Physics Notes - Reaction Energies and Spontaneity PDF

# Reaction Energies and Spontaneity - We have learned how to describe chemical reactions (equations) and how to describe the conservation of mass (balanced). - We now move on to how to describe the conservation of energy. ## First Law of Thermodynamics - **The total energy of a given system must r...

# Reaction Energies and Spontaneity - We have learned how to describe chemical reactions (equations) and how to describe the conservation of mass (balanced). - We now move on to how to describe the conservation of energy. ## First Law of Thermodynamics - **The total energy of a given system must remain constant.** - How do we define energy in this class? ### Work and Heat - **The brick is dropped** - The brick has potential energy. - Work is done on the brick. - Energy is converted to work. - **The brick is pushed across the floor** - Work is done on the brick. - Friction creates heat. - Work is converted to heat. - Work and heat combine to describe the total energy of the system. - Change in energy = heat added to system ΔE = q **How Do We Define Energy in this Class?** - **Heat:** Measured in joules (J). - It can be given off (exothermic). - It can be consumed (endothermic). - **Work:** Measured in Joules (J). **How do work and heat relate to energy and each other within a system?** - **The brick is picked up** - Work is done on the brick. - The brick has potential energy. - Work is converted to energy. **How do we define a system?** - Three different types of systems differentiated by how they interact with their surroundings. ## Isolated System (Least Common) * It does not exchange work, heat, or matter with its surroundings. ## Closed System * Systems walls permit the transfer of heat and work but not matter. ## Open System * It exchanges work, heat, and matter. ## Let's Use Bases and Bos Laws to explore work and heat in different systems ### Constant Pressure (Closed) **Add Heat:** * Heat has been added. * No work done on surroundings. ### Constant Volume **Add Heat:** * Heat has been added. * No work done on surroundings. ### Constant Temperature **Add Heat:** * Heat is being converted to work. * Work is done on the surroundings. ## In a Chemical System - **DE=E-W** defined by PAV - **DE=DH-PAV** defined by - Heat has been added. - Most often we are at constant pressure (1 atmosphere). - The system has done work on the surroundings - **W=PΔV** - **DE=DH-PΔY** @ constant pressure - **ΔE=ΔH** @ constant volume ## Let's talk a little bit more about E **DE (change in internal energy)** - **State function:** It does not depend on how the system arrives at a present point, only the characteristics at the point. ## What determines how heat added to a system impacts it’s kinetic energy? **1. Specific Heat Capacity:** * Quantity of heat required to raise the temperature of a substance by one degree Kelvin (Celsius). * Includes mass or number of moles. **Cp=mass(g) x ΔT/Amount(m) x ΔT** * **Units Cp:** J/(grams or moles) (K/°C) **2. Heat Capacity** * The quantity of heat a body of matter absorbs or releases when ΔT=1°K * Do not include mass or moles. * C= required heat/ ΔT. * **Units Cp:** J/(K/°C) **Energy comes in different forms** * **Remember our brick** * **Potential Energy:** The energy an object has because of its position, composition, or condition. * **Kinetic energy:** The energy an object has because of its motion. * We can convert between these two types of energy, but all energy in a system is either potential or kinetic. **Do you think heat is potential or kinetic energy?** Kinetic!