BIOB111_CHBIO - Tutorial activities for Session 4 General topics for the week Discussion of the effect of several factors on chemical equilibrium of selected reactions. Examples of effect of stress on equilibrium reactions are given and students are asked to complete the work sheet on this topic. Chemical reactions and rates of chemical reactions Instructions: Use the following link to learn more about Le Châtelier s principle: <http://www.chemguide.co.uk/physical/equilibria/lechatelier.html> (link verified 13 January 2017) Use the following link to understand Chemical Equilibrium: http://www.wwnorton.com/college/chemistry/chemistry3/ch/16/chemtours.aspx (link verified 13 January 2017) General tutorial questions: 1. I) What is a reversible reaction? Last Updated on 6-Feb-14 Page 1 of 9
II) Use the information from the diagram below to describe what happens at a chemical equilibrium with respect to rates of the forward and backward reaction? III) Use the information from the following diagram to describe what happens at a chemical equilibrium with respect to the concentrations of the reactants and products? 2. Fill in the blanks with the given words: Choose from: the same time, does not change, quickly. A reversible reaction is when the forward reaction and reverse reaction occur at A reversible reaction uses up the reactants until a point (the equilibrium) where concentration of the reactants A reversible reaction rapidly produces a large and rapid increase in the concentration of products until a point (the equilibrium) where concentration of the products 3. Fill in the blanks with the given words: Choose from: the concentration of reactants, temperature, catalyst, exothermic, slower, faster. A chemical reaction in which a substance combines with oxygen releasing a large amount of energy as heat is a (n) reaction The rate of a chemical reaction can be increased by increasing, and adding a(n) As reactants are used up and products accumulate the rate of forward reaction becomes Slower, while the rate of reverse reaction becomes Last Updated on 6-Feb-14 Page 2 of 9
4. Identify each reaction as either endothermic or exothermic: Chemical Reaction C(s) + O 2 CO 2 + 93.9 kcal 4Fe (s) + 3 O 2 (g) 2Fe 2O 3 +1.7MJ N 2(g) + O 2 (g) +21.6kcal 2NO (g) 2H 2S (g) 2H 2 (g) + S 2 (s)+ Heat Endothermic or Exothermic ΔH, change in heat energy ΔH = -93.9 kcal ΔH = - 1.7MJ ΔH = 21.6 k cal ΔH = negative 5. Whether a reaction is exothermic or endothermic is determined by: a. The activation energy b. The physical state of the reactants c. Whether energy is absorbed/released for the reaction d. Whether a catalyst is present 6. What happens when NH 3 is added to the chemical system (reaction) that is at equilibrium? N 2 + 3H 2 2NH 3 a. The rate of the forward reaction increases, as additional product is made b. The rate of the backward reaction increases, as additional product is made c. The rate of the backward reaction increases, as additional reactants are made d. The rate of the forward reaction increases, as additional reactants are made 7. What happens when N 2 is added to the chemical system (reaction) that is at equilibrium? N 2 + 3H 2 2NH 3 a. The rate of the forward reaction increases, as additional product is made b. The rate of the backward reaction increases, as additional product is made c. The rate of the backward reaction increases, as additional reactants are made d. The rate of the forward reaction increases, as additional reactants are made 8. What happens when H 2 is removed to the chemical system (reaction) that is at equilibrium? N 2 + 3H 2 2NH 3 a. The rate of the forward reaction increases, as additional product is made b. The rate of the backward reaction increases, as additional product is made c. The rate of the backward reaction increases, as additional reactants are made d. The rate of the forward reaction increases, as additional reactants are made Last Updated on 6-Feb-14 Page 3 of 9
Conceptual multiple choice questions: 9. Concept: Processes that occur during a chemical reaction Context: Reactants are converted to products through chemical reactions. Chemical bonds are formed and broken during a chemical reaction. Question: Which of the following accurately describes how a chemical reaction proceeds from the beginning to the end? A. After a collision between the reactant compounds some of the bonds within the reactants are broken, allowing atoms to be rearranged before new bonds are formed to generate the products B. In a chemical reaction, bonds form within the reactant compounds, before a collision between the product compounds, where some of the chemical bonds will be broken C. A collision between the reactant compounds begins the chemical reaction, where bonds are formed in the reactant compounds before they are converted into the product compounds, where bonds are broken D. To begin the chemical reaction, all of the chemical bonds that hold the reactant compounds together will break, which leads to a collision between the reactants, allowing new bonds to form which creates the product compounds 10. Concept: Collisions to begin chemical reactions Context: Chemical reactions rely on the collisions between the reactant compounds to begin the chemical reaction. Question: Which of the following accurately describes the chemical reaction below? NO + O 3 <---> NO 2 + O 2 A. Any collision between the reactants NO and O 3 will begin the chemical reaction, allowing some of the bonds within the reactants to be broken, so that new bonds can form to generate the products NO 2 and O 2 B. When the reactants NO and O 3 collide with sufficient force to meet the activation energy, new bonds are formed within the reactant compounds, which allows bonds to be broken in the products NO 2 and O 2 Last Updated on 6-Feb-14 Page 4 of 9
C. Any collision between the reactants NO and O 3 will begin the chemical reaction, allowing new bonds to form within the reactant compounds, which allows bonds to be broken in the products NO 2 and O 2 D. When the reactants NO and O 3 collide with sufficient force to meet the activation energy, some of the bonds within the reactants will be broken, allowing new bonds to form which generate the products NO 2 and O 2 11. Concept: Energy transfer in chemical reactions Context: Rearrangement of the atoms is required to change the chemical structure of a compound. Atom rearrangement occurs during chemical reactions, where reactant compounds are converted into products. Question: At what stage of a chemical reaction are chemical bonds broken and does this require or release energy? A. After collision between the products, bonds are broken within the product compounds which requires energy B. After collision between the reactants, bonds are broken within the reactant compounds which requires energy C. After collision between the products, bonds are broken within the product compounds which releases energy D. After collision between the reactants, bonds are broken within the reactant compounds which releases energy 12. Concept: Energy transfer in chemical reactions Context: Rearrangement of the atoms is required to change the chemical structure of a compound. Atom rearrangement occurs during chemical reactions, where reactant compounds are converted into products. Question: At what stage of a chemical reaction are chemical bonds formed and does this require or release energy? A. New chemical bonds are formed to create the products of a chemical reaction, which releases energy B. New chemical bonds are formed to create the reactants of a chemical reaction, which releases energy Last Updated on 6-Feb-14 Page 5 of 9
C. New chemical bonds are formed to create the products of a chemical reaction, which requires energy D. New chemical bonds are formed to create the reactants of a chemical reaction, which requires energy 13. Concept: Changing the rate of a chemical reaction Context: The rate of a chemical reaction can be sped up or slowed shown by adjusting the temperature or changing the concentration (amount) of the reactant or product compounds. Question: What temperature adjustment would increase the rate of the chemical reaction and why? A. By decreasing the temperature the reactant compounds are less likely to collide with each other to begin the reaction B. By increasing the temperature the reactant compounds are more likely to collide with each other to begin the reaction C. By increasing the temperature the reactant compounds are less likely to collide with each other to begin the reaction D. By decreasing the temperature the reactant compounds are more likely to collide with each other to begin the reaction 14. Concept: Changing the rate of a chemical reaction Context: The rate of a chemical reaction can be sped up or slowed shown by adjusting the temperature or changing the concentration (amount) of the reactant or product compounds. Question: What change to the concentration of the reactant compounds would increase the rate of the chemical reaction and why? A. Make the concentration of all of the different reactant compounds the same to increase the likelihood that they will collide to begin the reaction B. Decrease the concentration of all of the reactant compounds to increase the likelihood that they will collide to begin the reaction C. Double the concentration of all of the different reactants to increase the likelihood that they will collide to begin the reaction Last Updated on 6-Feb-14 Page 6 of 9
D. Increase the concentration of one of the reactants, but decrease the concentration of the other types of reactant compounds, to increase the likelihood that they will collide to begin the reaction 15. Concept: Activation energy Context: The energy barrier that must be overcome to begin a chemical reaction is called the activation energy. After a collision occurs between the reactant compounds, which exceeds the activation energy, the chemical reaction begins resulting in the conversion of reactants into products. Question: How does the amount of activation energy required for the chemical reaction impact on how often the reaction will occur (rate of the reaction)? A. Chemical reactions with low activation energies convert reactants into products occur more often than chemical reactions with high activation energies B. Only chemical reactions with a high activation energies can occur often without a large amount of input energy C. Chemical reactions with high activation energies convert reactants into products occur more often than chemical reactions with low activation energies D. Different chemical reactions occur at the same rate irrespective of their activation energies 16. Concept: Endothermic reactions Context: For an endothermic chemical reaction energy is absorbed by converting the reactants into the products. In contrast, for an exothermic chemical reaction, energy is released as heat by converting the reactants into the products. Question: Which of the following accurately describes the activation energy of an endothermic reaction and how often the reaction will occur? A. Endothermic reactions have high activation energies, meaning these chemical reactions do not occur often B. Endothermic reactions have low activation energies, meaning these chemical reactions occur often C. Endothermic reactions have high activation energies, meaning these chemical reactions occur often Last Updated on 6-Feb-14 Page 7 of 9
D. Endothermic reactions have low activation energies, meaning these chemical reactions do not occur often 17. Concept: Exothermic reactions Context: For an endothermic chemical reaction energy is absorbed by converting the reactants into the products. In contrast, for an exothermic chemical reaction, energy is released as heat by converting the reactants into the products. Question: Which of the following accurately describes the activation energy of an exothermic reaction and how often the reaction will occur? A. Exothermic reactions have low activation energies, meaning these chemical reactions do not occur often B. Exothermic reactions have high activation energies, meaning these chemical reactions occur often C. Exothermic reactions have high activation energies, meaning these chemical reactions do not occur very often D. Exothermic reactions have low activation energies, meaning these chemical reactions occur often 18. Concept: Equilibrium reactions Context: Reversible chemical reactions can proceed in both the forward and reverse directions. Question: What are the rates of the forward and reverse reactions in the reaction below when the reaction is at equilibrium? N 2 + 3H 2 <---> 2NH 3 A. Equilibrium reactions can only precede in a single direction B. The rate of the reverse reaction exceeds the rate of the forward reaction C. The rate of the forward reaction exceeds the rate of the reverse reaction D. The rate of the forward reaction is equal to the rate of the reverse reaction 19. Concept: Stresses on chemical equilibrium Context: Once a chemical reaction has reached equilibrium the equilibrium can be disrupted by stresses such as the addition or removal of the reactants or products. Last Updated on 6-Feb-14 Page 8 of 9
Question: What happens to a chemical reaction once a stress pushes the reaction out of equilibrium? N 2 + 3H 2 <---> 2NH 3 A. The chemical reaction remains in a non-equilibrium state B. The chemical reaction increases the rate of either the forward or reverse reaction to remove the stress, so that the chemical reaction can return to equilibrium C. The chemical reaction returns to equilibrium without adjusting the rates of the forward or reverse reactions D. The chemical reaction remains in a non-equilibrium state until another stress pushes the chemical reaction back into equilibrium Last Updated on 6-Feb-14 Page 9 of 9