Heat in Chemical Reactions

Theory of Heat  The theory of heat is based in how particles move  This theory is called Kinetic Molecular Theory  Essentially, the faster a particle is moving, the more heat the particle contains  The greater the kinetic energy of a particle, the greater the heat of the particle  Not all particles in a system are at the same heat so they do not all move at the same speed  The theory of heat is based in how particles move  This theory is called Kinetic Molecular Theory  Essentially, the faster a particle is moving, the more heat the particle contains  The greater the kinetic energy of a particle, the greater the heat of the particle  Not all particles in a system are at the same heat so they do not all move at the same speed

Theory of Heat  Temperature is an AVERAGE of every particles kinetic energy (heat) in a system  When you take the temperature of a liquid, you are finding the average speed the particles are moving in the liquid  Temperature is an AVERAGE of every particles kinetic energy (heat) in a system  When you take the temperature of a liquid, you are finding the average speed the particles are moving in the liquid

Theory of Heat  Heat is transmitted to other objects by particles striking each other and transferring kinetic energy  A system wants to reach Thermal Equilibrium  All of the particles in a system will collide and transfer energy until they all have the same kinetic energy  Heat is transmitted to other objects by particles striking each other and transferring kinetic energy  A system wants to reach Thermal Equilibrium  All of the particles in a system will collide and transfer energy until they all have the same kinetic energy

Theory of Heat  Thermal Equilibrium Example:  Imagine filling a cup with hot coffee. Immediately the cup heats up and the air above the coffee heats up. This is because those are the particles directly touching the hot coffee. The hot particles in the coffee strike the cup and the air above and transfer their kinetic energy. Eventually, the surrounding air continues to strike the cup and energy is transferred throughout the entire surrounding particles. This happens until the coffee, the air, and the cup all reach the same heat levels.  Thermal Equilibrium Example:  Imagine filling a cup with hot coffee. Immediately the cup heats up and the air above the coffee heats up. This is because those are the particles directly touching the hot coffee. The hot particles in the coffee strike the cup and the air above and transfer their kinetic energy. Eventually, the surrounding air continues to strike the cup and energy is transferred throughout the entire surrounding particles. This happens until the coffee, the air, and the cup all reach the same heat levels.

Theory of Heat  Because heat is kinetic energy an is transferred by particles striking and increasing the speed of others, we use an important convention in thermodynamics:  Heat ALWAYS flows from HOT to COLD  You can never let the cold out, you can only let the heat in!  Because heat is kinetic energy an is transferred by particles striking and increasing the speed of others, we use an important convention in thermodynamics:  Heat ALWAYS flows from HOT to COLD  You can never let the cold out, you can only let the heat in!

Heat – A TRANSFER of Energy  Heat is the transfer of energy. Note that heat always flows from hot to cold, never from cold to hot.  For example, imagine you had an ice cube in your hand. The heat from your hand gets transferred to the ice cube.  That is, fast moving particles hit the slow moving particles in the ice cube.  This interaction slows the particles down in your hand (making it cold) and speeds up the particles in the ice (making it warm).  Thus, thermal energy is transferred from your hand into the ice cube (coldness does not travel from the ice cube to your hand).  Heat is the transfer of energy. Note that heat always flows from hot to cold, never from cold to hot.  For example, imagine you had an ice cube in your hand. The heat from your hand gets transferred to the ice cube.  That is, fast moving particles hit the slow moving particles in the ice cube.  This interaction slows the particles down in your hand (making it cold) and speeds up the particles in the ice (making it warm).  Thus, thermal energy is transferred from your hand into the ice cube (coldness does not travel from the ice cube to your hand).

Bonds are Energy  Compounds are held together with bonds  These bonds are potential energy  When a bond is formed, energy is released  When a bond is broken, energy is absorbed  Compounds are held together with bonds  These bonds are potential energy  When a bond is formed, energy is released  When a bond is broken, energy is absorbed

Bonds in Chemical Reactions  During a chemical reaction, bonds are broken, formed, or both  In the reaction in the picture, some bonds are broken and some are formed CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O (g)  During a chemical reaction, bonds are broken, formed, or both  In the reaction in the picture, some bonds are broken and some are formed CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O (g)

Energy in Chemical Reactions  In the reaction on the side, the products have less energy than the reactants  An EXOTHERMIC REACTION is one that gives off energy  Energy is given off when the products have less energy than the reactants  In the reaction on the side, the products have less energy than the reactants  An EXOTHERMIC REACTION is one that gives off energy  Energy is given off when the products have less energy than the reactants

Graphing Exothermic Reactions  The energy of reactions is sometimes graphed  In an exothermic graph, the products are ALWAYS lower in energy than reactants  The difference between products and reactants is the Enthalpy  The energy of reactions is sometimes graphed  In an exothermic graph, the products are ALWAYS lower in energy than reactants  The difference between products and reactants is the Enthalpy

Energy in Chemical Reactions  In the reaction at the right, the products are at a higher energy than the products  An ENDOTHERMIC REACTION is one that takes in (absorbs) energy  The products are at a higher energy than the reactants  In the reaction at the right, the products are at a higher energy than the products  An ENDOTHERMIC REACTION is one that takes in (absorbs) energy  The products are at a higher energy than the reactants 2 H 2 O (l) → 2 H 2 (g) + O 2 (g)

Graphing Endothermic Reactions  When graphing an endothermic reaction, the products will always be higher in energy than the reactants

Enthalpy  Enthalpy is the energy difference between products and reactants  It is ALL of the energy (potential and kinetic)  Enthalpy is a measurement of energy and is measured in Kilojoules (kJ) (sometimes in calories like with your food)  Enthalpy has gets the symbol ∆H  Enthalpy is the energy difference between products and reactants  It is ALL of the energy (potential and kinetic)  Enthalpy is a measurement of energy and is measured in Kilojoules (kJ) (sometimes in calories like with your food)  Enthalpy has gets the symbol ∆H

Enthalpy in Equations  Enthalpy can be written in 2 ways 1.In a chemical reaction  For exothermic reactions, enthalpy is a PRODUCT  For endothermic reactions, enthalpy is a REACTANT  In both cases, it is ALWAYS written as a positive number  Enthalpy can be written in 2 ways 1.In a chemical reaction  For exothermic reactions, enthalpy is a PRODUCT  For endothermic reactions, enthalpy is a REACTANT  In both cases, it is ALWAYS written as a positive number CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O (g) kJ 2 H 2 O (l) kJ → 2 H 2 (g) + O 2 (g)

Enthalpy in Reactions  Enthalpy can also be written another way 2.Outside of a chemical reaction  For exothermic reactions, enthalpy is a NEGATIVE  For endothermic reactions, enthalpy is a POSITIVE  Enthalpy can also be written another way 2.Outside of a chemical reaction  For exothermic reactions, enthalpy is a NEGATIVE  For endothermic reactions, enthalpy is a POSITIVE CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O (g) ∆H = kJ 2 H 2 O (l) → 2 H 2 (g) + O 2 (g) ∆H = kJ

Stoich with Heat  Since we know molar ratios and the energy released/absorbed in a chemical reaction, we can use stoichiometry to calculate various things  Example  If 14.7g of methane are burned, how much heat is given off?  Since we know molar ratios and the energy released/absorbed in a chemical reaction, we can use stoichiometry to calculate various things  Example  If 14.7g of methane are burned, how much heat is given off? CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O (g) kJ