Chemistry (Version 8.4)

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Unit 4

Understand how Chemistry works

Rationale/Aims

Chemistry is the study of materials and substances, and the transformations they undergo through interactions and the transfer of energy. Chemists can use an understanding of chemical structures and processes to adapt, control and manipulate systems to meet particular economic, environmental and social needs.

Structure of Chemistry

In Chemistry, students develop their understanding of chemical systems, and how models of matter and energy transfers and transformations can be used to describe, explain and predict chemical structures, properties and reactions.

Links to Foundation to Year 10

The Chemistry curriculum continues to develop student understanding and skills from across the three strands of the F-10 Australian Curriculum: Science. In the Science Understanding strand, the Chemistry curriculum draws on knowledge and understanding from across the four sub-strands of Biological, Physical, Chemical and Earth and Space Sciences.

Representation of Cross-curriculum priorities

While the significance of the cross-curriculum priorities for Chemistry varies, there are opportunities for teachers to select contexts that incorporate the key concepts from each priority.

Achievement standards

Unit 3: Equilibrium, acids and redox reactions

Unit 3: Equilibrium, acids and redox reactions Description

The idea of reversibility of reaction is vital in a variety of chemical systems at different scales, ranging from the processes that release carbon dioxide into our atmosphere to the reactions of ions within individual cells in our bodies. Processes that are reversible will respond to a range of factors and can achieve a state of dynamic equilibrium. In this unit, students investigate acid-base equilibrium systems and their applications. They use contemporary models to explain the nature of acids and bases, and their properties and uses. This understanding enables further exploration of the varying strengths of acids and bases. Students investigate the principles of oxidation and reduction reactions and the production of electricity from electrochemical cells.

Through the investigation of appropriate contexts, students explore the ways in which models and theories related to acid-base and redox reactions, and their applications, have developed over time and through interactions with social, economic, cultural and ethical considerations. They explore the ways in which chemistry contributes to contemporary debate in industrial and environmental contexts, including the use of energy, evaluation of risk and action for sustainability, and they recognise the limitations of science in providing definitive answers in different contexts.

Students use science inquiry skills to investigate the principles of dynamic chemical equilibrium and how these can be applied to chemical processes and systems. They investigate a range of electrochemical cells, including the choice of materials used and the voltage produced by these cells. Students use the pH scale to assist in making judgements and predictions about the extent of dissociation of acids and bases and about the concentrations of ions in an aqueous solution.

Unit 3: Equilibrium, acids and redox reactions Learning Outcomes

By the end of this unit, students:

understand the characteristics of equilibrium systems, and explain and predict how they are affected by changes to temperature, concentration and pressure
understand the difference between the strength and concentration of acids, and relate this to the principles of chemical equilibrium
understand how redox reactions, galvanic and electrolytic cells are modelled in terms of electron transfer
understand how models and theories have developed over time and the ways in which chemical knowledge interacts with social, economic, cultural and political considerations in a range of contexts
use science inquiry skills to design, conduct, evaluate and communicate investigations into the properties of acids and bases, redox reactions and electrochemical cells, including volumetric analysis
evaluate, with reference to empirical evidence, claims about equilibrium systems and justify evaluations
communicate, predict and explain chemical phenomena using qualitative and quantitative representations in appropriate modes and genres.

Unit 3: Equilibrium, acids and redox reactions Content Descriptions

Science Inquiry Skills (Chemistry Unit 3)

Identify, research, construct and refine questions for investigation; propose hypotheses; and predict possible outcomes (ACSCH074)

Design investigations, including the procedure/s to be followed, the materials required, and the type and amount of primary and/or secondary data to be collected; conduct risk assessments; and consider research ethics (ACSCH075)

Conduct investigations, including using volumetric analysis techniques and constructing electrochemical cells, safely, competently and methodically for the collection of valid and reliable data (ACSCH076)

Represent data in meaningful and useful ways, including using appropriate graphic representations and correct units and symbols; organise and process data to identify trends, patterns and relationships; identify and distinguish between random and systematic errors, and estimate their effect on measured results; discuss how the nature of the procedure and the sample size may influence uncertainty and limitations in data; and select, synthesise and use evidence to make and justify conclusions (ACSCH077)

Interpret a range of scientific texts, and evaluate processes, claims and conclusions by considering the quality of available evidence, including confidence intervals in secondary data; and use reasoning to construct scientific arguments (ACSCH078)

Select, construct and use appropriate representations, including half-equations, balanced chemical equations, equilibrium constants and expressions, pH, oxidation numbers, standard electrode potentials and cell diagrams, to communicate conceptual understanding, solve problems and make predictions (ACSCH079)

Select and use appropriate mathematical representations to solve problems and make predictions, including calculating cell potentials under standard conditions, using the mole concept to calculate moles, mass, volume and concentrations from volumetric analysis data, determining the yield of incomplete reactions, and calculating the pH of solutions of strong acids and bases (ACSCH080)

Communicate to specific audiences and for specific purposes using appropriate language, nomenclature, genres and modes, including scientific reports (ACSCH081)

Science as a Human Endeavour (Units 3 & 4)

ICT and other technologies have dramatically increased the size, accuracy and geographic and temporal scope of data sets with which scientists work (ACSCH082)

Models and theories are contested and refined or replaced when new evidence challenges them, or when a new model or theory has greater explanatory power (ACSCH083)

The acceptance of scientific knowledge can be influenced by the social, economic, and cultural context in which it is considered (ACSCH084)

People can use scientific knowledge to inform the monitoring, assessment and evaluation of risk (ACSCH085)

Science can be limited in its ability to provide definitive answers to public debate; there may be insufficient reliable data available, or interpretation of the data may be open to question (ACSCH086)

International collaboration is often required when investing in large-scale science projects or addressing issues for the Asia-Pacific region (ACSCH087)

Scientific knowledge can be used to develop and evaluate projected economic, social and environmental impacts and to design action for sustainability (ACSCH088)

Science Understanding

Chemical equilibrium systems

Examples in context

Support materials only that illustrate some possible contexts for exploring Science as a Human Endeavour concepts in relation to Science Understanding content.

Chemical balance in wine

The production of wine, along with that of many other food products, relies on the successful control of a range of reversible reactions in order to maintain the required chemical balance within the product. For wine, this balance includes the acidity, alcohol concentration, sugar levels and the colour of the wine. Techniques such as auto titration, gas chromatography and infrared spectroscopy are used to measure the chemical composition of wine. Data from these methods, including the analysis of multivariate data, has enabled scientists to identify how the concentrations of the various chemicals in the wine are related, both to each other and the observable properties of wine such as taste and aroma (ACSCH082). Sulphur dioxide is used to maintain chemical balance in wine, as it binds with acetaldehyde. ‘Sulphite calculators’ are available so that wine makers can predict the amount of sulphur dioxide required. However decisions as to how the sulphur dioxide is added to the wine, including how much to use, will depend on preferences of the winemaker, especially for those producers who market wine as ‘organic’ or ‘preservative free’ (ACSCH084).

Carbon dioxide in the atmosphere and hydrosphere

The levels of carbon dioxide in the atmosphere have a significant influence on global systems, including surface temperatures. The oceans contribute to the maintenance of steady concentrations of atmospheric carbon dioxide because the gas can dissolve in seawater through a range of reversible processes. The uptake of anthropogenic carbon dioxide by the oceans is driven by the difference in gas pressure in the atmosphere and in the oceans, and by the air/sea transfer velocity. Because carbon dioxide is increasing in the atmosphere, more of it moves into the ocean to balance the oceanic and atmospheric gas pressures, causing a change in the equilibrium point. Dissolved carbon dioxide increases ocean acidity, which is predicted to have a range of negative consequences for ecosystems, including direct impacts on oceanic calcifying organisms such as corals, crustaceans and molluscs because structures made of calcium carbonate are vulnerable to dissolution under at lower pH levels (ACSCH088). The United Nations Kyoto Protocol and the establishment of the Intergovernmental Panel on Climate Change aim to secure global commitment to a significant reduction in greenhouse gas emissions over the next decades (ACSCH087).

Development of acid/base models

Lavoisier, often referred to as the father of modern chemistry, believed that all acids contained oxygen. In 1810, Davy proposed that it was hydrogen, rather than oxygen, that was common to all acids (ACSCH083). Arrhenius linked the behaviour of acids to their ability to produce hydrogen ions in aqueous solution, however this theory only related to aqueous solutions and relied on all bases producing hydroxide ions. In 1923 Brønsted (and at about the same time, Lowry) refined the earlier theories by describing acids as proton donators (ACSCH083). This theory allowed for the description of conjugate acid-bases, and for the explanation of the varying strength of acids based on the stability of the ions produced when acids ionise to form the hydrogen ions. This concept has been applied to contemporary research into ‘superacids’, such as carborane acids, which have been found to be a million times stronger than sulphuric acid when the position of equilibrium in aqueous solution is considered.

Chemical systems may be open or closed and include physical changes and chemical reactions which can result in observable changes to the system (ACSCH089)

All physical changes are reversible, whereas only some chemical reactions are reversible (ACSCH090)

Over time, physical changes and reversible chemical reactions reach a state of dynamic equilibrium in a closed system, with the relative concentrations of products and reactants defining the position of equilibrium (ACSCH091)

The reversibility of chemical reactions can be explained by considering the activation energies of the forward and reverse reactions (ACSCH092)

The effect of changes of temperature on chemical systems at equilibrium can be explained by considering the enthalpy changes for the forward and reverse reactions (ACSCH093)

The effect of changes of concentration and pressure on chemical systems at equilibrium can be explained and predicted by applying collision theory to the forward and reverse reactions (ACSCH094)

The effects of changes of temperature, concentration of chemicals and pressure on equilibrium systems can be predicted using Le Chatelier’s Principle (ACSCH095)

Equilibrium position can be predicted qualitatively using equilibrium constants (ACSCH096)

Acids are substances that can act as proton (hydrogen ion) donors and can be classified as monoprotic or polyprotic depending on the number of protons donated by each molecule of the acid (ACSCH097)

The strength of acids is explained by the degree of ionisation at equilibrium in aqueous solution, which can be represented with chemical equations and equilibrium constants (Ka) (ACSCH098)

The relationship between acids and bases in equilibrium systems can be explained using the Brønsted-Lowry model and represented using chemical equations that illustrate the transfer of hydrogen ions (ACSCH099)

The pH scale is a logarithmic scale and the pH of a solution can be calculated from the concentration of hydrogen ions; K_w can be used to calculate the concentration of hydrogen ions from the concentration of hydroxide ions in a solution (ACSCH100)

Acid-base indicators are weak acids or bases where the acidic form is of a different colour to the basic form (ACSCH101)

Volumetric analysis methods involving acid-base reactions rely on the identification of an equivalence point by measuring the associated change in pH, using chemical indicators or pH meters, to reveal an observable end point (ACSCH102)

Oxidation and reduction

Examples in context

Support materials only that illustrate some possible contexts for exploring Science as a Human Endeavour concepts in relation to Science Understanding content.

Breathalysers and measurement of blood alcohol levels

The level of alcohol in the body can be measured by testing breath or blood alcohol concentrations (ACSCH085). These analysis techniques rely on redox reactions. Police first used breath testing for alcohol in the 1940s. Currently, a range of other detection methods are available to police, and commercially to drivers who are now able to test themselves before driving. Some meters use infrared spectroscopy to determine the amount of alcohol present, which can be converted to blood alcohol concentration (BAC). Electrochemical cells form the basis of ‘alcosensors’ which can also be used to measure BAC. These cells work by recording the electrical potential produced by the oxidation of the ethanol at platinum electrodes. Although science can provide information about the effect of alcohol on our bodies in relation to the ability to drive, decisions about ‘safe’ levels of BAC for driving (including those used to write legislation) take into account other factors, such as the experience of the driver, and can vary from country to country (ACSCH086).

Fuel cells and their uses

Redox reactions that occur spontaneously can be used as a source of electrical energy. These include wet cells (such as car batteries), dry cells, and alkaline batteries. Fuel cells are electrochemical cells that use up a ‘fuel’, such as hydrogen. Fuel cells were first demonstrated in the 1840s, but were not commercially available until the late twentieth century. Currently, small fuel cells are designed for laptop computers and other portable electronic devices; larger fuel cells are used to provide backup power for hospitals; and wastewater treatment plants and landfills make use of fuel cells to capture and convert the methane gas they produce into methane (ACSCH088). Fuel cells are a potential lower-emission alternative to the internal combustion engine and are already being used to power buses, boats, trains and cars (ACSCH088). International organisations such as the International Partnership for Hydrogen and Fuel Cells in the Economy (IPHE) have been created to foster international cooperation on research and development, common codes and standards, and information sharing on infrastructure development (ACSCH087).

Electrochemistry for clean water

Electrochemistry has a wide range of uses, ranging from industrial scale metal extraction to personal cosmetic treatments. A new application has been in the treatment of mineral rich bore water. New Zealand scientists have trialled a system that uses electrochemistry to remove the iron and manganese ions present in bore water, which currently make the water undrinkable. An electric current converts chloride ions to chlorine, which then oxidises and precipitates out the metal contaminants, as well as disinfecting the water. The electric current passing through the water also dramatically increased the effectiveness of the chlorine in killing organisms in the water. The process requires minimal current and can be provided by a 12-volt car battery, which makes it a cheap and relatively ‘low tech’ solution suitable for use in rural areas of developing countries (ACSCH087).

A range of reactions, including displacement reactions of metals, combustion, corrosion, and electrochemical processes, can be modelled as redox reactions involving oxidation of one substance and reduction of another substance (ACSCH103)

Oxidation can be modelled as the loss of electrons from a chemical species, and reduction can be modelled as the gain of electrons by a chemical species; these processes can be represented using half-equations (ACSCH104)

The ability of an atom to gain or lose electrons can be explained with reference to valence electrons, consideration of energy, and the overall stability of the atom, and can be predicted from the atom’s position in the periodic table (ACSCH105)

The relative strength of oxidising and reducing agents can be determined by comparing standard electrode potentials (ACSCH106)

Electrochemical cells, including galvanic and electrolytic 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) (ACSCH107)

Galvanic cells, including fuel cells, generate an electrical potential difference from a spontaneous redox reaction; they can be represented as cell diagrams including anode and cathode half-equations (ACSCH108)

Fuel cells can use metal nanoparticles as catalysts to improve the efficiency of energy production (ACSCH109)

Cell potentials at standard conditions can be calculated from standard electrode potentials; these values can be used to compare cells constructed from different materials (ACSCH110)

Electrolytic cells use an external electrical potential difference to provide the energy to allow a non-spontaneous redox reaction to occur, and can be used in small-scale and industrial situations (ACSCH111)