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Redox Reactions with Lollipops Use a Lollipop to Activate Colour-Changing Redox Reactions in This Simple but Eye-Catching Activity

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Redox Reactions with Lollipops Use a Lollipop to Activate Colour-Changing Redox Reactions in This Simple but Eye-Catching Activity TEACH | Chemistry The chemical chameleon demonstration produces vivid colour changes as a result of redox reactions. IanRedding/Shutterstock.com Colourful chemistry: redox reactions with lollipops Use a lollipop to activate colour-changing redox reactions in this simple but eye-catching activity. By Marisa Prolongo and Gabriel Pinto Teaching oxidation-reduction (redox) reactions is part of from a lollipop is used as the reducing agent. When glucose all secondary-school chemistry curricula. In this article, we is added to a solution containing OH– ions, the alcohol groups describe a vivid colour-changing demonstration to illustrate in the glucose donate electrons, giving rise to ketone groups: a chain of redox reactions, whereby electrons are transferred between different compounds and ions. The activity is –C(H)(OH)– + 2OH– à –C(=O)– + 2H O + 2e– suitable as a teacher demonstration, or older students could 2 (secondary alcohol) (ketone group) carry out the experiment themselves. In our experiment, glucose is added to a permanganate Oxidising and reducing agents solution together with sodium hydroxide (NaOH), so electrons from glucose (C6H12O6) are first donated to – A redox reaction is any chemical reaction in which a permanganate ions (MnO4 ). The oxidation products of the molecule, atom or ion loses or gains electrons, altering its reducing sugar are mainly glucuronic acid (C6H10O7,), plus oxidation state. An oxidising agent gains electrons (and is some arabinonic acid (C5H10O6) and formic acid (CH2O2). reduced in the reaction) and a reducing agent loses electrons If the lollipop is made from fructose, which is an isomer of (and is oxidised in the reaction). In this experiment, glucose glucose, the main product is fructonic acid (also C6H10O7). www.scienceinschool.org I Science in School I Issue 43 : Spring 2018 I 41 Chemistry | TEACH In a series of redox reactions, electrons are donated continually from glucose to successive compounds of manganese. At each step in the chain, a colour change is visible. Manganese is ideal for this experiment, as it has more stable oxidation states than any other transition metal (from +2 to +7), each of which has a different colour. You may be familiar with the classic ‘chemical chameleon’ demonstrationw1, of which this experiment is an adaptation. In the original version, you begin with a potassium permanganate and glucose solution, which changes colour upon mixing with a spatula. By using a lollipop instead, the glucose is added more gradually to the solution, Marisa Prolongo which makes it easier to follow the colour changes. Using a miniature electric whisk means the lollipop is stirred faster than by hand. Figure 1: Materials required for the lollipop experiment Materials Procedure Chemistry You will need the following materials The activity is suitable for a single Redox reactions (see figure 1): lesson. The experiment takes only about · 15 minutes and can be followed up by a Potassium permanganate (KMnO4) Ages 16–19 crystals set of discussion questions. · Spherical lollipop containing glucose The steps are as follows: The redox chemistry of (or other reducing sugar, 1. Fill the flask or beaker with 200 ml manganese is a fascinat- e.g. fructose) of distilled water. ing aspect of transition · 3–4 sodium hydroxide (NaOH) metal chemistry. This 2. Stir in the NaOH pellets with the pellets (approximately 0.5 g) simple practical exercise spoon until they have dissolved · helps students to famil- 200 ml distilled water completely. iarise themselves with the · 250 ml conical flask or beaker 3. Using the spatula, add a few variable oxidation states (glass or plastic) REVIEW potassium permanganate crystals of manganese and their · Spoon and spatula (not too many, or the colour will be respective colours. · Miniature electric whisk, too dark to see the changes). When Observing the different e.g. hand-held milk frother potassium permanganate (KMnO4) colours will elicit discus- · Adhesive tape is added to the alkaline NaOH sion and will be a point solution, it dissolves into potassium Safety note: of focus for understanding + – (K ) and permanganate (MnO4 ) what is happening in the A lab coat, gloves and safety goggles ions. should be worn. Teachers should redox steps in the reaction. 4. Attach the stick of the unwrapped follow their local health and safety lollipop to the mini electric whisk rules, in particular concerning the use Andrew Galea, chemistry using adhesive tape (see figure 1). of potassium permanganate and the lecturer, Giovanni Curmi disposal of the resulting solution. See 5. Insert the lollipop into the solution Higher Secondary School, also the general safety note on the and switch on the whisk to start Malta Science in School website and at the mixing. end of this print issue. 42 I Issue 43 : Spring 2018 I Science in School I www.scienceinschool.org TEACH | Chemistry As the lollipop dissolves into the 1. The first colour (purple) corresponds has an oxidation state of +5 and a – solution, you will observe colour to permanganate ions (MnO4 ). blue colour. changes for each redox reaction. The Manganese has the oxidation state – – MnO4 (aq) (purple) + 2e first two changes happen very rapidly +7 (figure 2). 3– → MnO4 (aq) (blue) (3–5 seconds), while further changes 2. The permanganate ions (MnO –) are 4 3. The manganate ions (MnO 2–), take a little longer. Students can take then reduced to manganate ions 4 which have an oxidation state photos (e.g. with the camera of a (MnO 2–). The oxidation state of 4 of +6, are further reduced to mobile phone) at various time points manganese changes from +7 to +6, manganese dioxide (MnO ), with to better compare and follow the and the colour changes from purple 2 an oxidation state of +4, causing changes in colour. A video from the to green (figure 4). a colour change from green to authors demonstrating the experiment is – – MnO (aq) (purple) + e w2 4 yellow-brown (figure 5). available in Spanish . 2– → MnO4 (aq) (green) MnO 2–(aq) (green) + 2 H O(l) + 2e– An intermediate blue stage occurs 4 2 → MnO (s) + 4OH–(aq) between steps 1 and 2 (figure 2 What happens in the (yellow-brown) 3). One explanation is that the experiment? mixture contains both the purple 4. Finally, when even more glucose – As the lollipop dissolves in the solution permanganate (MnO4 ) and the is incorporated into the solution, 2– containing manganese ions, at least five green manganate ions (MnO4 ), brown-black manganese dioxide different colours can be distinguished which combine to produce a blue (MnO2) forms a colloidal (as shown in figures 2–6), which solution. Another explanation is that suspension in alkaline solution, correspond to different oxidation states a part of permanganate is reduced which (if fairly dilute) can appear 3– of manganese. to hypomanganate (MnO4 ), which orange (figure 6). Figure 2: The first colour in Figure 3: Before the colour Figure 4: Permanganate ions Figure 5: Manganate ions Figure 6: The final colour the chain of redox reactions changes from purple to are reduced to manganate are reduced to manganese change is to orange, when is purple, which corresponds green, there is a blue ions, resulting in a colour dioxide, causing the colour to manganese dioxide forms a to the permanganate ions. intermediate stage. change to green. change from green to yellow- colloidal suspension in the brown. alkaline solution. Marisa Prolongo www.scienceinschool.org I Science in School I Issue 43 : Spring 2018 I 43 Chemistry | TEACH Variations in colour Permanganate ions (MnO -) Manganate ions (MnO 2-) Manganese dioxide (MnO ) The food colours used in lollipops do 4 4 2 not have a big impact on the colours you see in this experiment, but some other factors do play a role. Once the reactions start, there are always mixtures of ions in the solution, resulting in mixtures of colours that are not always easy to interpret (see figure 7). Another factor is that the colour of manganese ions in solution is Nicola Graf Nicola generally different from the colour of their corresponding solid salts. This is because manganese ions form complexes with water due to Figure 7: Colours of the three main manganese compounds (top row) and example mixtures of these compounds as the redox reactions proceed (bottom row) the electron-accepting capacity of their atomic d-orbitals. In addition, the tendency for molecules to accept electrons varies with pH and temperature, so if you change these Electron configuration and transition metals variables or the quantities of the chemicals, the colours will vary, and the Electrons are arranged in energy levels called shells. Each shell is divided colour changes will occur at different into subshells, which are made up of orbitals. Transition metals have one times between experiments. or more electrons in their outermost d-orbital. The difference in energy between individual d-orbital electrons is relatively small, so all transition metal cations have a variety of ways of forming chemical bonds involving different numbers of d-orbital electrons. This is why transition Discussion metals have several oxidation states. To link the lollipop demonstration to When electrons absorb certain frequencies of electromagnetic radiation, the chemistry of redox reactions, ask they jump to a higher energy level. In many transition metals, the your students some of the following difference in energy between d-orbitals corresponds to the energy questions: of radiation of the visible light spectrum. For example, the d-orbital · In the experiment, what is the electrons of permanganate ions absorb electromagnetic radiation from reducing agent that donates the yellow part of the visible spectrum, but what we see as the colour electrons in the redox reactions? of permanganate ions is the colour complementary to yellow – that is, This depends on which reducing purple. We see the colour of the remaining wavelengths that were not sugar you use, but in our experiment, absorbed (fgure 8).
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