Sample Report On How Does Temperature Affect The Reaction Rate?

Question

It is well known that chemical reactions proceed at different speeds under different conditions. One of the main factors known to determine the rates of chemical reactions is the temperature. In this exercise the effect of temperature on the rate of a simple reaction was investigated.

Hypothesis

For a chemical reaction to occur, the particles of the reactants must collide with sufficient energy. An increase of thermal energy (corresponding to a rise in temperature) means that the particles will move more rapidly and collisions will become more numerous. However, the main effect of temperature comes from the increase of the particle energy. Only those particles that collide with an energy higher than the so-called “activation energy” (Ea) will participate in a chemical reaction. Thus, it is expected that at higher temperatures more particles will have the energy necessary to react, and that the reaction rate will be higher at higher temperature.

Materials

- Antacid tablets
- Ice water
- Warm water
- Vinegar
- Beakers
- Stopwatch
- Thermometer

Procedure

1. Vinegar was poured into a beaker. An antacid tablet was dropped into the vinegar and using a stopwatch the time necessary for the tablet to dissolve was measured. This was taken as the control value – the dissolution of the antacid tablet at room temperature.
2. The vinegar was poured into a beaker, but now the beaker was placed into warm water and left inside until thermal equilibrium was established (i.e., until the temperature stabilized). The temperature of the vinegar was measured with a thermometer. The measured time and temperature were noted. The antacid tablet was dropped in the vinegar, and the time necessary for the tablet to dissolve was noted once again.
3. The same as in Step 2. was repeated, but the beaker with the vinegar was placed in ice water.
4. We then used the warm and ice water to bring the vinegar to different temperatures, and measured the time necessary for the antacid tablets to dissolve. By changing the temperature (the independent variable) the time necessary for the tablet to dissolve (dependent variable) changed as well.
The temperature in science is universally expressed in the absolute, or Kelvin scale. The temperature measured in °C or °F has to be transformed to this scale using the equations:
[K] = [°C] + 273.15
or
[K] = ([°F] + 459.67) × 5⁄9

The measured variables were inserted into Table 1.

Observations
When the antacid tablet is dropped into the vinegar the chemical reaction starts. Antacid tablets usually contain bicarbonates (HCO3-), which react with the acetic acid (CH3COOH) in the vinegar:
HCO3- + CH3COOH → CO2↑ + H2O + CH3COO-
The CO2 is released as a gas and the bubbling caused by this is visible, indicating that the reaction is occurring. When the bubbling stops it means that all the bicarbonate has reacted and the reaction ends, the stopwatch was stopped at this point and measured time was noted.
Generally, the tablets dissolved more quickly at higher temperatures, i.e., it dissolved more quickly in Step 2, than Step 1, and in Step 1 more quickly than Step 3. The release of the gas, and, consequently, the bubbling was more intense at higher temperatures.

Results

Discussion
The increase in temperature is known to increase the kinetic energy of particles. The energy of the particles, E, is proportional to the temperature of the system T, with the proportional factor being the Boltzmann constant, k (1.3806488 × 10-23 m2 kg s-2 K-1):

E ̴ kt

The energy distribution within a system is given by the Maxwell-Boltzmann distribution law (Peckham, McNaught 554), which forms the basis of the kinetic theory, and defines the distribution of particle energies for a fluid at a certain temperature.
Figure 1: The Maxwell-Boltzmann distribution. E – energy, N – number of particles, T1, T2 – temperature, T2>T1, Ea – activation energy.
Figure 1 shows the Maxwell-Boltzmann distribution at two different temperatures. We can see that an increase in temperature will result in a significant increase in the number of particles that possess energies higher than Ea. resulting in an increased reaction rate.

The reaction rate of a general chemical reaction such as:

A + B → C
is given by the equation (Laidler 277):
v = k·[A]·[B]
where v is the reaction rate, k - the rate constant, and [A] and [B] are the concentrations of the corresponding species. According to the Arrhenius equation, the rate constant of the reaction can be expressed as (Logan 1982):
k=A∙e-EaR∙T
where k is the rate factor, A is the pre-exponential factor, which is connected to the collision frequency, Ea - the activation energy, R – the universal gas constant (8.314 J K-1 mol-1), and T – temperature (in Kelvin, K).
While the change in temperature affects the pre-exponential factor as well, the main contribution comes from the exponential part that describes the energetics of the reaction. A rough approximation states that a rate of a chemical reaction doubles for every 10° increase.
Our results are in agreement with the expectations that the temperature will increase the rate of the chemical reaction. The time necessary for the antacid tablet was measurably shorter at higher temperatures, compared to lower, e.g., at _value?_ K the time necessary for the tablet to dissolve was _value?_s shorter than at _value?_K. This is significantly exceeds the error of the measurements (Δt = 1s, Δτ = 1°) and can be treated as reliable. Based on our results, the approximation that the rate of reactions doubles with every 10° can be seen as a very rough trend, useful only for making qualitative assumptions.

Conclusion

The time necessary for the reaction to complete and the antacid tablet to dissolve was shorter at higher temperatures. The gas formation was also visibly more vigorous. The results have indicated that the rate of chemical reactions increases with the increase of temperature, which is in agreement with the general expectations.

Bibliography

Laidler, Keith J.“Chemical Kinetics”, 3rd ed., New York: Harper & Row, 1987.
Logan, Stephen R. "The orgin and status of the Arrhenius Equation", J. Chem. Educ., 59 (1982): 279.
Peckham, Gabriel D.; McNaught, I.J.; “Applications of the Maxwell-Boltzmann Distribution”, J. Chem. Ed., 69 (1992): 554-558.