3.1.4 Energetics, A Level Chemistry, AQA
questions with complete solutions.
What is enthalpy change? pressure
ANS.Heat energy transferred in a reaction at constant
What are standard conditions for enthalpy change? temperature (normally 25 degrees celcius)
ANS.100kPa and stated
What values of deltaH do exothermic reactions have? -
ANS.Negative (less than 0)
What values of deltaH do endothermic reactions have? -
ANS.Positive (more than 0)
What do exothermic reactions do with energy? -
ANS.Give out energy
What do endothermic reactions do with energy? -
ANS.Absorb energy
Are oxidation reactions endo or exothermic? Is combustion endo or exothermic? -
ANS.Exothermic
ANS.Exothermic
Are thermal decomposition reactions endo or exothermic? Is photosynthesis endo or exothermic? What happens to reactant bonds? -
ANS.Endothermic
ANS.They are broken
ANS.Endothermic
What happens to product bonds? -
ANS.They are formed
Which bonds are broken: reactant or product bonds? -
ANS.Reactant
Which bonds are formed: reactant or product bonds? -
ANS.Product
Is energy taken in or released when you break bonds? -
ANS.Taken in
Is bond breaking endo or exothermic? -
ANS.Endothermic
What values of deltaH does bond breaking have? -
ANS.Positive (more than 0)
How does bond breaking energy change as the bond strength increases? takes more energy to break
Is energy taken in or released when you make bonds? Is bond making endo or exothermic? -
ANS.It
ANS.Released
ANS.Exothermic
What values of deltaH does bond making have? -
ANS.Negative (less than 0)
How does bond making energy change as the bond strength increases? release more energy when formed
ANS.They
If you need more energy to break bonds than is released when bonds are made, what
values of deltaH do you get? ANS.Positive (more than 0)
If you need less energy to break bonds than is released when bonds are made, what
values of deltaH do you get? ANS.Negative (less than 0)
What is bond enthalpy? -
ANS.The energy required to break bonds
What is the mean bond enthalpy? ANS.The average energy needed to break a
certain type of bond, over a range of compounds
Why are mean bond enthalpies not exact? ANS.They are an average of energies
needed to break the bond over a range of compounds
Why may a mean bond enthalpy in a data book be different from the mean bond
enthalpy in a specific molecule? ANS.The mean bond enthalpy in the data book is
the average for a bigger range of molecules than just the specific molecule.
Why are mean bond enthalpies always positive? endothermic
ANS.Breaking bonds is always
Is energy absorbed or given out when bonds are broken? -
ANS.Absorbed
Is energy absorbed or given out during bond formation? -
ANS.Given out
How do you calculate the enthalpy change of a reaction (in terms of energy)? ANS.Total energy absorbed - total energy released
How do you calculate the enthalpy change of a reaction (in terms of bonds)? ANS.Breaking bonds - Making bonds
Why aren't enthalpy changes for reactions, calculated using mean bond enthalpies, not
exact? ANS.Mean bond enthalpies are averages over a range of molecule and
hence aren't totally accurate for certain reactions
How do enthalpy changes calculated from mean bond enthalpies compare to those
calculated using Hess's Law (in terms of accuracy). ANS.Using mean bond
enthalpies makes your result slightly less accurate than using Hess's Law
What is the standard enthalpy of formation? ANS.The enthalpy change when 1
mole of a compound is formed from its elements in their standard states under standard
conditions
What is the standard enthalpy of combustion? ANS.The enthalpy change when 1
mole of a substance is completely burned in oxygen under standard conditions
What do you use to find out how much heat is given out by a reaction? ANS.Calorimetry
What experiment can you use to find enthalpy changes? -
ANS.Calorimetry
How do you perform a calorimetry experiment for the combustion of a flammable liquid?
ANS.You burn the flammable liquid in a calorimeter so it heats the water. You can
work out the heat energy that has been absorbed by the water.
What three things do you need to know about the water to calculate the enthalpy of
combustion of a flammable liquid? ANS.Mass of the water, the temperature change
and the specific heat capacity of water.
What happens in an ideal calorimetry experiment for the combustion of a flammable
liquid? ANS.All the heat given out by the fuel as it burns would be absorbed by the
water
Why is it hard to get an accurate result for a calorimetry experiment? ANS.You lose
heat to the surroundings, some of the combustion maybe in complete which gives less
energy out, or you may lose some fuel to evaporation due to the flammable liquid's
volatility.
Why would subtracting the starting temperature from the highest temperature recorded
not be very accurate for a calorimetry experiment? ANS.Because of the heat lost to
surroundings
When should you record the temperature for a calorimetry experiment? ANS.At
regular intervals, beginning a couple of minutes before you start the reaction
How can you use a graph to find an accurate temperature change for calorimetry? ANS.Draw a line of best fit through the points before the reaction started and one
through the points after it started. Extend both lines so they pass the time when the
reaction started and look at the distance between the two lines at the time the reaction
started (before any heat was lost).
How do you perform a calorimetry experiment for a neutralisation reaction in solution? ANS.Add a known volume of acid to an insulated container and measure the
temperature. Add a known volume/mass of alkali and stir, recording the temperature of
the mixture at regular intervals over a period of time.
Why do you stir the solution during a calorimetry experiment for a solution? ensure it is evenly heated
ANS.To
What do you assume for a calorimetry experiment for a solution? same density as water
ANS.It has the
How do you calculate the number of moles from concentration? concentration x volume
ANS.moles =
What formula do you use to calculate an enthalpy change from a calorimetry
experiment? ANS.q=mc(deltaT)
What is q in q=mc(deltaT)? pressure is constant)
ANS.Heat lost or gained/enthalpy change (if the
When is heat lost or gained the same as enthalpy change? constant
What are the units for q in q=mc(deltaT)? What is m in in q=mc(deltaT)? calorimeter
ANS.Joules (J) or Kilojoules (kJ)
ANS.Mass of water (or other solution) in the
What are the units for m in q=mc(deltaT)? What is c in q=mc(deltaT)? -
ANS.If the pressure is
ANS.Grams (g)
ANS.Specific heat capacity of water
What are the units for c in q=mc(deltaT)? -
ANS.J g^-1 K^-1
What is the specific heat capacity of water (definition)? ANS.The amount of heat
energy it takes to raise the temperature of 1g of water by 1K
What is the specific heat capacity of water (numerical value)? What is deltaT in q=mc(deltaT)? solution
ANS.4.18 J/g/K
ANS.The change in temperature of the water or
What are the units for deltaT in q=mc(deltaT)? -
ANS.Kelvin (K0)
How can you calculate the enthalpy of combustion (in kJ mol-1)? ANS.Use
q=mc(deltaT) to calculate the amount of heat given out by the fuel and change to kJ by
dividing by 1000. Calculate the moles of fuel used using moles = mass/Mr. Divide the
heat given out by the number of moles and make sure to add a minus sign to the final
result.
Why is the enthalpy of combustion from calorimetry negative? ANS.The reaction is
exothermic, as evidenced by it raising the temperature of the water/solution.
What is Hess's Law? of the route taken
ANS.The total enthalpy change of a reaction is independent
What does delta r H mean? -
ANS.The enthalpy change of a reaction
What is the enthalpy change of formation for elements? -
ANS.Zero
At which temperature are all standard enthalpy changes measured at? -
ANS.298 K