Energetics Flashcards
Define Enthalpy change
Change in energy
The amount of heat energy taken in or given out during any change in a system provided the pressure is constant
Describe the enthalpy change in an exothermic reaction (2)
- Energy is transferred from the system(chemicals) to the surroundings
- The products also have less energy than the reactants
What is the energy change in exothermic reactions and give examples of exothermic reactions (3 + 1)
- Negative
- Combustion
- Respiration
- oxidation
What happens if there is an enthalpy change (2)
*Energy is transferred between the system and the surroundings
*The system is the chemicals and the surroundings is everything outside the chemicals
Describe the enthalpy change in an endothermic reaction (2)
*Energy is transferred from the surroundings to the system (absorb energy)
* Requires an input of heat energy
What is an example of an endothermic reaction (3) and what is the energy change in endothermic reactions
- The thermal decomposition of calcium carbonate
- The products have more energy than the reactants
- Enthalpy change is positive
What is the standard enthalpy change of formation
When 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa) all reactants and products being taken in their standard states
What is the standard enthalpy change of combustion
The enthalpy change that occurs when one mole of a substance is completely combusted in oxygen under standard conditions all reactants and products being in their standard stages
What are the 4 standard conditions
- 100kPa
- 298K (25C)
- Solutions at 1moldm^3
- All substances should have their normal state symbols at 298
What does incomplete combustion lead to
Soot( carbon)
Carbon monoxide
Water
It will be less exothermic than complete combustion
What is the equation used for a reaction in a solution?
Energy change (J)=Mass of solution (g) x Heat capacity (Jg^-1 K^1) x Temperature change (K)
Q= Mx Cp x Delta T
What is the general method of the Calorimetric method (10)
- Wash the equipment (cup and pipettes etc) with the solutions to be used
- Dry the cup after washing
- Put polystyrene cup in a beaker for insulation and support
- Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
- Clamp thermometer into place making sure the thermometer bulb is immersed in solution
- Measure the initial temperatures of the solution or both solutions if 2 are used. Do this every minute for 2-3 minutes
- At minute 3 transfer second reagent to cup. If a solid reagent is used then add the
solution to the cup first and then add the solid weighed out on a balance. - If using a solid reagent then use ‘before and after’ weighing method
- Stir mixture (ensures that all of the solution is at the same temperature)
- Record temperature every minute after addition for several minutes
What are the errors in the calorimetric method?
- Energy transfer from surroundings (usually loss)
- The method assumes all
solutions have the heat capacity of water. - Neglecting the specific heat capacity of the calorimeter- we ignore any
energy absorbed by the apparatus. - Reaction or dissolving may be incomplete or slow.
- Density of solution is taken to be the same as water.
Explain why bond breaking is endothermic and why bond forming is negative
- Energy is needed to break bonds so enthalpy change is positive (endothermic)
- Energy is released when bonds are formed so when the enthalpy change is negative it is exothermic
What is mean bond enthalpy
Bond enthalpies are always positive
The average energy required to break a certain type of bond
Bond enthalpies are always positive
How would you calculate the overall enthalpy chang of a reaction
Total energy absorbed- total enery released
What is calorimetry used for
To find how much heat is given out by a reaction by measuring a temperature change
How do you find the enthalpy of co,bustion of a flammable liquid
Burn it in a calorimeter
How do you find the enthalpy change using a calorimeter
1) Burn it in a calorimeter
2)as the fuel burns it heats the water–>allowing you to work out the heat energy that has been absorbed by the water if you know the mass of the water, teh temp and the specific heat capcity
Why can you not work out the exact enthalpy change ofcombustion
Heat is lost to the surroundings no matterhow insulated the calorimeter is
Also when you burn a fuel some of the combustion may be incomplete e.g. in flammable liquids as they are volatile
How can you use calorimetery to measyre the enthalpy change in solution (e.g. neautralisation)
Add a known volume of acid to an insulated container (polystyrene cup) and measyre the temperature
Then add a known volume of alkali and record the temp of the mixture at regular intervals over a period of time
Find the temp change for the experiment use it co calculate the nethalpy change of the reaction
If your trying to find the energy change per mole of reactant you might need the formula moles= conc x volume (dm3)
(you can assume that all solutions have the same density as water . e.g. 50 cm3 is 50g)
How can you use a graph to find an accurate temp change
record the temp at regular intervals beginning a couple mins before the you start the reaction
Plot a graph of the results draw two lines of best fit one through the points when u started and then another after you started
Extend both lines so they both pass the time when the reaction started
The distnace between the two lines at the time the reaction started is the accurate temp change for the reaction
Formula for enthalpy chanhge
q=mxcx(delta)T
q= heat loss or gained in joules
m=Mass of water in the calorimeter
c=Specific heat capcity
t=change in temp
What does Hess’s Law state
The total enthalpy change of a reaction is independent of the route taken
