# Entry Task: May 22nd 23rd Block 1

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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 worksheet
In-class (small notes) on Sec. 3 HW: 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 endothermic reactions?

What are exothermic reactions?
Exothermic reactions release energy (heat) to the surroundings when the reaction occurs. What are exothermic reactions?

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

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. How does it work?

q= cmΔT 9750 J = (4.184 J/g˚C)(335g)(X) 9750 (4.184)(335) 6.96 ˚C is the temperature difference = ˚C If 335g of water at 65.5˚C loses 9750 J of heat, what is the final temperature of the water?

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

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

Remember this? Chaos Most energy Taking in energy (endothermic)
Releases energy (exothermic) Taking in energy (endothermic) Releases energy (exothermic) Taking in energy (endothermic) Order Less energy Less order More energy Releases energy (exothermic)

What is the difference between the system, surroundings and universe?
The system is the reaction The surroundings is the everything other than the reaction. The universe is the system AND surroundings. Surroundings UNIVERSE System What is the difference between the system, surroundings and universe?

ΔHrxn = Hfinal – Hinitial
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 What is the difference between enthalpy and the enthalpy (heat) of reaction?

The sign of the enthalpy reactions
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_______________. released smaller negative negative

Notice the phase change
Provide the enthalpy change reaction for Iron/oxygen reaction 4Fe (s) + O2 (g) 2Fe2O3 (s) ΔH= kJ Notice the phase change (s) + (g)  (s) Gas to solid loses energy- exothermic Disorder to order = lose energy

The sign of the enthalpy reactions
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_______________. absorbed larger positive positive

Notice the phase change
Provide the enthalpy change reaction for cold pack with NH4NO3 NH4NO3(s) NH4+ (aq) + NO3- (aq) ΔH= 27 kJ Notice the phase change (s)  (aq) + (aq) Solid to liquid gains energy- endothermic Order to disorder gains energy

Endothermic Exothermic Exothermic Endothermic Exothermic
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 C A F D E G

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

Thermochemical equations
Are balanced chemical equations that include physical states of all reactants and products AND the energy change ∆H. NH4NO3(s) NH4+ (aq) + NO3- (aq) ΔH= 27 kJ

Types of enthalpy ∆H(comb) ∆H(vap) ∆H(fus)
Change in heat with combustion Change in heat with vaporization Change in heat with fusion

Thermochemical equations for changes in state
Consider: H2O(l)  H2O(g) ΔH(vap)= 40.7 kJ What is happening? Its vaporizing. What ∆H would you use? Would it be + or -? Positive- endothermic

Thermochemical equations for changes in state
Consider: H2O(s)  H2O(l) ΔH(fus)= 6.01 kJ What is happening? Its melting. What ∆H would you use? Would it be + or -? Positive- endothermic

Pay attention to the signs
∆Hvap = -∆Hcond ∆Hfus = -∆Hsolid So -∆Hcond for water is H2O(g)  H2O(l) ΔH(cond)= kJ So -∆Hsolid for water is H2O(l)  H2O(s) ΔH(fus)= kJ Makes sense- both are exothermic -∆H

Calculating energy released in a reaction
How much heat is released when 54.0 g of glucose (C6H12O6) is burned? C6H12O6 (s) + 6O2(g)  6CO2(g) + 6H2O(l) ∆H comb= kJ ∆H comb= kJ is per mole of glucose Convert gram to mole 54.0g glucose 1 mole glucose = 0.3 mol 180 g glucose = 0.3 mol X ∆H comb =842 kJ

You Try!! 1. Calculate the heat required to melt 25.7 g of solid methanol at its melting point. Which ∆H would you use? ∆H (fus) because its melting. So 3.22 its is. ∆H(fus) be positive or negative? ∆H (fus) be positive. ∆H (fus)= 3.22 per mole.

You Try!! 1. Calculate the heat required to melt 25.7 g of solid methanol at its melting point. ∆Hfus= 3.22 kJ is per mole of methanol Convert gram to mole 25.7g methanol 1 mole methanol = mol 32 g methanol = mol X ∆H fus =2.58 kJ

You Try!! Which ∆H would you use? ∆H(vap) be positive or negative?
2. How much heat is evolved when 275g of ammonia gas condenses to a liquid at its boiling point? Which ∆H would you use? ∆H (vap) because its condensing. So 23.3 its is. ∆H(vap) be positive or negative? ∆H (vap) be negative. ∆H (vap)= per mole.

You Try!! 2. How much heat is evolved when 275g of ammonia gas condenses to a liquid at its boiling point? ∆Hvap= kJ is per mole of ammonia Convert gram to mole 275g ammonia 1 mole ammonia = 16.2 mol 17 g ammonia = 16.2 mol X ∆H vap=377 kJ

You Try!! 12880 ÷ -891 =14.5 moles of methane 14.5mol methane
3. What mass of methane must be burned in order to liberate 12880kJ of heat? Methane (CH4) ∆Hcomb= kJ/mol ÷ =14.5 moles of methane 14.5mol methane 16 g methane = 232 g 1 mole methane

You Try!! 2. How much heat is evolved when 275g of ammonia gas condenses to a liquid at its boiling point? ∆Hvap= kJ is per mole of ammonia Convert gram to mole 275g ammonia 1 mole ammonia = 16.2 mol 17 g ammonia = 16.2 mol X ∆H vap=377 kJ

Tonight Finish backside w.s AND Pre-lab Questions