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

so when we write reactions we can write them like this!!!

2H2 + O2 --> 2H2O +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

2O3 --> 3O2 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.

2H2 + O2 --> 2H2O ΔH = -300J

If I react 400 grams of Hydrogen, how much heat is given off?

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)

2H2 + O2 --> 2H2O ΔH = -300KJ

2 H20 = O2 + 2H2

2O3 --> 3O2 ΔH + 500 Kj

what is the heat change for the reaction that takes

2 H20 + 2 O3 ⇒2 H2 and 4 O2

800 j4 H20 + 4 O3 ⇒4H2 and 8 O2

TiCl4 + 2Mg = Ti +2 MgCl2 ΔH - 478

createTiO2 + CCl4 + 2Mg = Ti + CO2 + 2 MgCl2

TiCl4 + CO2 = TiO2 + CCl4 ΔH=155

calorimetry- the study of the energyand heat of food!!!

calorimetry experiments are those that measure the heat change of foods by measuring the temp. change of a calorimeterthings 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

specific 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

How do we use this in the calorimeter?ti = temp initial!!! starting temptf = temp. final ( the temp at the end of the burning of the sample)

qrxn - the energy in joules of the reactionqsurr - 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)

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 joulesIf we are talking about food we measure it in calories.

so how does food or chemicals "give off" energy?

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

Let's review!!!how many words do you

remember from this chapter?enthalpy calorimetr

y

calorimetry

exothermic

specific heat

enthalpy

heat of a reaction

Δtchange in temperature from beginning to end of a calorimetry experiment

reaction that gives off heatheat needed to increase 1 g of a substance 1 ℃

the heat in joules, taking into acount pressure and volumes

experiments that determine heat given off by substances, often food

q rxn

calorimetry

exothermic

specific heat

enthalpy

heat of a reaction

Δtchange in temperature from beginning to end of a calorimetry experiment

reaction that gives off heatheat needed to increase 1 g of a substance 1 ℃

the heat in joules, taking into acount pressure and volumes

experiments that determine heat given off by substances, often food

q rxn

how about these?

q rxn

endothermic

Δheat

hess's law

heat capacity

+ ΔH

- ΔH

shows endothermic reaction, energy going into reaction. indicates products have more energy than reactants

the energy of a reaction, done in calorimetry to see how much heat a substance or reaction gives off

a reaction that takes in heat, often feels cold. less often spontaneous. ice melting

the change in the amount of heat between the beginning and end of a reaction

states that reactions may occur in steps, and that the total heat is found by summing the steps. you can also change factor or reverse the direction of reactions

the ability of something to transfer heat. does not include a grams unit, but looks and heat to increase temp of a substance

used to indicate exothermic reactions, heat of reactants was higher that heat of products, and products feel warm. most reactions are exothermic. see heat and light. explosions

2C2H10 + 9 O 2 = 4 CO2 + 10H2O ΔH -6660

given 5.5 grams of oxygen, how much heat will this explosion give?

2C2H10 + 9 O 2 = 4 CO2 + 10H2O ΔH -6660

given 5.5 grams of oxygen, how much heat will this explosion give?

5.5g O2 (1 mole) 31.99 o2

2C2H10 + 9 O 2 = 4 CO2 + 10H2O ΔH -6660kjgiven 5.5 grams of oxygen, how much heat will this explosion give?

5.5g O2 (1 mole) 31.99 o2

(-6660 kj)

9O2=

2C2H10 + 9 O 2 = 4 CO2 + 10H2O ΔH -6660kjgiven 5.5 grams of oxygen, how much heat will this explosion give?

5.5g O2 (1 mole) 31.99

o2

(-6660 kj)

9O2= -127 kj

2C2H10 + 9 O 2 = 4 CO2 + 10H2O ΔH -6660kjCO 2 + CaO = CaCO3 ΔH + 990

what is the heat change for this reaction?

calorimetryq surr = - q

rxnq surr = mass in calorimeter x specific heat of calorimeter x Δt

q surr = m x C x Δt

in a calorimeter I used 350 ml of water. there were 10 marbles each with a mass of 5.6 grams. the marbles started at a temp of 102℃ and the water started at 45℃, the marbles were placed in the water, and the final temp of both marbles and water was 65℃. what is the total heat from the reaction?