Energy Changes in Chemical Reactions

Unit Overview

This unit provides students with opportunities to understand the enthalpy changes that accompany chemical reactions. The unit should be treated both qualitatively and quantitatively. The use of analogies, models, and kinetic molecular theory may help students to appreciate how energy changes relate to what changes are taking place at the atomic level.

Students need to be able to interpret information from charts, tables, and graphs. They should compare the determination of enthalpy change by use of bond energy data, tables of heats of formation, and the application of Hess's Law, and discuss any discrepancies observed.

The consideration of enthalpy effects in chemical reactions should be clearly linked to the combustion of carbon-based fuels to provide energy for our society. The stability of the CO₂ and H₂O molecules, which makes the burning of hydrocarbon molecules so exothermic and therefore so attractive, and the contribution of CO₂ to global warming should be discussed.

Discussion of the entropy, free energy and the mathematical determination of the spontaneity of reactions is optional.

Factors of scientific literacy which should be emphasized

• A3 ..........holistic
• A4 ..........replicable
• A5 ..........empirical
• A7 ..........unique

• B1 ..........change
• B2 ..........interaction
• B5 ..........perception
• B8 ..........quantification
• B9 ..........reproducibility of results
• B13 ..........energy-matter
• B33 ..........entropy

• C3 ..........observing and describing
• C5 ..........measuring
• C6 ..........questioning
• C8 ..........hypothesizing
• C9 ..........inferring
• C10 ..........predicting
• C11 ..........controlling variables
• C12 ..........interpreting data
• C15 ..........analyzing
• C17 ..........using mathematics

• D1 ..........science and technology
• D4 ..........science, technology, and the environment
• D8 ..........limitations of science and technology

• E3 ..........using equipment safely
• E5 ..........computer interaction
• E7 ..........manipulative ability
• E8 ..........measuring time
• E9 ..........measuring volume
• E10 ..........measuring temperature
• E11 ..........measuring mass
• E12 ..........using electronic instruments
• E13 ..........using quantitative relationships

• F3 ..........search for data and their meaning
• F5 ..........respect for logic
• F7 ..........demand for verification

• G3 ..........continuous learner
• G6 ..........response preference

Foundational Objectives for Chemistry and the Common Essential Learnings

Examine the relationships between heat energy and reactions.

• Recognize that energy changes are associated with chemical reactions.
• Relate enthalpy change in a reaction to bond energy and stability.
• Differentiate between endothermic and exothermic reactions.
• Compare the energy changes in phases changes and chemical reactions.
• Explain the difference between heat and temperature.
• Identify reactions which are used to produce useful heat.
• Consider the environmental and social effects of the use of heat energy by our society.

Understand the quantitative description of enthalpy change.

• Measure some energy changes in chemical reactions.
• Investigate how tables of standard heats (enthalpies) of formation are created and used.
• Express the enthalpy change of a chemical reactions as a term in the equation for the reaction, or as a heat of reaction (H).
• Use tables, graphs, or diagrams and an application of Hess's Law to infer enthalpy changes in reactions.

Optional: Understand the reasons why entropy and enthalpy effects are important.

• Identify how entropy effects influence chemical reactions.
• Consider the interaction between enthalpy and entropy in determining whether a reaction is spontaneous.
• Use the concept of free energy to express the quantitative relationship between entropy and enthalpy.
• Predict spontaneity of reactions using
  G^o = H^o - TS^o

Understand and use the vocabulary, structures and forms of expression which characterize chemistry. (COM)

• Incorporate vocabulary such as bond energy, enthalpy, endothermic and standard heat of formation into their speaking and writing about energies of reactions.
• Use tables and graphs in interpreting, estimating and explaining the energy effects of chemical reactions.
• Relate the theoretical aspects of the study of energies of reactions to daily, practical experiences with energy produced by and consumed by reactions.

Strengthen understanding of chemistry through applying knowledge of numbers and their interrelationships. (NUM)

• Read, and interpret meaning from, graphs, charts and tables.
• Collect, organize and analyze quantitative information.
• Use graphs, charts and tables to help explain concepts and ideas about energy changes.
• Understand and explain to others (orally or in writing) how temperature change measurements can be used to infer the extent and type of bond rearrangements during a reaction.

Develop an understanding of how knowledge is created, evaluated, refined and changed within chemistry. (CCT)

• Make careful observations of energy effects in reactions, and explain how those effects can be used to make inferences about the atomic and molecular rearrangements.
• Reflect on the importance of theory in creating a framework by which reactions are viewed, and the place of theory with respect to the observations.

Appreciate the value and limitations of technology within society. (TL)

• Explore the distribution and uses in home, school and community of technologies making use of the exothermic or endothermic nature of chemical reactions.
• Assess the benefits and risks accruing from technologies which exploit the exothermic or endothermic nature of chemical reactions.
• Use technological devices to help measure heats of reaction.

Suggested activities and ideas for research projects

• Add a few pellets NaOH_(s) to about 50 mL dilute H₂SO_4(aq) or HCl_(aq). Stir, and note the temperature change.

• Spread about 1 g anhydrous CuSO_4(s) in a thin layer on a piece of paper. Anhydrous CuSO_4(s) can be formed by grinding bluestone crystals (CuSO₄•5H₂O) and drying in a 100^oC oven or under a heat lamp. Put a small drop of water on part of the layer of chemical. Observe the effect of the water. Describe the differences between the anhydrous CuSO4(s) and the hydrated form.

• Mix Ba(OH)₂·8H₂O_(s) and NH₄SCN_(s) in a 2:1 mass ratio. Stir the mixture, noting the temperature change.

• Use the case study "History of Modern Ideas About Heat" from Science: Process and Discovery (Field, 1985). 22 questions and problems for investigation accompany the study.

• Brainstorm to produce a list of uses made of the heat effects of chemical reactions. Some are:
  • space heating by combustion of fuels
  • "instant cold" compresses
  • oxidation of glucose in the body to maintain constant temperature

• Identify locations where thermal pollution exists. Analyze sources, effects and reasons why such pollution occurs.

• Consider the local and global environmental implications of burning fossil fuels.

• Pick s chemical reaction. Design a procedure to determine whether that reaction produces heat.

• When equal volumes of 0.10M HCl_(aq) and 0.10M NaOH_(aq) are mixed, heat is produced. Will twice as much heat be produced if equal volumes of 0.20M HCl_(aq) and 0.10M NaOH_(aq) are mixed? How about if equal volumes of 0.20M HCl_(aq) and 0.20M NaOH_(aq) are mixed?

• Identify ways of producing heat other than by chemical reaction. For what purposes is such heat currently used? List places where heat produced by chemical reaction is now used. Could the other sources of heat identified be substituted for heat produced by chemical reaction? Could heat produced by chemical reaction substitute for any of the other sources?

• Using an insulated cup calorimeter, dissolve 3.00 g KNO_3(S) in 50 mL of water. Record the temperature change. Rinse the calorimeter and repeat, using 3.00 g NH₄Cl_(S) in 50 mL water. Record the temperature change. Compare the two temperature changes. How many grams of NH₄Cl_(S) must be mixed with 3.00 g KNO_3(S) so that when the mixture is added to water, no temperature change is noticed?

• Determine the molar heat of combustion of paraffin by heating a beaker of water on a stand with a paraffin candle. Use a metal can, open at both ends, with a few vent holes on the side as a chimney to reduce heat loss from the burning candle.

  Based on the mass lost by the burning candle, and the temperature change of the known volume of water, calculate the molar heat of combustion of paraffin.

  Burn a beeswax candle, using the same appartus and procedure, to get data to compare the heat of combustion of paraffin and beeswax.

• Here is a set of reactions which can be used to illustrate Hess's Law. Measure the temperature change of each reaction and use that data to calculate the H for each. Use NaOH pellets which have not been exposed to the air. To know the exact mass of the NaOH_(S) is important, but it is not important that it be exactly 2.00 grams. The volumes of solutions and water have been adjusted to give a constant volume of 100 mL.

  2 g NaOH_(S) + 100 mL H₂O_(l)

  50 mL 1.0M NaOH_(aq) + 50 mL 1.0M HCl_(aq)

  2 g NaOH_(S) + 100 mL 0.5M HCl_(aq)

• Burning fossil fuels produces most of the heat that we use in North America. What are other sources of heat used? What percentage does each supply? What is the most common fossil fuel? For solid and liquid fossil fuels, compare their efficiency of heat production on the basis of kJ per mole and kJ per gram burned. For gaseous fossil fuels, compare their efficiency of heat production on the basis of kJ per mole and kJ per litre of gas at SATP.

  How much energy does it cost to extract, refine, transport and distribute each fossil fuel? (Express the energy cost in $ per million kJ.)

Sample ideas for evaluation and for encouraging thinking

• 3C_(s) + 2Fe₂O_3(s) + 463.1 kJ 4Fe_(s) + 3CO_2(g)

  Rewrite this equation using H notation for one mole of carbon dioxide product.

• How can energy be released when a bond is formed? If energy is released when bonds form, why aren't all reactions exothermic?

• What would happen if energy was released when bonds were broken and absorbed when they are formed? What would happen if energy was released when bonds were broken and formed?

• Why is the law of conservation of energy considered to be valid?

• A plateau in the heating curves of a liquid can indicate the boiling point of the liquid. What becomes of the heat being added to the system during the time period of the plateau, where the temperature doesn't rise?

• 2Fe_(s) + 1½O_2(g) Fe₂O_3(s)

  H= -824 kJ/mol Fe₂O₃

  C_(s) + O_2(g) CO_2(g)

  H= -393.5 kJ/mol CO₂

  Comment on the relative merits of burning Fe_(s) and C_(s) as fuels.

• C₃H_8(g) + 5O_2(g) 3CO_2(g) + 4H₂O_(l)

  H= -2220 kJ/mol C₃H₈

  CH_4(g) + 2O_2(g) CO_2(g) + 2H₂O_(l)

  H= -890 kJ/mol CH₄

  Since propane (C₃H_8(g)) gives off 2½ times as much heat per mol of fuel, why is natural gas (CH_4(g)) a more popular fuel?

• To measure the heat of reaction between Zn_(s) and HCl_(aq) , it would be better to use 6.54 grams of zinc and 250 mL of 1.0 M HCl(aq) than to use 6.54 grams Zn_(s) and 25 mL of 10.0 M HCl_(aq). Why?

• Use charts of thermodynamic data to calculate what percentage of potential heat loss there is when natural gas burns with insufficient oxygen to form H₂O_(l) , and equal mols of C_(s) and CO_(g) , compared to when it burns to form CO_2(g) and H₂O_(l).

  To make this question easier, the equations

  2CH_4(g) + 2½O_2(g) C_(s) + CO_(g) + 4H₂O_(l) and

  CH_4(g) + 2O_2(g) CO_2(g) + 2H₂O_(l) could be given.

• Energy is absorbed during an exothermic reaction. Where does the energy absorbed come from? Where does it go? Can it ever be recovered?

• How does sweating keep us cool during hot weather or after strenuous exertion? The heat which makes hot weather originates in a nuclear reaction. The heat responsible for heating us during strenuous exercise (and other times too) originates in a chemical reaction. What reactions are involved in each of these cases?

• Why do chemical reactions produce or consume heat as they occur? Where does the heat energy come from? Where does it go when it gets into the air?