Ch 16 Reaction Energy

 Standard: –7.d. Students know how to solve problems involving heat flow an temperature changes, using known values of specific heat.  Objective: –We will define heat, give its units, and perform specific-heat calculations.

Thermochemistry   Thermochemistry: the study of the transfer of energy as heat that occurs during chemical reactions and changes in state.   Heat: q, is energy transferred from one object to another because of a temperature difference between them.   Heat always flows from a warmer object to a cooler object and will continue to flow until they are in equilibrium.

 Endothermic Process: one that absorbs heat from the surroundings (+q).  Exothermic Process: one that releases heat to it’s surroundings (-q).  Calorimeter: the insulated device used to measure the absorption or release of heat in chemical or physical processes.

Specific Heat  Heat flow is measure in joules (J).  One joule of heat raises the temperature of 1g of pure water °C  Specific Heat: the amount of heat needed to increase the temp of 1 g of the substance 1°C. –The specific heat is the heat capacity of an object divided by its mass in grams.

Calculating Specific Heat (C)  Divide the heat input, q (Joules) by the temperature change, ΔT (°C) times the mass of the substance, m (g). C = q = heat m x ΔT mass x change in temp

Example  The temperature of a 95.4 g piece of copper increases from 25.0°C to 48.0°C when the copper absorbs 849 J of heat. What is the specific heat, C, of copper? q = 849 J m = 95.4g ΔT = (48.0°C -25.0°C)=23.0°C C = q = 849 J m x ΔT 95.4 g x 23.0°C C = J/(g x °C)

Example 2  How much heat, q, is required to raise the temperature of g of silver 45°C? The specific heat of silver is 0.24 J/(g x °C). ΔT = 45°Cm = g C= 0.24 J/(g x °C) q = C x m x ΔT q = 0.24 J/(g x °C) x g x 45°C q = 4320 J

Homework Notes  Table 1 is on page 533.  Kelvin and Celsius are interchangeable because they are on the same scale.  c p is the same as C  #7 solve for q  #8 solve for ΔT, add that to 20°C and then convert to Kelvin.  ΔT = q /(C x m)  #9 solve for C  #10 change mol to g and solve for C

Chapter 17 Reaction Kinetics

Ch 17.1 Reaction Kinetics

 Standard: –8.a. Students know the rate of reaction is the decrease in concentration of reactants or the increase in concentration of products with time.  Objective: –We will use the collision theory to interpret chemical reactions and define activated complex. We will draw energy diagrams.

Collision Theory  Collision Theory: atoms, ions, and molecules can react to form products when they collide with each other, provided that the colliding particles have enough kinetic energy.  Think about two balls of clay. If you throw them together gently, they won’t stick together, but if there is enough energy, they will stick together.

 Rate: a measure of the speed of any change that occurs within an interval of time.  In chemistry, the rate of chemical change or the reaction rate is usually expressed as the amount of reactant changing per unit time.  Activation Energy: the minimum energy that colliding particles must have in order to react.

 Activated Complex: an unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier.  This is also called the Transition Rate.

Sample Problem A page 566 and Practice Problem 2 page 567

Ch 17.2

 Standard: –8.b. Students know how reaction rates depend on such factors as concentration, temperature and pressure. –8.c. Students know the role a catalyst plays in increasing the reaction rate.  Objective: –We will discuss the factors that influence reaction rate and define a catalyst.

Factors Affecting Reaction Rates  The rate of a chemical reaction depends upon temperature, concentration, particle size, and the use of a catalyst. TEMPERATURE  Raising the temperature speeds up the reaction and lowering the temperature slows down the reaction.

CONCENTRATION  The higher the concentration, the more likely collisions will take place, which increases the reaction rate. PARTICLE SIZE  The smaller the particle size, the more surface area, which increases the reaction rate.

CATALYSTS  Adding a catalyst will increase the rate of reaction, in some cases, better than increasing the temperature.  Inhibitor: a substance that interferes with the action of a catalyst. –These will slow down or even stop a reaction

Nature of Reactants  The nature of the reactants involved in the chemical reaction also play a part. But just remember the first four.