The Rate and Extent of Chemical Change

  • Created by: India.02
  • Created on: 10-06-19 10:38

Different Rates of Reaction

- Rate of a chemical reaction is how fast the reactants are changed into products

- One of the slowest is the rusting of iron - another slow reaction is chemical weathering (acid rain)

- A moderate speed reaction would be the metal amgnesium reacting with an acid to produce a gentle stream of bubbles

- Burning is a fast reaction, but explosions are faster and release a lot of gas - explosive reactions are over in a fraction of a second

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Rate of Reaction Graphs

- Speed of a reaction can be found by recording the amount of product formed, or the amount of reactant used up, over time

- The steeper the line on the graph, the faster the rate of reaction - over time the line becomes less steep as the reactants are used up

- The quickest reactions have the stepest lines and become flat in the least amount of time

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Collision Theory

- Rate of chemical reaction depends on the collision frequency of reacting particles (how often they collide) - the more collisions there are, the faster the reaction - doubling the frequency of collisions will double the rate of reaction

- Rate of chemical reaction also depends on the energy transferred during a collision - particles have to collide with enough energy for the collision to be successful - successful collision is ne that ends in particles reacting to form products

- Activation energy is the minimum amount of energy needed for particles to react - particles need this much energy to break the bonds in reactants and start the reaction

- Factors that increase the number of collisions or the amount of energy particles collide with will increase the reate of reaction - temperature, catalyst, surface area, concentration of solution or pressure of gas

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Temperature, Concentration and Pressure

- When temperature is increased, the particles have more kinetic energy so they move faster - when they move faster, they will collide more frequently - because they have more energy, more of the collisions will have enough energy to make the reaction happen

- If a solution's concentration is increased, it means there are more particles in the same volume of solvent - this makes collisions between the reactant particles more frequent

- When the pressure of a gas is increased, it means the same number of particles occupy a smaller space - collisions become more frequent

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Surface Area and Catalyst

- If one of the reactats is a solid, then breaking it up into smaller pieces will incerase the surface area to volume ratio - for the same volume of solid, the particles around it will have a larger area to react with, therefore making collisions more frequent

- A catalyst is a substance that speeds up a reaction, without being used up in the reaction itself - different catalysts are needed for different reactions, but they all work by decreasing the activation energy needed for the reaction to occur - they do this by providing an alternate reaction pathway, with a lower activation energy

- Enzymes are biological catalysts, which catalyse reactions in living things

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Precipitation and Colour Change

- Record the visual change in a reaction if the initial solution is transparent and the product is a precipitate, which clouds the solution - changes the turbidity

- Observe a mark through the solution and measure how long it takes for it to disappear - the faster the mark disappears, the fast the reaction

- If the reactants are coloured and the products are colourless (or vice versa), you can time how long it takes for the solution to lose or gain colour

- Results are very subjective becuase different people might not agree over the exact point when the mark 'disappears' or the solution changes colour

- Using this method means you can't plot a rate of reaction graph with the results

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Change in Mass

- Measuring speed of a reaction that produces a gas can be carried out using a mass balance

- As the gas is released, the mass disappearing is measured on the balance

- The quicker the reading on the balance drops, the faster the reaction

- If you tak measurements at regular intervals, you can plot a rate of reaction graph and find the rate easily

- This is the most accurate of the three methods because the mass balance is very accurate

- A disadvantage is the gas is released straight into the room

- You can put cotton wool in the top of the flask as it lets the gas escape but stops the acid spitting out

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The Volume of Gas Given Off

- This involves the use of a gas syringe to measure the volume of gas given off

- The more gas given off during a given time interval, the faster the reaction

- Gas syringes usually give volumes accurate to the nearest cm*3 so they are quite accurate

- You can take measurements at regular intervals and plot a rate of reaction graph using this method

- Take precautions if the reaction can be vigorous because the plunger in the syringe can be blown out of the end

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- As reactants form, their concentrations fall - so the forward reaction will slow down, but as more products are made, and their concentrations rise, the backward reaction will speed up

- After a while, the forward reaction will be going at the same rate as the backward reaction - the system is then at equilibrium

- At equilibrium, both reactions are still happening, but there is no overall effect (dynamic equilibrium) - concentrations of reactantsand products have reached a balance and won't change

- Equilibrium is only reached if the reversible reaction takes place in a closed system - this means that no reactants or products can escape, and nothing else can get in

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Position of Equilibrium

- When a reaction reaches equilibrium, it doesn't mean that the amounts of reactants and products are equal

- If the equilibrium lies to the right, the concentration of the products is greater than that of the reactants

- If the equilibrium lies to the left, the concentration of the reactants is greater than that of the products

- Position of equilibrium depends on temperature, pressure and concentration

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Endothermic and Exothermic

- In reversible reactions, if the reaction is endothermic in one direction, it will be exothermic in the other direction

- The energy transferred from the surroundings by the endothermic reaction is equal to the energy transferred to the surroundings during the exothermic reaction

- A good example is the thermal decomposition of hydrated copper sulfate

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Le Chatelier's Principle

- This principle is the idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract the change

- It can be used to predict the effect of any changes you make to a reaction system

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Changes to Temperature

- If you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat - this means you will get more products for the exothermic reaction and fewer products for the endothermic reaction

- If you raise temperature, the equilibrium will move in the endothermic direction to try and decrease it - this means you will get more products for the endothermic reaction and less products for the exothermic reaction

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Changes to Pressure

- Changing the pressure only affects an equilibrium involving gases

- If you increase the pressure, the equilibrium tries to reduce it - it moves in the direction where there are fewer molecules of gas

- If you decrease the pressure, the equilibrium tries to increase it - it moves in the direction where there are more molecules of gas

- You can use the balanced symbol equation to understand how many molecules are on each side of the reaction

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Changes to Concentration

- If you change the concentration of either the reactants or the products, the system will no longer be at equilibrium

- The system will respond to bring itself back to equilibrium again

- If you increase the concentration of the reactants, the system tries to decrease it by making more products

- If you decrease the concentration of the products, the system will try to increase it by reducing the amount of reactants

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