Students should learn that:
- that energy changes are involved in chemical reactions
- what is meant by exothermic and endothermic reactions
- that energy changes in a chemical reaction can be measured.
- state a definition of exothermic and endothermic reactions
- list one example of an exothermicreaction and one of an endothermic reaction
- recognise an endothermic or anexothermic reaction when data are given
- describe how energy change in a reaction can be monitored.
- explain in detail the difference between exothermic and endothermic reactions.
An exothermic reaction is one that transfers energy to the surroundings. Examples of exothermic reactions include combustion, many oxidation reactions andneutralisation. Everyday uses … . [C2.5.1 b)]
An endothermic reaction is one that takes in energy from the surroundings.Endothermic reactions include thermal decompositions. Some sports … . [C2.5.1 c)]
Controlled Assessment: AS4.5 Analyse and interpret primary and secondary data.[AS4.5.4 d)] / Practical: Exothermic and endothermic reactions
Lesson structure / Support, Extend and Practical notes
Starters
Sherbet – Give students a sherbet sweet before they enter the room. Ask students to detail what their observations are as they eat it. Then, using questions and answers, get feedback from the students and ask them if they think the reaction is chemical or physical and exothermic or endothermic. Support students by having these words already defined on the board for them to refer to. Extend students by telling them the reactants for this reaction (citric acid and sodium hydrogencarbonate), and ask them to write a word equation for this reaction and classify the reaction [citric acid + sodium hydrogencarbonate → sodium citrate + carbon dioxide + water, neutralisation]. (5 minutes)
Cut and stick – Give students photographs of different exothermic or endothermic processes, such as a fire burning, a sports cold pack being used and a match burning. They first need to make themselves aware of the definitions of ‘exothermic’ and ‘endothermic’. They could then cut up the pictures and arrange them in a table to detail the energy changes shown in the reactions. (10 minutes)
Main
The energy changes of a reaction can be recorded using a coffee-cup calorimeter. Explain to the students that most reactions show their energy change by getting hotter or colder and that the reaction needs to be insulated to prevent energy loss to the surroundings.
Then ask students to complete the displacement reaction between zinc powder and copper sulfate solution. Students should design their own results table and record their results in order to draw a graph. They should be reminded that the scales do not have to start at zero (and the y-axis will probably start at about 15 °C). Students may struggle in drawing the line of best fit for this reaction; you could show them how to do this on the board. There are many whole investigations that this can be developed into to extend the Controlled Assessment concepts already covered. Consider concentrating on evaluating methodology in terms of the reproducibility and repeatability of the evidence generated.
Give the students an A5 sheet of blue paper. They should write a definition of ‘endothermic’ on it and include examples of endothermic reactions. A similar poster could then be created for exothermic reactions on red paper.
Plenaries
Exo-/endothermic – Give the students a blue card with the word ‘endothermic’ written on, and a red card with the word ‘exothermic’ printed on. Then read out these reactions and ask the students to decide if they are exo- or endothermic, displaying the card to represent their answer:
- Thermal decomposition of marble. [Endothermic]
- Combustion of methane. [Exothermic]
- Neutralisation of hydrochloric acid and sodium hydroxide. [Exothermic]
- Rusting of an iron nail. [Exothermic]
- Thermal decomposition of copper carbonate. [Endothermic]
Demonstration – Use a datalogger to plot the temperature changes in a neutralization reaction. Display the temperature graph using a digital projector. In small groups, students should decide whether the reaction is exothermic or endothermic and say how they could tell. Choose a few students to feed back to the class.(10 minutes) / Support
Give students information to incorporate into their posters about exothermic and endothermic reactions. However, they would need to decide which poster the information is referring to before copying it into their work.
Extend
Introduce the idea that exothermic and endothermic reactions are usually monitored by temperature changes and these indicate that energy is given out or taken in. Encourage students to consider other ways of monitoring a reaction for energy change (e.g. light or temperature sensor).
Practical support
Investigating energy changes
Equipment and materials required
Polystyrene coffee cup, polystyrene lid with two holes in, a mercury thermometer (0–50 °C), 1 mol/dm3 copper sulfate solution (harmful), zinc powder (highly flammable), spatula, balance, measuring cylinder, stopwatch, stirrer, eye protection.
Details
Wear eye protection and measure 25 cm3of copper sulfate solution into the coffee cup. Measure the temperature every 30 s for 5 min. Then add 1 g of zinc to the cup and quickly put on the lid and stir constantly. Take the temperature every 10 s for 10 min.
Safety: Make students aware that they are using a mercury thermometer for accuracy but that this involves a risk and they should be careful not to leave it by the edge of the bench. You should be aware of where the mercury spillage kit is and how to use it. CLEAPSS Hazcard 27C Copper sulfate – harmful; 107 Zinc powder – highly flammable.
Demonstration of neutralisation reaction
Equipment and materials required
Burette, measuring cylinder, burette holder, stand, 1 mol/dm3sodium hydroxide (corrosive), 1 mol/dm3 hydrochloric acid, universal indicator (highly flammable), magnetic stirrer, conical flask, magnetic stirrer bar, temperature probe, interface, computer, digital projector, white tile, filter funnel, eye protection (chemical splashproof goggles).
Details
Measure 25 cm3of sodium hydroxide into a conical flask and add a few drops of indicator. Place the flask on to the magnetic stirrer and add the bar. Fill the burette with hydrochloric acid using the filter funnel. Position the burette over the conical flask and add the temperature probe to the flask, taking care that it doesn’t hit the stirrer. Set the graph to take data for about 2 min and begin stirring. Start the data collection. Turn on the flow of acid to the flask and observe. This activity can be extended by adding a pH probe and comparing the temperature rise with the pH of the solution.
Safety: CLEAPSS Hazcard 91 Sodium hydroxide – corrosive; 47A Hydrochloric acid – corrosive; 32 Universal indicator – highly flammable/harmful.
Course / Subject / Topic / Pages
Additional science / Chemistry / C2 4.7 Exothermic and endothermic reactions / Pages 138–139
Learning objectives / Learning outcomes / Specification link-up / Kerboodle
Students should learn:
- what happens in the energy transfers in reversible reactions.
- recognise that if the forward reaction is exothermic, the reverse reaction will be endothermic
- recognise that if the forward reaction is endothermic, the reverse reaction will be exothermic
- recognise that the same amount of energy is taken in or released in either direction.
- explain why the same amount ofenergy is taken in or released in either direction.
[C2.5.1 d)]
Lesson structure / Support, Extend and Practical notes
Starters
Reversible reaction – Show students a solution of potassium dichromate(VI), K2Cr2O7(oxidising/very toxic), in a beaker, then add sodium hydroxide solution. Explain that thesolution changes colour from orange to yellow as potassium chromate(VI), K2CrO4, forms,then restore the orange colour by adding dilute hydrochloric acid. Add the alkali and acidagain to show that the reactions are reversible. Ask the students to write equations torepresent the reactions. Support students by giving them a list of compounds present sothey can write the word equations. Extend students by asking them to write the balancedsymbol equations. (5 minutes)
Questions – Give each student an A4 whiteboard (or laminated sheet of paper), a washable pen and eraser. Then ask them the following series of questions. The students should note down their answers and show you for immediate assessment. If students are unsure of the answer, they could refer to the Student Book or wait for other students to hold up their answer and then use these responses to inform their answer.
- What is the symbol to show a reversible reaction? []
- Give an example of a reversible reaction. [Hydration of anhydrous copper sulfate, thermal decomposition of ammonium chloride.]
- What does ‘exothermic’ mean? [Energy is given out in the reaction.]
- What happens to the temperature in an endothermic reaction? [Temperature decreases.] (10 minutes)
The students can experimentally complete the reversible reaction of hydration/dehydration of copper sulfate. Before the experiment is completed, encourage the students to think about how they will record their results (table, diagram, flow chart, paragraphs, bullet points, etc.).
Once the practical is complete, show the exemplar work to the rest of the class and explain why it is a good way to record the results. You may wish to set up a flexi-cam or video camera. This can be used to show exemplar work quickly and easily to the rest of the class.
Ask students to imagine that a top publisher has commissioned them to create a GCSE science revision book. Show students a selection of revision materials and ask them to discuss in groups what they like and dislike about the material.
Explain that they have an A4-page spread in such a book to explain energy and reversible reactions. They must include a worked examination question and an extra question for the reader to attempt, with the answers upside-down on the page.
Students could work in small teams to complete this, allowing them to distribute the tasks as they desire.
Plenaries
Objectives – Ask students to try to answer the questions posed by the objectives.
(5 minutes)
Crossword – Create a crossword with the answers taken from this double-page spread (words could include: ‘reversible’, ‘endothermic’, ‘exothermic’, ‘energy’, ‘hydrated’, ‘water’). There are many free sites on the internet that can be used to create your own crossword. Then ask students to complete the crossword. Students could be supported by being given both the words and the clues, so they match them up. (10 minutes) / Support
Give students the word equation for the reversible dehydration of hydrated copper sulfate. Ask them to annotate the equation to explain the observations and what the equation shows. You could support them further by giving them the labels and making this a ‘cut-and-stick’ exercise.
Extend
Ask students to discover other examples of energy changes in reversible reactions.
Practical support
Energy changes in a reversible reaction
Equipment and materials required
Hydrated copper sulfate (CLEAPSS Hazcard 27C – harmful), spatula, Bunsen burner and safety equipment, dropping pipette, water, boiling tube, boiling-tube holder, eye protection.
Details
Eye protection should be worn throughout this practical. Put a spatula of copper sulfate crystals into a boiling tube. Using the boiling-tube holder, hold the boiling tube just above the blue flame of the Bunsen burner. The tube should be held at an angle and pointing away from people’s faces. Do not overheat. Once the visible change is complete, allow the tube to cool. Add a few drops of water. Be aware that water added directly to the boiling tube may crack it.
Demonstration of making cobalt chloride paper
Equipment and materials required
Filter paper, cobalt chloride (toxic), 50 cm3beaker, stirring rod, wash bottle and water and a spatula, Bunsen burner, (desiccator), tweezers.
Details
Add half a spatula of cobalt chloride crystals to the beaker. Add water and stir until the crystals dissolves. Soak some filter paper in the solution. Take care drying the paper, using a yellow Bunsen flame. The paper will become blue (dehydrated), add water and it will become pink (hydrated). Explain that the paper should be kept in a desiccator because the air contains water and will turn the paper pink. A desiccator could be shown to the students and they could research how it works.
Safety: Wear chemical splashproof eye protection. Keep cobalt chloride off skin (avoid handling papers with fingers). Wash hands after use. (See CLEAPSS Hazcard 25.)
Course / Subject / Topic / Pages
Additional science / Chemistry / C2 4.8 Energy and reversible reactions / Pages 140–141
Learning objectives / Learning outcomes / Specification link-up / Kerboodle
Students should learn that:
- how the energy from exothermic reactions can be used
- how the cooling effect from endothermic reactions can be used
- that there are advantages and disadvantages of using energy changes in a chemical reaction.
- state a use for an exothermic reaction
- state a use for an endothermicreaction
- list advantages and disadvantages of using energy changes from a chemical reaction.
- explain in detail how an exothermic reaction can be used
- explain in detail how an endothermic reaction can be used
- evaluate the advantages and disadvantages of using energy changes from a chemical reaction.
Revision podcast: Collision theory
Test yourself: Rates and energy
On your marks: Rates and energy
Examination-style questions: Rates and energy
Answers to examination-style questions: Rates and energy
Lesson structure / Support, Extend and Practical notes
Starters
Hand warmers – Give each group a hand warmer and ask them to pop the metal clip inside and observe. Ask students to share their observations with the whole class. (5 minutes)
Sports packs – Ask for a volunteer and give him or her a cooling sports pack. Allow the volunteer to break the inner bag and mix the chemicals. Encourage the student to describe what he or she is doing and observing. Allow the cool pack to be passed around the class. Explain to students that the change is between ammonium nitrate and water and that it creates a solution. Extend students by asking them to use their Key Stage 3 knowledge as well as GCSE knowledge to identify the solute, solvent and name of solution and draw a particle model of the solution. [The solute is ammonium nitrate, the solvent is water and the solution is ammonium nitrate mixed with water.] (10 minutes)
Main
Split the class into small groups. Give each group a self-heating can and ask them to think about how it could be self-heating. Ask groups to feed back their ideas to the rest of the class. Show students one example of a used self-heating can cut open – complete this using appropriate protective clothing as the chemicals are hazardous.
Give each table of students a different commercial product that uses an exothermic or endothermic reaction. Ask the students to classify the reaction type that has been used and suggest some chemicals that would produce the desired effect. Then ask students to brainstorm the advantages and disadvantages of this commercial use.
Demonstrate the crystallisation of a supersaturated solution (see ‘Practical support’). Link this demonstration with the ‘Hand warmers’ Starter. Explain to students that this is a physical change and ask them to suggest how they could easily reverse it [reheat the crystals].
Plenaries
Other uses of exothermic/endothermic reactions – Ask students to brainstorm other uses of exothermic or endothermic reactions – e.g. combustion is an exothermic reaction used to cook food. Encourage volunteers to share their ideas. (5 minutes)
Mark scheme – Give students an examination question and ask them to create the mark scheme rather than answer the question. Students should consider all acceptable answers and also answers that would not be worthy of credit. Use a question from the same tier of entry as the students in the class. You could further support students by allowing them to work in small groups. (10 minutes) / Support
You can support students by giving them the advantages and disadvantages of using chemical reactions to generate an energy change. Students could then use these to generate a table to list the advantages and disadvantages for specific products such as the self-heating can.
Extend
You can extend students by asking them to write balanced symbol equations for some of the chemical reactions that they have studied in this double-page spread.
Practical support
Demonstration: crystallisation of a supersaturated
solution
Equipment and materials required
700 g sodium ethanoate, 50 cm3of hot water, 250 cm3glass conical flask, a crystal of sodium ethanoate, tweezers, magnetic hot plate, magnetic stirrer bar, eye protection.
Details
Eye protection should be worn throughout this practical. Put 50 cm3of hot water into the conical flask. Add 700 g of sodium ethanoate and a magnetic stirrer bar. Stir on a warm hot plate until all the solid has dissolved. Remove the flask from the heat and allow it to cool to room temperature. Avoid ‘shocking’ – for example, violently moving it – or continued stirring of the liquid or it will start to crystallise at once. Seed the solution with one crystal and see how it solidifies, releasing the heat of crystallisation – an exothermic physical change. You could alternatively demonstrate crystallisation by pouring the hot solution into a Petri dish on an overhead projector, and seeding the crystals there.
Safety: Refer to CLEAPSS Hazcard 38A.
Course / Subject / Topic / Pages
Additional science / Chemistry / C2 4.9 Using energy transfers from reactions / Pages 142–145
Text © Ruth Miller, Geoff Carr, Darren Forbes, Sam Holyman 2011