RQP 2: Measurement of an enthalpy change

Question 1

How do you convert q into enthalpy change

Answer 1

Divide by 1000 and divide by the moels used

Question 2

What is the equaiton we use to calculate enthalpy change experimentally

Answer 2

q=mc/\T

Question 3

Describe how to measure the enthalpy change of a reaction using calorimetry

Answer 3

• wash the equipment with the solutions to be used
• dry the cup after washing
• put polystyrene cup in a glass beaker for insulation and support
• measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
• clamp thermometer into place, making sure the thermometer bulb is immersed in solution
• measure the initial temperatures of the solution or both solutions if two are used for every thirty second for 4 mins before mixing
• after 4 mins mix the two reagents
• stir mixture
• record temperature every thrity seconds for another 6 mins
• plot a cooling curve and extrapolate back to the initial temperature change

Question 4

What are the errors that may occur in the calorimetry method

Answer 4

• heat loss to surroundings
• approximation in the specific heat capacity of a solution- the method assumes all solutions have the specific heat capacity of water
• neglecting specific heat capacity of the calorimeter- you ignore any heat absorbed by the apparatus
• the reaction may not go to completion
• the density of the solution is taken to be the same as water

Question 5

Describe how to work out the enthalpy change of a reaction from experimental data

Answer 5

1. Use q=mc/\T
2. work out the moles of the reactants used
3. Divide q by the moles of the reagent not in excess
4. Divide by 1000 to convert to KJmol-1

Question 6

What is the specific heat capactiy of water

Answer 6

4.18 Jg-1K-1

Question 7

What is the density of water

Answer 7

1gcm^-3

Question 8

What are the errors in the measurement of enthalpies of combustion using flame calorimetry

Answer 8

• energy loss/heat loss from calorimeter
• incomplete combustion of fuel
• evaporation of fuel after weighing
• heat capactiy of calorimeter not included
• measurements not carried out under standard conditions

Question 9

What three measurements do you need to take when doing flame calorimetry

Answer 9

1. mass of spirit burner before and after
2. temperature change of the water
3. volume of water in the cup

Question 10

In what ways is the flame calorimeter an improved version of the simple calorimeters used to measure enthalpy change

Answer 10

• there is a spiral chimney made of copper wich aims to reduce heat loss
• the flame is enclosed to reduce heat loss
• the fuel bruns in pure oxygen rather than air, avoiding incomplete combustion taking place