energetics Flashcards
in an endothermic reaction,
energy is taken in from the surroundings
in an exothermic reaction,
energy is released into the surroundings
in an exothermic reaction, temperature of the surroundings
increases
what are 2 examples of exothermic reactions
combustion and neutralisation
in endothermic reactions, temperature of the surroundings
decreases
what is an example of an endothermic reaction
thermal decomposition
how do you remember if energy goes in or out in exo and endo reactions
EXO = EXits
ENDO- in
what is the law of conservation of energy
energy cannot be created or destroyed but it can be transferred
displacement reactions are endo or exo
can be both
neutralisation reactions are endo or exo
always exothermic
combustion reactions are endo or exo
always exothermic
what are the 2 types of calorimetry experiments
enthalpy changes of reactions in solution
enthalpy changes for combustion
to calculate the amount of energy produced by a chemical reaction in solutions what do we do
- measure the temperature change when the solutions are mixed together
- the solutions need to be mixed together in an insulated container to prevent heat loss
the method for reactions in solutions can be used for (3)
- neutralisation reactions
- dissolving solids
- displacement reactions
what is the specific heat capacity of water
4.2
what is the density of water
1 g/cm³
in calorimetry experiments, what do we take for granted / assumptions (4)
- the specific heat capacity of the solution is the same as pure water (4.2)
- density of solution is same as pure water (1 g/cm³)
- reaction is complete
- specific heat capacity of the container is ignored
A calorimeter can be made up of and why
a polystyrene drinking cup, a vacuum flask or a metal can
because they’re insulating materials
how do you carry out a simple calorimetry experiment
- have a polystyrene cup with a plastic lid to prevent heat loss
- add one reagent
- have a thermometer there and measure initial temperature
- add the second reagent and stir continuosly
- record the maximum temperature it reaches and record the rise
- record energy released using Q=mcΔT
equation for energy released
Q = m x c x ΔT
what does Q stand for and units of measurement
heat energy change, J
what does m stand for and units of measurement
mass of the substance being heated, g
what does c stand for and units of measurement
specific heat capacity, J/g/°C
what does ΔT stand for and units of measurement
temperature change, °C
principle of a combustion calorimetry experiment
to use the heat released by a combustion reaction to increase the heat content of water
method for enthalpy of combustion
- fill copper calorimeter with 100cm³ of pure water
- measure initial temp
- weigh spirit burner
- place spirit burner under calorimeter ( under specific distance)
- light the spirit burner and leave for 2 mins
- after 2 mins measure the final temp of the water
- re-weigh the spirit burner
In combustion calorimetry experiment the main sources of error are (2)
Heat losses
Incomplete combustion
difference between the combustion experiment and the neutralisation etc.. experiment
combustion requires a spirit burner to burn
what is enthalpy
energy
what is molar enthalpy change
heat energy change per mole of substance
how do you calculate molar enthalpy change
ΔH= Q / n
= heat energy change / number of moles
molar enthalpy units
kJ/ mol
enthalpy units
joules
when calculating molar enthalpy change, what do you do to the enthalpy (Q) ( think of the units used)
need to divide by 1000, because molar enthalpy is measured in kilojoules while enthalpy is in joules
moles can be figured out using 2 equations
mass / Mr
concentration x volume
in molar enthalpy change, if the temperature of solution or water increased then the ΔH will be positive or negative and what reaction will it be
ΔH will be negative
exothermic reaction
in molar enthalpy change, if the temperature of solution or water decreased then the ΔH will be positive or negative and what reaction will it be
ΔH will be positive
will be an endothermic reaction
for an energy level diagram for exothermic reactions, how will the reactant and product lines look
as energy is being released, the reactant will be higher than the products
draw an exothermic energy level diagram
reactant will be higher than the products, add the arrow pointing down in the middle of then saying that energy is released
for an energy level diagram for endothermic reactions, how will the reactant and product lines look
products will have more energy than the reactants so the line will be higher up
draw an endothermic energy level diagram
reactant line will be low
arrow be pointing up saying energy is absorbed
product line will be high
what are the two labels ( x and y) for an energy level diagram
energy - y
progress of reaction - x
when drawing energy level diagrams, should you state the name of the products and reactants on the line
YES
bond breaking is an _______ reaction because
endothermic reaction,
energy must be supplied to existing bonds in order to break them
bond forming is a _______ reaction because
exothermic
energy is released
total change for enthalpy for a reaction can be calculated if we know
the bond energies of all the species involved
enthalpy change calculation
energy taken in - energy given out
each bond has a ___ energy
bond
what are bond energies
amount of energy needed to break the bond or the amount of energy given out when the bond is formed
for exothermic reactions, enthalpy change will be positive or negative
why
negative
product will have more energy than reactant
for endothermic reactions, enthalpy change will be positive or negative
why
positive
product will have less energy than reactant
an exothermic reaction will occur if ( bond energies)
more energy is released when new bonds are formed than what was needed to break them
an endothermic reaction will occur if ( bond energies)
if the energy taken in to break the bonds exceeds the energy when new bonds are formed
INVESTIGATE temperature changes
Using a measuring cylinder, place 25 cm3 of the NaOH solution into the calorimeter
Measure and record the temperature of the solution
Add 5 cm3 of the dilute HCl and stir
Measure and record the highest temperature reached by the mixture
Repeat steps 1 – 4 increasing the amount of acid added by 5 cm3 each time
