CPrac F - Enthalpy

Describe briefly one further experiment (NOT series) to find order of reaction with respect to propanone 

• Repeat the experiment above by replacing 25cm^3 FA3 with 12.5cm^3 of FA3 and 12.5cm^3 of deionized water to halve [FA3] 

• Plot volume of FA2 against time 

• Find gradient to find ROR -> gradient should be halved

Must include…

Include: 

• Limiting reagent, excess 

• Total volume SAME 

• NEW polystyrene cup for new experiment 

State procedure to identify 1 mol dm–3 NaOH, 1 mol dm–3 HCl and 2 mol dm–3 HCl

• Using a measuring cylinder, add 25 cm3 of FA1 into a polystyrene cup and record its initial temperature using a thermometer.

• Using another measuring cylinder, measure 25 cm of FA2 and record its initial temperature

• Add the 25 cm3 of FA 2 into the polystyrene cup containing FA1, stir gently with the thermometer and record the highest temperature reached, if any

• Rinse the cup and measure cylinders.

• Repeat steps 1 to 4, this time replacing FA 2 with FA 3

• Repeat steps 1 to 3, this time replacing FA 1 with FA 3.

• From the above experiment, the pairs of solutions with no temperature change must be the two acids. The remaining solution must be NaOH.

• Using a measuring cylinder, measure 50 cm' of NaOH and pour it into a polystyrene cup. Record its initial temperature.

• Using a measuring cylinder, measure 25 cm' of one of the acids, and pour it into the cup. Record the highest temperature obtained.

• Rinse the polystyrene cup and measuring cylinder.

• Repeat steps 8 and 9 with the other acid.

State safety precaution for identify 1 mol dm–3 NaOH, 1 mol dm–3 HCl and 2 mol dm–3 HCl

Safety precaution

Polystyrene cup should be placed in a beaker to prevent it from toppling

State 2 areas to ensure reliability for identify 1 mol dm–3 NaOH, 1 mol dm–3 HCl and 2 mol dm–3 HCl

2 areas to ensure reliability

Reaction mixture is stirred upon mixing to ensure thorough mixing of the two solutions and faster completion of reaction. Experiments are conducted under draught–free conditions to prevent excessive heat loss to the surroundings

Describe 2 enthalpy errors (the 1st 2)

1. Some solid may not dissolve completely within the given timeframe, leading to an underestimation of the heat change -> Use finer powder as the smaller particles dissolve more easily due to increased surface area

2. Stirring inconsistencies can also cause uneven dissolution of solid and uneven distribution of heat, leading to an inaccurate heat change -> Use a magnetic stirrer to stir at constant rate so heat in the solution will be distributed more evenly

Describe 2 enthalpy errors (the last 2)

1. Excessive heat loss resulting in lower measured T may due to: Insufficient lagging of the cup OR Presence of draughts -> Switch off the fans and stand the apparatus in a metal shield to reduce heat loss due to draughts

2. Slow response time of thermometer results in inaccuracy in T measured -> Use a data logger with a temperature probe for more accurate measurement of ∆T

Why must readings be taken at regular time intervals before adding solid into the solution? 

• This is to allow drawing of a best-fit line to obtain an initial temperature 

Why is it not possible to measure the temperature at the third minute when solid is added to solution? 

• The temperature reading is changing rapidly as the dissolution is still occurring 

Explain how you can tell [Fe(H2O)6]3+ ions are catalyst, using evidence from Table 1 and mechanism above 

• The yellow colour of FA 2 remains unchanged at the end of the reaction as [Fe(H2O)6]3+ is used in step 1 and regenerated in step 3

• Vigorous effervescence observed after the addition of FA 2, showing that FA 2 increases the rate of reaction with production of O2 in step 3

• The colour changes during the reaction indicate the formation of different species, suggesting a different pathway 

Why must weighing bottle be dry 

To prevent contamination of the ammonium chloride. Some of the FA 1 will dissolve in the water before the experiment starts -> calculated enthalpy change will be inaccurate

Why must the solution be stirred immediately after mixing?

• Stirring immediately ensures the reaction is completed in the shortest time to obtain maximum temperature change 

• Stirring ensures the distribution heat evenly in the solution, increasing accuracy of temperature measurements 

Is thermometric titration or convectional titration more accurate to calculate enthalpy change of neutralisation

• Thermometric titration LESS accurate volume of neutralisation than using indicator in conventional titration (NOT the extrapolation to point of mixing graph) 

• This involves plotting a graph and estimating the point of neutralisation from the temperature change -> extrapolation can lead to greater inaccuracy in Vneut

• An indicator gives a clear colour change at exact point of neutralisation, without a need for data manipulation