D. Energy Changes and Rates of Reaction · Energy Changes and Rates of ReactionEnergy Changes in Physical, Chemical and Nuclear Processes
D3.1 D2.1 A1.12About 65 minutes
Learning goals- I can describe a change in terms of a system and its surroundings, and classify systems as open, closed or isolated.
- I can classify physical, chemical and nuclear changes as exothermic or endothermic.
- I can compare the sizes of the energy changes in physical, chemical and nuclear changes.
- I can explain why mass is treated as conserved in chemical reactions but not in nuclear reactions.
Words to know
system
The part of the universe being studied, usually the reacting substances.
surroundings
Everything outside the system, including the water, container, air and thermometer.
open, closed and isolated systems
Open: matter and energy can cross the boundary. Closed: only energy can. Isolated: neither can (an ideal).
exothermic
Describes a change in which the system releases energy to the surroundings, so the surroundings warm up.
endothermic
Describes a change in which the system absorbs energy from the surroundings, so the surroundings cool down.
nuclear change
A change in the nucleus of an atom, such as fission (splitting a heavy nucleus) or fusion (joining light nuclei).
A cold morning, three kinds of energy
It is a January morning in Sudbury and −25 °C outside. Inside, a natural gas furnace burns methane to keep the house warm. On the stove, a kettle is boiling water for tea. And the lights are on, partly thanks to Ontario's nuclear stations, where uranium nuclei split apart.
These are three very different changes: a physical change (boiling water), a chemical change (burning methane) and a nuclear change (fission). All three involve energy moving in or out. In this unit you will learn to measure, calculate and predict those energy changes, and then study how fast reactions happen.
Remember from earlier grades: in Grade 10 science (SNC2D) and in SCH3U (Chemical Reactions) you learned that exothermic reactions give off heat and endothermic reactions take it in. In Grade 11 Physics (SPH3U, Thermal and Nuclear Energy) you used Q = mcΔT and compared fission and fusion. Now we make all of this precise and quantitative.
System and surroundings
Chemists split the universe into two parts:
- The system is the part we are studying: usually the reacting chemicals (or the substance changing state).
- The surroundings are everything else: the water the chemicals are dissolved in, the beaker, the air, your hand.
Energy that leaves the system goes into the surroundings, and energy that enters the system comes from the surroundings. The total is conserved (the law of conservation of energy). There are three kinds of system:
- Open: both matter and energy can cross the boundary (an open beaker, a campfire).
- Closed: energy can cross but matter cannot (a sealed flask, a sealed instant cold pack).
- Isolated: neither matter nor energy can cross. A perfect isolated system does not exist, but a well-insulated calorimeter comes close.
Exothermic and endothermic, from the system's point of view
- In an exothermic change, the system releases energy to the surroundings. The surroundings get warmer. Examples: burning, freezing water, condensing steam, neutralization, fission.
- In an endothermic change, the system absorbs energy from the surroundings. The surroundings get cooler. Examples: melting ice, boiling water, dissolving ammonium nitrate in an instant cold pack, photosynthesis, decomposing limestone.
A thermometer is always in the surroundings. If the thermometer reading rises, the system gave energy away: exothermic. This is the single most common source of confusion in this unit, so say it out loud: the thermometer measures the surroundings.
Exo = exit
In an exothermic change, energy exits the system. In an endothermic change, energy goes in (endo means "within"). The system and the surroundings always change in opposite directions.
Physical, chemical and nuclear changes
Physical changes (changes of state, dissolving) involve the forces between particles. Melting and boiling are endothermic because energy is needed to pull particles apart against their attractions; freezing and condensing are exothermic. These energies are small: about 6.01 kJ to melt one mole (18 g) of ice and about 40.7 kJ to boil one mole of water at 100 °C.
Chemical changes break and form chemical bonds within and between atoms. The energy changes are larger, typically tens to thousands of kilojoules per mole. Burning one mole of methane (16 g) releases about 890 kJ. Bleaching a stain is a chemical change too: the bleach breaks bonds in the coloured molecules, changing them into colourless ones.
Nuclear changes change the nucleus itself, so they involve the strong nuclear force. The fission of one mole of uranium-235 (235 g) releases about 1.9 × 10¹⁰ kJ, around twenty million times more than burning a mole of methane. Fusing deuterium and tritium (about 5 g for one mole of reactions) releases about 1.7 × 10⁹ kJ. Gram for gram, fusion releases even more energy than fission.
In nuclear changes the energy comes from a measurable loss of mass, given by E = mc². In chemical changes the mass change is far too small to detect, so we treat mass as conserved.
| Change | Type | Energy change (per mole) | Exo or endo? |
|---|
| H₂O(s) → H₂O(l) (melting ice) | physical | +6.01 kJ | endothermic |
| H₂O(l) → H₂O(g) at 100 °C (boiling) | physical | +40.7 kJ | endothermic |
| H₂O(g) → H₂O(l) (condensing) | physical | −40.7 kJ | exothermic |
| CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) | chemical | −890 kJ | exothermic |
| 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(s) + 6O₂(g) (photosynthesis) | chemical | +2803 kJ | endothermic |
| fission of U-235 | nuclear | about −1.9 × 10¹⁰ kJ | exothermic |
Worked exampleExample 1: Who gains, who loses?
An instant cold pack contains water and solid ammonium nitrate in separate pouches. You squeeze it, the inner pouch breaks, the solid dissolves and the pack becomes cold. Identify the system and the surroundings, and classify the change.
- The system is the ammonium nitrate dissolving in water: NH₄NO₃(s) → NH₄⁺(aq) + NO₃⁻(aq).
- The surroundings include the water in the pack (which is where your hand feels the temperature), the plastic and your hand.
- The pack feels cold, so the surroundings lost energy. That energy went into the system.
- Dissolving is a physical change (no new substance is formed; evaporating the water would return the solid), and the pack is a closed system (sealed, but heat can flow through the plastic).
The dissolving is endothermic: the system absorbs energy from its surroundings, so the surroundings (and your hand) get colder.
Worked exampleExample 2: Comparing energy per kilogram
Using the values in the table, compare the energy involved when 1.00 kg of each is changed: (a) water boiled at 100 °C, (b) methane burned, (c) uranium-235 undergoing fission. Use molar masses 18.02, 16.05 and 235.0 g/mol.
- (a) n = 1000 g ÷ 18.02 g/mol = 55.49 mol. Energy = 55.49 mol × 40.7 kJ/mol = 2.26 × 10³ kJ (absorbed).
- (b) n = 1000 g ÷ 16.05 g/mol = 62.31 mol. Energy = 62.31 mol × 890 kJ/mol = 5.55 × 10⁴ kJ (released).
- (c) n = 1000 g ÷ 235.0 g/mol = 4.255 mol. Energy = 4.255 mol × 1.9 × 10¹⁰ kJ/mol = 8.1 × 10¹⁰ kJ (released).
- Ratio (c) ÷ (b) = 8.1 × 10¹⁰ ÷ 5.55 × 10⁴ ≈ 1.5 × 10⁶.
Burning methane releases about 25 times more energy per kilogram than boiling water absorbs, and fission releases about 1.5 million times more than burning methane. That is why a nuclear station's fuel bundles last so long.
Worked exampleExample 3: Where does nuclear energy come from?
Use E = mc² (c = 3.00 × 10⁸ m/s) to find the mass that disappears when (a) one mole of U-235 undergoes fission (1.9 × 10¹⁰ kJ, which is 1.9 × 10¹³ J) and (b) one mole of methane burns (890 kJ = 8.90 × 10⁵ J).
- Rearrange: m = E ÷ c², and c² = (3.00 × 10⁸)² = 9.00 × 10¹⁶ m²/s².
- (a) m = 1.9 × 10¹³ J ÷ 9.00 × 10¹⁶ m²/s² = 2.1 × 10⁻⁴ kg = 0.21 g.
- (b) m = 8.90 × 10⁵ J ÷ 9.00 × 10¹⁶ m²/s² = 9.9 × 10⁻¹² kg, which is about 1 × 10⁻⁸ g.
- A good lab balance reads to 0.001 g, so the mass change in (b) is about 100 000 times too small to detect.
Fission of a mole of U-235 converts about 0.2 g of mass into energy, a measurable amount. In a chemical reaction the mass change is undetectable, so the law of conservation of mass still works perfectly for chemistry.
Hands-on activityIce, salt and a thermometer
You need: a kitchen thermometer (a digital cooking thermometer is ideal), two glasses, crushed ice or small ice cubes, table salt, a spoon, a timer and a notebook.
- Fill both glasses with the same amount of crushed ice and a little cold water. Stir and record the temperature of each every minute for 3 minutes. It should settle near 0 °C.
- Add 3 tablespoons of salt to one glass and stir well. Keep recording both temperatures every minute for 8 more minutes.
- Hold each glass in your hand for a few seconds. Notice the frost that may form on the outside of the salty glass.
Questions: (a) Melting ice is endothermic. Where does the energy come from in each glass? (b) The salt makes the ice melt faster at a lower temperature, so the mixture must draw energy from its own water, the glass and the air. Explain why the temperature falls below 0 °C. (c) Is each glass an open, closed or isolated system?
What to look for: the plain ice water stays close to 0 °C; the salty mixture usually drops to somewhere around −5 °C to −15 °C (it depends on how much salt and ice you use). Melting absorbs energy, and the energy comes from the surroundings, which cool down. Both glasses are open systems. This is the same idea as an old-fashioned ice-cream maker.
Just for funA chemist is asked how her day was. "Exothermic," she says. "I let off a lot of heat and my surroundings got warmer." Her friend says, "Mine was endothermic. I absorbed everybody else's energy."
Your turn
1Define system and surroundings in a chemical reaction carried out in a beaker of water.
The system is the reacting chemicals; the surroundings are everything else, including the water they are dissolved in, the beaker and the air.
2Water vapour condenses on a cold window. Is this exothermic or endothermic, and is it physical, chemical or nuclear?
Exothermic and physical: the water releases energy to the window as its molecules come together into a liquid; no new substance forms.
3A thermometer in the solution rises from 20.4 °C to 31.2 °C during a reaction. Is the reaction exothermic or endothermic? Explain.
Exothermic. The thermometer is in the surroundings, which warmed up, so the system released energy.
4Classify each as open, closed or isolated: (a) a campfire, (b) a sealed can of pop warming in the sun, (c) an ideal sealed and perfectly insulated bottle.
(a) open (matter and energy leave); (b) closed (energy enters, matter cannot); (c) isolated (neither crosses, an ideal).
5Put in order of increasing energy change per mole: burning ethanol, freezing water, fusion of hydrogen nuclei.
freezing water < burning ethanol < fusion. Physical changes are smallest, nuclear changes are largest.
6Why is photosynthesis endothermic?
Plants absorb light energy to build glucose and oxygen from carbon dioxide and water; the products store more energy than the reactants.
7How much energy is absorbed when 90.1 g of ice melts? (6.01 kJ/mol, M = 18.02 g/mol)
n = 90.1 ÷ 18.02 = 5.00 mol; energy = 5.00 × 6.01 = 30.1 kJ absorbed.
More practice: warm-up
1What does exothermic mean?
The system releases energy to its surroundings.
2What does endothermic mean?
The system absorbs energy from its surroundings.
3Is melting ice exothermic or endothermic?
4Which type of change involves the nucleus of the atom?
A nuclear change (fission or fusion).
5In a closed system, which can cross the boundary: matter, energy or both?
More practice: on your own
1Hand warmers contain iron powder that reacts with oxygen to form iron(III) oxide. Classify the change (physical, chemical or nuclear; exo or endo) and explain how you know.
Chemical (a new substance, iron(III) oxide, forms) and exothermic (the warmer gets hot, so energy is released to the surroundings).
2How much energy is released when 4.00 kg of steam at 100 °C condenses to water at 100 °C? (40.7 kJ/mol)
n = 4000 ÷ 18.02 = 222.0 mol; energy = 222.0 × 40.7 = 9.03 × 10³ kJ released. That is why a steam burn is worse than a hot-water burn at the same temperature.
3A student says, "An endothermic reaction gives off cold." Correct this statement.
There is no such thing as "cold" flowing. An endothermic system absorbs heat from its surroundings, so the surroundings lose energy and their temperature drops.
4How much methane must be burned to release the same energy as boiling 1.00 kg of water (2.26 × 10³ kJ)?
n = 2.26 × 10³ kJ ÷ 890 kJ/mol = 2.54 mol; mass = 2.54 × 16.05 = 40.8 g of methane (ignoring heat loss).
5Bleaching a stain and dissolving sugar both make a colour or a solid "disappear". Which is chemical, and why?
Bleaching is chemical: bonds in the coloured molecules are broken and new colourless substances form. Dissolving sugar is physical: the sugar molecules are unchanged and can be recovered by evaporating the water.
Challenge
1The Sun converts about 4 × 10⁹ kg of mass into energy every second. Estimate the Sun's power output in watts. (c = 3.00 × 10⁸ m/s)
E = mc² = 4 × 10⁹ kg × 9.00 × 10¹⁶ m²/s² ≈ 4 × 10²⁶ J each second, that is about 4 × 10²⁶ W.
2Calculate the energy released per gram for (a) fission of U-235 (1.9 × 10¹⁰ kJ/mol, 235 g/mol) and (b) D–T fusion (1.7 × 10⁹ kJ per 5.03 g of fuel). Which releases more per gram, and by roughly what factor?
(a) 1.9 × 10¹⁰ ÷ 235 = 8.1 × 10⁷ kJ/g. (b) 1.7 × 10⁹ ÷ 5.03 = 3.4 × 10⁸ kJ/g. Fusion, by a factor of about 4.
3Explain, using the idea of attractions between particles, why boiling one mole of water takes much more energy (40.7 kJ) than melting one mole of ice (6.01 kJ).
Melting only loosens the particles enough to slide past each other; most hydrogen bonds remain in the liquid. Boiling must separate the molecules completely so they are far apart in the gas, overcoming almost all of the attractions. So far more energy is needed.
Quiz
1. A reaction in a beaker makes the beaker feel cold. Which statement is correct?
2. Which change involves the largest energy change per mole?
3. A sealed bottle of water warming in the sun is best described as
4. Which of these is an exothermic physical change?
5. Why do chemists treat mass as conserved in chemical reactions?
6. During a reaction, the thermometer in the solution rises. The thermometer is measuring the temperature of
Big ideas (D3.1, D2.1): every physical, chemical and nuclear change either releases energy (exothermic) or absorbs it (endothermic). Physical changes involve the smallest energies, chemical changes larger ones and nuclear changes vastly larger ones. Chemists separate the system (what reacts) from the surroundings (everything else, including the thermometer).
How to help: When you cook together, ask "What is the system and what is the surroundings?" (water boiling in a pot: the water is the system, the burner supplies energy from the surroundings). Ask your teen to explain why sweating cools you (evaporation is endothermic and draws energy from your skin).
Common difficulties:
- Thinking that a rising thermometer means the reaction gained heat. The thermometer is in the surroundings.
- Thinking cold "flows". Only energy flows, always from warmer to cooler.
- Mixing up physical and chemical changes (dissolving and changing state are physical).
- Scientific notation when comparing nuclear and chemical energies: encourage a calculator check of each power of ten.
Activity safety: salt-and-ice mixtures can get cold enough to cause frostbite-like injuries if held against skin for a long time. Touch the glass only briefly.