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Chemical Reactions

Investigation 2 – Concept Day

 

 

 

 

 

 

 

Chemical Reactions: Investigation 2

Concept Day

SLIDE CHEM-2-1

  • During the second Investigation, you will continue to examine the common characteristics of simple chemical reactions.

Note: During Lab, you will follow a new chemical reaction between the metal magnesium and hydrochloric acid, this time following the reaction by gas formation. You will also begin to consider factors that can help chemical reactions along, such as reagent concentration, temperature, and stirring. Finally, you will conduct an experiment in which the concentration of one of two reactants is increased and the reaction products are measured.

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SLIDE CHEM-2-2

  • This slide uses the reaction from Investigation 1 Lab, the reaction between ammonium hydroxide and hydrochloric acid to yield water and ammonium chloride.
  • The upper half of the slide shows molecular models of each reactant and product to illustrate how atoms of the individual reactant molecules must be rearranged to form the product molecules.
  • The lower half of the slide shows that the reactant molecules must come into contact with each other in order for the atomic rearrangements to occur.

Note: This slide introduces the concept that certain conditions can help facilitate chemical reactions by increasing the likelihood or rate that reactant molecules interact with each other. This concept is further explored in the next slide.

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SLIDE CHEM-2-3

  • This slide illustrates three common ways to promote chemical reactions, all of which do so by increasing the rate at which reactant molecules interact with each other.
  • Increasing the concentration of reactants increases the chances of molecular collisions by simply providing more molecules in the same volume to interact.
  • Imagine how long it would take one molecule of each reactant to chance upon each other in a beaker or flask – a long time!
  • Both stirring and increasing temperature increase the kinetic energy of the reaction, thus increasing molecular motion and the rate at which reactant molecules collide and interact with each other. That is why stirring helps increase the rate of chemical reactions.

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SLIDE CHEM-2-4

Note: This slide focuses on reactant concentration in somewhat more detail. In particular, it considers the reaction that occurs if one reactant is kept constant and the other reactant concentration is increased.

  • In the example reaction, A and B are hypothetical reactants and C and D are hypothetical products.
  • The graph shows the formation of the product on the y-axis against increasing concentrations of reactant A on the x-axis.
  • By the third reactant A concentration all of reactant B is likely used up in the reaction process and no more products can be made. This is why the graph flattens out at this time.
  • If we were able to determine which molecules are found in the reaction at this point, we would find the products C and D as well as unreacted reactant A. Reactant B, on the other hand, would be absent from the reaction.
  • Adding additional reactant B at this point results in an upward deflection of the graph because unreacted reactant A can now react with the added reactant B to form additional products. This increase in product would continue until one or both of the reactants is used up.

Note: The concepts presented on this slide may appear somewhat abstract. However, Trial 6 of the Lab for this Investigation directly demonstrates this phenomenon. It may, therefore, be beneficial for you to return to this slide in PostLab when discussing experimental results.

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SLIDE CHEM-2-5

  • This slide simply introduces the chemical reaction that will be studied in the Lab in Investigation 2.
  • Remember that a chemical equation can be read as a sentence.
  • Remember that he exact same number of each atom is present on both sides of the chemical equation.
  • Matter is neither created nor destroyed in this or any other chemical reaction.

Note: The chart below shows the accounting of the atoms in the reactants and products:

CHEM2 atom table

 

 

 

 

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SLIDE CHEM-2-6

  • In the lab, you will follow the reaction by observing hydrogen gas (H2) formation.
  • You will use a rather crude but rapid method to measure the relative amount of gas production.
    • A balloon is fitted over the reaction flask and traps the escape of H2 gas. In the process, the balloon inflates to varying degrees in relation to the amount of the product is formed.
    • The actual amount of gas formed is then estimated by determining the diameter of the balloon after the various Trials.
  • You will use the simple formula at the bottom of the slide to convert the measured balloon diameter to the volume of the hydrogen gas.

Note: As the balloon does not form a perfect sphere, the application of the diameter into the volume equation is certainly a source of experimental error but may be expected to be an error of similar magnitude for each Trial.

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SLIDE CHEM-2-7

  • Caution must be taken when working with caustic chemicals such as hydrochloric acid.
  • Remember that you must wear lab coats, goggles (or safety glasses), and gloves throughout this Investigation.