Rates and Energy

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  • Created by: Chiara
  • Created on: 06-04-13 18:49

How fast?

The rate of chemical reactions tell us how fast reactants turn into products. Reaction rate is important in our body but also in industry. The amount of money it costs to make a product made be as cheap as possible, so that they can make a profit. If it takes too long to make the product it will be hard to make a profit, so the rate must be fast enough to make it quickly and safely.

How can find out the rate?  

Some reactions are fast like fireworks exploding but some are slow like iron rusting. There are two ways to work out the rate; one is we can find out how quickly the reactants are used up to make the products. The another is to find out how quickly the products are made.

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Collision theory and surface area

There are 4 main factors that affect the rate of a reaction; temperature, surface area, concentration of solutions or pressure of gases and presence of a catalyst.

In order for the reactants to come together and react, they need enough energy to react when they collide. This is collision theory. Activation energy is the smallest amount of energy particles must have before they can react. So reactions are more likely to happen if; we increase the chance of reacting particles colliding with each other and we increase the energy that they have when they collide.  

Surface area and reaction rate

When a solid reacts in a solution, the size of the pieces affect the rate. The particles inside the solid have to wait for the particles on the surface to react first. So if there are lots of smaller lumps then more particles are exposed, increasing the rate of reaction.

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The Effect of Temperature

We use fridges, when the temperature is decreased to slow down the rate, so that the food takes longer to go off. So, to speed up the rare we want to do the opposite. Collision theory explains why increasing temperate increase the rate; particles collide more often and particles collide with more energy.

Particles collide more often: when we heat the substance, energy is transferred to its particles. In solutions and gases this means that the particles move around faster, meaning that they collide more. So this increase the chance of them reacting.

Particles collide with more energy: the particles are moving more quickly so they have more energy. So any collisions they have will be more energetic. A higher proportion of the particles will have energy greater than the activation energy. So they there will be more collisions with more energy increasing the rate.

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The Effect of Concentration or Pressure

There are many buildings that are built from limestone or marble., and they have been around for centuries. But they are crumbling away at a greater rate than before. This is because both limestone and marble are mainly calcium carbonate. This reacts with acid, leaving it soft and crumbly. The rate of reaction has increased because the concentration of acids in rainwater has been increasing.   Increasing the concentration of the reactants in a solution, increases the rate because there are more particles moving round the same volume of solution. When it becomes more crowded, it makes it more likely that they will collide. So there is a faster reaction.

Increasing the pressure in gases, has the same effect. It pushes the particles closer together. This increases the chance that they will collide and react. So increasing the pressure increases the rate.

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The Effect of Catalysts

Catalysts speed up reactions. It is a substance, but is not chemically changed itself at the end of the reaction. In the reaction it does not get used up, so it can be used over and over again. There are different types of catalyst for different reactions. Many use in industry involve transition metals. For example iron is used to make ammonia. We normally use them in the form of powders, pellets or fine gauzes. This gives them the biggest possible surface area.

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Exothermic and endothermic reactions

Ectothermic reactions transfer energy from the reacting chemicals to the surroundings. It often heats up the surroundings, meaning that the temperature rises. An example is burning fuels, like when methane is burned it gets oxidised and releases energy. Respiration is another example, it is a special kind of oxidation. Neutralisation reactions between acids and alkalis are also exothermic. We can measure the temperature rise, to know that energy is being released.

Endothermic reactions transfer energy from the surroundings to the reacting chemicals. So they take in energy from the surroundings. They cause a drop in temperature and it absorbs the energy. They are less someone than exothermic reactions. An example is thermal decomposition reactions, like the thermal decomposition of calcium carbonate. When heated it forms calcium oxide and carbon dioxide. It takes in a great deal of energy from the surroundings.

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Energy and reversible reactions

As reversible reactions is when the products can react to form the reactants. For example A and B react together to form C and D. Then C and D can react to reform A and B. When A and B react it releases energy and so is an exothermic reaction. When C and D react to form A and B again, the reaction must be endothermic. Also the amount of energy released it the same as the amount absorbed.

An example is when we heat blue copper sulfate crystals. The crystals contain water as part of a lattice, so we say that the copper sulfate is hydrated. . Heating the copper sulfate drives off the water, producing white anhydrous. This is an endothermic reaction.

When we add water to anhydrous copper sulfate we form hydrated copper sulfate, the reaction is in the direction of exothermic.

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Using energy transfers from reactions: warming up

Body warmers and other products use exothermic reactions.soe. Hand warmer s can only be used once, an example of this type uses the oxidation of iron to release energy. Iron turns into hydrated iron oxide. Sodium chloride is used as a catalyst. It is then disposable but lasts for hours.

Others can be reused. These are based on the formation of crystals from solutions of a salt. The salt used is often sodium ethanoate. A supersaturated solution is prepared, this can be done by dissolving a much of the salt as possible in hot water. The solution is then allowed to cool.

In the plastic case there is a small metal disk, which I used to start the reaction. When you press it small particles of metal are scraped off. This starts off the crystallisation, and then the crystals spread throughout the solution, giving off energy. They work for about 30 minutes. They can be reused by boiling them in hot water, meaning the crystals re-dissolve. When it is cool it can be reused.

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Using energy transfers from reactions: cooling dow

Endothermic reactions are used to cool things down. For example chemical cold pack. They usually contain ammonium nitrate and water. When the ammonium nitrate dissolves it takes in energy from the surroundings, making them colder. They can be used as emergency treatment for sports injuries.

The ammonium nitrate and water are kept separate in the pack. When squeezed or struck, the bag inside the water breaks and releases the ammonium nitrate. The instant cold packs work for 20 minutes. They can only be used once. The same endothermic change can also be used to chill cans of drinks.

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