9. Energy Changes in Chemical Reactions

Because a weight falling the height of a room warms a kilogram of water by only about a tenth of a degree. He repeated the fall many times and read thermometers to about a two-hundredth of a degree Fahrenheit. Even the warmth of the room or of his own hand could spoil a result, so he took great care over it.

One calorie warms one gram of water by one kelvin, which is 4.184 J. A food "Calorie" (capital C) is 1000 of those, a kilocalorie, which is what a kilogram of water would need to warm by one kelvin. The joule (J) is the SI unit and the one used in chemistry.

The sign is from the point of view of the system you are watching. Heat flowing into it is positive and heat flowing out is negative. A reaction that warms the water around it is losing energy, so its q is negative (exothermic). One that cools the water around it, like a cold pack, takes energy in (endothermic).

Stirring brings the whole of the water to one temperature, so the thermometer reads the temperature of everything. Foam is a poor conductor, so little heat leaks to the room; the experiment treats the cup as a perfect wall. In a real measurement the cup itself absorbs a little heat and one calibrates for it (the calorimeter constant).

At 4.18 J/(g K) its specific heat is among the highest of common substances, because hydrogen bonds absorb energy as well as the motion of the molecules. Coastal weather is mild for the same reason: a large body of water takes a long time to warm or cool.

For most solid metals at room temperature, yes, to within about ten per cent. It fails for light, hard elements such as diamond, beryllium and boron, whose atoms are bound so stiffly that they cannot soak up heat that easily at 25 °C. Quantum theory explained the failure only after 1900.

Because the energy of a substance is a fixed property of its state (what it is, how much, how hot, at what pressure), not of its history. Two paths between the same start and the same finish change the energy by the same amount. A quantity like this is called a state function. Heat and work separately are not: how much of each you get depends on the path, but their sum does not.

If making CO₂ from carbon and oxygen releases 393.5 kJ, then splitting CO₂ back into carbon and oxygen must take back exactly 393.5 kJ, or one could cycle round and create energy from nothing.

Yes. About 850 kJ is released for every two moles of iron produced, and nothing carries the heat away from the small amount of material, so the iron comes out molten, above 2000 °C. It has been used to weld railway track since the 1890s.

Condensing water vapour to liquid releases about 44 kJ per mole (−285.8 compared with −241.8). That is why the heat of burning a fuel depends on whether the water ends up as vapour or as liquid. Tables that give the lower value assume the water leaves as steam.

It is defined that way: the standard state of an element is its most stable form at 1 bar and 298 K, and the formation of an element from itself involves no change at all. Zero is a zero of the scale, like sea level. Only differences matter, so any consistent zero will do.

No. Limestone breaking down takes in 178 kJ per mole, yet a lime kiln does it every day. Heat alone does not decide which way a reaction goes; the freedom of the particles to spread out matters as well. That is for a later course. For now, the enthalpy tells you how much heat is given out or taken in, not whether it will happen.