Rate of Reaction + Scientific method review

Chemical Kinetics

the study of reaction rates

Reaction rate

the rate at which a chemical reaction occurs; the rate at which concentration changes over time.

What happens over time as a chemical reaction occurs? Why?

As a reaction proceeds, the reaction rate decreases. The change in reaction rate is due to the decreasing concentration of reactants. As reactant concentration decreases, fewer and fewer collisions occur between two reactants. As number of collisions per second decrease, the reaction rate decreases.

Collision Theory

The collision theory of reaction rates states that, in order for a chemical reaction to take place, reactant particles must collide, have more than the minimum amount of energy and have proper orientation.

Explain why particles must collide.

The particles must be close enough to both break existing bonds and to form new ones.

Why does reaction rate increase with concentration?

Higher concentrations mean more particles are present. The more particles present, the more collisions will occur, the higher the rate of reaction. Therefore, the higher the concentration, the higher the rate of reaction.

Why does reaction rate increase with surface area?

With more surface area, more particles are exposed to the other reactant particles and therefore more collisions occur. Therefore the higher the surface area, the higher the rate of reaction.

Explain why particles must have more than the minimum amount of energy.

The particles must collide with enough energy, otherwise they will merely bounce apart. The energy is first used to collide, but then also in the bonds of the particles. To cause a chemical reaction, some bonds must be first broken and then new ones must be formed again, processes which both require energy. If the particles only have enough energy to collide, there will be insufficient energy to break the bonds and to form new bonds, therefore resulting in no chemical reaction.

Activation energy

Activation energy, denoted Ea, refers to the minimum amount of energy needed to allow for reactants to undergo chemical reaction and form products.

In essence, it is an energy barrier between the reactants and the products. If the energy level is met with the reactants, they rise above the energy barrier and a chemical reaction will take place. If it is not met, they rise partially but fall back again, the reactants simply collide and bounce apart as reactants.

Why does reaction rate increase with temperature?

Temperature translates as a measure of the average kinetic energy of the reactant particles. By increasing the temperature, the kinetic energy increases meaning more particles surpass the activation energy and collide.

Explain why particles must have proper orientation.

Particles can be orientated in many different ways, which affect the collision of the particles together. The particles must collide with each other in the proper orientation, otherwise they will merely bounce apart.

what is a catalyst and how does it work?

Catalysts refer to substances that accelerate the rate of reactions while remaining constant.

Although not all reactions have catalysts, the reactions are catalysed by specific substances. To witness the effects of catalysts, only a small amount of the substances is needed.

The accelerated reaction occurs as a result of a new pathway for the reaction to occur. This pathway has a lower activation energy and therefore more particles surpass the activation energy and collide with each other.

Give an example of a catalyst in a chemical reaction

iron acts as a catalyst during the synthesis of ammonia.

Enzymes

biological catalysts that exist within our own bodies as proteins. They have very specific purposes, unlike other non-biological enzymes.

Give an example of an enzyme

one of the fastest known enzymes is known as carbonic anhydrase. It works in reactions between carbon dioxide and water that helps keep pH balance throughout the blood and tissue, catalysing up to 1 000 000 reactions a second.

Propose an improvement for the concentration experiment.

there were no measurement specifications. The most major oversight was the lack of specification on when the solution has finished reacting - just wait until the solution had finished bubbling. However, this is not very reliable, and would have a lot of random errors. For the future use a test tube with a side arm, and then either using the limewater test or putting the tube in water to test when the gas would form. In the future, also measure the mass to have quantitative data rather than qualitative - then calculate the concentration in mol L^-1  or g/100ml.

Propose an improvement for the temperature experiment

• Include measurements for reactants

• Use thermometer for quantitative data

• Add more trials and temperatures for a better understanding of the trend

Propose an improvement for the surface area experiment.

• severe lack of specifications and details.

  • no specification about what type of marble chip,

    • used calcite marble chips (CaCO3(s)) for the solute

  • what acid,

    • HCl

  • how much of the marble and acid need to be used

  • how to crush the chip.

    • used a mortar and pestle to crush the marble chips.

• The crushed marble did not take long to dissolve, but the intact chips took a very long time.

  • For future experiments, the marble chips should be weighed to ensure that both trials have a constant mass of marble and there should not be that much marble to begin with (about 1g).

  • It is very important that the mass of marble and the volume of hydrochloric acid remain constant, to ensure that the experiment is valid and that it assesses only surface area rather than concentration.

Evaluate your validity.

the experiment has proven its validity in addressing the aim of the experiment. Its validity is also emphasised through the controlled variables, ensuring the molarity of each reacting substance remained constant, as well as the volumetric ratio, with 1.0 M hydrochloric acid reacting with 0.25 M sodium thiosulfate in a volumetric ratio of 1:5. This is important to ensure that the first part of the collision theory, relating to collisions and the amount of particles present, remains constant.

Evaluate your accuracy.

The data set was accurate, displaying values congruent with research and theoretical concepts. The thermometer and stopwatches had an ideal amount of significant figures for this purpose. By using an immersion thermometer, the degree of systematic error was reduced. However, regarding the thermometer and measuring cylinder, the measurements may have been subjected to parallax error, therefore reducing the accuracy.

Evaluate your reliability

moderate reliability. The lack of precision is a result of random errors such as human errors in relation to the stopwatch, measuring cylinder and thermometer. Human error is also experienced when deciding when the reaction has been completed, with slight variation on when to stop the stopwatch. In addition, the random errors may have arisen in the time between heating/cooling the sodium thiosulfate and placing it on the paper and pouring in the hydrochloric acid. In this small fraction of time, the temperature varies from its desired value, dropping from the warmer temperatures and rising from the cooler temperatures. This small variation in temperature across trials negatively impacts both the validity and reliability of the data set. Measures were taken to minimise this random error, most notably the repetitions. By repeating the experiment, there was a greater degree of precision and therefore reliability.

suggests improvements for your experiment.

While the stopwatch was mostly appropriate at this stage, more precise instruments and/or methods such as recording should be considered for future experimentation to limit the aforementioned human error. To ensure the volumetric ratio is accurate, pipettes should also be considered to reduce any systematic errors concerning the current measuring cylinder method. Instead of the liquid-in-glass thermometer, using a digital thermometer would produce more precise results free from systematic error. The issues regarding determining an endpoint can also be resolved further by using a light sensor or colorimeter, setting the endpoint at a specific percentage of light blocked by the sulfur, or with simple recording. This would help create more accurate, reliable and valid measurements. For future experiments, using a thermostatic circulating bath would remove the variance across trials and ensure the proper temperature, minimising both human and random errors. This would therefore create a higher degree of precision and become more reliable.

Propose future experiments

After the exploration of temperature and reaction rates, it would be interesting to investigate the other components of collision theory and catalysts. It may also be beneficial to perform a similar experiment to calculate the activation energy.

endothermic