Entry Task: May 22 nd 23 rd Block 1 Question: What is temperature change of 50.0 grams of water absorbs 35kJ of energy. (specific heat 4.18 J/gC) You have.

Entry Task: May 22nd 23rd Block 1

Question:What is temperature change of 50.0 grams of water absorbs 35kJ of energy. (specific heat 4.18 J/gC)

You have 5 minutes!

Agenda:

Discuss Ch. 16.sec 2 and worksheetIn-class (small notes) on Sec. 3HW: Look at the specific heats of metal Lab

What are endothermic reactions?

Endothermic reactions absorb energy (heat) from the surroundings for the reaction to occur.

What are exothermic reactions?

Exothermic reactions release energy (heat) to the surroundings when the reaction occurs.

Define a calorimeter

A calorimeter is an insulated device used for measuring the amount to heat absorbed or released during a chemical or physical process.

How does it work?

A known mass of water is placed in an insulated chamber.

The chamber contains the reaction- any heat absorbed in the reaction will drop the temperature of the water.

Any heat released in the reaction will raise the temperature of the water.

If 335g of water at 65.5˚C loses 9750 J of heat, what is the final temperature of the water?

q= cmΔT9750 J = (4.184 J/g˚C)(335g)(X) 9750 (4.184)(335) 1401.646.96 ˚C is the temperature difference65.5 - 6.96 = 58.5 ˚C

The temperature of a sample of water increases from 20.0˚C to 46.6˚C as it absorbs 5650 J of heat. What is the mass of the sample?

q= cmΔT5650 J = (4.184 J/g˚C)(Xg)(26.6˚C) 5650 (4.184)(26.6) 111.29= 50.8 g

Define thermochemistry

Thermochemistry is the study of heat changes that accompany chemical reactions and phase changes

Remember this?

Order

Less energy

Less order

More energy

Chaos

Most energy

Taking in energy

(endothermic)Releases energy

(exothermic)

Taking in energy

(endothermic)

Releases energy

(exothermic)

Relea

ses en

ergy

(exo

ther

mic)Ta

king

in e

nerg

y

(end

othe

rmic)

What is the difference between the system, surroundings and universe?

The system is the reactionThe surroundings is the everything other

than the reaction.The universe is the system AND

surroundings.

System

Surroundings

UNIVERSE

What is the difference between enthalpy and the enthalpy (heat) of reaction?

Enthalpy is the heat content in a system at constant pressure.◦*its almost impossible to know the absolute

amount of energy held in a substance.Enthalpy (heat) of reaction is the change

of energy- one substance to another.

◦ΔHrxn = Hfinal – Hinitial

For an exothermic reaction, heat energy is _____________.

When the Hreactants is subtracted from the ________Hproduct, a ________ value for ΔHrxn is obtained.

Enthalpy changes for exothermic reactions are always_______________.

The sign of the enthalpy reactions

released

smaller negative

negative

4Fe (s)

Provide the enthalpy change reaction for Iron/oxygen reaction

+ O2 (g) 2Fe2O3 (s) ΔH= -1625 kJ

Notice the phase change

(s) + (g) (s)

Gas to solid loses energy- exothermicDisorder to order = lose energy

For an endothermic reaction, heat energy is _____________.

When the Hreactants is subtracted from the ________Hproduct, a ________ value for ΔHrxn is obtained.

Enthalpy changes for endothermic reactions are always_______________.

The sign of the enthalpy reactions

absorbed

larger positive

positive

NH4NO3(s)

Provide the enthalpy change reaction for cold pack with NH4NO3

+NH4+ (aq) NO3

- (aq)

ΔH= 27 kJ

Notice the phase change

(s) (aq) + (aq)

Solid to liquid gains energy- endothermicOrder to disorder gains energy

Identify the reaction as endothermic or exothermic-PAY ATTENTION TO PHASE CHANGES

C2H5OH (l) C2H5OH(g)

NH3(g) NH3(l)

Br2(l) Br2(s)

NaCl (s) NaCl (l)

C5H12 (g) + 8O2 (g) 5CO2 (g) + 6H2O(l)

Endothermic

Exothermic

Exothermic

Endothermic

Exothermic

B

CAFDEG

8. A scientist is studying the solution in the flask. What is the system?

9. What are the surroundings?

10. What is the universe?

The solution

The Flask and everything around the flask.

Solution + surroundings

NH4NO3(s)

Thermochemical equations

+NH4+ (aq) NO3

- (aq)

ΔH= 27 kJ

Are balanced chemical equations that include physical states of all reactants and products AND the energy change ∆H.

Types of enthalpy

Change in heat with combustion∆H(comb)

∆H(vap)

∆H(fus)

Change in heat with vaporization

Change in heat with fusion

H2O(l) H2O(g)

Thermochemical equations for changes in state

Positive- endothermic

What is happening?

Consider:

Its vaporizing.

What ∆H would you use? Would it be + or -?

ΔH(vap)= 40.7 kJ

H2O(s) H2O(l)

Thermochemical equations for changes in state

Positive- endothermic

What is happening?

Consider:

Its melting.

What ∆H would you use? Would it be + or -?

ΔH(fus)= 6.01 kJ

∆Hvap = -∆Hcond

Pay attention to the signs

∆Hfus = -∆Hsolid

H2O(g) H2O(l) ΔH(cond)= - 40.7 kJ

So -∆Hcond for water is

So -∆Hsolid for water is

H2O(l) H2O(s) ΔH(fus)= - 6.01 kJ

Makes sense- both are exothermic -∆H

How much heat is released when 54.0 g of glucose (C6H12O6) is burned?

Calculating energy released in a reaction

C6H12O6 (s) + 6O2(g) 6CO2(g) + 6H2O(l) ∆H comb= -2808 kJ

Convert gram to mole

∆H comb= -2808 kJ is per mole of glucose

54.0g glucose

180 g glucose

1 mole glucose = 0.3 mol

= 0.3 mol X -2808 ∆H comb =842 kJ

1. Calculate the heat required to melt 25.7 g of solid methanol at its melting point.

You Try!!

∆H (fus) because its melting. So 3.22 its is.

Which ∆H would you use?

∆H(fus) be positive or negative?∆H (fus) be positive. ∆H (fus)= 3.22 per mole.

1. Calculate the heat required to melt 25.7 g of solid methanol at its melting point.

You Try!!

Convert gram to mole

∆Hfus= 3.22 kJ is per mole of methanol

25.7g methanol

32 g methanol

1 mole methanol = 0.803 mol

= 0.803 mol X 3.22 ∆H fus =2.58 kJ

2. How much heat is evolved when 275g of ammonia gas condenses to a liquid at its boiling point?

You Try!!

∆H (vap) because its condensing. So 23.3 its is.

Which ∆H would you use?

∆H(vap) be positive or negative?∆H (vap) be negative. ∆H (vap)= -23.3 per mole.

2. How much heat is evolved when 275g of ammonia gas condenses to a liquid at its boiling point?

You Try!!

Convert gram to mole

∆Hvap= -23.3 kJ is per mole of ammonia

275g ammonia

17 g ammonia

1 mole ammonia = 16.2 mol

= 16.2 mol X -23.3 ∆H vap=377 kJ

3. What mass of methane must be burned in order to liberate 12880kJ of heat? Methane (CH4) ∆Hcomb= -891 kJ/mol

You Try!!

14.5mol methane 16 g methane

1 mole methane

= 232 g

12880 ÷ -891 =14.5 moles of methane

2. How much heat is evolved when 275g of ammonia gas condenses to a liquid at its boiling point?

You Try!!

Convert gram to mole

∆Hvap= -23.3 kJ is per mole of ammonia

275g ammonia

17 g ammonia

1 mole ammonia = 16.2 mol

= 16.2 mol X -23.3 ∆H vap=377 kJ

TonightFinish backside w.s

ANDPre-lab Questions