Energy Changes and Energy Profile Diagrams for Chemical Reactions
Senior 4 · Chemistry
Energy Changes and Energy Profile Diagrams for Chemical Reactions
Lesson 1 of 4
25 min
Pour hot tea into an open cup and into a vacuum flask. An hour later the cup is cold and some of its water has evaporated; the flask is still hot. Squeeze an instant cold pack and it goes icy in your hand; strike a match and it flares hot. In every case energy is moving, and to keep track of it chemists draw an imaginary line round the part they are studying. Inside the line is the system; everything outside is the surroundings. This lesson sets out that language, which the rest of the unit depends on.
Definition
System: the part of the universe being studied, for example the chemicals reacting in a beaker. Surroundings: everything else, including the beaker, the air and the thermometer. Boundary: the real or imaginary surface between them. System + surroundings = the universe.
Open system: can exchange both energy and matter with its surroundings. Closed system: can exchange energy but not matter. Isolated system: exchanges neither energy nor matter.
State function: a property that depends only on the present state of the system (its temperature, pressure, amount and composition), not on how it got there. Examples: internal energy U, enthalpy H, temperature, volume.
Figure 17.1: Open, closed and isolated systems. A real vacuum flask is only nearly isolated: it slows heat transfer greatly, but cannot stop it completely.
Energy can enter or leave a system in only two ways:
Chemists look at energy from the point of view of the system:
| Exothermic reaction | Endothermic reaction | |
|---|---|---|
| Energy flow | from system to surroundings | from surroundings to system |
| Temperature of surroundings | rises (feels hot) | falls (feels cold) |
| Sign of ΔH | negative | positive |
| Examples | combustion of methane in a biogas stove; respiration; neutralisation; rusting | photosynthesis; thermal decomposition of limestone; dissolving ammonium nitrate (cold packs) |
Imagine walking from the bottom of a hill to the top. The height you gain is fixed by where you start and finish: it is like a state function. The distance you walk, and how tired you get, depend on the path you take. In chemistry, the change in internal energy (ΔU) or enthalpy (ΔH) between reactants and products is fixed, whatever route the reaction takes; but the separate amounts of heat and work can depend on the route. This idea becomes Hess's law in Senior 5.
Worked example 1: naming the system
Classify each system as open, closed or (nearly) isolated, and say whether the process is exothermic or endothermic for the system: (a) magnesium ribbon reacting with dilute hydrochloric acid in an open test tube; (b) a sealed glass ampoule of nitrogen dioxide being warmed in a water bath; (c) hot soup in a closed vacuum flask.
Solution. (a) Open: hydrogen gas (matter) escapes and heat flows out; the tube gets warm, so the reaction is exothermic. (b) Closed: energy enters as heat but no matter can escape; the system gains energy (endothermic for the system). (c) Nearly isolated: very little energy or matter crosses the boundary, so the soup stays hot.
Worked example 2: following the energy
A reaction in a beaker warms 100 g of water (the surroundings) by 5.0 °C. The specific heat capacity of water is 4.18 J g⁻¹ °C⁻¹. How much energy did the system lose, and is the reaction exothermic or endothermic?
Solution. Energy gained by the water: Energy is conserved, so the system lost the same amount: . The surroundings warmed up, so the reaction is exothermic. (Lesson 3 shows how to turn this into ΔH per mole.)
Warning
Common mistakes. (1) Saying an endothermic reaction "gives out cold": it takes in heat from its surroundings, which is why they cool. (2) Mixing up points of view: when the beaker feels hot, the system has lost energy and ΔH is negative. (3) Calling a stoppered flask "isolated": heat still passes through the glass, so it is closed. (4) Treating heat and temperature as the same thing: heat is energy transferred; temperature measures how hot something is.
Safety. Exothermic reactions can boil and spit: add reactants in small amounts and wear goggles. Never seal a reaction that produces gas in a closed container: the pressure can burst it. Do not cut open a cold pack; its contents can be harmful if swallowed.
Tip
Stand inside the system. Energy coming in is positive (endothermic, surroundings cool); energy going out is negative (exothermic, surroundings warm). Open: energy and matter cross; closed: energy only; isolated: nothing.
The Organic Chemistry Tutor: Open, Closed and Isolated Systems (3 min). Watch for an everyday example of each kind of system and what can cross each boundary.
Practice
Key points
Sources and credits
Quick check
A stoppered flask of hot solution cools on the bench. What kind of system is the solution?
