HS-PS1-4: Modeling Energy Change in Reactions with Bond Energy
In plain English: HS-PS1-4 is the NGSS high school performance expectation that asks students to develop a model showing that whether a chemical reaction releases or absorbs energy depends on the change in total bond energy. Breaking bonds absorbs energy, forming bonds releases it, and the energy is conserved between the reacting system and its surroundings. It is usually taught in Chemistry or Physical Science.
Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
Clarification Statement: Emphasis is on the idea that a chemical reaction is a system that affects the energy change. Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved.
Assessment Boundary: Assessment does not include calculating the total bond energy changes during a chemical reaction from the bond energies of reactants and products.
Next Generation Science Standards · Disciplinary Core Idea: Matter and Its Interactions · Official standard
Students build and use models to explain why some reactions warm their surroundings and others cool them. The central idea is an energy account for the reaction system: breaking the bonds in the reactants absorbs energy, forming the bonds in the products releases energy, and the difference between the two decides whether the system releases or absorbs energy overall. Students draw molecules, sketch energy diagrams, measure temperature changes in safe classroom reactions and trace where the energy goes so the total is conserved.
The NGSS Clarification Statement says: "Emphasis is on the idea that a chemical reaction is a system that affects the energy change. Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved." The Assessment Boundary says: "Assessment does not include calculating the total bond energy changes during a chemical reaction from the bond energies of reactants and products." This page follows that limit: students count and compare bonds, read energy diagrams and analyze temperature data, but never add up bond energies. All lab data are invented for teaching, and every lab uses classroom-safe materials with goggles.
Learning Objectives
By the end of this lesson, students will be able to:
Explain that breaking chemical bonds absorbs energy and forming chemical bonds releases energy
Develop a molecular-level drawing of a reaction that conserves atoms and identifies the bonds broken and formed
Draw and interpret energy diagrams that show the relative energies of reactants and products for exothermic and endothermic reactions
Use temperature data from a reaction to decide whether the reaction system released or absorbed energy
Build a representation that shows energy is conserved as it moves between a reaction system and its surroundings
Prior Knowledge Required
Students should already be comfortable with:
Atoms are rearranged and conserved in a chemical reaction MS-PS1-5
Some chemical processes release thermal energy and others absorb it MS-PS1-6
Reading chemical formulas and balanced equations with coefficients
Covalent bonds as shared pairs of electrons, including single, double and triple bonds
Pass around air-activated hand warmers that were opened a few minutes ago. Students feel them but do not cut them open. Tell students the pouch holds iron powder, salt, water and carbon, and that air gets in through the fabric.
Warm-Up Prompt
"Nothing is burning, yet the pouch keeps getting warmer. Draw what you think the particles inside are doing. Where does the thermal energy come from, and where does it go? Is any energy created?"
Collect a few drawings. Many students say energy is "stored in" the iron and "let out". Keep the drawings: at the end of the lesson students revise them using bonds broken and bonds formed.
Direct Instruction15-20 minutes
The energy account of a reaction. Treat the reacting particles as the system and everything else (water, cup, air, your hand) as the surroundings. Pulling bonded atoms apart always takes energy, so breaking bonds absorbs energy. When atoms bond, energy is released, so forming bonds releases energy. If the bonds formed in the products release more energy than the bonds broken in the reactants absorb, the system gives the extra energy to the surroundings (exothermic: the surroundings warm). If breaking takes more than forming gives back, the system takes energy from the surroundings (endothermic: the surroundings cool, or energy must be supplied). Energy is never created or destroyed; it moves between the system and the surroundings. The ΔH values below are standard values from reference tables.
Molecular drawing: hydrogen and chlorine
H2 + Cl2 → 2HCl. Draw one H-H molecule and one Cl-Cl molecule on the left and two H-Cl molecules on the right.
Result: Atoms: 2 H and 2 Cl on each side. Bonds broken: 1 H-H and 1 Cl-Cl. Bonds formed: 2 H-Cl. The reaction releases about 185 kJ for every 2 mol of HCl formed, so forming the two H-Cl bonds releases more energy than breaking the reactant bonds absorbs.
Energy diagram: burning methane
CH4 + 2O2 → CO2 + 2H2O releases 890 kJ per mole of methane. Sketch the energy diagram.
Result: Products sit 890 kJ below the reactants. Bonds broken: 4 C-H and 2 O=O (6 bonds). Bonds formed: 2 C=O and 4 O-H (6 bonds). The same number of bonds, yet energy is released: the product bonds are stronger, so counting bonds is not enough.
Energy diagram: an endothermic reaction
N2 + O2 → 2NO absorbs 180.5 kJ. It happens in lightning and hot engines.
Result: Products sit 180.5 kJ above the reactants. Breaking the very strong N≡N triple bond and the O=O bond absorbs more energy than forming the bonds in two NO molecules releases, so the system must take in energy.
Energy is conserved: a lime kiln
CaCO3 → CaO + CO2 absorbs 178 kJ per mole of calcium carbonate. The kiln burns fuel to heat the limestone.
Result: Energy flows from the burning fuel into the reaction system, and it ends up in the new arrangement of bonds in CaO and CO2 (with some lost to the kiln walls and air). The reaction stops when the heating stops, a sign of an endothermic reaction.
Show Diagram 1 and ask: "Which panel has products below reactants, and what happens to the thermometer in the surroundings?" Then show Diagram 2, a step model for H2 + Cl2: the up arrow is energy absorbed to pull the atoms apart, the down arrow is energy released as new bonds form, and the net drop is the energy the system gives to the surroundings. Say clearly that the page does not ask students to calculate that net value from bond energies (beyond this standard).
Guided Practice15 minutes
Pairs analyze the invented temperature data below. The first two reactions ran in an insulated cup with 50 mL of solution and were run by the teacher (copper(II) sulfate is an irritant, so students do not handle it); for the third, the thermometer was held against the surface of a hand warmer pouch.
Guided practice temperature data (invented)
Reaction
Start (°C)
End (°C)
Zinc powder + copper(II) sulfate solution
21.0
33.5
Citric acid solution + baking soda
22.0
13.5
Iron powder + oxygen (hand warmer, surface)
20.5
48.0
Find the temperature change for each reaction and say whether the surroundings gained or lost energy.
Classify each reaction as exothermic or endothermic.
For each reaction, sketch an energy diagram with the reactants and products at the right relative heights.
For one reaction, write a sentence that compares the energy absorbed breaking bonds with the energy released forming bonds, and one sentence that says where the energy went.
Expected results: +12.5 °C and +27.5 °C (exothermic), -8.5 °C (endothermic). Point out that the thermometer measures the surroundings, not the reacting particles.
Independent Practice10-15 minutes
Each student builds a three-part model for burning propane in a gas grill: C3H8 + 5O2 → 3CO2 + 4H2O, which releases energy.
Molecular drawing: show that 3 C, 8 H and 10 O atoms appear on each side.
Bond list: bonds broken are 8 C-H, 2 C-C and 5 O=O (15 bonds); bonds formed are 6 C=O and 8 O-H (14 bonds).
Energy diagram and energy flow: products below reactants, with an arrow from the system to the surroundings.
Then answer: "The reaction forms fewer bonds than it breaks, yet it releases energy. Use your model to explain how." (The bonds formed are stronger on average, so they release more energy than the weaker bonds broken absorb.)
Closure5 minutes
Exit ticket: "In a reaction, forming the product bonds releases more energy than breaking the reactant bonds absorbs. Does a thermometer in the surrounding water rise or fall? Sketch the energy diagram." (It rises; products are drawn below reactants.) Students then revise their warm-up drawing of the hand warmer using the words system, surroundings, bonds broken and bonds formed.
Differentiation Strategies
For Struggling Students
Give a two-column organizer, "Bonds broken (energy in)" and "Bonds formed (energy out)", and have students fill it from a model-kit build before drawing any diagram
Provide energy-diagram templates with the axes, a reactant line and blank space, so students only decide whether products go above or below
Use the magnet model first: pulling two magnets apart takes effort, letting them snap together gives off a click, then connect this to bonds
For Advanced Students
Ask students to explain why the same reaction run in reverse must absorb exactly the energy the forward reaction releases
Have students compare two reactions that break and form the same number of bonds but release different amounts of energy, and explain the difference with bond strength
Ask students to design a better-insulated calorimeter from cups and lids and explain how it makes the temperature data a fairer measure of the energy released
Assessment Guidance
What to Look For
Listen for students who say that breaking bonds releases energy: this is the key misconception to correct. Check that molecular drawings conserve atoms and name the bonds broken and formed, that energy diagrams place products below reactants for exothermic reactions and above for endothermic ones, and that students explain a temperature change as energy moving between the system and the surroundings. A strong model always accounts for the energy on both sides so none is created or lost.
02
Classroom Activities
3 Activities
1
Hot and Cold Reactions Lab
30 minGroups of 3-4
Groups run three classroom-safe reactions in foam cups or a jar, record the temperature change of the surroundings, and build a model for each: a molecular drawing, an energy diagram and an energy-flow arrow between system and surroundings.
Procedure
Safety first: everyone wears safety goggles for the whole lab, and gloves when handling steel wool. Use only 3% household hydrogen peroxide, never a stronger solution. Work in a tray, do not taste anything, and keep all flames and heaters out of the room. Pour liquids down the sink with plenty of water; steel wool goes in the trash.
Station A: pour 20 mL of 3% hydrogen peroxide into a foam cup, record the temperature, add half a teaspoon of active dry yeast (the yeast speeds up the breakdown of hydrogen peroxide into water and oxygen and is not used up), stir and record the highest temperature.
Station B: pour 30 mL of vinegar into a foam cup, record the temperature, add one teaspoon of baking soda and record the lowest temperature.
Station C: soak a pad of fine steel wool in vinegar for one minute to remove its coating, squeeze it out, wrap it around the thermometer bulb in a small jar and record the temperature for five minutes. The iron slowly reacts with oxygen.
Sample Data (invented)
Station A: 21.4 °C to 30.1 °C. Station B: 21.8 °C to 17.6 °C. Station C: 21.6 °C to 24.9 °C after five minutes.
Discussion Questions
Station A shows the largest temperature change of the three. Which way did the energy move, from the system to the surroundings or the reverse?
At Station B the thermometer fell. Did the reaction system gain or lose energy, and where did that energy come from?
Station C rose less than Station A. Does that prove each reacting iron atom releases less energy? Name two other reasons the rise could be smaller.
For each station, which is larger: the energy absorbed breaking bonds or the energy released forming bonds?
Modification for Distance Learning
Students analyze the sample data, watch a teacher video of each station, and upload their three models. A family can safely try Station B at home with goggles.
2
Model Critique Gallery
20 minPairs
Four invented student models hang around the room. Each has one flaw. Pairs rotate, write a critique on a sticky note, and then revise one model completely.
The Four Models
Model 1: "Burning methane releases energy because breaking the bonds in methane releases the energy stored in them."
Model 2: an energy diagram for the vinegar and baking soda reaction with the products drawn below the reactants, labeled "endothermic".
Model 3: a molecular drawing of the breakdown of hydrogen peroxide with two H2O2 molecules on the left and two H2O molecules plus two O2 molecules on the right.
Model 4: an energy-flow drawing of a hand warmer with an arrow of energy leaving the pouch and the label "the energy is used up", with nothing showing where it goes.
Discussion Questions
Model 3 has 4 oxygen atoms on the left but 6 on the right. What does that tell you, and how do you fix it?
Which model has the direction of energy flow right but breaks the rule that energy is conserved?
Rewrite Model 1 so it uses both bonds broken and bonds formed.
Challenge Variation
Each pair writes a new flawed model for a reaction from the lab and trades it with another pair to critique.
3
Snap Apart, Snap Together: Building Bond Models
20 minGroups of 3-4
Groups use ceramic magnets as a physical model of a bond, then build the burning of ethanol with molecular model kits and track the bonds broken and formed.
Procedure
Pull two ceramic magnets apart and let them snap together. Record what you had to do to separate them (supply energy) and what you noticed when they joined (a click: energy given off as sound and a little thermal energy). Use ceramic magnets, not strong neodymium magnets, which can pinch fingers.
Build C2H5OH + 3O2 with the model kit. Take the reactants apart one bond at a time and tally each bond broken, then build 2CO2 + 3H2O and tally each bond formed.
Tally: broken 5 C-H, 1 C-C, 1 C-O, 1 O-H and 3 O=O (11 bonds); formed 4 C=O and 6 O-H (10 bonds). Check that 2 C, 6 H and 7 O atoms appear on each side.
Ethanol burning releases energy. Draw its energy diagram and an energy-flow arrow.
Discussion Questions
How is the magnet model like a chemical bond, and where does it fail? (In a reaction, the energy released by forming bonds leaves mostly as thermal energy and light.)
The reaction forms one fewer bond than it breaks. What must be true about the strength of the bonds formed?
Modification for Distance Learning
Use a free online molecule builder, or paper cut-outs of atoms and bonds, and share a photo of each stage of the build.
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: Energy Diagrams for an Exothermic and an Endothermic Reaction
Two invented reactions, drawn to scale in arbitrary energy units. In Reaction A the products are 80 units below the reactants, so the system releases 80 units to the surroundings. In Reaction B the products are 70 units above the reactants, so the system absorbs 70 units. The hump in each curve is the energy needed to start breaking bonds.
Diagram 2: A Step Model of Bonds Broken and Bonds Formed for H₂ + Cl₂ → 2HCl
Arrow lengths are drawn to scale from standard average bond energies (H-H 436, Cl-Cl 242 and H-Cl 431 kJ/mol). The model shows why the reaction releases about 185 kJ: the down arrow for forming two H-Cl bonds is longer than the up arrow for breaking the reactant bonds. Students read the model; calculating the net value from bond energies is beyond this standard.
04
Homework Assignment
~30 min
HS-PS1-4 Homework: Modeling Energy in Reactions
Directions: All data are invented for practice. For each problem, draw or describe a model, and explain energy changes with bonds broken and bonds formed. You do not need to calculate with bond energies.
Part 1: Molecular Models (Problems 1-2)
Hydrogen burns in oxygen to form water: 2H2 + O2 → 2H2O, and the reaction releases energy. (a) Draw a molecular model and show that atoms are conserved. (b) List the bonds broken and the bonds formed. (c) Use your model to explain why the reaction releases energy.
Passing electric current through water breaks it into hydrogen and oxygen: 2H2O → 2H2 + O2. The reaction continues only while the current flows. (a) List the bonds broken and formed. (b) Is the reaction exothermic or endothermic? Use your bond list as evidence. (c) How does the energy this reaction absorbs compare with the energy released in Problem 1 for the same amounts? Explain using conservation of energy.
Part 2: Energy Diagrams and Data (Problems 3-4)
Two reactions are run in insulated cups, each with 50 mL of water. Reaction X: the water goes from 20.0 °C to 26.4 °C. Reaction Y: the water goes from 19.8 °C to 14.1 °C. (a) Find the temperature change for each. (b) Classify each reaction. (c) Sketch an energy diagram for each. (d) For which reaction does forming the product bonds release more energy than breaking the reactant bonds absorbs?
An energy diagram shows the reactants at 30 units, the top of the hump at 95 units and the products at 80 units. (a) Is the reaction exothermic or endothermic? (b) How much energy does the system gain or lose overall? (c) Which is larger for this reaction: the energy absorbed breaking bonds or the energy released forming bonds? (d) Would a thermometer in the surroundings rise or fall?
Part 3: Conservation and Arguments (Problems 5-6)
A classmate writes: "Cellular respiration, C6H12O6 + 6O2 → 6CO2 + 6H2O, gives cells energy because glucose has high-energy bonds that release energy when they break." (a) Show that atoms are conserved. (b) Explain what is wrong with the claim. (c) Rewrite it as a correct explanation that uses bonds broken, bonds formed and the surroundings.
A self-heating can of soup has an outer chamber where calcium oxide reacts with water: CaO + H2O → Ca(OH)2, and the reaction releases energy. In a test, the soup gains 20 kJ of thermal energy and the can loses 3 kJ to the air. (a) Show that atoms are conserved. (b) At least how much energy did the reaction system release? (c) Draw an energy-flow model with the system, the soup and the air, and label each arrow.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
Molecular Models
Atoms conserved, and the bonds broken and formed are listed correctly
Model drawn with one counting or bond error
Energy Diagrams and Data
Products placed correctly relative to reactants, and temperature data interpreted as energy moving between system and surroundings
Classification correct but the diagram or the data interpretation is incomplete
Explanations
Explains the energy change using both bonds broken (energy in) and bonds formed (energy out), and accounts for all the energy
Uses only one side of the bond account, or loses track of where the energy goes
05
Quiz: 20 Questions
Interactive, with answers
Instructions
All data are invented. You will not need bond-energy values. Your score updates as you answer, and Reset quiz clears everything so you or your students can try again.
Multiple choice: pick an option to check it. Short answer: write your answer, then reveal the model answer.
0 of 20 answered · 0 correct
Question 1 of 20 · Multiple Choice
Which statement about chemical bonds is correct?
Answer: B
Pulling bonded atoms apart always takes energy, and energy is released when atoms bond. Choice A is the reverse, a frequent misconception that comes from phrases like "energy stored in bonds". Choice D ignores that every bond change involves energy.
Question 2 of 20 · Multiple Choice
A reaction runs in a cup of water. The water goes from 22.0 °C to 18.5 °C. What do the data show?
Answer: D
The water (the surroundings) cooled by 3.5 °C, so energy moved from the surroundings into the reacting system: endothermic. Choice A confuses the temperature of the surroundings with energy released. Choice C ignores the reaction, which is the only change in the cup.
Question 3 of 20 · Multiple Choice
Four reactions are run in insulated cups of water. Which one released energy to its surroundings?
Answer: C
Only Reaction 3 warms its water (+3.8 °C), so only it released energy to the surroundings. Reactions 1, 2 and 4 cool the water by 1.8, 2.5 and 1.9 °C, so they absorbed energy; choosing Reaction 2 confuses the largest temperature change with energy released.
Question 4 of 20 · Multiple Choice
An energy diagram shows the products of a reaction higher than the reactants. Which statement must be true?
Answer: A
Products above reactants means the system ends with more energy, so it absorbed energy overall: breaking bonds took more than forming bonds gave back. Choice B describes an exothermic reaction. Choice C is the opposite of what happens, and choice D breaks conservation of energy.
Question 5 of 20 · Multiple Choice
Ammonia forms from nitrogen and hydrogen: N2 + 3H2 → 2NH3. Which list gives the bonds broken and formed?
Answer: D
One N2 molecule has one triple bond and each of the 3 H2 molecules has one H-H bond. Each NH3 has 3 N-H bonds, and there are 2 molecules, so 6 N-H bonds form. Choice C counts the bonds in only one NH3. Choice B reverses broken and formed.
Question 6 of 20 · Multiple Choice
An exothermic reaction runs in a sealed, perfectly insulated container. What happens to the total energy of the reaction system plus its surroundings inside the container?
Answer: B
Energy is conserved: the energy the system releases is gained by the surroundings, so the total does not change. Choice A counts the released energy as new energy. Choice C treats the released energy as destroyed.
Question 7 of 20 · Multiple Choice
A student says: "The products of an exothermic reaction have more energy than the reactants, because energy came out." Which response is best?
Answer: C
In an exothermic reaction the system loses energy to the surroundings, so the products sit lower on the energy diagram. Choice A mixes up the system and the surroundings. Choice D ignores bond strength and the energy diagram.
Question 8 of 20 · Multiple Choice
An invented energy diagram shows reactants at 75 units and products at 20 units. How much energy does the system release overall?
Answer: C
The system drops from 75 to 20 units, so it releases 75 - 20 = 55 units. Choice D adds the two levels instead of finding the difference. Choices A and B read one level only.
Question 9 of 20 · Multiple Choice
A reaction breaks 4 bonds and forms 4 bonds. What can you conclude about its energy change from this information alone?
Answer: D
Bonds differ in strength, so equal numbers of bonds broken and formed can still give a release or an absorption of energy. Choice C assumes every bond has the same energy, which is not true.
Question 10 of 20 · Multiple Choice
Photosynthesis, 6CO2 + 6H2O → C6H12O6 + 6O2, needs a constant supply of light energy. Which energy diagram fits it?
Answer: A
The reaction runs only while energy is supplied, so the system absorbs energy and the products end higher. Choice B describes an exothermic reaction such as respiration. Choice D is false: light carries energy.
Question 11 of 20 · Multiple Choice
A reaction in an insulated flask releases 12 kJ. Measurements show the solution gains 10.5 kJ. Where is the rest of the energy?
Answer: B
By conservation, 12 - 10.5 = 1.5 kJ went to the other parts of the surroundings (the flask and the air). Choice A breaks conservation of energy. Choice C adds instead of subtracting.
Question 12 of 20 · Multiple Choice
Which model best shows that energy is conserved in an exothermic reaction?
Answer: D
A conservation model must account for every bit of energy: what the system loses, the surroundings gain. Choices A and C show matter but not energy. Choice B shows the system's energy but not where the energy goes.
Question 13 of 20 · Multiple Choice
Baking soda breaks down in a hot oven: 2NaHCO3 → Na2CO3 + H2O + CO2. The reaction stops when the oven is turned off. What is the best explanation?
Answer: A
A reaction that runs only while energy is supplied is absorbing energy overall. Choice B would let the reaction keep going on its own. Choice C breaks conservation of energy, and choice D is false: atoms are always conserved (2 Na, 2 H, 2 C and 6 O on each side).
Question 14 of 20 · Multiple Choice
A match has to be struck before it burns, even though burning releases energy. Which explanation fits the bond model?
Answer: C
The start of a reaction needs energy to begin breaking bonds (the hump on an energy diagram). Once product bonds form, they release more energy than was absorbed, so the reaction keeps going. Choice A contradicts the energy released, and choice B breaks conservation of energy.
Question 15 of 20 · Short Answer
Hydrogen reacts with bromine: H2 + Br2 → 2HBr, and the reaction releases energy. List the bonds broken and formed, and use them to explain the energy release.
Bonds broken: 1 H-H and 1 Br-Br (energy absorbed). Bonds formed: 2 H-Br (energy released). Because the reaction releases energy, forming the two H-Br bonds must release more energy than breaking the H-H and Br-Br bonds absorbs; the extra energy goes to the surroundings. Atoms: 2 H and 2 Br on each side.
Question 16 of 20 · Short Answer
A reaction runs in 100 mL of water, and the water temperature goes from 24.0 °C to 17.5 °C. Classify the reaction, explain it with bonds, and describe its energy diagram.
The temperature change is -6.5 °C, so the surroundings lost energy to the system: the reaction is endothermic. Breaking the reactant bonds absorbs more energy than forming the product bonds releases. On the energy diagram, the products sit above the reactants.
Question 17 of 20 · Short Answer
A student writes: "In an endothermic reaction, the energy is used up." Use conservation of energy to correct the statement.
Energy is not used up. In an endothermic reaction, energy moves from the surroundings into the reaction system, where it ends up in the new arrangement of bonds (the products have more energy than the reactants). The surroundings lose exactly as much energy as the system gains, so the total stays the same.
Question 18 of 20 · Short Answer
An energy diagram shows the reactants of a reaction at 50 units and the products at 10 units. How much energy would the reverse reaction absorb, and why?
The forward reaction releases 50 - 10 = 40 units. The reverse reaction starts at 10 and ends at 50, so it absorbs 40 units. It breaks the bonds the forward reaction formed and forms the bonds it broke, so the energy account is the same size with the opposite sign.
Question 19 of 20 · Short Answer
Plan how you would use a thermometer to decide whether a reaction between two classroom-safe solutions is exothermic or endothermic. Name one control and one safety step.
Put a measured volume of one solution in an insulated foam cup, record its starting temperature, add the second solution (also at room temperature), stir and record the highest or lowest temperature. A rise means the system released energy (exothermic); a fall means it absorbed energy (endothermic). Control: start both solutions at the same temperature, or run a trial mixing the solution with plain water to show the change comes from the reaction. Safety: wear goggles and do not taste anything.
Question 20 of 20 · Short Answer
Reaction 1 has products 60 units below its reactants. Reaction 2 has products 25 units below its reactants. The same amounts react in identical cups of water. Which warms its water more, and for which is the gap between energy released forming bonds and energy absorbed breaking bonds larger?
Reaction 1 for both. Its system drops 60 units, compared with 25 units for Reaction 2, so it gives more energy to the water and warms it more. The drop is the extra energy released by forming bonds beyond what breaking bonds absorbed, so the gap is larger for Reaction 1 (60 units against 25).
0 of 20 answered · 0 correct
06
Frequently Asked Questions
10 Questions
What does HS-PS1-4 mean?
HS-PS1-4 asks students to develop a model showing that a reaction releases or absorbs energy because of the change in total bond energy. Students show that breaking bonds absorbs energy, forming bonds releases energy, and the balance of the two decides the energy change. The code means High School, Physical Science, core idea 1 (Matter and Its Interactions), performance expectation 4.
Is HS-PS1-4 taught in chemistry or physical science?
It is usually taught in a high school Chemistry course, often in a unit on reactions or thermochemistry. It also fits Physical Science and the chemistry unit of an integrated science course. It links to biology through photosynthesis and cellular respiration.
What is the assessment boundary for HS-PS1-4?
Assessment does not include calculating total bond energy changes from the bond energies of reactants and products. Students can count bonds, compare which side has stronger bonds, read energy diagrams and analyze temperature data. Adding up tables of bond energies to get a numerical ΔH is beyond this standard, although some chemistry courses teach it later.
Does breaking a chemical bond release energy?
No. Breaking a bond always absorbs energy, because work must be done to pull the atoms apart. Energy is released when new bonds form. The phrase "energy stored in bonds" leads many students to think the opposite, so it helps to talk about the energy of the whole system instead.
What is the difference between exothermic and endothermic reactions?
An exothermic reaction releases energy to its surroundings, so the surroundings get warmer and the products sit below the reactants on an energy diagram. An endothermic reaction absorbs energy from its surroundings, so they cool, or energy must be supplied; the products sit above the reactants. In bond terms, the difference is whether forming bonds releases more or less energy than breaking bonds absorbs.
What kinds of models can students use for HS-PS1-4?
The Clarification Statement lists molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved. In class this means ball-and-stick drawings or model kits with a tally of bonds, energy diagrams, and energy-flow drawings with arrows between the reaction system and its surroundings.
How do you show that energy is conserved in a chemical reaction?
Draw the reaction as a system with surroundings, and account for every transfer: the energy the system loses is gained by the water, container and air, or the energy the system gains comes from them or from a source like light or electricity. A good check is that the arrows into and out of the system balance.
What safe labs show energy changes in chemical reactions?
Classroom-safe options include 3% hydrogen peroxide with yeast (warms), vinegar with baking soda (cools), citric acid solution with baking soda (cools) and vinegar-soaked steel wool in a jar (warms slowly). Students wear goggles, work in trays and never taste chemicals. Reactions that need open flames or stronger chemicals are better shown by the teacher or by video.
How is HS-PS1-4 different from HS-PS1-5?
HS-PS1-4 is about how much energy a reaction releases or absorbs overall, explained with bonds. HS-PS1-5 is about how fast a reaction goes, explained with the number and energy of collisions between particles. The hump on an energy diagram connects the two: it is the energy needed to start breaking bonds.
What comes after HS-PS1-4?
HS-PS1-5 uses collisions to explain reaction rates, and HS-PS1-6 asks students to change conditions to get more product at equilibrium, where the energy change of the reaction matters when temperature changes. In biology, HS-LS1-7 applies the same bond model to cellular respiration.
07
Related Standards
6 standards
These standards connect to HS-PS1-4: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
MS-PS1-5Prerequisite
Model how atoms are conserved in a chemical reaction, so mass is conserved
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