HS Chemistry - Working With Chemical Reactions

Unit Summary

Since particles are too small to measure individually, we use moles to keep track of how many there are.
- A mole is a counting unit, not a unit of mass. There are as many Hydrogen atoms in 1 mole of Hydrogen atoms as there are Uranium atoms in 1 mole of Uranium atoms. This means that 1 mole of Uranium atoms does not have the same mass as 1 mole of Hydrogen atoms, as a Uranium atom has much more mass than a Hydrogen atom. Moles are also used as a counting unit for molecules and ions.
- 1 mole of something = 6.02 × 10²³ of that thing $for example, 1 mole of Hydrogen atoms is 6.02 × 10^23^ Hydrogen atoms$ .
- 6.02 × 10²³ is known as Avogadro's constant. 6.02 × 10²³ amu = 1 gram
If a chemical molecule has a mass of y amu, then 1 mole of that substance has a mass of y grams.

Given how much of a substance is present in grams, we can calculate the amount of moles of that substance present.

Chemical reaction equations represent how a reaction occurs, with the reactants combining to form the products.
- It is generally of the form A + B → C + D, where A & B are the products, and C & D are the products.
- The arrow shows the direction of the pathway - it starts with the reactants, which combine and then form the products.
- In a chemical reaction equation, no mass is lost. The left-hand side must equal the right-hand side in terms of both the types of atoms present, how many there are of each atom, and the total charges.
The molecular formula for a molecule, metal atom, or ionic compound shows what atoms are present inside 1 molecule of that substance.

Closely related to the molecular formula is the empirical formula, the ratio of atoms in a molecule. This is often different from the molecular formula, and as such, different molecules can have the same empirical formula. Thus, it is not interchangeable with the empirical formula.

When we know the mass in grams we have of each of the reactants, we can calculate the amount of moles of reactants present. This allows us to calculate the amount, in moles, of products produced, which allows us to calculate the mass of the products produced in the chemical reaction.
1. Convert the mass $in grams$ to moles of substance, for each product, by dividing the mass we have by the mass of one mole of that reactant.
2. Find the amount, in moles, of product created.
3. Convert the number of moles of product to mass in grams.
To convert grams to moles, divide the mass $in grams$ of the substance by its molar mass.

Sometimes, a reaction doesn't completely turn all of its reactants into products because there is too little of one reactant, causing the reaction to stop.
- The reactant we have too little of is called the limiting reagent.
- The reactant $s$ we have too much of is/are called the excess reagent $s$ .
- Some of the excess reagents are left unused. These are the leftovers.
Often, when doing an experiment, not all the reactants will mix with one another, even though in the chemical reaction, they will.
- The amount of product produced in the chemical reaction is the maximum amount of product that can be created in the reaction. It is called the theoretical yield.
- The actual amount experimentally created is called the actual yield.
- The percentage yield is the actual yield divided by the theoretical yield $converted to a percentage value$ .