Thermochemistry Chemistry One B

Energy and Heat Heat energy can either be needed to start a chemical reaction or it can be given off during a chemical reaction. Thermochemistry is the study of energy changes that occur during chemical reactions. Energy- the capacity for doing work or producing heat Temperature- Measurement of the average kinetic energy of the particles within a substance.

Energy and Heat Heat (q)-energy that transfers from one object to another because of temperature differences between them. Heat always flows from a warmer object to a cooler object. EX: Ice in hand

Potential and Chemical Energy Potential Energy- energy that can do work that is stored in chemical bonds Chemical Energy is available to do work when bonds are broken Work – The product of energy usage to make something happen

Endothermic and Exothermic Reactions Essentially all chemical reactions and change of state reaction involve either Absorption of heat (Endothermic) Release of heat (Exothermic)

Endothermic and Exothermic Reactions In studying heat changes, think of defining these two parts: The system- the part of the universe on which you focus your attention The surroundings- includes everything else in the universe Universe= system + surroundings A major goal of thermochemistry is to examine the flow of heat between the system and its surroundings.

Endothermic and Exothermic Reactions The Law of Conservation of Energy states that in any chemical or physical process, energy is neither created or destroyed. If the energy of the system decreases, the energy of the surroundings must increase by the same amount so that the total energy of the universe remains unchanged.

Endothermic Reaction Read and underline key ideas in the paragraph below. Sometimes a reaction or process requires heat to proceed and the systems will take heat from the surroundings. Since heat is being removed from the surroundings, the reaction vessel will feel cool. This means that the entalpy of the products is higher than the enthalpy of the reactants and heat is one of the reactants. This is called an endothermic reaction (endo=into). A common example of an endothermic process is the melting of ice. Solid water (ice) needs heat energy to help break apart the forces holding it together as a solid. The heat flowing into the ice will cool the surroundings.

Endothermic and Exothermic Reactions Heat flowing into a system from its surroundings: Defined as endothermic q (Δ H) has a positive value System gains heat as the surroundings cool down

Exothermic Reactions Read and underline key ideas in the paragraph below. When a system releases heat to the surrounding environment during a reaction the temperature of the surroundings increases and the reaction vessel, which is part of the surroundings will feel warm. For this type of reaction, the enthalpy of the products is lower than the enthalpy of the reactants and heat is one of the products. This is called an exothermic reaction (Exo=out of).

Endothermic and Exothermic Reactions Heat flowing out of a system into its surroundings: Defined as exothermic q (Δ H) has a negative value System loses heat as the surroundings heat up

Endothermic and Exothermic Reactions Every reaction has an energy change associated with it Exothermic reactions release energy, usually in the form of heat Endothermic reactions absorb energy, usually in the form of heat

Endothermic and Exothermic Reactions ΔH = Heat of Reaction (also known as Enthalpy) ΔH is measure in KJ or Kilo Joules Endothermic Rxns: Heat enters the reaction as a reactant ΔH (heat of reaction) is positive Exothermic Rxns: Heat exits the reaction as a product ΔH (heat if reaction) is negative At constant pressure heat (q)= ΔH

Endothermic and Exothermic Reactions NaCl + F2 + 108 KJ  NaF + Cl2 1. Is heat a reactant or product? 2. Is this endothermic or exothermic? 3. Would ΔH be + or -? 4.Re-write the equation:

Endothermic and Exothermic Reactions CH4 + O2  CO2 + H2O + 309 KJ 1. Is heat a reactant or product? 2. Is this endothermic or exothermic? 3. Would ΔH be + or -? 4.Re-write the equation:

Heat Capacity and Specific Heat A calorie is defined as the quantity of energy needed to raise the temperature of 1g of pure water 1 degree Celcius. Calorie is written with a lower case c except when referring to the energy contained in food. The dietary calorie is written with a capital C, refers to energy in food. 1 C= 1000 c

Heat Capacity and Specific Heat The calorie is also related to the Joule, the SI unit of heat and energy. 4.184 J = 1 cal Specific Heat Capacity (C) – the amount of the heat it takes to raise the temperature of 1g of the substance 1 degree Celcius Specific heat for short

Heat Capacity and Specific Heat Water has a high specific heat value compared to other chemicals For water, C= 4.18J/g ° C C= 1.00 cal/g ° C Thus for water: It takes a lot of heat to raise the temperature It take a long time to cool off

Heat Capacity and Specific Heat To calculate specific heat, use the formula: C = q m x Δ T Heat is abbreviated a q Δ T= change in temperature (Tf – Ti) C= Specific Heat of the substance Units are either in J/(g deg C) or cal/(g deg C) m= mass

Calorimetry Calorimetry- the accurate and precise measurement of heat flow into or out of a system for chemical and physical processes. The device used to measure the absorption or release of heat in chemical or physical processes is called a calorimeter.

Calorimetry Foam cups are excellent heat insulators and are commonly used as simple calorimeters.

Summary Thermochemistry is the study of . Heat and temperature are NOT the same thing. Heat is a form of that can be transferred from one object to another. Heat flows from to . When you hold an ice cube in your hand, your hand feels cold because the heat is . Temperature is a measure of the of all the atoms and molecules in a substance. The higher the kinetic energy, the the molecules move and the temperature Reactions that are endothermic heat and feel when you touch them. Reactions that are exothermic heat and feel when you touch them.