Rates and equilibrium

• Rate of a chemical reaction is how fast the reactants are changed into products
• One of the slowest rates of reaction is the rusting of iron. other slow reactions include chemical weathering
• An example of a moderate speed reaction would be metal magnesium reacting with an acid to produce a gentle stream of bubbles
• the Steeper the line on the graph the faster the reaction. over time the line becomes less steep as the reactants are used up. The quickest reactions have the steepest lines and become flat in the least time
• The rate of a chemical reaction depends on the collision frequency of reacting particles. The more collisions there are faster the reaction is. eg doubling the frequency doubles the rate
• The rate of a chemical reaction also depends on the energy transferred during a collision. particles have to collide with enough energy for collisions to be successful
• The minimum amount of energy that particles need to react is called activation energy. particles need this much energy to break the bonds in the reactants and start the reaction
• Increasing the temperature increases the rate because when the temperature is increased the particles all move faster. If their moving faster their going to collide more frequently. also the faster they move the more energy they have, so more of the collisions will have enough energy to make the reaction happen.
• Increasing the concentration or pressure increases the rate because if a solution is made more concentrated it means there are more particles knowing about in the same volume of water or other solvent. when the pressure of a gas is increased it means that the same number of particles occupies a smaller space. this makes collisions between reactant particles more frequent
• Increasing the surface area increases the rate because if one of the reactants is a solid then breaking it into smaller pieces will increase its surface area to volume ratio. this means that for the same volume of solid the particles around it will have More area to work on so there will be more frequent collisions.
• Using a catalyst increases the rate because a catalyst is a substance that speeds up a reaction without being used up in the reaction itself. this means it’s not part of the overall reaction equation. 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 alternative reaction pathway with a lower activation energy
• Rate of a reaction= amount of reactant used or amount of product formed/ time
• • you can record the visual change in the reaction if the initial solution is transparent and the product is opaque.
  • you can observe a mark through the solution and see how long it takes for it to disappear
  • if the reactants are coloured and the products are colourless or vice verse you can time how long it takes for the sol to lose or gain its colour
  • the results are very subjective
• measuring The 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. this is the most accurate way of measuring the rate of a reaction as the mass balance is very accurate but it has a disadvantage of releasing the gas straight into the room
• use 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
• magnesium and HCl react to produce H2 gas

  Start by adding a dilute hydrochloric acid to a conical flask and carefully place on a mass balance. now add some magnesium ribbon to the acid and quickly plug the flask with cotton wool. start the stopwatch and record the mass. take readings at regular intervals. plot results in a table and work out mass lost for each reading. you can now plot a graph with time on the x axis and loss of mass on the y axis. repeat with more concentrated acid solutions. amount of magnesium ribbon and volume of acid kept the same only change the acids concentration
• Sodium thiosulfate and HCl produce a cloudy precipitate 


  Add a set volume of dilute sodium thiosulfate To a conical flask. place flask on a piece of paper with black cross drawn on it. add some dilute HCl to the flask and start the stopwatch. now watch the black cross disappear through the cloudy sulfur and time how long it takes go.
• Mean Rate of reaction= overall change in the y value/ total time taken
• As the reactants react their concentrations will fall so the forward reaction will slow down but as more and more products are made their concentrations rise so the backward reaction speeds up. After a while the forward reaction will go at the same rate as the backward reaction - the system is at Equilibrium. at equilibrium both reactions are still happening but there’s no overall effect (it’s at dynamic equilibrium). this means the concentrations of the reactants and products have reached a balance and won’t change.
• If the equilibrium lies to the right the concentration of products is greater than that of the reactants. if the equilibrium lies to the left the concentration of reactants is greater than that of the products
• The position of equilibrium depends on the following Conditions: temperature - ammonia chloride = ammonia + hydrogen chloride. heating the reaction moves the equilibrium to the right (more ammonia and hydrogen chloride) and cooling it moves it to the left (more ammonium chloride)

  
  

  the pressure

  the concentration
• Hydrated copper sulfate = anhyodrous copper sulfate + water

  if you heat hydrated copper sulphate crystals it drives the water off and leaves white anhydrous copper sulphate powder. this is endothermic. if you then add a couple drops of water to the white powder you get the blue crystals back again. this is exothermic
• Le chateliers principle is the idea that if you can change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change
• Changes To the 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 the temperature the equilibrium will move in the endothermic direction to try and decrease it. Now you will get more products for the endothermic reaction and fewer products for the exothermic reaction
• Change to pressure

  Changing the pressure only affects The equilibrium involving gases. If you increase the pressure the equilibrium tries to reduce it - it moves in the dir 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
• Changes to concentration

  if you change the concentration of either the reactants or products the system will no longer be at equilibrium. So the system responds to bring itself back into 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 products the system tries to increase it again by reducing the amount of reactants