Rate Of Reaction
Visão Geral
Leave an iron gate out in the rain and it will take years to rust through. Grind the same iron into a fine powder, tip it into the air, and it catches light and is gone in a flash. Same metal, same oxygen, same chemical reaction. The only thing that has changed is how fast it happens, and the difference runs to millions of times over. A chemist who understands why can slow a reaction to a crawl or drive it to completion in seconds, and that control is worth more to industry than almost anything else on this course.
This lesson gives you the four handles you can pull on any reaction, plus a fifth that works differently from the rest. You will learn how to put an actual number on a rate from a balance reading or a gas syringe, how to read the story a rate graph is telling before you calculate anything, and why the single most common answer written in examinations about concentration is only half right. You will also meet the substance that speeds up a reaction, comes out at the end exactly as it went in, and can be used again tomorrow.
Objetivos
- The rate of a chemical reaction can be found by measuring the amount of a reactant used or the amount of product formed over time: Rate of reaction = amount of reactant used / time Rate of reaction = amount of product formed / time Students need to be able to interpret graphs showing the amount of product formed (or reactant used up) with time, in terms of the rate of the reaction. Knowledge of specific reactions other than those in the subject content is not required, but students will be expected to have studied examples of chemical reactions and processes in developing their skills during their study of this section.
- Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy.
- Increasing the temperature increases the speed of the reacting particles so that they collide more frequently and more energetically. This increases the rate of reaction.
- Increasing the pressure of reacting gases increases the frequency of collisions and so increases the rate of reaction.
- Increasing the concentration of reactants in solutions increases the frequency of collisions and so increases the rate of reaction.
- Increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction. Required practical: Investigate factors affecting the rate of a reaction.
- Catalysts change the rate of chemical reactions but are not used up during the reaction. Different reactions need different catalysts. Knowledge of named catalysts other than those specified in the subject content is not required, but students should be aware of some examples of chemical reactions and processes that use catalysts.
- Catalysts are important in increasing the rate of chemical reactions used in industrial processes to reduce costs.
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Nota de Aula
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- A reaction produces 48 cm3 of gas in 40 s. What is the mean rate of reaction? A. 0.83 cm3 per second B. 1.2 cm3 per second C. 8.0 cm3 per second D. 1920 cm3 per second Answer: B
- What is meant by the activation energy of a reaction? A. The energy given out when the reaction takes place. B. The minimum energy that colliding particles must have in order to react. C. The energy stored in the bonds of the products. D. The total energy of all the particles in the mixture. Answer: B
- Which change increases the rate of a reaction only by making collisions more frequent, without changing the proportion of collisions that are successful? A. Raising the temperature of the mixture. B. Adding a catalyst. C. Grinding the solid reactant into a powder. D. Warming the flask in a water bath. Answer: C
- Which statement about a catalyst is correct? A. It is used up during the reaction. B. It is chemically unchanged at the end of the reaction. C. The same catalyst works for every reaction. D. It increases the activation energy of the reaction. Answer: B
- Increasing the pressure of a mixture of reacting gases increases the rate of reaction. Why? A. The particles gain energy from the pressure. B. There are more particles in the same volume, so collisions are more frequent. C. The activation energy of the reaction is lowered. D. The bonds in the reactant particles become weaker. Answer: B
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