Kinetics and Thermodynamics The focus of this unit is threefold: – Heat energy and chemical reactions – Enthalpy and chemical reactions – Gibb’s free energy: Entropy vs. Enthalpy Heat Energy and Chemical Reactions Energy is involved in every chemical reaction. Often energy is supplied from an external source to ‘encourage’ the reaction. Ex. Coal is used for heating but energy must be supplied to encourage the coal to release its energy. A question to consider is “Why do some chemical reactions occur but other do not?”

Let’s answer the preceding question Please recognize that energy changes are associated with chemical reactions. There is a relationship between enthalpy and bond energies and stability. –ENTHALPY – is the amount of heat energy either used or released by a system at a constant pressure. Remember there is a difference between phase changes and chemical reactions. Phase changes do not result in a new compound being produced but chemical reactions do make new products.

You should be able to explain the difference between heat and temperature. You should also be able to identify those reactions that produce ‘useful’ heat. Different forms of energy in chemical reactions Kinetic energy is the energy of motion and in chemistry we have to consider 3 different types of energy of motion: 1)Energy of translation – refers to the motion of a molecule through space. 2)Energy of rotation – refers to the end-over-end or rotating motion of a molecule through space. 3)Energy of vibration – refers to the vibration of the atoms within a molecule.

Other forms of energy Chemical energy – this is the potential energy stored within the bonds of compounds. This potential energy is related to the attractive forces acting between molecules and within atoms within the molecule. The attractive forces between molecules is the smallest in a gaseous state, increases when the molecules are in a liquid state, and is the greatest between molecules in a solid. Heat energy – aka thermal energy

We will examine thermal energy in a fair amount of detail as it plays a major role in reactions, and is often the end product of other forms of energy – like nuclear energy. HEAT IS THE PRODUCT OF MOTION – as long as a molecule is in motion it contains thermal energy. The exception to this is when a molecule is frozen at absolute zero. Absolute zero (0 K or -273°C) is when all molecular motion ceases. Heat will always flow from an area of high heat to an area of low heat. An example of this is the flow of heat from inside your house to the cold outside during the winter. There is no way to prevent this but insulation will slow the movement.

Difference between heat and temperature Heat is a measure of the intensity of thermal energy, Temperature is a measure of average kinetic energy of its particles. A thermometer is used to measure the temperature of an object. The most common temperature scale is the Celsius scale. The Celsius scale uses the boiling point and the freezing point of water as reference points. The Kelvin scale (K) uses absolute zero (0 K) as the reference point. Heat is measured in joules (SI unit) and, if you live in the US, calories.

Specific Heat Specific Heat – the number of joules required to raise the temperature of 1 kilogram of a substance 1 degree Kelvin (or 1 degree Celsius) is called the Specific Heat capacity for that substance. The specific heat capacity will depend on the physical state of the substance (vapor, liquid, or solid) and on the substance itself. For example, the specific heat capacity for water vapor is different from liquid water, and from ice.

How to calculate the specific heat capacity The units for specific heat capacity are: Joules per kilogram per degree Celsius J/kg. °C The quantity of heat a substance can hold is determined by the formula: Q = mcΔT Q = quantity of heat in J m = mass of the substance in kg c = specific heat capacity for the substance ΔT = change in temperature in degrees C

The specific heat of a substance will indicate its heat storing ability. A substance with a high specific heat will be able to store more energy per unit of mass than a substance with a low specific heat. Example 1 – Determine the amount of heat required when 3.4 kg of water is heated from 8.00°C to 100°C. Specific heat for water is 4184 J/kg-°C m = 3.4 kg c = 4184 J/kg-°C ΔT = 92°C (ΔT = T f – T i ) Q = (3.4 kg)(4184 J/kg-°C)(92°C) = J or 1310 kJ

Example 2 – Determine the amount of heat given off by g of coffee as it cools from 80.0°C to 27.0 °C. Q = mcΔT Q = ? m = 0.5 kg c = 4184 J/kg-°C ΔT = - 53°C (remember the T final – T initial ) Q = (0.5 kg)(4184 J/kg-°C)(-53°C) = J or -111 kJ

A few heat questions Determine the amount of heat required to raise the temperature of 4.5 kg of Copper from 0°C to 240°C. The specific heat for Copper is 386 J/kg °C Determine the amount of heat lost when a 30 kg block of Aluminum (900 J/kg °C) is cooled from an initial temperature of 890 °C down to 25 °C.

Determine the final temperature if 50 kg of water absorbs 4345 kJ of heat. The initial temperature of the water is 3 °C Determine the final temperature of a 2 kg block of Copper absorbs 4000 kJ of heat. The initial temperature is 20°C. Determine the mass of water that was heated from 3°C to 98°C and absorbed 5600 KJ of heat. The specific heat for water is 4184 J/kg °C