Energy Changes Flashcards
How do you find the mass of a solution?
Volume x density (assume density is 1 unless told otherwise)
What is the specific heat capacity
The amount of energy that is needed to raise the temperature of a solution by 1 degree
What is the specific heat capacity of water?
4.2 J/g/•C
When do you not add the masses together and when do you?
Don’t add the mass of a solid reactant to the mass of the solution, but if two solutions then add them together
What is the equation for molar enthalpy change?
Molar enthalpy change (kJmol-1) = - heat energy transferred / no. of moles
Delta H (triangle H) = -Q / n
Equation to find energy change
Energy change (J) = mass of solution (g) x specific heat capacity (J/g/•C) x temp change
Q= m x c x Delta T
How does temperature, energy, and molar enthalpy change in an exothermic reaction
Temperature increases
Heat energy transferred is positive
Molar enthalpy change is negative
How does temperature, energy, and molar enthalpy change in an endothermic reaction
Temperature decreases, heat energy is negative, molar enthalpy is positive
What are exothermic reactions and what is the energy of their products like in relation to the reactants
Exothermic reactions release energy into their surroundings
Products have a lower energy than the reactants
What are endothermic reactions and what are their products like in relation to their reactants
Endothermic reactions absorb energy from their surroundings
Products have a higher energy than the reactants
Example of an exothermic reaction
N2 + 3H2 = 2NH3 (Haber process)
Example of an endothermic reaction
H2 + I2 = 2HI
KNOW HOW TO DRAW ENERGY CHANGE DIAGRAMS
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What is the bond energy
Bond energy is the energy required to break one mole of covalent bonds.
Equation for molar enthalpy change from bond energies
Molar enthalpy change = Sum of bonds broken - sum of bonds made
What happens when bonds are broken
It requires energy and is an endothermic process
What happens when bonds are made
Energy is produced, it’s an exothermic process
What is the relative atomic mass (Ar)
The weighted average sum of all the isotopes of an element
Equation for relative atomic mass
Ar = sum (isotope mass x abundance) / 100
Equation for Mr
Sum of Ar
Equation for moles
Mass / Mr
What is Avogadro’s Law
Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. This means that one mole of any gas will occupy the same volume at the same temperature and pressure.
What is the “molar volume” of one mole of any gas?
24dm3 (24000cm3)
Equation to work out moles of a gas
Mols of a gas = volume (cm3) / 24000 (cm3)
What is a redox reaction?
A reaction in which one species is oxidised and another is reduced
What is oxidation
The loss of electrons, loss of hydrogen, addition of oxygen
What is reduction
The gain of elections, addition of hydrogen, loss of oxygen
What is the full equation for magnesium + copper oxide
Mg + CuO = MgO + Cu
What is the ionic equation for magnesium + copper oxide
Mg + Cu 2+ = Mg 2+ + Cu
In magnesium + copper oxide what is the reducing agent?
Magnesium is oxidised so is therefore the reducing agent
In magnesium + copper oxide what is the oxidising agent
Copper is reduced and therefore is the oxidising agent
What are the half equations in magnesium + copper oxide
Mg = Mg 2+ + 2e-
Cu2+ + 2e- = Cu
In a half equation what does e- mean?
“One mole of electrons”
What is the faraday constant and what is its value
The faraday constant is the charge carried by 1 mole of electrons. It has a value of about 96500C/mol
What is the equation for the moles of electrons?
Mol of e- = charge (C) / faraday constant
What is the equation for charge
Charge (C) = current (A) x time (s)
What does an electrolyte contain
Positive and negative ions
What happens at the anode
Negative ions (anions) move to the positive electrode (anode) and lose electrons. This is oxidation.
What happens at the cathode
Positive ions (cations) move to the negative electrode and gain electrons. This is reduction.
What happens at the anode and cathode with molten salts (PbBr2)
(The equations)
PbBr2 (l) = Pb (l) + Br2 (g)
Anode: 2Br- = Br2 + 2e- (oxidation)
Cathode: Pb 2+ + 2e- = Pb (reduction)
What happens at the anode and cathode with concentrated aqueous solutions (NaClaq)
(The equations)
NaCl(aq) ions: Na+, Cl-, H+, OH-
Anode: 2Cl- = Cl2 + 2e-
Cathode: 2H+ + 2e- = H2
What happens at the anode and cathode with dilute aqueous solutions (NaClaq)
(The equations) AND WHY
Anode: 4OH- = 2H2O + O2 + 4e-
Cathode: 2H+ + 2e- = H2
Because there is so much more water than sodium chloride
What are the rules for electrolysis at the anode
If you have a reasonably concentrated solution of halides you get the halogen
With other common negative ions, OH- is oxidised
What are the rules of electrolysis at the cathode
If the metal is high in the reactivity series, you get hydrogen
If the metal is below hydrogen, you get the metal
How would you work out this question:
4g of H2 produced, what mass of chlorine is produced?
(Just write what steps you would do)
1) Write out half equations
2) work out mols of hydrogen
3) work out ratio of moles
4) find mass of chlorine
How would you work out this question:
What mass of elements would be formed from Pb2+ if 1930C of charge was used?
1) write out half equation
2) work out mols of e-
3) work out ratio of moles of Pb (product) : e-
4) find mass of lead
How would you work out this question:
1930C of charge liberates 0.01 mol of cobalt from cobalt nitrate solution, find the charge on the cobalt ion
1) work out the mols of e- for cobalt
2) find ratio of moles of e-: Co
3) write out half equation
4) charge is how many moles of electrons
How would you work out this question:
For how long would a current of 1A need to flow to liberate 0.001 mol of silver from silver ions (Ag+) at the cathode?
1) write out half equation
2) work out charge using faraday constant equation
3) work out time using charge and current equation
What direction is the arrow in an energy level diagram of an exothermic reaction and why?
Down, because negative enthalpy change
What direction is the arrow in an energy level diagram of an endothermic reaction and why?
Up, because positive enthalpy change
How do you measure enthalpy change?
Using calorimetry experiments
Describe the calorimetry experiments for dissolving, displacement and neutralisation reactions
To measure the amount of energy transferred in these reactions:
1) Take the temperature of the reagents (making sure they’re the same)
2) Mix them in a polystyrene cup
3) Measure the temperature at the end of the reaction
How to reduce heat loss in calorimetry experiments for dissolving, displacement and neutralisation reactions
1) polystyrene is an insulator
2) put the polystyrene cup into a beaker of cotton wool to give more insulation
3) put a lid on the cup to reduce heat loss by evaporation
Describe the calorimetry experiment for combustion (to measure the amount of energy produced when a fuel is burnt)
1) To reduce heat loss use a screen to act as a draught excluder
2) Put 50g of water in a copper can and record it’s temperature
3) Weigh the spirit burner and lid
4) Put the spirit burner underneath the can and light the wick. Heat the water, stirring constantly, until the temperature reaches about 50 degrees
5) Put out the flame using the burner lid, and measure the final temperature of the water
6) Weigh the spirit burner and lid again
7) Use the measurements to calculate the molar enthalpy change
Experimental points for the calorimetry experiment for combustion
1) Use a medal can (eg copper) because it is a good heat conductor
2) Reduce heat loss by reducing draughts - by using a screen as a draught excluder and by not doing the experiment near an open window
Apart from burning, name 3 simple exothermic reactions:
- the reaction of metals with acids
- neutralisation reactions
- adding water to calcium oxide
In an exothermic energy diagram where are the reactants/products?
The reactants start with a higher energy than the products then draw a sudden arrow down at 90 degrees until it reaches the lower energy of the products. The arrow can be labelled ‘heat evolved’
what is on the x and y coordinates of an energy diagram
progress of the reaction on x
energy on y
In an exothermic reaction is Delta H negative or positive?
negative because the reactants are losing energy as heat, which is transferred to the surroundings
What is Delta H measured in?
kJ mol-1
What is thermal decomposition?
Breaking something up by heating it
What do carbonates split up to give when they are heated?
Most carbonates split up to give a metal oxide and a carbon dioxide
Give the products of the thermal decomposition of copper carbonate
Copper (II) carbonate is a green powder which decomposes on heating to give black copper (II) oxide.
Give the products of the thermal decomposition of zinc carbonate
zinc carbonate is a white powder which decomposes to give zinc oxide, yellow when hot, white when cooled
Give the products of the thermal decomposition of calcium carbonate
takes a LOT of heat but this reaction is commercially important as it turns calcium carbonate (limestone) into calcium oxide (quick lime)
In an endothermic energy diagram where are the reactants/products?
reactants are lower than the products with an arrow at 90 degrees upwards to the reactants labelled heat absorbed leading to the products.
Why do reactions give out or absorb heat?
During chemical reaction bonds in the reactants have to be broken and new ones formed. Breaking bonds needs energy and energy is released when new bonds are made.
