Topic 6 - The rate and extent of chemical change

• Rates of reaction are crucial in the chemical industry as faster reactions lead to quicker production of chemicals and ultimately more profit
• The rate of a chemical reaction is how fast the reactants are changed into products
• Examples of reaction rates:

  • Slow reactions - the rusting of iron and chemical weathering (like acid rain damage to limestone buildings)
  • Moderate reactions - magnesium reacting with an acid to produce a gentle stream of bubbles
  • Fast reactions - burning is a fast reaction, nut explosions are even faster and release a lot of gas. Explosive reactions are all over in a fraction of a second
• You can find the speed of a reaction by recording the amount of product formed or the amount of reactant used up over time
• The steeper the line on a rate of reaction 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 steepest lines on a rate of reaction graph and become flat in the least time
• Graph 1 represents the original reaction

  
• Graphs 2 and 3 represent the reaction taking place quicker, but with the same initial amounts of reactants. The slopes of the graphs are steeper than for graph 1

  
• Graphs 1, 2, and 3 all converge at the same level, showing that they all produce the same amount of product although they take different times to produce it

  
• Graph 4 shows more product and a faster reaction. this can only happen if more reactant(s) are added at the start

  
• The plot below uses the amount of product formed over time to show how the speed of a particular reaction varies under different conditions

  
• Reaction rates are explained by collision theory. It says the rate of a chemical reaction depends on:

  • The collision frequency of reacting particles (how often they collide). The more collisions there are the faster the reaction is
  • The energy transferred during a collision. Particles need to collide with enough energy for the collision to be successful
• Activation energy is the minimum amount of energy that particles need to react
• Particles need the activation energy to break the bonds in the reactants and start the reaction
• Factors that increase the number of collisions (so that a greater proportion of reacting particles collide) or the amount of energy particles collide with will increase the rate of reaction
• The 4 factors affecting rates of reaction:

  • Temperature
  • The concentration of a solution or pressure of gas
  • Surface area - this changes depending on the size of the lumps of a solid
  • The presence of a catalyst
• All 4 methods of increasing the rate of a reaction can be explained by increasing the number of successful collisions between reacting particles
• When temperature is increased, particles move faster, leading to more frequent collisions. Additionally, 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 of a solution or pressure of a gas leads to more frequent collisions between reactant particles. For increasing concentration this is because there are more particles in the same volume of solvent. For increasing the pressure this is because there is the same number of particles in a smaller space (volume)
• 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
• 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 alternate reaction pathway with a lower activation energy
• Enzymes are biological catalysts - they catalyse reactions in living things
• Three ways to measure the rate of a reaction:

  1. Precipitation and colour change
  2. Change in Mass (Usually Gas Given Off)
  3. The Volume of Gas Given Off
• When the product or reactant is a gas, measure the amount in cm³; if it's a solid, use grams (g).You measure time in seconds (s). You can also measure the amount of product or reactant in moles (mol)
• Units for rate may be in cm³/s, g/s, or mol/s
• Gas syringes are used to measure the volume of gas given off in the Volume of Gas Given Off method
• The rate of a reaction can be observed either by how quickly the reactants are used up or how quickly the products are formed:

  Amount of reactant used

  or amount of product formed

  Rate of reaction= ----------------------------

  Time
• Precipitation and colour change:

  • You can record the visual change in a reaction if the initial solution is transparent and the product is a precipitate which clouds the solution (it becomes opaque)
  • You can observe a mark (e.g. a cross) through the solution and measure how long it takes for it to disappear - the faster the mark disappears, the quicker the reaction
  • If the reactants area coloured and the products are colourless (or vice versa) you can time how long it takes for the solution to lose or gain its colour
• With observing a precipitate forming or colour change the results are very subjective - different people might not agree over the exact point where the mark 'disappears' or the solution changes colour. Also, if you use this method, you can't plot a rate of reaction graph from your results
• Change in mass (usually a gas given off):

  • 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 rate of reaction. If you take measurements in regular intervals, you can plot a rate of reaction graph and find the rate
  • This is the most accurate of the three methods of measuring rate because the mass balance is very accurate. However, it has the disadvantage of releasing the gas straight into the room
• 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 cm3, so they're quite accurate. You can take measurements at regular intervals an plot a rate of rection graph using this method too. You have to be quite careful though - if the reaction is too vigorous you can easily blow the plunger out of the end of the syringe
• A posh way of saying that the cloudiness of a solution changes is to say that its 'turbidity' changes
• It's important to learn the methods of these experiments as they investigate factors affecting reaction rate, but be prepared for different experiments in exams
• Le Chatelier's Principle states that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change
• Experiment 1: Magnesium and HCl reaction to produce H2 gas:

  • Add a set volume dilute hydrochloric acid to a conical flask and 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 on the balance. Take readings of the mass at regular intervals
  • Work out the mass lost at each reading and plot mass lost against time on a graph. Repeat with more concentrated HCL. Variables such as the amount of magnesium ribbon and the volume of acid used should be kept the same
• The three graphs show that a higher concentration of acid gives a faster rate of reaction. You could measure the mass using a balance or a gas syringe

  
• Experiment 2: Sodium Thiosulfate (colourless) and HCl (also colourless) reaction to produce a cloudy precipitate:

  • Add a set volume of dilute sodium thiosulfate to a conical flask. Place the flask on paper with a black cross drawn on it. Add dilute HCl to the flask and start the stopwatch. Then observe the black cross disappear through the cloudy sulfur and record the time taken for it to disappear
  • The reaction can be repeated with solutions of either reactant at different concentrations (only change the concentration of one reactant). The depth of the liquid must be kept the same each time
• These results show the effect of increasing the concentration of HCl on the rate of reaction, when added to an excess of sodium thiosulfate. The higher the concentration, the quicker the reaction and therefore the less time it takes for the mark to disappear. This data doesn’t give any graphs (unless you draw a graph of concentration against 1/time which will give you an approximate rate

  
• A rate of reaction graph shows the amount of product formed or amount of reactant used up on the y-axis and time on the x-axis. So to find the mean rate for the whole reaction just work out the overall change in the y-value and then divide this by the time taken for the reaction. you could also use the graph to find the mean rate of reaction between any two pints in time (see below). If your asked to find the mean rate of reaction for the whole reaction, remember the reaction finishes as soon as the line on the graph goes flat