How are diamonds made?
Natural diamonds are mined from sites around the world. However, the price of natural diamonds is carefully controlled, so other sources for diamonds are being explored. Several different methods for producing synthetic diamonds are available, usually involving treating carbon at very high temperatures and pressures. The diamonds produced are now of high quality, but are primarily used in industrial applications. Diamonds are one of the hardest materials available and are widely used for cutting and grinding tools.
Calculating Heat of Reaction from Heat of Formation
An application of Hess’s law allows us to use standard heats of formation to indirectly calculate the heat of reaction for any reaction that occurs at standard conditions. An enthalpy change that occurs specifically under standard conditions is called the standard enthalpy (or heat) of reaction and is given the symbol
The symbol Σ is the Greek letter sigma and means “the sum of”. The standard heat of reaction is equal to the sum of all the standard heats of formation of the products minus the sum of all the standard heats of formation of the reactants. The symbol “
|C (s, graphite)||0||NO(g)||90.4|
|C (s, diamond)||1.90||NO2(g)||33.85|
Sample Problem: Calculating Standard Heat of Reaction
Calculate the standard heat of reaction
Step 1: List the known quantities and plan the problem.
ΔHf∘ for NO(g)=90.4 kJ/mol ΔHf∘ for O2(g)=0 (element) ΔHf∘ for NO2(g)=33.85 kJ/mol
First write the balanced equation for the reaction. Then apply the equation to calculate the standard heat of reaction for the standard heats of formation.
Step 2: Solve.
The balanced equation is:
Applying the equation form the text:
The standard heat of reaction is -113 kJ.
Step 3: Think about your result.
The reaction is exothermic, which makes sense because it is a combustion reaction and combustion reactions always release heat.
- Standard heats of reaction can be calculated from standard heats of formation.
- Is a Hess’s Law calculation a direct determination of a standard heat of reaction?
- What is the purpose of the
nin the equation?
- What does