BlogChemistryThe Ultimate Guide to QCAA Chemistry Unit 3: Equilibrium, Acids and Redox Reactions

The Ultimate Guide to QCAA Chemistry Unit 3: Equilibrium, Acids and Redox Reactions

Laboratory Researcher - QCE Chemistry Unit 3

Have you started studying Unit 3 of QCAA Chemistry in class but are a little confused about what you’re meant to be learning? No worries, we’re here to help!

To make things easier for you, we’ve broken down what you’ll be learning in each of the topics for Unit 3 of QCAA Chemistry and what assessments you’ll be completing.

So what are you waiting for? Let’s get started!

What is Unit 3 in QCAA Chemistry all about?
Topic 1: Chemical Equilibrium Systems
Topic 2: Oxidation and Reduction
QCAA Chemistry Unit 3 Assessments
Study Tips

What is Unit 3 in QCAA Chemistry all about?

Unit 3 is the first of your two formative senior units for QCAA Chemistry. This means that the assessments you complete for this unit will count towards your ATAR.

The unit is split into two topics: chemical equilibrium systems, and oxidation and reduction, which cover a variety of subtopics from equilibrium constants to electrolytic cells. 

We’ll break down each of these topics so you know exactly what you’ll be studying!

Topic 1: Chemical Equilibrium Systems

Chemical Equilibrium 

This subtopic covers: 

  • open VS closed chemical systems
  • forwards VS reverse reactions 
  • reversibility/irreversibility of physical VS chemical changes 
  • what it means for a system to have reached a dynamic equilibrium 
  • observable chemical and physical changes of a chemical system and what they mean at an atomic and molecular level

When studying this area, it may be helpful to revise the following topics: 

  • Unit 1 Topic 3: Chemical reactions: reactants, products and energy change
  • Unit 2 Topic 3: Rates of chemical reactions

Factors that Affect Equilibrium 

This subtopic covers:

  • the effect that increasing or decreasing temperature, pressure or concentration can have on a particular chemical system at equilibrium 
  • understanding and applying Le Châtelier’s principle

Factors that affect equilibrium

Factors that affect equilibrium 2

Equilibrium Constants 

This subtopic covers: 

  • understanding that the equilibrium constant (KC) is a ratio of the product and reactant concentrations at equilibrium
  • understanding how to apply the KC formula
Relevant Formulas

Relevant Formulas

Properties of Acids and Bases

This subtopic covers: 

  • understanding that acids dissociate to produce H+ ions 
  • understanding that bases dissociate to produce OHions 
  • monoprotic VS diprotic VS polyprotic donors
  • strong acids/bases: completely dissociate (e.g. hydrochloric acid)
  • weak acids/bases: partially dissociate (e.g. ethanoic acid) 
  • concentrated acids/bases: have many acid/base molecules
  • dilute acids/bases: have few acid/base molecules

Learn more about the strength of acid bases here on QCETogether!

pH Scale

This subtopic covers: 

  • understanding that KW can be used to calculate the concentration of hydrogen ions from the concentration of hydroxide ions in a solution
  • understanding that the pH scale is a logarithmic scale
  • using appropriate formulas and convention to complete relevant calculations
Relevant Formulas
  • self-ionisation of water: KW = [H+][OH] 
  • pH = –log10[H+]

pH table

Brønsted-Lowry Model 

This subtopic covers: 

  • the Brønsted–Lowry model definition of acids and bases 
  • conjugate acids vs conjugate bases and how to identify them
  • amphiprotic species 
  • what buffer solutions are and how they respond to the addition of hydrogen and hydroxide ions

Brønsted–Lowry Model

Dissociation Constants 

This subtopic covers: 

  • understanding that that the strength of acids is explained by the degree of ionisation at equilibrium in an aqueous solution
  • calculating dissociation constants (𝐾𝑎 and 𝐾𝑏) as well as concentrations of reactants and products via the appropriate formulas
  • understanding that a high 𝐾𝑎 value correlates to more product dissociated, stronger acidity, lower pH, higher [H+]
  •  and lower pKa and vice versa
Relevant Formulas 

Relevant Formulas

Acid-Base Indicators 

This subtopic covers: 

  • understanding that an acid-base indicator is a weak acid or a weak base and why the colour change occurs at a certain pH
  • understanding that the relationship between the pH range of an acid-base indicator and its pKa value 
  • understanding that indicators change colour when pH = pKa
  • determining appropriate indicators for titrations given the equivalence point and the range of the indicator 

Acid-Base Indicators

Image sourced from Oxford Chemistry Units 3 & 4 

Acid-Base Indicators 2

Image sourced from Chemistry 2019 Formula and data book 

Volumetric Analysis 

This subtopic covers: 

  • equivalence point vs endpoint 
  • significance of equivalence point and how it is measured 
  • sketching titration curves and identifying/labelling intercept with pH axis, equivalence point, half-equivalence point, end point and buffer region
  • calculating moles, volume and concentration from volumetric data analysis 

Volumetric Analysis

Image sourced from Lumen Learning

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Topic 2: Oxidation and Reduction

Redox Reactions 

This subtopic covers: 

  • understanding that redox reactions involve oxidation of one substance and reduction of another substance 
  • types of redox reactions e.g. combustion, displacement of metals, corrosion
  • understanding how an atom’s position in the periodic table relates to its ability to gain or lose electrons 
  • identify oxidised and reduced species in redox reactions 
  • applying Oxidation Number Rules to deduce oxidation states of atoms in an ion or compound

QCE Chemistry Unit 3 Redox Reactions

QCE Chemistry Unit 3 Redox Reactions 2

Electrochemical Cells 

This subtopic covers: 

  • understanding that electrochemical cells consist of oxidation and reduction half-reactions connected via an external circuit that allows electrons to move from the anode (oxidation reaction) to the cathode (reduction reaction)

Even though this seems like a tonne of content, it’s pretty easy to get the hang of once you learn how to take effective science study notes!

Galvanic Cells 

This subtopic covers: 

  • understanding that galvanic cells generate an electrical potential difference from a spontaneous redox reaction 
  • understanding that oxidation occurs at the negative electrode (anode) and reduction occurs at the positive electrode (cathode)
  • describing and labelling the components of a galvanic cell on a diagram, including: the oxidation and reduction half-cells, the positive and negative electrodes and their solutions of their ions, the flow of electrons, the movement of ions and the salt bridge
Things to Remember
  • anode is negative (-)
  • cathode is positive (+)
  • spontaneous reaction occurs
  • does not require external energy source
  • converts chemical energy into electrical energy 
  • EMF is positive (+)

QCE Chemistry Unit 3 Galvanic Cells

Image sourced from Oxford Chemistry Units 3 & 4 

Standard Electrode Potential 

This subtopic covers: 

  • understanding that the standard electrode potential is the electrical potential than an electrode generates under standard conditions 
  • determining the relative strength of oxidising and reducing agents by comparing standard electrode potentials
  • understanding that the limit of standard electrode potentials is that they can only be applied to aqueous equilibrium 
  • using appropriate formulas and convention to complete relevant calculations

Section of the Electrochemical Series Table 

QCE Chemistry Unit 3 Electrochemical Series Table

Image sourced from Oxford Chemistry Units 3 & 4 

Relevant Formulas 
  • EMF = E°reduction half-cell — E°oxidation half-cell

Electrolytic Cells 

Things to Remember
  • anode is negative (-)
  • understanding that electrolytic cells use an external electrical potential difference to provide the energy to allow a non-spontaneous redox reaction to occur
  • predicting and explaining the products of the electrolysis of a molten salt and aqueous solutions of sodium chloride and copper sulfate
  • describing and labelling the components of a galvanic cell on a diagram, including source of electric current and conductors, positive and negative electrodes and the electrolyte
Things to Remember
  • cathode is negative (-)
  • anode is positive (+)
  • non-spontaneous reaction occurs
  • requires external voltage source 
  • converts electrical energy into chemical energy
  • EMF is negative (-)

QCE Chemistry Unit 3 Electrolytic Cells

Image sourced from Oxford Chemistry Units 3 & 4 

QCAA Chemistry Unit 3 Assessments

Want to know the marks you need to achieve to score your ATAR goal? Check out our QCE Cohort Comparison Tool!

#1: IA1 – Data Test

It will assess your ability to interpret and analyse data from various data sets and constitutes 10% of your overall grade. For IA1 practice questions, see QCAA Unit 3 Chemistry Data Test IA1 – Practice Questions!

#2: IA2 – Student Experiment

It will assess your ability to create a research report based on an experiment that you conduct. It constitutes 20% of your overall grade. To find out how to write your student experiment, see The Definitive Guide to Writing a Student Experiment Report for QCAA Chemistry!

#3: EA – External Assessment

It is an exam that consists of both Unit 3 and 4 content and constitutes 50% of your overall grade. To find out how to best prepare for the external assessment, see How to Ace Your External Assessment for QCAA Chemistry.

For practice questions, see:

Check out this exhaustive list of all relevant articles for Term 1 of Year 12 to help you ace every subject and assessment!

Tips for Study 

#1: Create a revision table like the one below using the syllabus dot points to guide you 

Chemistry External Assessment Revision Table

#2: Get visual!

Use diagrams, flowcharts and other visual forms of displaying information where you can when making your notes. 

#3: Don’t forget examples

Include examples of each concept you learn about. For example, when learning about buffer solutions, make sure to include examples of a few relevant buffer solutions.

#4: Manage your time well

The syllabus has a guide to how much time should be spent on each subtopic. Although this guide is technically for teachers and their lesson planning, you as a student can also use it to help guide your study (i.e. to figure out what topics may be more important and therefore require more study time) 

QCE Chemistry Unit 3 Syllabus

Also studying Unit 3 of Physics? Check out our guide on Unit 3: Gravity and Electromagnetism!

Ready to jump over to Unit 4 of Chemistry? We have an Ultimate Guide for QCAA Chemistry Unit 4 as well!

Are you looking for some extra help with revising QCAA Chemistry Unit 3?

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Yalindi Binduhewa is an Art of Smart tutor based in Queensland and was part of the very first cohort to go through the ATAR system, so she knows exactly how fun and enjoyable it can be. She is currently studying a Bachelor of Medical Imaging (Honours) at QUT and is loving it. When she’s not doing uni-related stuff or tutoring, she’s hanging out with her friends, rewatching a show for the 100th time, or trying out new crafty projects and discovering that she doesn’t have a talent for everything. 

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