Chemistry: Light Energy and the Rate of Reaction

As we found out during our undertaking of this practical from a passing teacher the reaction between yeast and sugar does not derive any of its energy from light at all as we first thought but the energy required to break and reform the bonds is sourced from the reactants alone. This would means that the light and different wavelengths of light that were impact on the conical flask with the reaction taking place should have had no effect on the reaction rate. As seen in the results the reaction rate varied from 4.5 cm per 10 minutes of reaction to 21 cm per 10 minutes of reaction.

The large difference in the rate of the reaction is most likely due to the heat generated from the globes that were being used. The IR had the highest rate of reaction due to the intense amounts of heat that were very direct onto the conical flask. The heat was so intense that the yeast denatured and the reaction ceased once that temperature was reached. This means that the actual rate of reaction for the fermentation of glucose is the control, which is 4.5 cm of CO₂ per 10 minutes of reaction.

The sugar was used to reaction glucose with the yeast making it ferment.

This reaction is

C₆H₁₂O₆ --------------> 2CO₂ + 2C₂H₅OH

This reaction has an enthalpy change of -13.5 KJ mol^-1 as worked out by using the average bond enthalpies and the equation below.

ΔH = Bonds Broken – Bonds Formed

This is a relatively small enthalpy change indicating that very little energy was released during this reaction. This means that nearly the entire rise in the temperature during the reaction was because of the lamps heat incident on the conical flask. 

Control

1. Rate of reaction

= 13.5-9 cm

=4.5 cm per 10 minutes

2. Rate = Scratched

3. Rate= 4.5 cm per 10 minutes

4. Rate = 9 cm per 10 minutes (outlier)

Average = 4.5 cm per 10 minutes

UV Light

1.Rate = 8.7 cm per 10 minutes

2. Rate= 1 cm per 10 minutes (outlier) water leaked into tube

3. Rate = 12 cm per 10 minutes

Average= 10.4 cm per 10 minutes

IR Light

1.Rate= 21 cm per 10 minutes

Orange Light

1. Rate =10.5 cm per 10 minutes

Green Light

1. Rate = 18 cm per 10 minutes

Blue Light

1. Rate = 19.5 cm per 10 minutes

Red Light

1. Rate = 19 cm per 10 minutes

Purple Light

1. Rate = 18 cm per 10 minutes