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