What makes reactions happen

Site powered by Webvision Cloud. Skip to main content Skip to navigation. Lesson plans. No comments. Learning objectives Students will know and understand that: Disorder increases when spontaneous chemical reactions occur.

An increase in disorder can be represented by wider distribution of energy or particles. Sequence of activities Introduction The issue of why chemical reactions happen can be used to introduce the session. List views about this from the students, with little discussion.

Retain the list for review at the end. Organise students into groups of three or four. Circulate and support as the groups: Consider their responses to the statements. Discuss the statements to reach agreement. Agree an answer to the question. Select a spokesperson to feedback to the whole group. Plenary 1 In a plenary: Hear the responses from each group.

Ask for explanations supporting their views. Do not comment on the answers at this stage. After the discussion, introduce the next activity. Demonstration Direct the students to make careful observations during the demonstration. Help students to notice: Reaction 1 — gas, liquid and solid products are formed; temperature decreases.

Reaction 2 — heat and light are produced; iodine gas may be formed; temperature increases. Pose questions to develop their ideas from their observations: Were their predictions correct? How can one reaction be endothermic and the other exothermic? Consider chance as a driving force for events.

Consider how chance applies to chemical reactions. Select a spokesperson to feedback to the class. Plenary 2 In a plenary: Invite responses from each group.

Introduce the point that odds predict the most likely event to occur, unless there is some special factor preventing it, eg sportsman taking drugs, cheating on the lottery, stopping smoking, a football team getting a better coach or players etc.

Explain that this is not to do with exothermic or endothermic, but must be something else. When completed, ask questions to draw some meaning from the role play: What happened to the energy? Could the energy be used again after the reaction? If this were the environment, what would have happened to its temperature?

Which reaction does this help to explain? Introduce the notion that the energy has become disordered as a result of the reaction. Send the students back to their groups to look again at the first reaction. Tell them to try to work out what has become more disordered. Check for understanding that energy and particles can become more disordered.

Introduce the term entropy as a measure of disorder. Check that students realise entropy always increases when chemical reactions occur. All alternatives should be considered with no resolution at this stage. A starting activity could be observing the burning of a candle and discussing the changes that take place.

Here the distinction can be made between the melting of the wax and the appearance of new materials. Questions posed could include:. Activities which provide problems to be explored and challenge existing ideas are useful in encouraging students to seek new explanations for things they observe.

Students should investigate a number of changes and ask questions similar to those above. In all of these students should be encouraged to observe the changes that take place and to identify what products are formed. Discussion can also centre on how these are different from the starting materials. Some examples could include:. Other activities can involve chocolate making.

There are many other similar chemical changes that can be investigated - further cooking activities can include: making a chocolate cake, melting and browning cheese, making honeycomb, baking bread, poaching eggs and making toast. Other changes can include the setting of two component glues like Araldite and mixing steel wool and a solution of copper sulfate available from plant nurseries.

Oxygen is a very important reactant in many chemical reactions and students can investigate changes involving this component of air. It is important at this stage to clarify and consolidate what students have observed and to focus on what happens in a chemical reaction which is different from melting, boiling and freezing.

To achieve this students could be asked in groups to make mini posters which show the changes that take place in the one or more of the reactions they have seen, particularly comparing the products with the starting materials and demonstrating how they are different. Students then present their posters to the class. Resulting class discussion should bring out student ideas, examine alternatives and move to more accepted scientific views about chemical reactions.

Activities should be carried out which test the usefulness of the chemical reactions model and further consolidate student ideas about what constitutes a chemical reaction. Students can be further encouraged to compare the products with the starting materials.

To further develop students' appreciation of the role of chemical change in their lives, they could research the production of metals from ores such as aluminium and steel or the production of plastics and synthetic fibres. The emphasis in this exploration is on the importance of chemical change in producing the materials we use every day.

Another factor has to do with the relative bond strengths within the molecules of the reactants. For example, a reaction between molecules with atoms that are bonded by strong covalent bonds will take place at a slower rate than would a reaction between molecules with atoms that are bonded by weak covalent bonds. This is due to the fact that it takes more energy to break the bonds of the strongly bonded molecules.

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Skip to main content. Chemical Kinetics. Search for:. Factors that Affect Reaction Rate. Learning Objective Explain how concentration, surface area, pressure, temperature, and the addition of catalysts affect reaction rate. Key Points When the concentrations of the reactants are raised, the reaction proceeds more quickly. This is due to an increase in the number of molecules that have the minimum required energy.

For gases, increasing pressure has the same effect as increasing concentration. When solids and liquids react, increasing the surface area of the solid will increase the reaction rate. This is due to an increase in the number of particles that have the minimum energy required.

The reaction rate decreases with a decrease in temperature.