Rate of Reaction

• rate of a chemical reaction


  How fast the reactants are changed into products
• Example of a slow reaction

  rusting of iron
• Example of a moderate speed reaction


  Metal magnesium reacting with an acid to produce a gentle stream of bubbles
• Example of a fast reaction

  burning

  explosions
• find the speed of a reaction...

  By recording the amount of product formed or the amount of reactants used up over time
• The steeper the line on the graph


  the faster the rate of reaction
• What does the rate of a chemical reaction depend on?

  The collision frequency-the more collisions there are the faster the reaction is

  The energy transferred during a collision-particles have to collide with enough energy for the collision to be successful
• activation energy


  the minimum amount of energy particles require to start a chemical reaction
• 4 things rate of reaction depends on

  Temperature, Concentration of solution or pressure of gas, Surface Area, Catalyst
• More collisions means


  higher rate of reaction
• How does increasing the temperature increase the rate of reaction?

  When the temperature is increased, the particles all move faster.

  If they're moving faster, they're 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.
• How does increasing the concentration or pressure increase the rate of reaction?

  If a solution is made more concentrated, it means there are more particles colliding in the same volume of water. Similarly, when the pressure of a gas is increased, it means that the same number of particles occupy a smaller space, making collisions between the reactant particles more frequent.
• How does increasing the surface area increase the rate of reaction?

  If one of the reactants is a solid, then breaking it up into smaller pieces will increase its surface area to volume ratio. This means that for the same volume of the solid, the particles around it will have more area to work on so there will be collisions more frequently.
• How does using a catalyst increase the rate of reaction?

  Decrease the amount of activation energy needed by providing an alternative pathway with a lower activation energy
• Rate of reaction equation


  amount of reactant used up or amount of product formed / time
• Three ways to measure rate of reaction

  Precipitation/colour change, change in mass, volume of gas given off
• Precipitation and colour change

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

  
  

  If the reactants are coloured and the products are colourless you can time how long it takes for the solution to lose or gain its colour.

  
  

  The results are subjective - different people might not agree over the exact point the mark disappears or the solution changes colour. Also can't plot a rate of reaction graph.
• Change in mass


  1)measuring the speed of a reaction that produces a gas can be carried out using a mass balance

  2)as the gas is released, the mass disappearing is measured on the balance

  3)the quicker the reading on the balance drops the faster the reaction

  4)if you take measurements at regular intervals you can plot a rate of reaction graph and find the rate quite easily.

  5)this is the most accurate of the three methods described on this page beacuse the mass balance is very accurate,but release gas straight into the room
• Volume of gas given off

  - involves using a gas syringe

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

  - a graph of gas volume against time elapsed could be plotted to give a rate of reaction graph

  - gas syringes usually give volumes accurate to the nearest millimetre
• Two rates experiments - MAGNESIUM + HCL -> H2 GAS

  1) set up equipment as shown

  2) add a set volume of dilute HCL acid to conical flask and carefully place on mass balance

  3) now add Mg ribbon to acid

  4) quickly plug flask with cotton wool

  5) start stopwatch and record mass on the balance (take readings of mass at regular intervals)

  6) plot results in table, calculate mass lost + plot graph of loss of mass

  7) repeat with more concentrated acid solutions

  8) variables such as the amount of magnesium ribbon and volume of acid should be kept the same . The graph shows that a higher concentration of acid gives a faster rate of reaction.
• Two rates experiments - SODIUM THIOSULFATE + HCL -> CLOUDY PRECIPITATE

  1) add a set volume of dilute sodium thiosulfate to a conical flask

  2) place flask on piece of paper with black cross drawn on

  3) add some dilute HCL acid and start start the stopwatch

  4) watch black cross disappear through the cloudy sulfur and record time required

  5) repeat with different concentrations keep the depth of the liquid same.

  6) the higher the concentration, the quicker the reaction (quicker the cross disappears). No graph
• How to calculate the mean reaction rate from a graph

  You work out the overall change in the y value (amount of reaction used up) and divide this by the time taken for the whole reaction
• How to find the reaction rate at a particular point

  Draw a tangent at that point and work out the gradient
• What happens to the forward reaction when the reactants react?

  It slows down as a result of concentration falling
• What happens to the backwards reaction when more products are made?

  It will sped up as a result of concentration rising
• What happens at equilibrium?

  the forward and reverse reactions are proceeding at the same rate
• dynamic equilibrium


  Both reactions are happening but there's no overall effect,the concentrations of reactants and products have reached a balance and won't change
• when is equillibrium reached?

  If the reversible reaction takes place in a closed system
• equilibrium lies to the right

  The concentration of products is greater than that of the reactants
• Equilibrium lies to the left

  The concentration of reactants is greater than that of the products
• What does the position of equilibrium depend on?

  Temperature

  Pressure

  Concentration
• In reversible reactions, if the reaction is endothermic in one direction


  it will be exothermic in the other direction
• Thermal decomposition of hydrated copper sulfate

  If you heat blue hydrated copper sulfate crystals it drives the water off and leaves white anhydrous copper sulfate powder. This is endothermic.

  If you then add a couple of drops of water to the white powder you get the blue crystal back again. This is Exothermic.
• Le Chatelier's Principle


  if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change
• What can Le Chatelier's principle be used to predict?


  The effect of any changes you make to a reaction system
• Effect of temperature on reversible reactions at equilibrium

  If you decrease the temperature the equilibrium will move in the Exothermic direction to produce more heat . This mean you'll get more products for the Exothermic reaction and fewer product for the endothermic reaction.

  If you raise the temperature the equilibrium will move in the endothermic direction to try and decrease it. You'll now get more products for the endothermic reaction and fewer products for the Exothermic reaction
• Effects of pressure on reversible reactions at equilibrium

  changing the pressure only affects the equilibrium involving gases. If you increase pressure the equilibrium tries to reduce it-it moves in the direction where there are fewer molecules of gas.If you decrease pressure the equilibrium tries to increase it-it moves in the direction where there are more molecules of gas
• Effects of concentration on reversible reactions in equilibrium

  if you change the concentration of either the reactants or products the system will no longer be in equilibrium. So the system responds to bring itself back to equilibrium. If you increase concentration of reactants the system tries to reduce it by making more products. If you decrease the concentration of products the system tries to increase it by reducing the amount of reactants