Chemistry SM-1131 Week 3 Lesson 2

Chemistry SM-1131Week 3 Lesson 2

Dr. Jesse ReichAssistant Professor of Chemistry

Massachusetts Maritime AcademyFall 2008

Class Today• Friday• Wiki• Matter- physical v. chemical, physical changes

v. chemical changes• Separating Matter• Wednesday = Energy and Temperature, maybe

some problems

Friday• I misspoke. We do have class on Friday, but

I’m going to “cancel” it.• Class on Friday is meant to be spent working

on your Wikis. Get at least one scientist on your wiki!

Density• Density = Mass volume

d = m v

Let’s calculate density• I have a ball of metal that takes up 3ml and

weighs 12.6g. What is it’s density?d = m v

d= 12.6g= 4.2 g/ml, but 3ml only has 1 sig fig 3mlOK so, finally 4 g/ml would be the answer!

Matter• Definitions: vocab. words to memorize• Matter: Anything that has mass and takes up

space.• Elements: The most basic different types of

matter. You can’t break it down into other substances.

• Atoms: The smallest distinguishable unit of an element.

• Molecules: 2+ more atoms bonded together.

Solid, Liquid, Gas

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Elements, Compounds, Mixtures

• If there is only 1 atom type or molecule type in a given space we call it a pure substance

• If there are 2 or more atom types or molecules in a given space we call it a mixture.

Mixture• When you combine at least two pure

elements, at least two pure compounds, or at least 1 pure element and 1 pure compound into the same space.

Homogeneous Mixture

Heterogeneous Mixture

New Material• Chemical v Physical• Energy

Properties• Properties: The characteristics we use to

distinguish one substance from another.• Chemical vs. Physical properties

Physical Properties• Physical Properties don’t change what it’s

made up of.• Temperature changes, physical location, color,

appearance, melting point, boiling point, density, cutting it into smaller pieces, smushing, location changes.

• If you change phase solid-> liquid it’s still the same compound. So its physical.

Chemical Properties• A chemical property is displayed when its

composition is changed.• Can it be burned? Is it corrosive? Is it acidic?

Toxic?• When something burns it turns into different

chemicals.• When something rusts the chemical

composition changes!

Chemical Reaction• When matter undergoes a composition

change we call it a chemical reaction.• We write them like this:Reactants Products

Reactants are the substances before the change.Products are what are formed after the reaction.

Evidence of Chemical RXNs

• Odor• Color Changes• Heat and Light• Sound• Bubbling• Explosions

Separating Mixtures• Decanting: pour off liquid leaving solids• Distillation: evaporate off a material that boils

more quickly (Volatile) than the one it’s mixed with.

• Filtration: Solids are separated from a liquid by pouring both through a porous material.

Conservation of Mass• There is a chemical law:Mass is neither created nor destroyed in a

chemical reaction.

Energy• “The capacity to do work”• Energy is conserved: ie it is neither created

nor destoryed

Kinetic Energy• Energy in motion

Potential Energy• Stored energy that could “potentially” be

released

Electrical Energy• Energy associated with the flow of electrons.

Chemical Energy• Energy stored when compounds are formed

and released when compounds react.

Units• Joule: the SI equivalent of a calorie, 1 cal =

4.184 J• calorie (cal) the amount of energy required to

raise the temperature of 1g of water 1 degree C.

• Calorie (Cal) = 1000 cals• Kilowatt-hour (kWh) = 3.6e6 j

You should practice converting units

• 23 Cal into cal• 45 joules into Cal• 3 kWh to cal

Energy during changes• Chemists tend to think of compounds having

potential energy. If they have a lot of stored energy we normally think they are high energy. If they are un-reactive we consider them low energy.

Endothermic and Exothermic

Endothermic Exothermic

Requires Energy Releases Energy

Temperature• How much thermal energy something has.• K = C + 273• C = (F -32) 1.80C = Freezing Water 100C = boiling water32F = freezing water 212F= boiling water 96F body

temp0K means there is no thermal energy at all. There is no

lower temperature possible than 0K.

Heat Capacity• The amount of energy needed to change the

temperature of a given amount of it 1C.• When the mass of the compound is expressed

in grams then the term is “specific heat capacity” or “specific heat”

• Water has a high heat capacity. So it takes a lot of energy to make it hot. Boiling water takes a long time. Showering uses a lot of energy.

Calculating Heat Capacity

• Heat = Mass x Specific Heat Capacity x Temp Change q = m x C x DT

DT = Tfinal - Tinitial

This is a 4 variable problem. How many values do you have to know to find one of them?

Can you solve for m? C? or DTIf q is positive it means the temperature goes upIf q is negative it means the Temp went down.

Homework• Copy the example problems 3.10 and 3.11

from the book• Make sure you HW answers use sig figs and

scientific notation• You will get this one chance and only this one

chance to fix your HW from chapter 1&2 and turn it in for credit.

Homework• Get your wiki up. Put in a couple of scientists• Get your homework in by Monday

Extra Time• Heat = Mass x Specific Heat Capacity x Temp

Change q = m x C x DT• What amount of heat is needed to 250g of

water from 55C to 85C if the heat capacity of water is 4.18J

gC• q = 250g x 4.18J x 30C =31350J = 3.1e4 J gC