Energy & Heat

Energy – ability to produce heat

Heat - energy in the process of flowing from a warmer object to a cooler object. In chemical reactions this flow can be in either direction depending upon the type of reaction.

Remember that a reaction is called a system. Anything outside the system is called the surroundings.

Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings. In other words, heat is produced (and released).

Endothermic process is any process in which heat has to be supplied to the system from the surroundings. In other words, heat is absorbed (or required).

2H2 (g) + O2 (g) 2H2O (l) + energy

H2O (g) H2O (l) + energy

energy + 2HgO (s) 2Hg (l) + O2 (g)

energy + H2O (s) H2O (l)

EnthalpyJust as we could calculate how much heat could be produced, we can also see how much heat is within the system (or reaction). Enthalpy is the heat content of a system at constant pressure. (aka “q” at a constant pressure is H) We do this by measuring the enthalpy (heat) of reaction (H reaction)

H reaction = H products - H reactants

If H is negative, the reaction is exothermicH reactants > H products

Reactants are at a higher energy level

If H positive, the reaction is endothermicH products > H reactants

Products are at a higher energy level

Entropy• Entropy (S) is a measure of the disorder or

randomness of the particles that make up a system.• The Law of Disorder state that spontaneous processes

always proceed in such a way that entropy of the universe increases.

• Change in the amount of disorder is determined much like the change in enthalpy.

• Ssystem = Sproducts - S reactants

• If entropy increases, then S is positive• If entropy decreases, then S is negative

Five Rules of Predicting Entropy 1. Entropy changes associated with changes in state can be predicted.

Entropy increases as energy increases (i.e. solid to liquid) 2. The dissolving of a gas in a solvent always results in a decrease in

entropy. 3. Assuming no change in physical state, the entropy of a system

usually increases when the # of gaseous product particles is greater than # of gaseous reactant particles.

4. With some exceptions, you can predict the change in entropy when a solid or a liquid dissolves to form a solution. Usually it increases.

5. An increase in the temperature of a substance is always accompanied by an increase in entropy.

The specific heat (Cp) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius.

Heat (q) absorbed or released:

q = mCpt

t = tfinal - tinitial

Specific Heat

Reaction Rates

• Activation Energy "Ea"-The Energy required to initiate a chemical reaction. Both endothermic and exothermic reactions require activation energy.

• Activated complex - an unstable combination of reacting molecules that is intermediate between reactants and products.

• Potential energy – stored energy

Practice

1. Does the graph represent an endothermic or exothermic reaction1. Determine the heat of reaction, ∆H, for this reaction. 2. Determine the activation energy, Ea, for this reaction. 3. What is the energy of the activated complex for this

reaction? 4. Determine the reverse activation energy, Ea for this

reaction.

Chemical Kinetics

• The study of reaction rates is called chemical kinetics.

• This concept has to do with the number of atom/molecule collisions and whether or not they are effective.

Factors Affecting Reaction Rates: Concentration

As the concentration of the reactants increases, the reaction rate increases. WHY?

According to the collision theory, the rate of a reaction is directly proportional to the number of effective collisions per second between the reactant molecules.

Effective Collisions - the fraction of total collisions that actually result in the formation of the product(s).

If the concentration of the reactants increases (i.e. particles per given volume) the greater the number of total collisions. The greater the frequency of total collisions, the greater the frequency of effective collisions. If the frequency of effective collisions increases, so does the reaction rate.

Factors Affecting Reaction Rates: Surface Area

As the surface area of the reactants increases, the reaction rate increases. WHY?

– Increasing the surface area of the reactants results in a higher number of reaction sites.

Reaction sites - specific sites on molecules at which reactions occur.

– Increasing the number of reaction sites increases the number of total collisions. – The greater the frequency of total collisions, the greater the frequency of effective collisions. – If the frequency of effective collisions increases, so does the reaction rate.

As the temperature of a system increases, the reaction rate increases. WHY?

Temperature (T) - A measure of the average kinetic energy (KEavg) of the particles of a substance. Increasing T increases KEavg. At higher T, the fraction of molecules with energies greater than the activation energy (Ea) increases.

Factors Affecting Reaction Rates: Temperature

The presence of a catalyst increases the reaction rate by lowering the activation energy of a reaction.

Catalyst - A substance that increases the rate of a reaction but is not consumed in the reaction.

For equilibrium reactions, both the forward and reverse reaction rates are affected by the catalyst. i.e. the Ea for both directions is decreased.

Therefore, the equilibrium constant is not changed by the presence of a catalyst.

The relative concentrations of the reactants and products is not changed.

Factors Affecting Reaction Rates: Catalysts