Commentary Informal peer and self assessment is inherent throughout the group discussions that promote listening, discussion and development of coherent ideas , the role play that requires team working and cooperation , the demonstrations and the plenaries when teacher questioning and review are essential.
Practical notes Demonstration 1: hydrated barium hydroxide and ammonium chloride This reaction is endothermic. Set the beaker on the water. Mix the chemicals in the beaker, stirring with the thermometer or probe. Keep the beaker on the mat. As the temperature falls, the water will freeze so the beaker can be lifted with the mat underneath.
Demonstration 2: aluminium and iodine This reaction is exothermic. Mix the iodine and aluminium together in the mortar. Do not grind them together. Set the dish on the mat in a fume cupboard. Pile the mixture in the centre of the dish. Gently add a few drops of water to the mixture. Close the fume cupboard. Wait a few seconds. Health, safety and technical notes Read our standard health and safety guidance. Wear eye protection. It is the responsibility of the teacher to carry out appropriate risk assessments for the demonstrations.
Other equipment A multi-pack of crisps for the role play — the number of packs within the multipack can be varied according to the number of students in the class, although there is no need for everyone to be involved.
To make an endothermic reaction happen, heat is needed. Exothermic reactions store energy. People setting the odds assess the likelihoods of the events — there is greater flexibility of outcomes in some events than others. An athlete training hard, or in a specific way, or cheating; revising thoroughly; a government minister behaving in a way that influences electors etc.
Download all. Additional information This lesson plan was originally part of the Assessment for Learning website, published in Acknowledgement V. Dunstable: Folens, Level years. Use Demonstrations Formative assessment. Category Group work Lesson planning Higher-order thinking and metacognition Thermodynamics Reactions and synthesis. Fuels and heats of reactions 5. Exothermic and endothermic reactions and changes of state. The greater the degree of disorder, the greater the entropy.
Related articles. Ideas When things go wrong in the classroom, a video can help TZ Practical demo gone wrong in the chemistry lab? Ideas Reuse remote teaching resources TZ During lockdown, teachers worked so hard to create engaging remote resources.
Resource Sustainability contexts for primary science Find out how to teach science curriculum topics through engaging sustainability contexts. Load more articles. No comments yet. You're not signed in. Before diving into kinetics, let me present another quick example of a thermodynamically-favorable process.
An energy diagram of an Sn2 substitution reaction in this case. What does this mean? That the reaction is thermodynamically favorable, and in principle it will not take place the other way around.
But does this mean that, mixing hydroxide with chloromethane at any temperature will lead to the immediate formation of tert -butyl alcohol and chloride? Of course not! The rate at which the reaction proceeds will depend directly on the temperature, and if the temperature is low enough, the reaction will not take place at all, even though the process is thermodynamically favorable.
Chemical or reaction kinetics is the branch of physical chemistry that studies the rates or speeds of chemical reactions. In summary, thermodynamics determines in what direction a chemical reaction proceeds, and kinetics determines the speed or rate at which that process occurs.
Of course, in the last scheme of the previous section, there was something missing. In a chemical reaction, reactant A does not simply transform into product B. Reactions take place through what we call transition states. Transition states are intermediate structures between reactants and products of a chemical reaction step. They are usually higher in energy less stable than both the reactants and the products, and the energy difference between the reactants and the transition states, also known as activation energy, is the barrier necessary to overcome for a thermodynamically-favorable reaction to take place.
See below a now complete version of the free-energy diagram of the Sn2 substitution reaction. As you can see, the process is thermodynamically favorable, but a barrier or activation energy of The larger the activation energy, the lower the speed or rate of a reaction at any given temperature.
Easy enough to remember. This allows us to make a clarification between stability and inertness as properties of chemicals. Stability is a thermodynamic concept, while inertness is a kinetic concept. A compound can be both unstable and inert. That is why we can handle and store thermodynamically unstable primary alkenes such as 1-propene without them isomerizing to more-stable secondary alkenes such as cis- or tran spropene see the first two schemes. But we can trick kinetics! Catalysts can be used to lower the activation energy of chemical transformations, allowing them to proceed more rapidly, or simply to proceed at all!
Now that we know why chemicals react, let me explain how we chemists try to override the system and make activation barriers lower. A catalyst is a chemical entity a molecule, a salt, a coordination complex… which speeds up a chemical reaction. It also can unlock new reactivity pathways and make reactions work that would not be possible otherwise.
The electrophilic aromatic substitution of benzene with molecular bromine Br—Br. This reaction is traditionally carried out using a Lewis acid as catalyst, such as iron tribromide.
But let us imagine first a catalyst-free version of the process, which I am certain can occur if you mix together benzene and bromine, and heat it up enough.
The first step of this reaction is the formation of the well-known Wheland intermediate. An intermediate not to be confused with a transition state , which rather connects intermediates together is a reactive chemical species which is formed in one of the steps in the middle of a chemical reaction of A leading to B, as intermediate point. For benzene to be transformed into bromobenzene, it has to pass through this intermediate species.
Intermediates can rarely be isolated, since they usually are both thermodynamically unstable and kinetically reactive. By adding a catalyst to the mixture, we can access new transitions states, which are more stable, and hence lower in energy. And what happens when the transition state of the rate-limiting step of a reaction is lower in energy? That the activation barrier of the whole process is much lower!
This is basically the role of FeBr 3 the catalyst of this reaction: stabilizes transition states and intermediates. Also, note that FeBr 3 is recovered unreacted with the products. Because so many reactions children know about are involved in things like cooking and burning, they assume that heat is always necessary for reactions to occur.
Hence students may regard chemicals as a group of substances found in laboratories rather than seeing all the substances in foods for example as chemicals. All materials are made of chemicals. Chemical reactions involve interaction between chemicals such that all reactants are changed into new materials.
The properties of the new materials are different from those of the reactants. This is distinct from other changes such as evaporation, melting, boiling, freezing and mixing where changes involve no new substances. While heat is often necessary to start reactions, this need not be the case. Chemical reactions involve breaking chemical bonds between reactant molecules particles and forming new bonds between atoms in product particles molecules.
The number of atoms before and after the chemical change is the same but the number of molecules will change. Although many chemical reactions proceed quickly, small, slow changes such as rusting or biological processes can take place over much longer periods of time. Chemical reactions are reversible a fact often omitted in many science texts but in practice most differ from other changes children observe, such as melting, by being very difficult to reverse.
Humans use chemical reactions to produce a wide range of useful materials; the breakdown of waste materials also involves chemical reactions that occur naturally in the environment. For some human made wastes, there are no such reactions and they cause problems as a result. In teaching about chemical reactions at this level the emphasis should be on improving student understanding of the importance of chemical reactions in our lives in producing many of the things we take for granted as well as improving their recognition and understanding of what is involved in a chemical change.
It is not necessary at this stage to talk about particles such as atoms or molecules or chemical bonds. In learning about chemical reactions students will need to describe various substances, which at this level will be materials they are familiar with the kitchen and changes involving cooking are very good starting points. They will need to be able to identify changes in these substances with the purpose of eventually recognising when new chemicals have been produced i.
As mentioned above, this is can be difficult as students often fail to see the difference between an egg white going through a change from liquid to solid as it is cooked and changes such as melting chocolate or boiling water which do not involve chemical change. Teaching will need to be focused on what happens when new substances are formed. These ideas are also explored in the focus idea Problems with classifying. Environmental effects of chemical reactions can also be considered, for example how we dispose of some chemicals once they are produced, in forms such as plastic bags.
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