Energetics: Calorimetry

Question 1

What is a calorimetry experiment?

Answer 1

• Any experiment that measure the heat given out by reactions

Question 2

What is the equation for heat change/enthalpy change?

Answer 2

• q = mc∆T

• q = heat change/enthalpy change in joules (J)
• m = mass of substance being heated (usually water) in grams (g)
• c = specific heat capacity in joules per Kelvin per gram (4.18 JK–1g–1 for water)
• ΔT = change of temperature in Kelvin (K)

Question 3

What practical can we carry out to measure the enthalpy change of a reaction in solution? (Practical 2)

Answer 3

• Weigh solution and put the reactants in a container
• Use a thermometer to measure the temperature of the mixture at regular intervals - allows us to measure temperature change
• Use a polystyrene beaker to reduce the amount of heat lost or gained through the sides

Question 4

What practical can we carry out to measure the enthalpy change of a combustion reaction?

Answer 4

• To find the enthalpy of combustion of a flammable liquid, burn it in a calorimeter
• Measure the temperature of water at regular intervals
• Some heat will be lost to surroundings
• When burning a fuel, some of the combustion that takes place may be incomplete - less energy is given out
• Flammable liquids are often volatile so some fuel may be lost to evaporation

Question 5

How do you calculate an accurate temperature change from a calorimetry experiment?

Answer 5

• During the experiment, record the temperature at regular intervals, beginning a couple of minutes before you start the reaction
• Plot a graph of your results
• Draw two lines of best fit: one going through the points from before the reaction started and one going through the points from after it started
• Extrapolate (extend) both line so they both pass the time when the reaction started
• The distance between the two lines at the time the reaction started is the accurate temperature change for the reaction

Question 6

Why is subtracting the starting temperature from the highest temperature an inaccurate way of calculating temperature change?

Answer 6

• It’s inaccurate because of the heat lost from the calorimeter to the surroundings

Question 7

How do you calculate the standard enthalpy change of combustion?

Answer 7

• Calculate the heat change using q = mc∆T
• Change values of q from joules to kilojoules
• Calculate the number of moles of fuel used in reaction using n=mass/Mr
• Calculate the standard enthalpy change of combustion using ∆cHƟ=q/n

Question 8

What is the definition of enthalpy of neutralisation?

Answer 8

• The enthalpy of neutralisation is the energy change when one mole of water is formed by the reaction of an acid and an alkali

Question 9

How do you calculate the enthalpy of neutralisation?

Answer 9

• Calculate heat change of reaction using q = mc∆T
• You can assume the solutions have the same mass and specific heat capacity as water
• Add masses of both solutions together
• Work out moles for limiting reagent
• Calculate enthalpy of neutralisation using q/n