Topic 6 - The rate and Extent of Chemical Change

• What is the rate of a reaction?

  the rate of a reaction is how fast the reactants are changing into products
• Give 4 factors that affect the rate of reaction

  TemperatureConcentration (or pressure for gases)Surface areaCatalysts
• According to collision theory what two things does the rate of reaction depend on?

  The collision frequency of reacting particlesThe energy transferred during a collision
• How does temperature affect the rate of reaction?


  When the temperature is increased the particles all move quicker. If they are moving quicker then they are going to collide more often and more often collisions means a faster rate of reaction Temperature also increases the energy of collisions which also means a faster rate of reaction
• How does concentration (or pressure) affect the rate of reaction?

  If a solution is made more concentrated (or a gas more pressurised) the particles are closer together so the frequency of collision will increase and that will increase the rate of reaction
• How does surface area affect the rate of reaction?


  If one of the reactants is a solid then breaking it into pieces will give it a larger surface area to volume ratio. This means that there is still the same amount of solid but there is more area for particles to collide with it so the frequency of collision and therefore the rate of reaction increases
• What is activation energy?

  The minimum energy particles must have when they collide to be able to react
• Rate and proportionality

  Rate of reaction is directly proportional to the frequency of succesful collisions
• What is a catalyst?

  A catalyst is a substance which can speed up a reaction without being changed or used up in the reaction
• How do catalysts work?

  Catalysts decrease the activation energy needed for a reaction to occur by providing an alternative reaction pathway with a lower activation energy
• What is an enzyme?

  A biological catalyst
• What equation is used to find the mean rate fo reaction?
• Give three ways that the formation of products during a reaction can be measured


  precipitationchange in massvolume of gas given off
• How can you use precipitates to calculate the rate of reaction?

  Observe a mark placed on the botom of the flask and measure how long it takes for the mark to disapear. The quicker it dissapears the faster the rate of reaction
• How can the rate of a reaction be found by change in mass?


  Place a conical flask with the reactants inside with cotton wool in its neck on scales. Start a stopwatch and take measrements of the time and mass at regular intervals. A graph of mass against time can then be plotted
• How can the volume of gas given of be used to measure the rate of reaction

  Use a gas syringe to measure the volume of gas given off. Take measurements at rregular intervals and plot a graph of volume given off over time
• Give two issues involved with finding the rate of reaction form precipitation

  The results are very subjective- people will disagree over where the cross dissapearedYou cannot plot a rate of reaction graph from the results
• Give an issue with finding the rate of reaction from change in mass

  The gas being produced is released directly into the room which is bad if it is a dangerous gas
• What represents the rate of reaction on a graph showing the amount of product formed/ reactant used against time?

  The gradient of the line. The hger the gradient (steeper) the faster the rate
• Why are lines curved on graphs of product formed/ reactant used against time?

  Reactions start quickly and then slow down and eventually stop
• How can you find the point where a reaction finished on a product formed/reactant used against time graph?

  Where the line begins to be staight
• Why do reactions slow down?

  As the reactants get used up, the number of reactant particles decreases so collisions become less frequent and the reaction slows down
• How do you calculate the mean rate of reaction from an amount of substance against time graph?


  Chnage in y / change in x
• What is a tangent?

  A straight line that touches the curve at a particular point without touching it
• How do you draw a tangent?

  place a ruler at the point of the curve you wish to measure the rate so that its just touching the curveadjust the ruler so that the space between the ruler and the curve is equal on both sides of the pointDraw a line along the ruler to complete the tangentThe rate of reaciton can then be calculated by change in y/change in x of the tangent line
• How do you calcluate the rate of reaction at a particular point on a curved line?

  By drawing a tangent and calculating its gradient
• How do you calculate rate of reaction from a tangent
• Investigating the effect of concentration on rate of reaction Measure out 50cm3 of dilute hydrochloric acid using a measuring cylinder and add this to a conical flaskAdd some magnesium ribbon t the dilute hydrochloric acid and quickly attatch an empty gas syringe to the flaskStart a stopwatchTake readings of the volume at regular intervals until the volume has not changed for three readings in a rowPut the results in a table. Plot a graph with time on the x-axis and volume of gas produced on the y-axisRepeat with more concentrated acid colutions. Variables such as mass and surface area of the magnesium ribbon, the temperature of the reaction and the volume of acid used should be kept the same only change the acid's concentration
• Give two experiments that can be used to observe the effects of concentration on rate of reaction

  measuring gas given off from hydrochloric acid and magnesium reactionTiming how long it takes for a black cross to dissapear due to a precipitate in the reaction of sodium thiosulfate and hydrochloric acid
• Investigating the effects of concentrarion on rate of reaction Add 50 cm3 of dilute sodium thiosulfate solution to a flaskPlace the slask on a piece of paper with a black cross drawn on it. Add 10cm3 of dilute hydrochloric acid to the flask, give the flask a brief swirl to mix the reactants and start the stopwatchWatch the black cross through the flask and time how long it takes to dissapear. Record the results in a tableRepeat the reaction in exactly the same way twice, using the same concentrations of reactantsand calculate a mean time for the cross to dissapearThe reaction can then be repeated with solutions of either reactant at differing concentrations The same person should observe the cross each time
• What is a reversible reaction


  A reaction where the products of the reaction can themselves react to produce the original products
• What is equilibrium

  When amounts of the reactants and products reach a balance in a reversible reaction - their concentrations stop changing
• Under what condition does a reversible reaction reach equlibrium

  If it is in a closed system it will always reach equlibrium
• What is a closed system

  None of the reactants or products can escape and nothing new can be introduced
• Misconseption about equlibrium in reversible reactions

  It does not mean that the amounts of products and reactants are equal
• When has a reaction reached equlibrium

  When the forward and reverse reactions occur at exactly the same rate
• Energy transfer in reversible reactions

  In reversible reactions, if the reaction is endothermic in one direction it will be exothermic in the other direction. The energy absorbed by the endothermic reacction is equal to the energy released by the exothermic reaction
• Wjat is the le chatliers principle

  The idea that if the conditions are changed for a reversible reaction at equlibrium the position of equlibrium will alter to counteract those changes
• Implication of le chateliers principle

  the effect of any changes to a system can be predicted. altering the temperature, pressure or concentration of the reactants can alter the yield of the reaction
• Three conditions that cause a predictable change in the yield of a reversible reaction

  TemperaturePressureConcentration