Thermochemistry and Reaction Rates and 18.1

Energy Transformations So far in our reactions we have been ignoring energy– in this chapter we will study how it changes. There are two forms of energy – _________________(Stored) – ______________(Heat or Work)

Chemical Potential Energy In chemistry the only potential energy we care about is _________________ _________________________ _________________________ _________________________ It is released into kinetic energy when the bond is broken Examples… – Gasoline powering an engine – Food being digested (like glucose) Fuels such as charcoal are hydrocarbons. When a fuel burns in air, the hydrocarbons break up into simpler compounds. The chemical potential energy they contain is then released as heat energy.

Heat or Energy? When chemical energy is transformed it can become heat or light. Usually both result in different amounts – Consider a car Energy from the gas leaves as both heat and light– lets see how…

Endothermic vs Exothermic Endothermic – Heat __________ from the surroundings (_________ to the system) – Products _____ stable than reactants – Non-spontaneous Exothermic: – Heat is _________ from the system (___________ to the surroundings) – Products _______ stable than reactants – _______________

This is both endothermic and exothermic depending on how you define the system. Explain in your own words in your notes Answer:.

The flow of heat can be measured Described as ____________ ______________ calorie: the amount of heat needed to raise the temperature of __________ _______________________ – When we talk about food we talk about __alories (with a capital __) – 1000 calories = 1 Calorie __________ can also be used to measure heat… they are related to calories based on the relationship – 1 cal = 4.184J

Heat Capacity and Specific Heat The energy required to increase the energy of a substance is measured as its HEAT CAPACITY – Depends on the _________(more _______ = more heat capacity= more __________________.) Drop of water heats up faster than a pot of water – Depends on the ___________________and ____________________ ________________________________ Metal heats up faster than concrete (cement is slower than blacktop) This difference is measured as SPECIFIC HEAT(C): the amount of energy it takes to raise the temp of the ______________________ _________ (how is this different from a calorie?)

Specific heat specifics… You can calculate the specific heat with the following equation… – C= (q) / (m x ∆T) C= q = m = ∆T = Example: The temperature of a 95.4 g piece of copper wire increases from 25 ˚C to 48.0 ˚C when the copper absorbs 849 J of heat. What is the specific heat of copper?

Challenge Problem… How much heat is required to raise the temperature of g of silver by 45 ˚ C?

Measuring Enthalpy Calorimetry is the measurement of _________ into or out of a system – ____________________________________________________ ________________________________________________ – Measure in a calorimeter (an insulated device used to measure heat flow) Can be measured at constant ____________ or constant _____________ Since our experiments happen in a lab– with constant pressure– we will consider only constant pressure calorimetry.

Calorimeters and Enthalpy Calorimeters need to be __________ so that all of the heat is contained At a constant pressure, the change in heat is equal to a term called __________ (H) – ___________

Calculating Enthalpy The reactants are dissolved into the calorimeter, then the initial and final temperatures are measured. The energy can be calculated by measuring how much the temp of the water increased, times the amount of water, times the specific heat of water. – ___________________________ So if the water (surroundings) gain heat, what happened to the system? – __________________________ – q (surr) = ____________ – q (sys) = _____________

Try one A lead mass is heated and placed into a foam cup calorimeter containing 40.0 mL of water at 17 ˚ C. The water reaches a temperature of 20 ˚ C. How many joules of heat were released by the lead? – q (sys) = - m x C x ∆T m = 40gFind q(sys) C= 4.18 J/g ˚ C ∆T = 3 ˚ C

Reaction Rates Section 18.1

Collision theory Most steps in a reaction mechanism require molecules to ______________________ ______________________ Collision theory describes _____ the molecules must collide in order to have an interaction – Reaction must ___________ _______ (_________ energy) – The molecules must hit in the ______________ so interacting atoms are actually in contact

Activation energy Activation energy is: the __________ amount of energy needed to complete a reaction H 2 and O 2 should, by means of free energy (∆G = kJ/mol) spontaneously form H 2 O Based on collision theory the reason that it doesn’t is due to the ___________________ H 2 and O 2 repel each other so, without the help of a __________ pushing them together H 2 O will not form

Activated Complex As the molecules collide there is a moment when the ______ ______________________________________________________ __ __________________________________________________ This in-between molecule is the _________________ (very unstable and just as likely to form reactants again as the product) In most cases the activated complex forms and changes so fast that it cannot be visualized… but it can be represented

Understanding reaction diagrams E a (pronounced activation energy): the amount of energy needed to start the foreword reaction E a ’ (pronounced activation energy prime) energy necessary to start the reverse reaction ∆E (∆G) (pronounced delta E or delta G) is the amount of energy change (released or absorbed in the reaction) __________________________ _________________________ *Difference means subtract Ea’Ea’

Factors Affecting Reaction Rates Temperature: – Higher temperature means the molecules move ______ – Which means more ______ – Which gives higher probability of them striking in the correct orientation (collision theory) Concentration: – More molecules means more ____________ – Which gives higher probability of them striking in the correct orientation (collision theory)

More factors… Particle Size (surface area): the more ________ you have, the more likely to __________ __________ with each other. – Lots of small sticks burn faster than one big log… smaller pieces = faster reaction. – You can accomplish this by grinding reactants into powder or dissolving them. Catalysts = ______________ ________________________. – Lowers the _____________________