core practical 2: determining enthalpy change of reaction

what is calorimetry?

• Calorimetry is a technique used to measure changes in enthalpy of chemical reactions

• A calorimeter can be made up of a polystyrene drinking cup, a vacuum flask or metal can

what is specific heat capacity?

The energy needed to raise the temperature of 1 g of a substance by 1 K

what is the principl of enthalpy changes for reaction in solution?

• The principle of these calorimetry experiments is to carry out the reaction with an excess of one reagent and measure the temperature change over the course of a few minutes

sample method for a displacement reaction?

1. Using a measuring cylinder place 25 cm³ of the 1.0 mol dm^-3 copper(II) sulphate solution into the polystyrene cup

2. Weigh about 6 g of zinc powder - as this is an excess there is no need to be very accurate

3. Draw a table to record the initial temperature and then the temperature and time every half minute up to 9.5 minutes

4. Put a thermometer or temperature probe in the cup, stir, and record the temperature every half minute for 2.5 minutes

5. At precisely 3 minutes, add the zinc powder to the cup (DO NOT RECORD THE TEMPERATURE AT 3 MINUTES)

6. Continue stirring and record the temperature for an additional 6 minutes

• For the purposes of the calculations, some assumptions are made about the experiment:

  • That the specific heat capacity of the solution is the same as pure water, i.e. 4.18 J g^-1 K^-1

  • That the density of the solution is the same as pure water, i.e. 1 g cm^-3

  • The specific heat capacity of the container is ignored

  • The reaction is complete

  • There are negligible heat losses

Why do we extrapolate graphs to find maximum temperature?

Because reaction is not instantaneous so it may lose heat to surroundings, so we don't know its true maximum temperature

The moles of which substance is used to caluculate heat transfer (Q=mcT)

moles of the limiting reactant

The steps to make a temperature correction graph are:

1. Take a temperature reading before adding the reactants for a few minutes to get a steady value

2. Add the second reactant and continue recording the temperature and time

3. Plot the graph and extrapolate the cooling part of the graph until you intersect the time at which the second reactant was added

enthalpy change of combustion experiment

• Fill copper can with 100cm³ of water, record initial temperature with thermometer

• Measure and record the mass of an empty spirit burner, add fuel and record mass

• light the wick, heat water (stirring constantly) for set time/until fuel is completely burnt. Record temeprature of water. Measure mass of burner + and remaining fuel. Subtract to find mass of fuel burnt

What is Hess's Law?

The total enthalpy change of a reaction is independent of the route taken.

How do you calculate enthalpy change?

Find the heat energy released (Q=mcAt)
Find the amount of substance reacted (n=m/Mr)
Calculate the enthalpy changer per mol (changeH=Q/n)

What are errors when calculating enthalpy change?

• 1. Some of the heat energy is transferred to the air and

  not the water.

  2. Some of the heat energy produced is transferred to

  the copper can and not to the water.

  3. Incomplete combustion of alcohol – less heat energy

  and also cause soot to form on the bottom of the

  copper can.

• -some alcohol evaporates

• heat cappacity of beaker not taken nto account

  4. The conditions are not standard. e.g. water vapour is

  produced instead of liquid water.

• only used mass of water not solution

• If formation of water in gaseous state then value would be less exothermic/lower as energy taken in to form gas is lower

effect of using polystyrene cup instead of glass beaker

minimum temperature would be lower (1)

 temperature would increase at a slower rate (> 90 s) (1)

 less heat (from the surroundings) would enter the solution

• if glass beaker usedThe heat loss will be greater Or • (Because) polystyrene is a better insulator Or • More energy is used to heat the container/ glass Or • (Because) the polystyrene cup has a low heat capacity

ways to increase accuracy of enthalpy of combustion

• 1. Put the lid on – to reduce heat loss and prevent evaporation of

  water

  2. Use draught shield – to reduce heat loss

  3. Reweigh spirit burner immediately – to reduce evaporation of

  methanol

  4. Use a copper calorimeter - to improve heat conduction to the water

why value mroe exothermic

-ethanol/water are not in standard stated for bond enthalpy caluclations

standard enthalpy change of combustion refers to ehtanol /water as liquids but bond energies are clauclated for gases

• Standard enthalpy change of combustion,

  ΔcH° trend

• When comparing the ΔHc for successive members of a

  homologous series such as alcohols there is a constant rise in

  the value as the number of carbon atoms increases. The experimental results are lower than the calculated ones because there will be heat loss and incomplete combustion which leads to less energy being released.

why stir solution

• so solid dissolves

• to create a unifrom temp

what do we assume when doing enthalpy change

 solution has a density of 1 g cm–3 (1)

 mass of ammonium chloride / solid is ignored (1)

 (specific) heat capacity of the solution is the same as water

why enthalpy chnage of combustion inaccurate

 incomplete combustion  evaporation of the ethanol/alcohol/fuel

give two reasons for the differenc btw book molar enthalpy change of combustion and bond enthalpy

• mean bond enthalpy are averages/ mean for different molecules

• butan-1-ol is a liquid and bond enthalpies refer to gases or mean enthalpy calcualtions don’t include changes of state

what would happen if larger Q used

• if larger SHC taken Q will be larger so lowers the error in the final value obtained

why increase in enthalpy change of combustion from butane to pentane very small

• butane is a gas and pentane is a liquid at 298K

• liquids vapourise before combustion

• pentane is a liquid so some of the enregy released by combustion is used to vaporise or more energy is needed to break intermolecular forces in pentane