Thermodynamics Lecture 1

A calorimeter is a device used to measure the heat energy given off during the course of a reaction Heat and temperature are not the same thing Temperature: the measure of the average kinetic energy of a sample Heat: the sum of the total kinetic energies of the particles in a sample A calorimeter surrounds a reaction with a known quantity of water. By measuring the change in temperature of the water, we can determine how much heat energy was given off or taken in during the course of a reaction

A reaction that gives off energy is said to be exothermic If the reaction gives off heat to the surroundings, it loses heat itself Therefore, an exothermic reaction results in products with less energy than the reactants A reaction that takes in energy from the surroundings is said to be endothermic This means the reaction gains heat Therefore, an endothermic reaction results in products that are higher in energy than the reactants

Specific heat is the amount of heat energy required to raise one gram of a substance by one degree Celsius (or Kelvin) cp= q m x DT cp is specific heat and the p means at constant pressure q is heat lost or gained m is mass in grams DT is the change in temperature in Celsius or Kelvin

Open your books to page 534 and we will work practice problem 2 If 980 kJ of energy are added to 6.2L of water at 291K, what will the final temperature of the water be? Question 2: If you had two beakers of water both at the same temperature, but one beaker has 100 cm3 of water and the other beaker has 50 cm3 of water. How much heat is present in the beaker with 100 cm3 of water?

Hess’s Law The overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process. General Rules: If a reaction is reversed, the sign of H is also reversed. Multiply the coefficients of the known equations so that when added together they give the desired thermochemical equation

Hess’s Law ΔH˚f = Σ[(ΔH˚f of products)(mol of products)] - Σ[(ΔH˚f of reactants)(mol of reactants)]

Enthalpy Diagrams The reactants have a certain amount of energy, as do the products All chemical reactions involve… 1) Breaking of Bonds 2) Making of new Bonds We represent the change in enthalpy by using diagrams… If the bonds in the products are stronger than the bonds in the reactants then the reaction is exothermic, as heat is given out to the surroundings. In endothermic reactions heat is absorbed from the surroundings because the bonds in the reactants are stronger than the bonds in the products. Is this an exothermic or endothermic reaction? EXOTHERMIC

Enthalpy Diagrams Cont. Enthalpy Diagrams also are used to predict the relative stability of the reactants and products H = Hproducts - Hreactants The natural tendency is for a reaction to proceed in a direction that leads to a lower state of energy The lower the enthalpy = the greater the stability, so…. Which results in more stable products, an endothermic or exothermic reaction? EXOTHERMIC

The measure of the degree of randomness of the particles in a system. Entropy The measure of the degree of randomness of the particles in a system. Naturally, the universe is always proceeding to a state of higher entropy, towards an increase in disorder. The more disordered a system becomes the more positive the value of S becomes Melting Ice Formation of a solution: degree of randomness increase, so entropy becomes more positive What type of reaction is it? H = ? S = ?

Entropy cont. S = ? 2 moles of gases vs. 1 mol of solid Look at this equation: NH3 (g) + HCl (g)  NH4Cl (s) Is this going to lead to more or less disorder? LESS NEGATIVE so… S = ? How about this… C2H4 (g) + H2 (g)  C2H6 (g) S = ? NEGATIVE

things that increase entropy a reaction that results in an increased number of moles a reaction that results in products with more disordered phases ie-solidliquidgas a solid dissolving in a liquid ie- solidaqueous

Free Energy Go = Ho - TSo So far we have talked about two factors: Enthalpy & Entropy Naturally, Enthalpy: tries to be more negative Entropy: tries to be more positive If these two factors oppose one another, the dominant factor determines the direction of change. Free Energy, G, assesses both enthalpy and entropy changes. Go = Ho - TSo Naturally, processes proceed in the direction that lowers the free energy of a system

Spontaneity (Spontaneous Rxns) A reaction is said to be spontaneous if it causes a system to move from a less stable to a more stable state. The only way to predict spontaneity is to take in to account both enthalpy and entropy  Free energy FOR A REACTION TO BE SPONTANEOUS, G MUST BE NEGATIVE

Putting it all together S G -value Exothermic + value disordering Always Negative exothermic - value Ordering (at low temp.) endothermic Disordering (at high temp) ordering Never