affecting the rate of photosynthesis | Nuffield Foundation.
In this experiment, rhubarb sticks, which contain oxalic acid, are used to reduce and consequently decolourise potassium manganate(VII) solution. The experiment can be used to show how the rate of reaction is affected by surface area or concentration and is available from the Nuffield Foundation [Additional Resource 1], which contains health and safety guidance especially cautioning against the use of rhubarb leaves, which contain too much oxalic acid and are harmful. To investigate the effect of surface area cut three 5cm lengths of rhubarb. Leave one complete and divide the others into two and four pieces respectively. Place the pieces into a beaker containing 50cm3 of acidified potassium manganate (VII) and start the timer. Once the purple colour disappears stop the timer. This can be repeated for each set of rhubarb pieces and more able students may be able to identify the number of pieces as the independent variable and the time taken as the dependent variable in order to plot a graph of the relationship. To investigate the effect of concentration make an extract of rhubarb by boiling in a beaker until the rhubarb falls to pieces. Allow it to cool and strain and filter the mixture keeping the solution you have extracted. Then conduct a similar reaction to the first experiment, initially adding one drop of the extract to 50cm3 of the potassium manganate (VII) solution and timing how long it takes to decolorise. Repeat for 2, 3, 4 and 5 drops plotting a graph of the results. The concentration of the potassium manganate (VII) solution is not critical for these experiments; it can be made by dissolving a few crystals in 1 M sulfuric acid, giving a light purple colour. By carrying out these experiments students should be able to observe that as the surface area or concentration of the rhubarb increases, so does the rate of the reaction. Higher ability students may observe (or be prompted) that putting in more drops of the rhubarb extract has increased the total volume. You may then like to discuss the implications of this with the students. If the drop volume is small enough compared to the total volume it should not have a significant effect on the relationship observed.
Photosynthesis | Nuffield Foundation 2017
Rate of reaction provides a link between the particle model students study in physics at the start of KS4 and how a chemical reaction takes place. Students enjoy practical chemistry and rate practicals extend students’ dexterity in manipulating laboratory equipment such as gas syringes. They are also adaptable for the less well-stocked department as upturned measuring cylinders are equally as effective and cheap to provide in class sets. Data generated in rate experiments is typically reliable enough to analyse mathematically and cross-curricular links to GCSE mathematics, in particular the gradient at different points on a curve, lend themselves well to team teaching between faculties. Living in the “Rhubarb Triangle” I succumb to any opportunity to get this leafy vegetable (yes, it is considered a vegetable not a fruit) into my lessons. The rate of reaction experiment in this lesson, using rhubarb, is one of my favourites. Rate of reaction is a key concept at KS4 and requires secure knowledge for students who progress onto A-level. It is also a topic that can be taught very practically and adapted for a range of abilities and is particularly suited to extending your gifted and talented students both chemically and mathematically. Students begin by investigating the factors that can affect the rate of a chemical reaction using their own bodies and use their discoveries to suggest ways of speeding up some basic practical reactions. Links with the chemical industry could be discussed in the context of controlling reactions that may be explosively fast as well as speeding up those reactions that would cost too much because they are too slow. The topic lends itself well to demonstrations of impressive catalysis in the case of the Genie in a Bottle as well as student led discovery learning included in the sequence of practicals in the main part of the lesson. Most rate practicals can easily be adapted to generate data that can be plotted as a graph extending student mathematical understanding to include the changing gradient of a curve.
Learners often approach this topic with some reservations as their interest in plant science is limited and as a result they find it difficult to engage with the content. Many of the concepts are not visible so to generate interest in the topic images, video clips and practical investigation should be used. Learners can conduct research and then plan investigations into the environmental effects on the rate of photosynthesis or the factors effecting enzyme activity therefore leading to a better understanding.