Physical Chemistry

The basics of physical chemistry

We’ve mentioned atoms above, but what actually are they?

in fact, atoms are the fundamental building blocks of all parts of chemistry, but there are some other pretty important terms you should know about:

• An element is a pure substance containing only atoms which all have the same number of protonsin their nucleus. That’s a mouthful, but it just means, for example, that all the atoms in carbon have exactly six protons - no more, no fewer. We also talked about protons above. An atom’s number of protons determines exactly which element it is a part of.
• If you put two atoms together, you get a molecule. A molecule is two or more atoms chemically bonded together.
• Molecules made from different elements are called compounds. A compound is simply two or more atoms from different elements chemically bonded together.
• In chemistry, we often refer to a species. A species is a group of identical entities, be they atoms, ions, molecules or particles.
• The periodic table is a chart showing all the different elements, arranged by their number of protons and by their properties. We call the columns groups and the rows periods.

Topics within physical chemistry

In physical chemistry, you’ll study a variety of topics. These range from atomic structure, where you’ll find out more about the particles inside an atom, to kinetics, where you’ll discover how reactions occur and how we can manipulate them. Other topics include acids and bases, amount of substance and equilibria.

Atomic structure

You should already know what an atom is. It is the smallest unit of an element that can’t be broken down further by any sort of chemical reaction. But that doesn’t mean that atoms don’t contain their own constituent parts. We explored above how atoms consist of subatomic particles known as protons, neutrons and electrons. In “Atomic Structure”, you’ll learn how these particles are arranged inside atoms. You’ll explore how changing their numbers changes the atom, and you’ll learn to define words such as ion, isotope and ionisation energy.

For example, what is the difference between a hydrogen atom and a helium atom? How can we tell them apart? Can we change from one to the other - and if not, why not?

Left: a hydrogen atom. Right: a helium atom. Can you spot the differences?commons.wikimedia.org

Amount of substance

Once you’ve learnt about atoms and elements, you can begin to look at how they react together. But before you do that, you need to understand the basics of reactions and how chemists work with them. In “Amount of Substance”, you’ll cover topics such as molecular mass, empirical formulae and the mole.

Moles have nothing to do with the animal - instead, a mole is a specific quantity of atoms or molecules. They make reaction equations much easier to work with.

There a 602 sextillion molecules in a mole of any substance. This is known as Avogadro's constant.StudySmarter Originals

For example, if we know values such as the mass of a sample and its chemical formula, we can work out not only the number of moles it contains, but how many moles of a product we expect a reaction to produce.

Perhaps you didn’t get as much of your product as expected. You could say that your reaction has a low percentage yield. In “Amount of substance”, you’ll also learn how to calculate this percentage yield and why it is sometimes so low.

Bonding

We know that atoms react by moving their electrons about. Atoms want to be in the most stable state possible, and they do this by losing or gaining electrons. Too many electrons? They’ll give some up. Not enough? They’ll try to gain a few.

In “Bonding”, you’ll explore some of the ways that atoms shunt their electrons around, from donating them to other atoms to sharing them amongst themselves. Moving electrons forms bonds, and you’ll learn about the different types of bonds between atoms. You’ll also define something called electronegativity.

This topic builds on your knowledge of atomic structure. It also incorporates knowledge about forces and attraction. For example, what forces hold a bond together? Why are some bonds much stronger than others?

After looking at bonds from a subatomic point of view, you’ll then consider them on a molecular level. You’ll again apply your knowledge of forces to explain why different molecules have different shapes, and consider how bonding between molecules gives substances such different properties.

Some examples of molecule shapes, Public domain

commons.wikimedia.org

Energetics and thermodynamics

What determines if two substances react together? We call this the feasibility of a reaction, and it is all to do with energy changes. In “Energetics” and “Thermodynamics”, you’ll learn about two related ideas known as enthalpy and entropy.

You’ll explore why some substances give out heat when they react and how we measure this heat in a process called calorimetry. After that, you’ll practise working out energy changes of reactions by drawing enthalpy diagrams. And once you finish this topic, you’ll be able to predict the feasibility of any reaction just by looking at a few simple enthalpy and entropy values.

An enthalpy diagram for an exothermic reaction[3], Brazosport College, CC BY-SA 3.0, via Wikimedia Commons

Kinetics and rate equations

Now that we know why atoms react from a chemical point of view, we can turn our attention back to physics. “Kinetics” is the study of the motion of particles and how this affects changing systems. Take iron and water, for example. They react to produce iron(III) oxide. At room temperature, this reaction is very slow. But if you instead react the iron with steam, the reaction happens much faster. Why is this the case?

In “Kinetics”, you’ll learn about the rates of different reactions and how we can manipulate them. Heat is one way of increasing the speed of a reaction, but you’ll also explore other factors such as surface area and concentration. You’ll expand this knowledge in “Rate Equations”. By the end of the topic, you’ll know how to work out the rate of reactions both experimentally and theoretically.

Equilibria

In “Thermodynamics”, you learnt about the feasibility of a reaction. In fact, some reactions are feasible in both directions - there is a forward reaction and a backward reaction.

Let’s look at the white solid ammonium chloride, \( NH_4Cl\) . It breaks down into ammonia gas and hydrochloric acid.

$$ NH_4Cl_{(s)}\rightleftharpoons NH_{3(g)}+ HCl_{(g)} $$

If you leave the reaction alone in a sealed system, there will always be some white solid left, no matter how long you leave it for. The reaction doesn’t go to completion. We say that this type of reaction is reversible - as some of the ammonium chloride breaks down, some is reformed from ammonia and hydrochloric acid. It forms an equilibrium.

But what if we don’t want any ammonium chloride? How can we influence the reaction to maximise our yield of ammonia and hydrochloric acid? In “Equilibria”, you’ll learn how changing reaction conditions shifts the reaction to one side or the other. You’ll meet Le Châtelier and find out more about his principle then apply it to industrial examples, such as the production of ethanol. We’ll also explore the equilibrium constants, Kp and Kc. You'll be able to use them to work out the composition of an equilibrium mixture.

Redox

Above, we mentioned how iron(III) oxide is formed from iron and water. But what do the roman numerals shown after the iron mean?

Well, they describe something called oxidation states. You’ll learn about these in “Redox”. Oxidation states show how many electrons an atom has lost or gained in a reaction. In oxidation reactions, atoms lose electrons whereas in reduction reactions, atoms gain electrons. We’ll return to the movement of electrons and practice writing half equations to represent this.

You’ll then explore electrochemical cells. Imagine hooking two different metals up to a wire and dipping them in a salt solution. What do you expect to happen? Why is a current produced and how do we predict which direction it will flow? We can use our knowledge of atomic structure and electronegativity to answer these questions.

An electrochemical cell.StudySmarter Originals

Acids and bases

For the final topic “Acids and bases”, instead of focusing on electron movement, we’ll look at proton movement. You’ll have learnt in “Atomic Structure” that the hydrogen ion, \(H^+\), contains just one proton and no electrons or neutrons, and so we call it a proton. When substances called acids and bases react in reactions known as neutralisation reactions, they move protons around. Neutralisation reactions are extremely common in everyday life - for example, toothpastes neutralise acidic compounds produced by bacteria in our teeth, and baking powder contains a mixture of acids and bases, that when dissolved in solution, produce gases that make cakes rise.

You’ll learn about the pH scale, invented by a Danish biochemist working for a brewing company. You will also find out how to work out the pH of substances before and after neutralisation. Finally, you’ll explore how pH changes in titrations.

The pH scale. Acidic substances have a low pH and alkaline substances have a high pH.StudySmarter Originals