C3.3 Types of chemical reactions
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C3.3 Types of chemical reactions
Mathematical learning outcomes:
CM3.3i arithmetic computation, ratio, percentage and multistep calculations permeates quantitative chemistry
Assessable content statements:
C3.3a explain reduction and oxidation in terms of loss or gain of oxygen, identifying which species are oxidised and which are reduced to include the concept of oxidising agent and reducing agent
C3.3b explain reduction and oxidation in terms of gain or loss of electrons, identifying which species are oxidised and which are reduced
C3.3c recall that acids form hydrogen ions when they dissolve in water and solutions of alkalis contain hydroxide ions
C3.3d describe neutralisation as acid reacting with alkali or a base to form a salt plus water
C3.3e recognise that aqueous neutralisation reactions can be generalised to hydrogen ions reacting with hydroxide ions to form water
C3.3f recall that carbonates and some metals react with acids and write balanced equations predicting products from given reactants
C3.3g use and explain the terms dilute and concentrated (amount of substance) and weak and strong (degree of ionisation) in relation to acids to include ratio of amount of acid to volume of solution
C3.3h recall that relative acidity and alkalinity are measured by pH
C3.3i describe neutrality and relative acidity and alkalinity in terms of the effect of the concentration of hydrogen ions on the numerical value of pH (whole numbers only) to include pH of titration curves
C3.3j recall that as hydrogen ion concentration increases by a factor of ten the pH value of a solution decreases by a factor of one
C3.3k describe techniques and apparatus used to measure pH
Linking the idea of gaining oxygen to being oxidised and the loss of oxygen to reduction is the first approach in enabling learners to understand redox reactions. The reactant providing the oxygen being the oxidant while the reactant receiving the oxygen is the reductant. Higher - level learners should also be aware that redox reactions are always accompanied by the loss (oxidation) and gain (reduction) of electrons thus enabling understanding of reactions (or half equations) not involving oxygen. The reactant losing electrons being the reductant while that gaining being the oxidant. Use the acronym OILRIG to identify which is being oxidised and which being reduced.
Learners need to be introduced to acid-base theory i.e. in aqueous solution those substances that release hydrogen ions are acidic while those that release hydroxide ions are alkaline. Learners should then be led to understand that the general reaction between acids and alkalis is the reaction between hydrogen ions and hydroxide ions to form water while the remaining (spectator) ions form a salt.
Learners should be made aware of the use of the term pH and the pH scale. This is useful, at higher level, to enable differentiation between the use of the words strength and concentration in relation to acids and bases. Weak bases are unable to provide solutions of pH at either end of the spectrum (i.e. 0-2 or 12-14) as they incapable of providing enough hydrogen or hydroxide ions, whereas even relatively weak solutions of strong acids and bases will have pH values approaching the extremes of the scale. In addition when the latter are diluted tenfold the pH value changes by a factor of one. Comparison should be made with tenfold dilution of weak acids and bases using a pH meter to make the difference in ionisation clear. All learners should be given the opportunity to see use a pH meter as well as use full range indicator paper or universal indicator solution.
Common misconceptions or difficulties learners may have
Learners have difficulty with the idea that the acid produces separate ions in solution and maintain the idea that hydrogen ions remain as part of the molecule. Understanding of pH is poor and learners tend to think that alkalis are less corrosive than acids. Learners also confuse the two ideas of concentration and strength of acids and bases. Common usage of the terms is interchanged often and it can be difficult to persuade learners that these terms are used in a more specific manner in chemistry.
Conceptual links to other areas of the specification – useful ways to approach this topic to set learners up for topics later in the course
Redox reactions are responsible for rusting and corrosion of metals thus this understanding links to 6.1 n and o. Discussion at this point of the faster reaction with oxygen of the more reactive metals will form the basis for understanding rusting later.
The strength of an acid used in determination of rate of reaction will determine the gradient of the line at t0 weaker acids will be shallower. Thus full understanding of the nature of hydrogen ion release will be needed for 5.2 a and c.
Approaches to teaching the content
Oxidation as defined by the addition of oxygen can easily be shown by simple combustion, the use of the thermite reaction is justified here, as it is a displacement reaction, in 'Redox' and 'Thermite demo'.
Taking the definition further to include the loss and gain of electrons is covered in 'Redox practical'.
The use of familiar household chemicals in 'pH simulation' gives learners a link to real-life, it also makes it clear that there are some very hazardous materials being stored and used in the home. 'Acid-base simulation' gives access to changing a number of variables and performing virtual tests in a wide variety of situations and should be used to promote discussion to ensure clear learning. This simulation accesses a number of methods for measuring pH and can look at the change in pH for a change in concentration.This discussion and its impact is checked and improved using 'Acid base worksheet'. The latter half of the film linked in 'Acids and bases' covers most of the necessary material regarding acids bases, pH and neutralisation.
All the instructions to carry out this exciting redox reaction.
To explore the transfer of oxygen between two metals.
A pair of microscale experiments providing a safe initial introduction to redox and the transfer of electrons.
Full documentation is provided for learners, teacher and technician.
Using common household materials to investigate the effect of concentration on pH.
This is a self-learning tool that can be accessed as a group in class or individually at school or at home. To use the tool you need to click on the red button to add the substance, drag the pH probe into the substance and drag out the valves on the water pipe to add water.
A simulation providing access to a wide range of situations and equipment allowing a great deal of discussion.
This is a self-learning tool that can be accessed as a group in class or individually at school or at home.
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