Quantitative chemistry can seem intimidating because of how maths-based it is. But it’s not as scary as it sounds, and being well-prepared will help! Revising quantitative chemistry for your chemistry GCSE will help you familiarise yourself with key concepts and reinforce your existing knowledge. It also allows you to digest the fundamental ideas of the subject, including stoichiometry and how substances can be quantified in terms of moles.
In this post:
Quantitative Chemistry In A Nutshell
When thinking about what quantitative chemistry is and how it’s used, the answer is in the name: it’s used to calculate (quantify) the amount of chemicals or materials in a sample. To really master this subject, you should first make sure that you have the fundamentals nailed down so that you know how to calculate mass:
- The Mass Number of an element is shown at the top of the chemical symbol, and it represents the total number of protons and neutrons in an atom.
- The Relative Atomic Mass is the average mass of an element’s atom compared to the mass of a carbon-12 atom (12.000).
- The Relative Molecular Mass can be calculated by adding together all the relative atomic masses of all of the atoms in an element. Calculating this will allow you to determine what masses of reactants to use and what mass of products will be formed.
- The Law of Conversion of Mass says that the total mass of reactants and total mass of products in a chemical reaction will be the same, as no atoms are destroyed or created in the process – instead, they just reform into new products.
- The Mole is a scientific unit used to measure things like atoms and molecules. You can calculate the mole of an atom by remembering that one mole has a mass in grams that’s equal to the relative atomic mass, or the relative molecular mass. For example:
- The relative atomic mass of oxygen is 16, so one mole of oxygen atoms weighs 16g
- The relative molecular mass of oxygen is 32, so one mole of oxygen gas weighs 32g
- Stoichiometry basically determines the quantitative relationship between the reactants and products in a chemical reaction. In Greek, it literally translates to measure of elements, and it involves the understanding of the relationships between the reactants and products, as well as how to balance equations.
When revising quantitative chemistry, you also need to look at the different types of reactions that occur between different elements so you’re able to form predictions on the products that can be formed and the proportions of the reactants.
Chemical reactions occur under certain parameters and limits. Chemical reactions are predictable and can be directed to achieve specific results because of the known laws of chemical combinations, such as the law of definite proportions, law of conservation of mass, and Avogadro’s law.
Reactions Of Metals
You can predict the chemical reactions of metals with other substances based on their locations and classifications in the periodic table of elements. The groupings here provide crucial clues on the reactivities and affinities between elements. For instance, the first column is the group of alkali metals, which are highly reactive.
Alkali metals don’t exist in elemental forms in nature because they can easily react with other substances. For instance, elemental sodium becomes highly explosive when it becomes wet as it violently ejects hydrogen from water. This exothermic reaction triggers the rapid oxidation of the liberated hydrogen, resulting in an explosive flame.
Metals are classified into four classes based on their reactivity. Here are the four classes of metals:
Class I Metals: The most reactive metals are in Groups IA and IIA in the periodic table. They’re abundant in the earth’s crust, where they occur as compound, mineral forms. Their pure elemental forms are only available in laboratories, where they’re kept in special storage to prevent them from reacting with other substances.
- Lithium (Li)
- Sodium (Na)
- Rubidium (Rb)
- Caesium (Cs)
- Calcium (Ca)
- Strontium (Sr)
- Barium (Ba)
Class II Metals: These are slightly less active metals that are able to naturally exist in their elemental forms. They react easily with acids and oxygen, but are sufficiently stable to exist as elements. Class II metals include:
- Magnesium (Mg)
- Aluminium (Al)
- Zinc (Zn)
- Manganese (Mn)
Class III Metals: These are known as the structural metals because they’re commonly used in structural constructions such as buildings, machines, vehicles, weapons, electrical grids, and electronics:
- Chromium (Cr)
- Iron (Fe)
- Tin (Sn)
- Lead (Pb)
- Copper (Cu)
Class IV Metals: These metals have been commonly used in manufacturing coins since ancient times – this is why they’re also known as coinage metals. They’re considered rare precious metals because they’re shiny and they don’t tarnish over time. These metals are ideal for coinage because they’re not reactive with most substances, even at high temperatures:
- Silver (Ag)
- Gold (Au)
- Platinum (Pt)
- Mercury (Hg)
You can predict the compounds that can be formed as well as the proportions of the reactants based on the reactivity of these metals. A metal with higher reactivity will almost always replace a metal in a compound that has lower reactivity. If it’s the reverse, no reaction will occur. Here’s an example:
- CuSO4 + Zn → ZnSO₄ + Cu
- MgSO4 +Zn → NO REACTION
In the first example, zinc replaced copper because zinc is more reactive than copper. Meanwhile, in the second example, no reaction occurred because zinc is less reactive than magnesium.
Useful chemistry GCSE revision guides on metals:
Acids, Alkalis And Salts
Acid and base reactions, particularly inorganic reactions, are among the simplest and easiest to predict in terms of reaction rate, energy involved, byproducts, and mass proportions of reactants.
Salts are the main byproducts of acid-base reactions. For instance, when an alkaline solution of sodium hydroxide reacts with a solution of hydrochloric acid, sodium chloride is produced. Here is the balanced chemical reaction:
NaOH + HCl → H2O and NaCl
As you can see, a completely balanced reaction of acid and base produces a salt solution. The pH level of this solution is neutral at pH 7. However, if the base or the acid reactant has a higher proportion at the start, the final solution can either be acidic or basic. Neutralisation can be achieved if you know the correct molar proportions.
It can also be experimentally achieved through a titration process (more on that below), wherein a slow drip of a known concentration of acid is allowed to mix with an unknown concentration of base solution, or vice versa, until a neutral pH level is achieved. You can use litmus paper, methyl orange or a pH meter to do this. However, you should have precise control of environmental factors like the temperature and humidity of the room.
Useful chemistry GCSE revision guides on acids and alkalis:
Quantitative Analytical Methods
There are several analytical methods used in quantitative chemistry. In general, though, these methods can be divided into two categories: physical methods and chemical methods. Physical methods are used to quantify physical properties, for example, density, electromotive force or the absorption of light. Physical methods include:
- Trace element analysis
- Atomic Emission Spectroscopy (AES)
- Energy Dispersive X-ray Spectroscopy (EDS)
Chemical methods of quantification are used for chemical reactions so that they can measure things like oxidation, precipitation, and neutralisation. Chemical methods include:
- Gravimetric analysis
- Volumetric analysis (e.g. titration)
- Wet chemistry tests
One quantitative analytical method for determining the concentration of a solution is the titration method. This is an experimental and highly precise method for analysing acid-base reactions. The basic laboratory setup involves a buret, a ring stand with clamp, and an Erlenmeyer flask.
The solution with known concentration of either acid, base or any other chemical is contained in the burette. Meanwhile, the other solution of unknown concentration is contained in the flask. A slow drip of the first solution is added to the second solution until neutralisation is achieved. Aside from acid-base reactions, titration is also applied in complexation, redox, and precipitation reactions.
As the name implies, precipitation titration involves the formation of precipitates when a certain level of chemical reaction is achieved. For instance, one of the pioneering precipitation titration applications developed was the analysis of K2CO3 and K2SO4 in potash. The analysis involves calcium nitrate (Ca(NO3)2) as the titrant that forms CaCO3 and CaSO4 as precipitates. The titration ends when no more precipitate is generated by adding more titrant.
Electrolysis is an electrochemical process that is used both in laboratory research and in industrial processes. It implements a direct electric current or DC to drive a chemical reaction. Electrolysis is used in electroplating, separation of elements from naturally occurring sources, and for analysing genetic materials.
The basic setup for electrolysis involves the following:
- DC power source: This could be from a battery, a converter or a generator
- Anode: This is the electrode from which the electrons flow and on which the negatively charged ions congregate
- Cathode: This is the electrode to which the electrons flow and on which the positively charged ions congregate
- Electrolyte: This is the chemical solution in which the electrodes are immersed and where the electrons flow. It serves as the bridge between the electrodes
- Container: This has to be something that is robust and non-reactive, or inert
A simple electrolysis process can split water into hydrogen and oxygen. Under normal conditions, water is stable, but when a current is passed through water that has some salt ions in it, the balanced chemical reaction is this:
2H2O (l) —> 2H2 (g) + O2(g)
As you can see, for every two molecules of water, four molecules of hydrogen and two molecules of oxygen are liberated.
Useful chemistry GCSE revision guides on electrolysis:
When revising for GCSE chemistry, you should prioritise subjects that aren’t yet very familiar to you. Hone your skills by answering the sample questions we’ve included in this article, and best of luck!
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