- The thermometer (scales up by one degree Celsius)
The temperature control is very important in this experiment. When heating up two chemicals, the heat source provides a great amount of energy for the reaction to take place. This increases the collision between particles, allows the reaction to speed up. Therefore, it is very important to measure the temperature at the exact degree Celsius.
- The weighing machine
The accuracy of measuring in mass scale: It is important to record the results up to at least 2 decimal places. It will have an effect on the accuracy of the final result.
The Science
Heat transfer:
Heat transfers directly from the reacting substances to the water in which they are dissolved. Combustion reactions must be performed outside the water. Heat transferred into the water by conduction. The water must therefore be held in a metal container for good conduction.
Combustion:
Combustion is an exothermic reaction between oxygen and hydrocarbon. This reaction is called oxidation. When the hydrocarbons burn, they react with the oxygen in the air
Formula for complete combustion:
Hydrogen + oxygen ➔ water + carbon dioxide + (heat)
CH4+ 2O2 ==> CO2 + 2H2O + (heat)
Enthalpy:
An enthalpy is the heat content of a chemical system. Enthalpy change is referred to heat energy changes under standard condition of temperature and pressure.
The enthalpy of combustion of a substance is the enthalpy change which occurs when one mole of the substance undergoes complete combustion in excess oxygen. Combustion reactions are always exothermic.
(In an exothermic reaction, the heat energy released increases the temperature of the water in the solution.)
Conduction, convection and radiation:
Conduction: Conduction occurs through the collisions between atoms and molecules in a substance. It is the transfer and distribution of heat energy from atom to atom within a substance. Conduction is most effective in solids. It can also occur in fluids.
Radiation: Radiation is by the emission of electromagnetic waves which carry energy away from the emitting object.
Convection: Convection is the flow of heat through a movement of air from a hot region to a cool region. As the air gets heated, the molecules spread out, therefore causes the region to become less dense than the surrounding. The movement of hot air will than into a cooler region is then said to transfer heat by convection. This is called the convection current.
Calculation
Energy gained can be calculated by the following formula:
Heat energy = mass of substance x specific heat capacity x temperature change
This calculation needs to be done for both the water and the metal container.
The total is assumed to be equal to the energy released by the burning fuel. In practice, a large, but unknown, proportion of this energy is lost to the atmosphere. Therefore results are highly inaccurate.
However, the proportion lost is roughly constant. The method can therefore be used to compare results for different substances.
Since the reaction is combustion, the energy change being measured or compared is called the enthalpy change of combustion, ΔHc. The difference between the weightings tells you the change in the mass of gas, but not the mass of gas inside the cylinder.
Similarly, we cannot measure enthalpy, H; but we can determine the enthalpy change, ΔH, when heat energy is lost or gained during a reaction.
Equipments
- spirit burner filled with a flammable liquid (PENTAN) (PROPAN)
- Tin can (Copper)
- 250 cm3 measuring cylinder
- -10 o to 110 oC thermometer
- Three bench mats to use as draught shields
- Clamp
- Goggles
- Weighing machine (mass of scale grams)
Procedure
Risk assessment
- Weigh the tin can (record)
- Add 200 cm3 of water, and weigh again (record)
- Weigh the spirit burner (record)
- Clamp the can above the spirit burner. Take the initial temperature of the water.
- Place the draught shields on two sides
- Ignite the burner
- Place the third draught shield
- Stir the water thoroughly until the temperature has risen by 30 oC.
- Put a cap on the burner to extinguish the flame
- Stir and record the highest temperature reached
- Reweigh the burner (record)
Calculation
Formula: [mw x cw x (T2 - T1)] + [mc x cc x (T2 - T1)] (kJ) = Q
mw = mass (in kg) of water
cw = specific heat capacity of water = 4.18 kJ kg-1 K-1
mc = mass (in kg) of the can
cc = specific heat capacity of the can = 0.45 kJ kg-1 K-1 (if steel)
T1 = starting temperature of can and water
T2 = final temperature after combustion
Energy released Q/m = kJ g-1
ΔHc =Q x M /m
n = number of moles of the compound burned (= m/M)
M = molar mass (in g mol-1) of the compound burned
Primary Alcohols
Methanol CH3OH 32 -726
Ethanol CH3CH2OH 46 -1367
Propan-1-ol C2H5CH2OH 60 -2021
Pentan-1-ol C4H9CH2OH 88 -3329
Hexan-1-ol C5H11CH2OH 102 -3984
Heptan-1-ol C6H13CH2OH 116 -4638
Octan-1-ol C7H15CH2OH 130 -5294
Above is a result table of how the final result should turn out
Evaluation
In this experiment there were many potential errors that could have affected the result. Theses included the accuracy of the measuring instruments such as the thermometer and the weighing machine. The result also could not be carried out under perfect lab conditions so the results will be inaccurate this is why in the equation I could not use ΔHө c because I could not replicate these conditions.
The hexan-1-ol results doesn’t conform to the rest of the results this may have been because I measured the values wrong i.e. human error or it could have been that the result variables I mentioned first effected it.
The results may also have been affected by energy that was wasted in the reaction because no experiment is 100% energy efficient. The energy would have been wasted in light from the flame and in a small amount of sound energy.
