Energetics Flashcards
Enthalpy change
Heat energy change measured under conditions of constant pressure
Standard enthalpy change of formation
Enthalpy change when 1 mole of a compound is formed from its elements in their standard states at 298K and 100KPa pressure
Standard enthalpy change of combustion
Enthalpy change when 1 mole of a substance is burnt in excess oxygen at 100KPa and 298K with all products and reactants in their standard states
Standard enthalpy change of neutralisation
Enthalpy change when 1 mole of water is produced by the reaction between an acid and an alkali
How do you write standard enthalpy change of formation equations
- Start on the RHS and put 1 mole of the compound formed.
- On the LHS, put the elements in their standard states
How do you write standard enthalpy change of combustion equations
- Start on the LHS and put 1 mole of the substance.
- On the RHS put the combustion products
What is an exothermic reaction
System loses heat energy to surroundings
Products have lower energy than reactants
Delta H is negative
Why are some reactions exothermic
More energy is given out when new bonds are formed in the products than is needed to break the bonds in the reactants, so the reaction is overall negative and hence exothermic
What is an endothermic reaction
System gains heat energy from surroundings
Products have higher energy than surroundings
Delta H is positive
Why are some reactions endothermic
The energy needed to break the bonds in the reactants is greater than the energy given out when new bonds are formed in the products, so the reaction is overall positive and hence endothermic
Describe how to use Hess’ law to calculate enthalpy changes from enthalpies of FORMATION
- Write the reactants on the LHS and products on RHS
- Label the reactants H1 and products H2
- Write the elements in their standard states underneath.
- Draw two arrows one on each side from bottom to top.
- Balance.
- Put the numbers in, looking at the balanced equation at the top
- Enthalpy change = -H1+H2
Describe how to use Hess’ law to calculate enthalpy changes from enthalpies of COMBUSTION when NO equation is given
- Write the elements in their standard states on LHS and compound+O2 on the RHS
- Write the combustion products underneath.
- Draw two arrows from top to bottom, one on each side.
- Balance everything
- Put in the values
- Enthalpy change = H1-H2
Describe how to use Hess’ law to calculate enthalpy changes from enthalpies of COMBUSTION when an equation is given
- Write the reactants on the LHS and products on the RHS.
- Write the combustion products below.
- Draw 2 arrows from top to bottom, one on each side and with O2 beside each arrow.
- Balance everything
- Put in the values
- Enthalpy change = H1-H2
Explain the term thermodynamic stability with respect to this reaction
The reaction is thermodynamically feasible because delta H is negative.
The products are thermodynamically stable with respect to the reactants.
Explain the term kinetic stability/inertness with reference to the reaction between aluminium and zinc oxide
The mixture of aluminium and zinc oxide is kinetically stable/inert because the activation energy is very high. Therefore the reaction does not take place at room temperature. The initial heat provides energy to overcome the high activation energy.
Suggest why the value for the standard enthalpy of formation of liquid antimony is not zero
The standard state of antimony is solid
Suggest why the calculated value for enthalpy change of combustion is different from the value given.
The calculation is based on liquid H2O. However, water is formed as steam which is not the standard state of water. Energy is absorbed when water changes from liquid to gas. Therefore the calculated value will be less negative than the data book value.
When would a reaction be thermodynamically feasible
When delta H is negative
Suggest why a reaction has to be heated to start but not thereafter
The initial heat provides the energy to overcome the high activation energy.
Existing bonds must first be broken before new bonds can form.
More energy is released when new bonds are formed, so the reaction is overall exothermic and provides the energy needed to break more bonds.
Why is the standard enthalpy of formation of oxygen zero
The standard state for oxygen is gas hence it is zero.
Is bond breaking + or -
+
Is bond making + or -
-
What is average/mean bond enthalpy
The energy needed to break 1 mole of bonds in gaseous molecules into gaseous atoms
Or
The enthalpy change to break 1 mole of ……….. averaged over a range of compounds
Why are bond enthalpies average values
They are found from a range of compounds
Why is the C-O bond enthalpy in ethanol slightly different from the C-O bond enthalpy in ethanoic acid
The C-O bond is in different chemical environments
Is the reaction exothermic or endothermic if there is a rise in temperature
Exothermic
Why is the use of mean bond enthalpy values not as accurate as use enthalpies of formation/combustion in finding the enthalpy change of a reaction
Mean bond enthalpy values are average values because they are found from a range of compounds.
Enthalpies of formation/combustion are specific to a given compound and take into account any special structural features
The value of the enthalpy change for the COMBUSTION of methanol can be calculated from the mean bond enthalpy values of the substances in the reaction.
Give 2 reasons why this value differs from the value obtained in the experiment, even after corrections are made for experimental error
- The calculation using mean bond enthalpy values refers to gases. However, when methanol burns it forms water vapour which is not the standard state of water. The experimental value is less negative than the actual value because energy is absorbed to change water from a liquid to a gas.
- Mean bond enthalpy values are average values because they are found from a range of compounds.
Energy transferred (Q) =
mass of solution (usually water or acid+base) x specific heat capacity of water (4.18) x change in temperature
Why is a lid placed on the spirit burned when not in use
It prevents the fuel from evaporating
Why might a copper can used instead of a glass beaker when measuring enthalpy change
Copper is a better conductor of heat than glass so more of the heat is transferred to the water.
How do you make measurement of enthalpy change a fair test
Use the same volume of water
Keep the same distance between the spirit burner and the can.
Keep the strength of the flame the same
Why might the experimental value be less negative/wrong when measuring the enthalpy change for a reaction
- Some heat is absorbed by the beaker.
- Some ethanol from the spirit burner is lost because it evaporates
- Fuel does not burn completely. Incomplete combustion could take place
- Reaction not carried out under standard conditions
- Some heat loss from calorimeter if no lid used
% uncertainity
((number of measurements x error) / actual measurement) x 100
How can you measure out 50g water without a balance
Water has a density of 1 gcm^-3 therefore a volume of 50cm^3 could be measured out using a burette eor measuring cylinder.
enthalpy change =
Energy transferred (Q) / moles
Why must a thermometer be used in calorimetry
So the temperature of the mixture is constant
How to reduce uncertainty in max temperature rise
Measure the temperature of the mixture more often
How would a student measure enthalpy change of combustion values of a fuel in a lab
- Record the temperature rise when a known solution of water is heated for a measured time.
- Calculate the energy transferred using Q = m x c x change in T
- Calculate the enthalpy change for the combustion of 1 mole of the fuel used.
Why might the reaction be carried out in a polystyrene cup rather than a glass beaker
Polystyrene is a better insulator than glass, hence less heat energy is lost to the surroundings, and the temperature change is more accurate
Why might a series of temp readings and a graph be used rather than simply the initial and final readings
- It can be used to eliminate errors due to heat loss which causes the value of enthalpy change to be less negative.
- You can calculate a more accurate temp change
- It allows a line of best fit to be drawn, which allows for the response time of the thermometer
Why must 2 solutions be allowed to stand before mixing
So they are the same temperature initially
Why does the temperature reach a maximum and then falls slightly on further addition of NaOH solution
Temp reaches max when reaction is complete and all the acid is neutralised. Further addition of NaOH solution which is cooler causes temp to fall.
Why is the mass of the metal not used in the calculation of the heat change
The specific heat change of the metal is low compared with that of the solution
What change in volume of water would cause a reduction in heat loss
Increase the volume of water so there is a smaller heat loss by evaporation
Describe the steps you would take to determine an accurate minimum temperature that is not influenced by heat from the surroundings.
Start a clock when …… is added to …….
Record temperature every subsequent minute or in regular intervals for ……. Minutes
Plot a temperature of temperature against time
Extrapolate back to time of mixing = 0 and determine temperature
