PAG

Question 1

for reactions involving acids and bases we measure…

Answer 1

• pH changes by carrying out titrations

- pH changes using a pH meter

Question 2

for reactions involving gases we can measure…

Answer 2

• change in volume or pressure

- loss in mass of reactants

Question 3

for reactions that produce visual changes we measure..

Answer 3

• formation of a precipitate
• colour change
  (can use a colorimeter)

Question 4

how does clock reaction work

Answer 4

• some reactions produce visible changes, measure the rate using a stop watch until you see a precipitation
• only measures the initial rate
• the time taken for a visible event to occur is inversely proportional to the initial rate, the shorter the time taken for the change the faster the reaction is
• expressed as 1/t, when the graph is plotted, 1/t is taken as a good approximation of the initial rate
• draw a cross underneath on a piece of paper and place it under a beaker, the reactants are then added in the beaker and the products form a cloudy solution as a precipitate is formed

Question 5

how do you determine concentrations at equilibrium

Answer 5

• titration

- using a coloriometer

Question 6

how do you use titrations to determine the concentrations at equilibrium

Answer 6

• allow us to find how much of the reactant or product is present
  drawbacks
• reactant or product has to be removed from the reaction mixture and this will effect the equilibrium position
• other substances may be present that can affect the results of titrations for example if an alkali is used to titrate against the equilibrium substance and an acid catalyst is used then both the equilibrium substance and acid catalyst will react with the alkali

Question 7

how do you use coloriometer to determine concentrations at equilibrium

Answer 7

• can be used to monitor the concentration of a reactant or product throughout, this works because coloured substances alter the amount of light that passes through a substance, more intense colours mean the substance has a higher concentration and absorbs more light

Question 8

what are the limitations when calculating the ph for weak acids

Answer 8

• it is assumed that little of the original acid has dissociated that the concentration of the acid at equilibrium is the same as the concentration of the original amount of acid
• there will be some acid that does dissociate even if only a small amount,
• if this amount is less than 5% of the total of HA undissociated then it is safe to use the approximation but if it is above 5% then the above approximation cann be used,
• this would be the case for weak acids with low Ka values

Question 9

how to use a pH meter

Answer 9

• probe is removed from storage solution and rinsed with deionised water
• the probe is blotted dry and then placed into a solution of a known pH, the starting with a solution of pH 4, the pH reading is allowed to settle before checking that a pH of 4 has been reigstered
• the process is repeated with other solutions of a known pH, and using prepared solutions of pH 7 and pH 10

Question 10

how do pH meters actually work

Answer 10

• pH meter can give an accurate reading for pH
• contain a protected electrode which is placed in the solution
• need to calibrate it first