# OCR AS/A Level Chemistry A

# Acids and redox

Navigate to resources by choosing units within one of the unit groups shown below.

## Introduction

### Overview

Delivery guides are designed to represent a body of knowledge about teaching a particular topic and contain:

- Content: a clear outline of the content covered by the delivery guide
- Thinking Conceptually: expert guidance on the key concepts involved, common difficulties students may have, approaches to teaching that can help students understand these concepts and how this topic links conceptually to other areas of the subject
- Thinking Contextually: a range of suggested teaching activities using a variety of themes so that different activities can be selected that best suit particular classes, learning styles or teaching approaches.

## Curriculum Content

### Overview

**2.1.4 Acids **

(a) the formulae of the common acids (HC*l*, H_{2}SO_{4}, HNO_{3} and CH_{3}COOH) and the common alkalis (NaOH, KOH and NH_{3}) and explanation that acids release H^{+} ions in aqueous solution and alkalis release OH^{–} ions in aqueous solution

(b) qualitative explanation of strong and weak acids in terms of relative dissociation

(c) neutralisation as the reaction of:

(i) H^{+} and OH^{–} to form H_{2}O

(ii) acids with bases, including carbonates, metal oxides and alkalis (water-soluble bases), to form salts, including full equations

(d) the techniques and procedures used when preparing a standard solution of required concentration and carrying out acid-base titrations

(e) structured and non-structured titration calculations, based on experimental results of familiar and non-familiar acids and bases.

**2.1.5 Redox **

(a) rules for assigning and calculating oxidation numbers for atoms in elements, compounds and ions

(b) writing formulae using oxidation numbers

(c) use of a Roman numeral to indicate the magnitude of the oxidation number when an element may have compounds/ ions with different oxidation numbers

(d) oxidation and reduction in terms of:

(i) electron transfer

(ii) changes in oxidation number

(e) redox reactions of metals with acids to form salts, including full equations (see also 2.1.4 c)

(f) interpretation of redox equations in (e), and unfamiliar redox reactions, to make predictions in terms of oxidation numbers and electron loss/gain.

The following activities can be used to assess students prior knowledge before embarking on the teaching of this content.

### Name that salt

To be used to review the products of neutralisation reactions from GCSE, or to consolidate understanding of salts once it has been re-taught at AS Level.

Mini whiteboard quiz where students guess the salt produced from the neutralisation reaction between the acid and base displayed in the Teacher Resource PowerPoint: Name that salt.

### Acid, bases and pH

### Neutralisation role play

## Thinking Conceptually

### Overview

**Approaches to teaching the content **

Students will be familiar with acids and alkalis, but they may not have a grasp of all the key concepts involved. However, redox will be new to most students and is sometimes a real sticking point.

Section 2.1.4 has four main parts:

- Formulae
- Neutralisation
- Preparing a standard solution
- Titrations.

Section 2.1.5 has four main parts:

- Oxidation numbers
- Formulae
- Meanings of oxidation and reduction
- Redox reactions.

In order for students to successfully grasp the content of these topics, start by teaching oxidation numbers; this will help students to know the charges of common ions, thus enabling them to deduce formulae. In addition to this, ensure that students commit to memory the following positive and negative ions: C*l*^{–}, SO_{4}^{2}^{–}, NO_{3}^{–}, OH^{–}, CO_{3}^{2}^{–}; this will further enable them to predict formulae. Confidence in writing formulae will aid in determining products in reactions, so with this in place, creating equations for neutralisation reactions will come with greater ease. Once able to deduce oxidation numbers and formulae, students can progress to the remainder of the acids or the redox sections. Interpreting redox reactions requires students to understand oxidation and reduction in terms of loss/gain of electrons and change in oxidation state as well as oxidising and reducing agents.

**Common misconceptions or difficulties students may have **

*Strength of acid *

Students struggle to distinguish between concentration and strength of acid. A combination of videos and chemical demonstrations can be used to reinforce the concept that strength is due to relative dissociation.

*Titration calculations – where to start? *

Some students will have had experience with burettes while some will not have. This technique is important as a competency in the practical.

When doing any calculation, students often know how to do each part of the calculation, like calculating moles, but do not know where to start. There are two possible approaches to this:

1) Steps – teach students steps to learn that will work for any titration calculation.

2) Grids – teach students to draw a table under the equation where they must fill in information they are given in the question about Moles, Concentration and Volume and use it to determine the final thing the question asks for.

*Oxidation numbers*

Do not assume students can interpret roman numerals; test them on their understanding before using roman numerals to indicate magnitude of the oxidation state. There is sometimes confusion over whether or not to consider the coefficient or the subscript when deducing oxidation state (e.g. 2 Cr_{2}O_{7}^{2–}) so teach students to determine oxidation states of species in chemical equations with coefficients.

*Definitions of oxidation and reduction*

Care must be taken when teaching students terminology relating to oxidation and reduction, as these keywords can be used in many ways. Students must understand oxidation and reduction in three separate forms:

a) in terms of loss/gain of electrons;

b) in terms of increase/decrease in oxidation number;

c) in terms of a reducing or oxidising agent.

For a) and b), choose a mnemonic to teach oxidation state in terms of number of electrons and stick to it, e.g. OIL RIG (Oxidation Is Loss Reduction Is Gain) or LEO goes GER (Loss of Electrons is Oxidation, Gain of Electrons is Reduction). Students will know that electrons are negative and that in general terms if an atom gains an electron, it will gain a negative charge; likewise, if an atom loses an electron, it will lose a negative charge, thus becoming less negative, e.g. if the oxidation state of Cr is +6, it is as if it has lost 6 electrons. Number lines can be useful in understanding the change in the number of electrons and oxidation number. Get students to determine oxidation and reduction from half equations so that they become more familiar with them before moving on to redox equations.

For c), reinforce the idea that an agent is something that makes something happen, e.g. mustard gas is a blistering agent; it isn’t blistered itself, but causes other things to blister. Allow students to use their knowledge of oxidation and reduction to deduce oxidising/reducing agents on a case-by-case basis.

*Writing redox equations*

The recommended method to use in writing redox equations from half equations:

1) If necessary add H_{2}O on one side to balance the number of oxygen atoms. If it’s not necessary, then proceed to step 3.

2) Add H^{+} to the opposite side to balance the hydrogen atoms.

3) Add electrons to balance the charges.

4) If the electrons either side of the equation are not the same, multiply the half equation so that they are the same.

5) Add equations together.

6) Cancel out.

### Writing chemical formulae template: The cross-over rule (TES)

A step-by-step guide to determining chemical formulae using the cross-over rule, with a template for students to use.

### Roman numerals quizzes (Roman-Numerals.org)

Various quizzes for students to practice their knowledge of roman numerals. Can be completed as homework or as part of a lesson by using computers for each student to individually access the site or mini-whiteboards with the quizzes projected from the screen.

### Gridlocks (Royal Society of Chemistry)

Available online, as a game, or downloadable as a worksheet. These are based on the Sudoku principle where students must use their knowledge to complete boxes with words, numbers or pictures. There are a range of these available for use as a means of consolidating knowledge within the lesson, or as homework.

### Gridlocks: Acids 2 (Royal Society of Chemistry)

Use this to consolidate students’ understanding of acid formulae and salts produced from neutralisation reactions.

### Gridlocks: Alkalis 2 (Royal Society of Chemistry)

Use this to consolidate students’ understanding of alkali formulae and salts produced from neutralisation reactions.

### Gridlocks: Concentrations of solutions 1 (Royal Society of Chemistry)

Could be used to review concentration calculations before starting carrying out titrations and subsequent titration calculations.

### Gridlocks: Oxidation number rules (Royal Society of Chemistry)

Use to consolidate understanding of oxidation number rules, before or whilst students calculate oxidation states.

### Distinguishing between weak and strong acids and bases video

### Titration calculation steps

This is a list of steps to be given to students, to follow when carrying out titration calculations. Due to changes in mathematical requirements, many titration calculations will be less structured than they have been in the past. This means that students will need strategies to tackle these questions. Encouraging students to come up with their own model answer/help sheet can help embed this learning.

1) Write out a balanced symbol equation for the reaction.

2) Calculate the number of moles in the known solution using Moles = Concentration (mol dm^{-3}) × Volume (dm^{3}).

3) Use the chemical equation to work out the stoichiometry/ratio between the known and unknown solutions. Do they react in a 1:1 ratio or a 2:1 etc.

4) Calculate the number of moles in the unknown solution by using the ratios you worked out in point 3. E.g. if you have 0.5 moles of the known solution and you know from the equation that for every 1 mole of known, you have 2 moles of the unknown solution, you will have 0.5 × 2 moles of the unknown solution.

5) Ensure that your volume is in the correct form (dm^{3}).

6) Calculate the concentration of the unknown solution by dividing the number of moles by the volume required to neutralise the known solution.

### Titration calculations – the grid method

### Oxidation and reduction number line

### Redox equations video

Video which uses the recommended method for writing redox equations from half equations. It is quite fast, so may be better set as a homework exercise in order for students to watch and pause in their own time.

### Reducing agents video

Several videos showing one after the other, explaining the workings of redox reactions. Helps to interpret redox reactions by using two examples and working through to assign oxidation and reduction agents to the appropriate species.

Video 1 (7 min 41 sec) Uses the acronym LEO goes GER (Loss of Electrons is Oxidation, Gain of Electrons is Reduction).

Video 2 (7 min 26 sec) Uses the acronym OIL RIG (Oxidation Is Loss Reduction Is Gain) and works through the examples at a slower pace. Be aware that the oxidation number of sulphide (-2) is used to determine the oxidation number of copper in copper sulphide. Students are not required to know this oxidation number.

## Thinking Contextually

### Overview

There are many possible contexts in which you could introduce or extend learners’ understanding of these topics.

**Redox titrations **

In order to stretch, challenge, consolidate and prepare, encourage students to determine which sub-shells the electrons have been lost or gained from. This will prepare them for the redox reactions of halogens and trends in ionisation energies.

**Transition metal complexes **

When teaching students how to calculate oxidation numbers, you could set some calculating oxidation numbers of complex ions as a stretch and challenge task.

**Acids, bases and indicators **

When doing titrations, allow students to choose which indicators to use when given information about the range of each one.

**pH calculations **

Dissociation of weak/strong acids/bases.

**Group 2 metal hydroxides **

When teaching neutralisation reactions, encourage students to learn the reactions of Ca(OH)_{2} in agriculture to neutralise acidic soil and Mg(OH)_{2} and CaCO_{3} as acids in treating indigestion.

When teaching acid-base reactions use:

- Relief of indigestion by neutralisation of stomach acid with antacids.
- Flue gas desulphurisation.
- The effect of acid rain on limestone statues.

Students can do a redox titration to determine the % of iron in iron tablets or the % of iron in steel wool.

### Determination of concentration of hydrochloric acid

### Determination of the molar mass of an acid

### Identification of an unknown carbonate

### Identifying unknowns 1

### Identifying unknowns 2

### Identifying unknowns 3

### Neutralisation of indigestion tablets practical (Royal Society of Chemistry)

Titration practical where the concentration of alkali in an indigestion tablet is found by titrating against hydrochloric acid. This helps students to consider the practical application of titrations, by relating them to an everyday remedy.

### Determination of Fe in dietary supplement through redox titrations (Collin College)

Practical where students use a redox titration to determine the purity of an iron tablet. This is useful in that it allows students to use their experimental techniques to investigate a household item.

### Range of redox practicals and demonstrations (Nuffield Foundation)

Use these as part of your lessons to show students the concepts in a context.

### Homework that prepares

In addition to setting homework that consolidates learning from the lesson, set homework that will prepare students for the following lesson. This can be GCSE questions about the topic, or open-ended tasks. Inform students that the more effort they put into their homework, the more they will get out of the next lesson. Open-ended tasks allow students to use their creativity, be more independent in their learning and enable differentiation by outcome. In many cases, the less structure you provide, the more independent the learning. Undoubtedly, it can mean that the task is more difficult to mark and that for certain topics, some may come armed with very complex chemistry, which will stretch and challenge them, but could test your memory!

If students are prepared with some knowledge before they enter each lesson, it will result in deeper understanding within the lesson. Ensure that part of the homework that you set before a new topic ensures that they revise what they have learned at GCSE and prepares them for the new content. See Teacher resource 2.

## Acknowledgements

### Overview

OCR’s resources are provided to support the teaching of OCR specifications, but in no way constitute an endorsed teaching method that is required by the Board and the decision to use them lies with the individual teacher. Whilst every effort is made to ensure the accuracy of the content, OCR cannot be held responsible for any errors or omissions within these resources. We update our resources on a regular basis, so please check the OCR website to ensure you have the most up to date version.

© OCR 2015 - This resource may be freely copied and distributed, as long as the OCR logo and this message remain intact and OCR is acknowledged as the originator of this work.