Thermochemistry Flashcards
Heat of reaction
the heat change that occurs when a reaction takes place according to a given balanced equation
exothermic reaction
heat is given out from the reaction to the surroundings (-)
endothermic reaction
heat is taken in from the surroundings (+)
ΔH = -
exothermic
ΔH = +
endothermic
Heat of combustion
the heat change that occurs when 1 MOLE of a certain substance is burned in an excess of oxygen
unit of heat of combustion
Kj/mol
if the quantity is not a mole and you want to find the heat of combustion
adjust it (divide or multiply)
when you are finding the heat of combustion which side of the equation does the substance go?
left (it is a reactant)
what is the function of the bomb calorimeter 2
to accurately measure the heat of combustion or the kilogram calorific value
bomb calorimeter:
what holds the substance
the crucible
bomb calorimeter:
what heats it and ignites the sample
wires that can be electrically heated
bomb calorimeter:
what provides the excess of oxygen
an inlet tube
2 things found in the water in the bomb calorimeter and what are their functions
thermometer to measure the heat change and stirrer to get an accurate reading
5 things you need to know about a bomb calorimeter to do the calculation
- amount of substance burned
- initial and final temperature of water
- specific heat capacity of water
- the heat capacity of the calorimeter
- the mass of water in KG
what substances always have the same result
those with a definite formula
what substances always have different results
peat, coal etc. depends on their composition
Kilogram calorific value
the heat that is produced when 1kg of the fuel is burned in an excess of oxygen
units of Kilogram calorific value
KJ/Kg
Heat of formation
the heat change that occurs when 1 MOLE of a substances is formed from its elements in their standard states
unit of Heat of formation
KJ/mole
where does the substance go in the equation when find the Heat of formation
on the left (it is a reactant)
why is it more difficult to find the heats of formation rather than the heats of combustion
as other products along with the main product will be formed also
Hess’ Law
if a chemical reaction takes place in several stages, then the sum of the individual stages equals the heat change if the reaction has been carried out in one single stage
what is Hess’ law an example of
the law of conservation of energy
The law of conservation of energy
energy cannot be created nor destroyed, it can only be converted from one form of energy to another
heat of neutralisation of strong acids
-57 KJ/mol
why are heats of neutralisation of strong acids all the same?
because essentially the same reaction is happening in all of them - the hydrogen ion from the acid reacts with the hydroxide ion from the base to form water
heats of neutralisation of weak acids and explain
less than -57 KJ/mol, extra heat needed for acid to fully disassociate and then extra is subtracted from -57 KJ/mol at the end
DETERMINATION OF HEAT OF REACTION BETWEEN HYDROCHLORIC ACID AND SODIUM HYDROXIDE
where is this experiment carried out and why
in polystyrene cups at they are insulators
DETERMINATION OF HEAT OF REACTION BETWEEN HYDROCHLORIC ACID AND SODIUM HYDROXIDE
what must be known about substances before experiment starts
known volume and molarity
DETERMINATION OF HEAT OF REACTION BETWEEN HYDROCHLORIC ACID AND SODIUM HYDROXIDE
first reading that is taken (separately)
the temperature of each
DETERMINATION OF HEAT OF REACTION BETWEEN HYDROCHLORIC ACID AND SODIUM HYDROXIDE
3 essential things when carrying this out
do it very quickly
stir briefly
sensitive thermometer
DETERMINATION OF HEAT OF REACTION BETWEEN HYDROCHLORIC ACID AND SODIUM HYDROXIDE
why must you do it very quickly?
to ensure that heat was not lost to the surroundings
DETERMINATION OF HEAT OF REACTION BETWEEN HYDROCHLORIC ACID AND SODIUM HYDROXIDE
why do you stir?
to ensure uniform heat distribution throughout the solution so that an accurate temperature reading can be obtained
DETERMINATION OF HEAT OF REACTION BETWEEN HYDROCHLORIC ACID AND SODIUM HYDROXIDE
why is the stirring only brief?
as not to encourage more heat loss
DETERMINATION OF HEAT OF REACTION BETWEEN HYDROCHLORIC ACID AND SODIUM HYDROXIDE
formula
E = mcθ
E
amount of energy produced by amount used
m
mass of solution in KG
c
specific heat capacity
θ
rise in temperature
density =
mass/volume
DETERMINATION OF HEAT OF REACTION BETWEEN HYDROCHLORIC ACID AND SODIUM HYDROXIDE
if the 2 temperatures at the start are unequal
get an average
bond broken
endothermic
requires energy
bond made
exothermic
releases energy
Bond energy
the energy requires to break 1 MOLE of covalent bonds and to separate the neutral atoms completely from each other
energy profile diagrams =
reaction profile diagrams
energy profile diagrams of endothermic reaction
products have more energy reactants
energy profile diagrams of exothermic reaction
products have less energy than the reactants
energy profile diagrams x axis
reaction coordinate
energy profile diagrams y axis
energy
