavier an average atom of a specific element is than 1/12th of the mass of carbon. It depends on the abundance (how much) 0f each isotope of that element exists.
Relative molecular mass
It is the combined relative atomic mass of all the atoms in a compound or molecule.
Relative formula mass for ionic compounds
Ionic compounds do not exist as molecules so when dealing with their mass, we use relative formula mass. Calculated same way as relative molecular mass.
Calculating reacting masses
In a balanced equation, the number of each element must be the same on both sides of the equation. This means that the total mass of the reactants equals the total mass of the products. You can predict the relative masses of products and reactants involved in a chemical reaction with appropriate data.
Calculating reacting masses (method)
- Create a balanced symbol equation for the reaction.
- Work out the relative atomic masses and relative molecular masses of all substances present. Add the total mass of the reactants together and check it equals the total mass of the products.
- Use the given data to solve the problem.
The Avogadro constant
In a chemical reaction there isn’t just one atom or molecule there is actually a huge number of them. These numbers a so big we refer to them as a mole.
Mole
I mole contains exactly 6.02 x 10^23 elementary entities. An elementary entity can be an atom, a molecule, an ion, etc. This number is also referred to as the Avogadro constant. Constant because the number never changes.
Mole (Relation to Ar/Mr)
1 mole of a substance is related to its Ar/Mr. I mole of a substance is it’s Ar/Mr in grams.
Mole (equation)
Mass (g) = Ar/Mr x Moles (mol)
Molar gas volume
Under the same temperature and pressure conditions, equal volumes of different gases would contain an equal number of molecules. 1 mole of a gas occupies a volume of 24 dm^3 (24 000 cm^3).
Mole (formula with gas)
The number of moles of a gas at room temperature and pressure can be calculated using the formula:
moles (mol) = gas volume (V)/24
Molar mass
The molar mass is the mass of one mole, so it is the same as relative molecular mass, Mr, expressed in grams:
moles (mol) = mass (g)/molar mass (g/mol)
Mass of chemical equation
The concept of the mole being a constant number of atoms to enable us to work out the mass of a substance formed during a chemical reaction.
Concentration
Method that shows the proportion of the amount of substance contained in a solution. Formula:
concentration (g/dm^3) = mass(g)/volume(dm^3)
Molar concentration
The amount of moles of a substance dissolved in 1 dm^3. Formula:
concentration (mol/dm^3) = moles (mol)/volume (dm^3)
Titration
A titration is performed to determine the concentration of a solution.
Molecular formula
Molecular formula indicates the exact number of atoms in the case of the Alkenes (example is ethane).
Limiting reagents
In chemical reactions sometimes there are reactants in excess. The limiting reagent is the element in the chemical reaction that is completely used up. The amount of product formed is limited by this element.
Percentage yield
The number of mole of product formed according to an equation is the theoretical yield. The actual yield is the number of moles (or total mass) of a substance obtained experimentally. The percentage yield is the percentage of theoretical yield that is actually obtained when the reaction is carried out during a controlled reaction:
percentage yield = actual yield/theoretical yield x 100
Percentage purity
percentage purity = mass of pure product/mass of impure product x 100
