 Section 5.2

The temperature of a substance increases when heated Heat capacity: the amount of heat energy required to raise the temperature of a substance by one Kelvin (or one degree Celsius) Better to have a measurement related to a unit mass

The amount of heat required to raise the temperature of a unit mass (usually a gram) by one Kelvin Is specified as “c” in formulas Often has units of J g -1 K -1 The lower the specific heat capacity of a substance, the greater its temperature rise for the same amount of heat absorbed

The following are the symbols used in formulas regarding heat capacity: Heat energy change: q Mass of the object: m Temperature change: Δ T Formula: q = mc Δ T

How much heat energy is required to increase the temperature of 20.0 g Ni (specific heat capacity of 440 J kg -1 K -1 ) from 50.0 ° C to 70.0 ° C? Consider: the specific heat capacity uses kg and your measurement is in grams Consider: the change in one degree Celsius is the same measurement as the change in one Kelvin See the board for the working out.

A beaker containing cm 3 of water is heated using a Bunsen burner. Over a period of 10 minutes, the temperature of the water increased from 15.0 ° C to 80.0 ° C. Determine the heat energy change of the water in the beaker. Consider: the density of water is 1.0 g cm -3 Consider: the specific heat capacity of water is 4.18 J g -1 ° C -1 (or 4.18 kJ kg -1 K -1 ) See the board for the working out

A Bunsen burner was used to heat a g block of copper for 10 minutes. The heat energy change was 27.2 kJ. Determine the highest temperature that the block of copper, initially at 15.0 ° C, would reach. The specific heat of copper is J g -1 ° C -1 Consider: you are solving for Δ T Consider: the heat energy change is in kJ, while the formula uses J See the board for the working out

Notice that the lower the specific heat capacity of copper compare with that of water enables the copper to be heated to a high temperature by the same amount of energy

Enthalpy changes are usually measured by their effect on a known volume of water in a container known as a calorimeter A chemical reaction occurs and the temperature change is measured A combustion reaction could occur and the temperature increase in a water bath is recorded

Simple Calorimeters

Reactions in solution can be performed in a simple calorimeter, which can be a styrofoam cup with a lid A combustion reaction can be measured using a can with water in it, while the fuel is used to heat the water in the can

All the heat absorbed or evolved changes the temperature of the calorimeter and its contents No heat is gained from, or lost to, the surroundings The solution is sufficiently dilute so its density and specific heat capacity are taken to be equal to water The reaction is fast enough so the maximum temperature can be achieved before the reaction mix begins to cool

Heat exchange with the surroundings, can be minimized with an insulated lid for the calorimeter In combustion experiments, loss of heat with the hot gas used to heat the liquid Uncertainty in the temperature change Heat lost because the reaction is too slow

The heat energy released when one mole of a pure substance is completely burned in excess oxygen under standard conditions Consider: one mole Enthalpy changes of combustion are always negative as heat is released during the process

A student filled a spirit burner with methanol and measured its mass. The spirit burner was then positioned under a tin can that had cm 3 of water in it, and the wick of the burner was lit. The temperature of the water rose by 34.5 ° C. The mass of methanol burned was 0.75 g. What is the enthalpy change of combustion of methanol? Methanol is CH 3 OH See the board for the working out

50.00 cm 3 of mol dm -3 silver nitrate solution was put in a calorimeter and g of zinc powder added. The temperature of the solution rose by 4.3 ° C. Calculate the enthalpy change for the reaction (per mole of silver nitrate that reacts). Assume that the density of the solution is 1.00 g cm -3 and the specific heat capacity of the solution is 4.18 J g -1 ° C -1. Ignore the heat capacity of the metals and dissolved ions. The zinc is in excess. See the working out on the board.

The enthalpy change is quoted for molar amounts in a particular chemical equation. 2Mg (s) + O 2(g) → 2MgO (s) ∆H = kJ mol - 1 This means that 1200 kJ of heat are evolved when 2 moles of magnesium reacts completely with 1 mole of oxygen molecules If g of Mg is burned, how much heat is produced? See the board for the working out.

50.00 cm 3 of 1.0 mol dm -3 HCl was added to cm 3 of 1.0 mol dm -3 NaOH solution. The temperature rose by 6.8 ° C. Calculate the heat of neutralization for this reaction. Assume that the density of the solution and the specific heat capacity is the same as water. What is the enthalpy change of neutralization?

The temperature change can be deduced from a graph. Assume the reaction occurred instantaneously with no heat loss. Draw a line tangent to the max temperature and extrapolate to the point on the graph where the reaction began. See the graph on the next slide.