Warm-up

Mg(s) + HCl(aq) H2(g) + MgCl2(aq)

How much H2(g) is produced (in grams) in the above reaction if we have a mass of 0.9 grams Mg and 1.87 grams of HCl?

Mg = 24.31g/mol

H = 1.01g/mol

Cl = 35.45g/mol

ObjectiveSWBAT calculate energy changes

in a thermodynamics problem set

Chemical Chemical ThermodynamicsThermodynamicsStandard 7

Specific Heat Specific Heat 7d7dThe specific heat of a

substance is the amount of energy needed to change the temperature of 1gram by 1˚C.◦Every substance has its own specific

heat.Specific Heats of Common Substances

Substance Specific Heat (J/g˚C)

Water (l) 4.184

Ethanol (l) 2.44

Aluminum (s) 0.897

Silver (s) 0.235

Gold (s) 0.129

Logical Thinking (don’t Logical Thinking (don’t write)write)Since it takes 4.184 Joules to

raise the temp of 1gram of water 1˚C, how much would it take to:

◦Raise 2 grams 1˚C?

◦Raise 1 gram 2˚C?

◦Raise 2 grams 2˚C?

Heat added here doesn’t change the temperature of the substance, it is causing the phase change.

Heat Flow ProblemsHeat Flow ProblemsTo calculate heat, use the following

formula:

◦Energy = mass * specific heat * temperature change

Always given to you!

Practice ProblemsPractice ProblemsIf 200.0 grams of water is heated

from 70.0° C to 100.0° C to make a cup of tea, how much heat must be added?

Q = (m)(c)(ΔT) )0.700.100)(184.4)(0.200( CCg

JgQ

25100Q Joules

Calculations:

4.184J/gC

Latent HeatLatent Heat 7d cont7d contLatent (hidden) heat is energy

added to a substance that doesn’t change the temperature.◦Instead, it is used to change the

phase solid liquid liquid gas

What is latent heat and what is it used for?

Energy Requirements for the Changes of State Changes of state

are physical changes ◦No chemical bonds

are broken

Energy Requirements for the Changes of State

Molar heat of fusion – energy required to melt 1 mol of a substance

• Molar heat of vaporization – energy required to change 1 mol of a liquid to its vapor

Practice ProblemsPractice ProblemsIf 200.0 grams of water is at

100.0° C, how much heat must be added to change it all to steam or gas?

Hvap = 40.6 kj/molkJ

OmolH

kj

OgH

OmolHgQ 451)

1

6.40)(

02.18

1)(0.200(

22

2

JxJkJQ 51051.4451000451

Calculations:

Putting it all togetherPutting it all togetherIf 200.0 grams of water is heated

from 70° C to 100.0° C and then a complete phase change is made to steam, how much energy was required?

kJkJkJQ 476)2(#451)1(#1.25

Calculations:

SummarySummaryWhat is temperatureWhat happens when you heat a substance?Describe endo/exothermic reactions.What is ΔH for endo/exothermic reactions?Where does the energy go during phase

changes?Be able to calculate heat problems.What is latent heat used for?