Heat in Chemical Reactions

Heat: the transfer of energy to a substance causing an increase in that substance’s average kinetic energy Temperature: a measure of the average kinetic energy of a material’s particles.

Number of Particles vs. Average Speed (Temperature):

Temperature Conversions K = C F = 1.8C + 32 C = (5/9)(F – 32)

Heat in chemical reactions Reactions that release heat are EXOTHERMIC C 3 H 8 + 5O 2  3CO 2 + 4H kJ How many grams of propane are necessary to react with excess oxygen to produce 18,000 kJ of heat?

Heat in chemical reactions Reactions that absorb heat are ENDOTHERMIC C + H kJ  CO + H 2 How much heat energy is required to react with excess carbon and water to produce 45 L of hydrogen gas at STP?

Specific Heat Capacity q = mC p ΔT q = Heat energy (J) m = mass of substance (g) C p = specific heat capacity ΔT = T f – T i (in Celsius)

Specific Heat Capacity C p (ice) = 2.1 J/g C p (water) = 4.18 J/g C p (steam) = 1.7 J/g

How much energy is required to heat 43 grams of water from 22 degrees Celsius to 85 degrees Celsius?

The combustion of glucose in cellular respiration produces 2603 kJ/mol glucose of energy. This reaction is being used to heat a container of 74 grams of water from 12 degrees to 93 degrees. How many grams of oxygen are being reacted with the glucose to cause this change in temperature?

A 47 gram chunk of hot copper is placed into 200 mL of water originally at 33 degrees Celsius. Afterwards, the water now has a temperature of 35.4 degrees Celsius. Assuming all heat lost by the copper was gained by the water, how hot was the copper before putting it into the water? The specific heat of copper is 0.38.

What happens to temperature as heat is added?

Molar Heat of Fusion The amount of heat absorbed by 1 mole of a substance to melt from a solid to a liquid. ◦For water, ΔH fus = 6.01 kJ/mol How much heat would be required to melt 30 grams of water at 0˚Celsius?

Molar Heat of Vaporization The amount of heat required to cause 1 mole of a liquid to vaporize ◦For water, ΔH vap = 40.7 kJ/mol How much heat would be required to vaporize 30 grams of water at 100˚C?

˚C. How much heat will be required to convert this amount of water to steam at a temperature of 125 ˚C? I have 67 grams of ice at -10˚C. How much heat will be required to convert this amount of water to steam at a temperature of 125 ˚C?

Enthalpy (H) The enthalpy of a substance is equal to its total energy plus a small added term that takes into account volume and pressure. It’s a measure of how much energy is stored in a substance.

Enthalpy changes (ΔH) All reactions have a standard enthalpy change ◦Found at 1 atmosphere of pressure and 25˚C ΔH = H products – H reactants

What does it all mean? If ΔH is positive, it’s an endothermic reaction and heat is absorbed. If ΔH is negative, the reaction is exothermic and heat is released.

Using Enthalpy How much heat is transferred when 9.22 g of glucose (C 6 H 12 O 6 ) in your body react with O 2 according to the following equation? C 6 H 12 O 6 + O2  6CO 2 + 6H 2 O ΔH˚ = kJ

Hess’s Law In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps.

Hess’s Law N 2 + O 2  2NOΔH = 180 kJ 2NO + O 2  2NO 2 ΔH = -112 kJ Net: N 2 + 2O 2  2NO 2 ΔH = 68 kJ

S(s) + 3/2 O 2 (g)  SO 3 (g) ΔH = kJ 2SO 2 (g) + O 2 (g)  2SO 3 (g) ΔH = kJ What is the ΔH for the following net reaction? S(s) + O 2 (g)  SO 2 (g) ΔH = ? How many grams of sulfur dioxide gas can be created if 350 kJ of heat is transferred?