Chemistry - Topic 3 - Quantitative Chemistry
Conservation of mass
In a chemical reaction, mass is ALWAYS conserved.This means that no atoms are created or destroyed,therefore, the same number of atoms, and the type is always present in each side of the reaction equation. Hence, mass is conserved. If it appears to change, a gas is usually involved.
By adding up the relative masses of both the reactants and products you can see that the mass is conserved. The equation must be balanced for this to work however.
Gas can sometimes affect the conservation of mass calculations as its not accounted for before the reaction as it floats in the air. This means when the products mass is calculated the gas has reacted and become part of the reaction vessel's contents.
E.g when oxygen reacts with metal in a unsealed container, the oxide has a greater mass as its the total mass of the metal and oxygen in the air.
If one of the reactants is a gas, then during the reaction, if the container isn't sealed it can escape. Therefore you cannot account for its mass. This decreases the overall mass or at least appears to.
Concentration Calculations
Gases and Solutions
Limiting Reactants
The Mole and Equations
Relative Formula Mass
The Mole
Atom Economy
Compounds Have a Relative Formula Mass, Mr
You Can Calculate the % Mass of an Element in a Compound
If you have a compound such as MgCl2 then it has a relative formula mass, Mr, which is just the relative atomic masses of all the atoms in the molecular formula added together.
Example: Find the percentage mass of sodium in sodium carbonate, Na2CO3.
Ar of sodium = 23, Ar of carbon = 12, Ar of oxygen = 16
Mr of Na2CO3 = (2 x 23) + 12 + (3 x 16) = 106
Percentage mass of sodium = 23 x 2 / 106 x 100 = 43%
Example: A mixture contains 20% iron ions by mass. What mass of iron chloride (FeCl2) would you need to provide the iron ions in 50 g of the mixture? Ar of Fe = 56, Ar of Cl = 35.5.
1) Find the mass of iron in the mixture.
The mixture contains 20% iron by mass, so in 50 g there will be 50 x 20/100 = 10 g of iron.
2) Calculate the percentage mass of iron in iron chloride.
Percentage mass of iron = 56 / 56 + (2 x 35.5) x 100 = 44.09...%
3) Calculate the mass of iron chloride that contains 10 g of iron.
Iron chloride contains 44.09% iron by mass, so there will be 10 g of iron in 10 / 44.09.../100 = 23 g
So you need 23 g of iron chloride to provide the iron in 50 g of the mixture.
Percentage mass of an element in a compound = Ar x number of atoms of that element / Mr of the compound x 100
“The Mole” is Simply the Name Given to an Amount of a Substance
Formula to Find the Number of Moles in a Given Mass:
One mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass for that substance.
For example, Carbon has an Ar of 12 so one mole of carbon weighs 12 g.
One mole of any substance is just an amount of that substance that contains an Avogadro number of particles. The particles could be atoms, molecules, ions or electrons.
Avogadro's constant is 6.02 x 10^23.
Example: How many moles are there in 66g of carbon dioxide (CO2)?
1) Calculate the Mr of carbon dioxide.
Mr of CO2 = 12 + (16 x 2) = 44
2) Use the formula above to find out the number of moles.
No. of moles = 66 / 44 = 1.05 mol
Number of moles = mass in g (of an element or compound) / Mr (of the element or compound)
You Can Use Moles to Calculate Masses in Reactions
You Can Balance Equations Using Reacting Masses
The big numbers in front of chemical formulas of the reactants and products tell you how many moles of each substance takes part or is formed during the reaction.
For example: Mg + 2HCl --> MgCl2 + H2. In this reaction, 1 mole of magnesium and 2 moles of hydrochloric acid react together to form 1 mole of magnesium chloride and 1 mole of hydrogen gas.
2) Divide the number of moles of each substance by the smallest number of moles in the reaction.
3) If any of the numbers aren't whole, multiply them all by the same amount so that they will become whole numbers.
1) Divide the mass of each substance by its relative formula mass to find the number of moles.
4) Write the balanced symbol equation for the reaction by putting these numbers in front of the chemical formulas.
If you know the masses of the reactants and products that took part in a reaction, you can work out the balanced symbol equation for the reaction.
The Amount of Product Depends on the Limiting Reactant
Reactions Stop When One Reactant is Used Up
3) Find out how many moles there are of the substance you know the mass of.
4) Use the balanced equation to work out how many moles there'll be of the other substance.
2) Work out relative formula masses of the reactant and product you want.
5) Use the number of moles to calculate the mass.
1) Write out the balanced equation.
2) The reactant that's used up is called the limiting reactant as it limits the amount of product formed.
3) The amount of product formed is directly proportional to the amount of limiting reactant.
1) Reactions stop when all of one of the reactants is used up. Any other reactants are in excess to make sure that the other one is used up.
4) This is because if you add more reactant there will be more reactant particles to take part in the reaction, which means more product particles.
When some magnesium carbonate is placed into a beaker of hydrochloric acid, you can tell a reaction is taking place as you see lots of bubbles of gas. After a while, the fizzing slows down and the reaction eventually stops.
You Can Calculate Volumes of Gases in Reactions
Concentration is a Measure of How Crowded Things Are
One Mole of Any Gas Occupies 24 dm3 at 20 °C
At the same temperature and pressure, equal numbers of moles of any gas will occupy the same volume.
At room temperature and pressure, one mole of any gas occupies 24 dm^3
You can use the volume of one gas to find the volume of another: How much carbon dioxide is formed when 30 dm^3 of oxygen reacts with carbon monoxide? 2CO + O2 --> 2CO2
1 mole of O2 --> 2 moles of CO2, so 1 volume of O2 --> 2 volumes of CO2.
So 30 dm^3 of O2 --> (2 x 30 d,^3) = 60 dm^3 of CO2.
A way of measuring a solution's concentration is by calculating the mass of a substance in a given volume of solution.
Concentration = mass of solute / volume of solvent
The more solute there is in a given volume, the more concentrated the solution.
Concentration can also be given in mol/dm^3: Concentration = number of moles of solute / volume of solvent
The amount of a substance in a certain volume of a solution is called its concentration.
Calculate the Concentration
Find the Concentration in mol/dm3
Titrations are experiments that let you find the volumes needed for two solutions to react together completely.
Percentage Yield
% of Reactants Forming Useful Products
High Atom Economy is Better for Profits and the Environment
The atom economy of a reaction tells you how much of the mass of the reactants is wasted when manufacturing a chemical and how much ends up as useful products: Atom Economy = relative formula mass of desired products / relative formula mass of all reactants x 100
100% atom economy means that all the atoms in the reactants have been turned into useful products. The higher the atom economy the 'greener' the process.
A lot of reactions make more than one product; some will be useful but others will be waste.
Low atom economy reactions aren't usually profitable - raw materials are expensive to buy and waste products can be expensive to remove and dispose of responsibly.
To solve this problem, we should find a use for these waste products.
Reactions with low atom economy use up resources very quickly and at the same time they make lots of waste materials that somehow have to be disposed of. That tends to make these reactions unsustainable.
Reactions with the highest atom economy are those with only one product. The more products there are, the lower the atom economy is likely to be.
Lots of waste = a problem
Other factors as well as atom economy that need to be considered when choosing which reaction to use are the yield, the rate of the reaction and the position of equilibrium for reversible reactions.
1) Not All Reactants React to Make a Product
2) There Might be Side Reactions
Yields are Always Less Than 100%
3) You Lose Some Product When You Separate It From the Reaction Mixture
Percentage Yield Compares Actual and Theoretical Yield
% yield = mass off product actually made (g) / maximum theoretical mass of product (g) x 100
% yield is always between 0 and 100%.
The amount of product you get is known as the yield. The more reactants you start with, the higher the actual yield. However, % yield doesn't depend on the amount of reactants you started with.
Industrial processes should have as high a percentage yield as possible to reduce waste and reduce costs.
In reality, you never get a 100% yield. Some product or reactant always gets lost along the way - and that goes for big industrial processes as well as school lab experiments. There are three common problems relating to what can go wrong...
For example, in the 0% yield = no reactants were converted into product., at the same as the reaction N2 + 3H2 --> 2NH3 is taking place, the reverse reaction is also happening.
In reversible reactions, the products can turn back into reactants, so the yield will never be 100%.
The reactants sometimes react differently to how you'd expect. They may react with gases in the air or impurities in the reaction mixture, so they end up forming extra products other than the ones you want.
If you want to keep the liquid, you'll lose the bit that remains with the solid and filter paper.
If you want to keep the solid, some of it'll get left behind when you scrape it off the filter paper.
When you filter a liquid to remove solid particles, you nearly always lose a bit of liquid or a bit of solid.
You'll also lose a bit of material when you transfer it from one container to another - even if you manage not to spill it.
Concentration = number of moles of solute / volume of solvent
100% yield = all the product you expected to get.
0% yield = no reactants were converted into product.
This means that the reaction never goes to completion, meaning the reactants don't all get used up.