Thermochemistry

Thermochemistry

By Mihir Paranjape

Heat

• Heat is Thermal Energy which is transferred due to a difference in Temperature.

• Heat is transferred from higher temperature to lower temperature.

• Heat is transferred by collissions between ​molecules. At higher temperature, molecules have more Kinetic Energy (as they move faster), these collide with molecules with lower Energy thereby transferring this energy.

• Units are Joules (J) or Calories (cal)​

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Temperature

• Temperature is the average Kinetic Energy of molecules within a system.

• Difference in Temperature causes Heat to Flow.​

• Can be measured using a Thermometer.

• Units - C, F, K, R

• K = C+273​

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Exothermic Processes

• Heat EXITS the system or flows out.

• ​Given a negative sign. Imaging Money flowing out of your account which is bad and hence negative.

• This heat goes to the surroundings, which feel war​mer.

• ​As heat leaves the system, molecules SLOW DOWN.

• Gas > Liquid​ > Solid is example of Exothermic Process as Molecules are slowing down.

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Endothermic Processes

• Heat ENDERS the system or flows in.

• ​Given a positive sign. Imaging Money flowing in your account which is good, will increase your balance and hence positive.

• This heat enters from surroundings, which feel colder.

• ​As heat enters the system, molecules SPEED UP.

• Solid > Liquid > Gas is example of Endothermic Process as Molecules are speeding up.

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Multiple Choice

During an exothermic reaction, heat content in the surroundings increases because

• heat energy is destroyed during reactions

• the reaction absorbs heat energy

• the energy contained in the reactants is lower then the products

• the energy contained in the products is lower than the reactants

Multiple Choice

ΔH value in an endothermic reaction is a positive number.

• True
• False

Phase Changes

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Multiple Choice

What is the phase change of a solid to a liquid?

• freezing

• melting

• boiling

• condensation

Multiple Choice

5. What is it called when a solid turns directly into a gas?

• Sublimation
• Condensation
• Liquid

Multiple Choice

What happens to particles when energy is added (heated)?

• They speed up and spread out

• They slow down and compress

• They stop moving

• They move closer together and speed up

Multiple Choice

The phase change from water vapor to liquid water is known as...

• evaporation
• precipitation
• condensation
• sublimation

Heating Curve

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• ​Start with Ice (SOLID) and heat it.

• Ice will rise in Temp till it reaches 0C.

• It will then melt to water (LIQUID). Temp does not change.

• Water(LIQUID) will then heat up as more heat is added.

• Water(LIQUID) will rise in Temp to 100C

• At 100C, ​Water changes to Steam.(GAS)

Multiple Choice

What state(s) of matter are present at D-E?

• Solid-liquid
• Liquid
• Liquid-gas
• Gas-Solid

Multiple Choice

Between which points is the temperature of the substance remaining constant?

• A-B only. 
• A-B, C-D, E-F
• B-C only. 
• B-C, D-E

Multiple Choice

Between which points is the temperature of the substance increasing?

• A-B only. 
• A-B, C-D, E-F
• B-C only. 
• B-C, D-E

Multiple Choice

This type of graph is called a:

• Phase diagram
• Heating curve
• State of matter
• Temperature chart

Heating Curve

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Slope Part: (Red)

Temp increases as Heat is added.

​

Q = m x c x Δ​T

Q = Heat (J)

m = mass (g)

c = Specific Heat (J/gC)

​Δ​T = Change in Temp (Tf-Ti)

Multiple Choice

What  is the formula to calculate heat energy required to raise the temperature of any substance?

• Q=mc∆t
• Q=mc
• Q= ½mv
• m=QC

Multiple Choice

How many joules of heat are needed to raise the temperature of 10.0 g of aluminum from 22.0°C to 55.0°C, if the specific heat of aluminum is 0.903 J/g°C?

• 298 Joules

• 0.003 Joules

• 297 J/g°C

• 0.003 J/g°C

Multiple Choice

The specific heat of aluminum is 0.9025 J/g°C. How much heat(Q) is released when a 10.0 g piece of aluminum foil is taken out of the oven and cools from 100.0° to 50.0°?

• 451 J

• 45.1 J

• 400 J

Multiple Choice

20.0 g of water. specific heat of water is 4.184 J/g°C. temperature changes from 25.0° C to 20.0° C, how much heat energy (Q) moves from the water to the surroundings?

• 418 Joules

• 209 J

• 83 J

• 4.18 J

Heating Curve

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Flat Part: (Blue)

Temp remains same as Heat is added.

All Heat is going towards Phase Change, called latent (lost) heat.

​

Q = m x Δ​H

Q = Heat (J)

m = mass (g)

​Δ​H = Latent Heat (J/g)

Multiple Choice

Latent heat is

• the heat absorbed or released at a boiling process during a change of phase

• the heat absorbed or released at a constant temperature during a change of phase

• the heat absorbed or released at a constant temperature during a change of phase of 1 kg object

• the heat absorbed or released by an object to increase or decrease its temperature by 1 degree Celcius

Multiple Choice

How much energy is required to freeze 150 g of water? $$\left[l_v=2260000\ J\ kg^{-1};\ l_f=334000\ J\ kg^{-1}\right]$$  

• 50 100 cal

• 339 000 J

• 339 000 cal

• 50 100 J

Multiple Choice

How much energy is required to change 5 kg of ice at 0 $$\degree$$ C into water at the same temperature? (latent heat of fusion is 3.36 X $$10^6$$ J/kg )

• 16.1 XundefinedJ

• 16.8 XundefinedJ

• 15.1 Xundefinedcal

• 16.8 Xundefinedcal

Multiple Choice

How much energy is required to melt down a 2000 g of mercury?(latent heat of fusion is 1.14 x  $$10^4$$ J/kg)

• 2.55  x undefinedJ

• 2.28 x undefinedJ

• 2,280 x undefinedJ

• 3.28  x undefinedJ