4.13

Two objects (A and B) of the same temperature are in contact with each other. Which statement is true?

Although the molecules collide, no heat transfers because there is no temperature difference between the objects.

How much heat is given off when 32 g liquid methanol (CH3OH) at its freezing point changes to solid methanol? (methanol ΔHf=3.16 kJ/mol)

3.16 kJ

In a calorimeter, 100 g of ice melts at 0oC. The enthalpy of fusion of the ice is 334 J/g. How much heat was absorbed?

33.4 kJ

In a calorimeter, 6.68 kJ of heat was absorbed by 20 g of ice. What is the enthalpy of fusion of the ice? q=mΔHf

334 J/g

Which statement is a correctly written thermochemical equation?

4Fe (s) + 3O2 (g)→2Fe2O3 (s) , ΔH = −3,926 kJ

A chemical reaction is carried out in a closed container. The energy absorbed by the chemical reaction is 50 kJ. What is the energy liberated from the air or the walls of the container?

50 kJ

What is the change in enthalpy when 250 g of water evaporates at 100oC? (ΔHv=40.67 kJ/mol)

565 kJ

Which statement is true?

In an exothermic reaction, the energy of the products is less than the energy of the reactants.

In which thermochemical equations would the ΔH be considered a heat of solution?

NH4NO3(s)→NH4+(aq)+NO3-(aq), ΔH=+25.7 kJ/mol

Two hundred grams of a substance requires 1.84 kJ of heat to raise its temperature from 25°C to 45°C. Use the table to identify the substance. q=mCΔT. Mass (m) is in grams. Temperature is in degrees Celsius.

iron

Which example is an exothermic reaction?

oxidation of iron

Which parameter is measured directly in a coffee-cup calorimeter?

temperature

You have a series of four chemical reactions (1→2→3→4) . How do you calculate the ΔH2 if you know the other ΔH values?

ΔH2=ΔH4−(ΔH1 + ΔH3)

What is likely to be true about the ΔH for this reaction? C6H6 (s) → C6H6 (l)

∆H>0

Consider these equations: 2S (s) + 3O2 (g)→2SO3 (g) , ΔH=−792kJ 2S (s) + 2O2 (g)→2SO2 (g) , ΔH=−594kJ 2SO2 (g) + O2 (g)→2SO3 (g) , ΔH=? What is the missing ΔH ?

−198 kJ

How much heat do you need to raise the temperature of 150 g of oxygen from −30 °C to −15 °C ?

2.07 kJ

What type of substance would be best for use in a heating system?

a liquid with a high specific heat

Which term measures the energy transferred from one object to another?

heat

This diagram would represent the enthalpy changes in which example?

hot pack

The reaction 2NO (g) → N2 (g) + O2 (g) has a ΔH =−180.5 kJ . How much heat will be required to produce 44.8 L of NO by the reverse reaction?

180.5 kJ