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SC1 Ratesof Reaction
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Introduction-‘The Aim of this Investigation’ We must produce a piece of coursework investigating the rates of reaction, and the effects different changes have on them. I will carry out a series of experiments and collect data of which I will analyse and evaluate. My experiments will show how changing a reaction variable has an effect on the rate of reaction as a whole. By collecting Secondary Source information I can investigate into the main variables and my chosen variable, and create a hypothesis (prediction) on what I think will and/or will not happen to the rate of reaction. To ensure that my experiments are carried out efficiently and safely, I must consider the other effecting variables, making sure they stay constant, the safety features involved when carrying out the experiment, and to investigate into how I can make my tests as fair as possible. Then by recording my results, working out the averages and rates of reaction and drawing graphs, I can prove whether or not my hypothesis (prediction) was correct by writing a detailed critical analysis. Secondary Source Information What is the Rate of Reaction? The rate of reaction is the rate of loss of a reactant or the rate of formation of a product during a chemical reaction. It is measured by dividing 1 by the time taken for the reaction to take place. What is a Chemical Reaction? The collision theory says that: Chemical reactions occur when particles of the reactants collide. They must collide with a certain minimum energy, called the activation energy. This activation energy is the minimum energy needed to break the chemical bonds in the molecules, and so when they collide a chemical reaction can take place (otherwise they will just bounce off of each other harmlessly). Example: Methane does not react with oxygen at room temperature (as like many other substances). In order to make them react, heat energy must be added to provide the molecules with activation energy, and this is why we ignite them (to increase temperature). Only a small percent of particles will overcome the activation energy barrier (the minimum energy needed for particles to react) and only those with enough energy to overcome the barrier will react after colliding. The following shows how the collision theory is used to explain the effect of surface area, concentration and temperature Surface Area Surface area of zinc low. Concentration Concentration of acid low. Temperature Temperature low. Key Zinc Atom Hydro -gen ion from acid Surface area higher- more zinc exposed to collisions Concentration higher- more chance of particles colliding. Temperature higher- particles collide with more energy. How Do You Know When A Chemical Reaction Is Taking Place? When a chemical reaction occurs, one substance changes to another and energy is transferred. Most chemical reactions give out heat, and are called exothermic reactions. However some reactions take in heat and we call them endothermic reactions. Exothermic reactions cause the temperature to rise and endothermic reactions cause the temperature to fall. Example: When sherbet is put in your mouth it mixes with the saliva, creating an endothermic reaction and causing it to draw heat from your mouth. Energy in chemical reactions can be transferred as heat, light, sound, electrical energy, and as odours. The five human senses eyes, nose, tongue, ears and skin can detect these energy transfers. Example: The chemical reactions in fireworks transfer energy as heat, light, and sound. This is why we can hear, see and feel the heat from them. The three main variables shown on the previous page are: Surface area Concentration Temperature I have chosen temperature as a variable for my experiment because I feel that it is the most practical to carry out, and the range of variability is large. Secondary Source Information- Effect of Temperature Breaking Bonds Substances are held together by chemical bonds. When chemical reactions take place these bonds are broken, and then re-made, changing one substance into another. It is the breaking and making of these bonds that causes the energy changes in chemical reactions. Chemical bonds hold atoms together. Breaking bonds involves pulling these atoms apart- and this needs activation energy, also known as heat. On the other hand, making new bonds gives out energy. Breaking bonds takes in energy. Making new bonds gives out energy. All reactions need activation energy to break the bonds and get them started. For some reactions the bonds are quite easily broken and so the activation energy is fairly low. Such reactions can start at room temperature, without heating. Example: As soon as sodium is put into water it reacts. Other reactions need a lot of energy (heat) to break the bonds and get them started. Example: Charcoal (Carbon) needs a lot of heating to get it burning. This is because the bonds holding the Carbon atoms together are very strong. Example: The Reaction of Methane with Oxygen HEAT When more heat is supplied to the particles they have more energy and move around faster, making bond breakage easier and creating more effective collisions in a certain time. The more heat supplied the stronger the collisions. Therefore at higher temperatures, the particles are moving faster, crashing harder and more effectively, creating a faster reaction. Raising the temperature: 1. makes particles collide more often in a certain time, and 2. makes it more likely that collisions result in a reaction. Preventing Bond Breakage To prevent a chemical reaction the bonds need to remain in their fixed positions. We already know that heat breaks bonds, and so it is logical to say that a decrease in temperature strengthens the bonds, holding them firm in place. Light also breaks bonds, however this process is much slower. Example of Preventing Bond Breakage: Many types of food are kept, wrapt up in a cold, dark fridge. This is to decrease the rate of reaction (bond breakage) and keep the food fresh. The food is: • Wrapt up- to decrease the rate of reaction with oxygen in the air. • Kept in the dark- to decrease the rate of reaction with light. • Kept in a cold environment- to decrease the rate of reaction with heat. Preliminary Experiment The following experiment will be carried out in order to observe the changes that take place when I mix equal volumes of the two clear liquids, ‘Hydrochloric Acid’ (1M) –HCl and ‘Sodium Thiosulphate Solution’ (40G/L) - Na2S2O3. This experiment was first of all carried out and demonstrated by our teacher during a lesson one morning. We were shown what happened when the two clear liquids contained in two clear beakers were simply poured from one into the other. Once added together, the two liquids started to change and within seconds there was evidence that a chemical reaction had occurred because we could see a newly formed cloudy, opaque mixture. Our teacher touched the beaker and told us that it was warm. This was also evidence that a chemical reaction had taken place as the mixture was giving out heat, indicating that it was an exothermic reaction. A precipitate of sulphur is given off as a result of this reaction and this is the reason for which the liquid turns cloudy. Aim I will now go about this preliminary experiment myself. The aim of this preliminary experiment will be to collect further information, providing me with a more detailed view on how I will go about changing the variable chosen (temperature), how I will measure and record my results accurately, and how to make my tests as fair and safe as possible when carrying out my main investigation.
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