17.2 - Introducing Calorimetry

beware density of water in data booklet

0.997g/mL

If the mass of food burned to produce energy is measured, the energy content of food (energy released per gram of food) can be calculated as follows

Energy content = energy transferred to water/change in mass of food during combustion = q/delta m

heat energy transferred to volume of water can be calculated by measuring (3) and then using formula

Initial temperature of water Highest temperature of water Volume of water Q = m x C x delta T Where q is the energy that's transferred to the water (J), m is the mass of the water (g), C is the specific heat capacity of the water and delta T is the change in temperature of water (C or K) Delta T = delta T final - delta T initial

ways to reduce heat loss during calorimetry (3)

Putting lid on container Insulating beaker of water Placing insulation around burning item, although sufficient oxygen must reach it for combustion to be complete

types of calorimeters (2)

Solution calorimeters are used for reactions taking place in a solution Bomb calorimeters are used for reactions taking place in a sealed bomb vessel

calculation of energy content assumes that

all energy is transferred form burning food to volume of water substantial heat loss occurs, so values calculated for energy content will be less than actual values

specific heat capacity

amount of energy required to increase temperature of 1 g of a substance by 1 C. the specific heat capacity of water is 4.18 J/Cg

which calorimeter reaction is quicker

bomb calorimeter

energy content of food is measured by

burning it in bomb calorimeter sufficient oxygen is required to completely combust food so that all available energy is released

why energy content differs between bomb and solution calorimeter

differs from a nutritional pov because fruits and vegetables contain indigestible carbohydrates in the form of cellulose, and some energy is converted to heat in the body

when some heat energy is transferred to surrounding air, temperature of water

doesn't increase as much as it would if all the energy was used to heat the water

calorimeters minimise

energy losses, enabling accurate results

function of stirrer

ensure temperature of water is uniform

what solution calorimeter can be used for

enthalpy change that occurs when acids react with bases and solids dissolve in water (food such as glucose)

what happens in calorimetry

heat energy released by a chemical reaction can be transferred to water in a separate container.

what happens when energy is transferred from another food across an open space

heat is lost to surroundings e.g. if no lid on container of water, heat will be lost from surface of water

calorimeter

instrument designed to measure energy changes in a reaction. made up of an insulated container of water in which the reaction occurs, with a stirrer and thermometer to measure the temperature change during the reaction. lid is an important part of the insulation.

bomb calorimeter

insulated container in which a sealed, oxygen filled reaction vessel is surrounded by a known volume of water. Combustion reactions are carried out in the reaction vessel and the heat from the reaction is transferred to the surrounding water

solution calorimeter

insulated container that hols a known volume of water and in which a reaction in solution, such as dissolution of a solid or a neutralisation reaction, can be carried out

effect of heat loss

lower change in temperature, delta T, of water results in lower energy value, q

calorimetry

method by which the heat energy released by the combustion reaction is measured

how to tell if reaction is exothermic (2) and vice versa

reaction has released heat energy water in calorimeter has absorbed energy, so temperature of water has increased

why solution calorimeter can't be used to measure energy content of food

reaction is combustion reaction in which food burns in oxygen

what happens when exothermic reaction is carried out underneath a container of water

some of heat released by combustion reaction is transferred to water

insulation prevents

transfer of heat to or from surroundings of calorimeter

in calorimeter, almost of all heat energy released or absorbed is

transferred directly to or from a measured volume of water

water is distinguished by

very high heat capacity, which is a consequence of hydrogen bonding

reaction vessel in bomb calorimeter is designed to

withstand high pressures that may build up during reaction