1. chapter 12 Chemical reactions that involve heat!!!!! thermochemistry- study of chemistry of heat ! endothermic- pull in heat to react. will feel cold. the products store more energy than the reactants. can be spontaneous, but often these are reactions that you have to feed energy to get to work. melting ( ice or chocolate) exothermic- reactions that give off heat. feel hot to touch, explosions, most reactions. generally spontaneous, the products have less stored energy, but are more stable. how do we measure heat change? Joules? or Kilojoules. generally talk about the change in heat, ( ENTHALPY) how much we gain or lose between reactants and products, use Δ H to represent

2. so when we write reactions we can write them like this!!! 2H 2 + O 2 --> 2H 2 O +300kJ ( exothermic is a product) or off to the side with a sign to tell you where it is going - is exothermic and + is endothermic 2O 3 --> 3O 2 ΔH + 500 Kj ( endothermic cause heat goes in) you can use this for stoiciometry, it applies the same. if you double the reaction you double the heat and etc.

3. 2H 2 + O 2 --> 2H 2 O ΔH = - 300J If I react 400 grams of Hydrogen, how much heat is given off?

4. Hess's law one last fun thing we do with reactions, add them!!! some times the product that you want has to be made from several steps. if that is the case, then the total energy to make that chemicals is the total of the energy of all the steps!! that is Hess's law! - if a series of reactions are added together, the total enthalpy change for the net reaction is the sum of the individual reactions added up!! several things can be done with Hess’s Law. reverse sign, change of factor, or sum of individual reactions)

5. 2H 2 + O 2 --> 2H 2 O ΔH = 300J 2 h20 = o2 and H2 2O 3 --> 3O 2 ΔH + 500 Kj what is the heat change for the reaction that takes 2 H 2 0 + 2 O 3 ⇒ 2 H 2 and 4 O 2 800 j

6. calorimetry- the study of the energy and heat of food!!! calorimetry experiments are those that measure the heat change of foods by measuring the temp. change of a calorimeter things to know- heat capacity- the ability of an item to increase the temperature of its surroundings heat capacity is defined as the heat needed to raise the temp of the object by 1 degree celcius specific heat- ability to raise one gram of a substance one degree celcius. Water has one of the highest specific heats of any common substance

7. s pecific heat of water- 4.184 J /g( C) = 1 calorie or you can say specific heat of water is 1 cal/g(C) depending on your other units if you use joules or cal. we like cal. better. 1000 calories= 1 Kilocalorie- or the Calories you eat!!! peanut m&m's have 200 Calories= 200,000 calories or 200,000* 4.184 Joules =836800 J

8. How do we use this in the calorimeter? t i = temp initial!!! starting temp t f = temp. final ( the temp at the end of the burning of the sample) q rxn - the energy in joules of the reaction q surr - energy in joules that the surroundings pick up, should be equal but opposite sign of qrxn m - mass of the water!!! C - specific heat of your substance, in our case water, so 4.184 joules = 1 little c calorie qsurr = m x C x (tf-ti) ( ΔT)

9. ok, so now you know that all sorts of chemicals, food and otherwise, can give off energy if we are talking about "regular " chemicals we measure the energy in joules If we are talking about food we measure it in calories. so how does food or chemicals "give off" energy?

10. breaking bonds- that's how when chemicals react, they break the bonds of the reactants and reform bonds of the products. the breaking of bonds releases energy it goes from being potential stored energy in the reactant moleucles to kinetic energy of movement. sometimes heat and light, etc heat is defined as the transfer of kinetic energy from a hotter object to a colder one