Energy Flashcards
What are energy level diagrams used for?
To show the relative energies of reactants and products in a chemical reaction, measured in kJ/mol
Describe an exothermic graph
- The reactants are at higher energy level than the products
- The energy change symbol is the energy given out to the surroundings
- The temperature will increase
- The DeltaH will be negative
What is an example of an exothermic reaction?
Combustion of methane, neutralisation as well
Describe an endothermic graph
- The reactants are at lower energy level than the products
- The energy change symbol is the energy taken from the surroundings
- The temperature will decrease
- The DeltaH will be positive
What is an example of an endothermic reaction?
Thermal decomposition of limestone and evaporation
What is the formula for calculating energy change?
Energy change (Q) = mass of water x 4.2 x temperature change
What is bond energy?
The energy required to break 1 mole of X-Y covalent bonds. An average energy value is associated with a particular bond
What type of process is making bonds?
Making bonds produces energy and is an exothermic process (MEXO)
What type of process is breaking bonds?
Breaking bonds requires energy and is an endothermic process (BENDO)
How do you calculate molar enthalpy change (DELTAH)?
DeltaH = -Q/no.moles (check if negative is needed if it works i.e. endo or exo reaction
What does DeltaH stand for?
- Molar enthalpy change
- Negative if exothermic reaction
- Positive if endothermic reaction
How do you use bond energies to calculate enthalpy change during a reaction?
DeltaH = bonds broken - bonds made
Always draw out display formulas
What is the bond energy?
The energy needed to break a bond
How can you find out enthalpy change?
Using calorimetry which allows you to measure the amount of energy transferred in a chemical reaction
What is one way of calorimetry with dissolving, displacement, and neutralisation reactions?
To measure the amount of energy transferred in these reactions (in solution) you can just take the temperature of the reagents (making sure they are the same), mix them and measure the temperature of the solution at the end of the reaction
1. To investigate the enthalpy change of dissolving, displacement or neutralisation reactions you can do it by mixing the reactants in a polystyrene cup
2. The biggest problem with energy measurements is the amount of energy lost to the surroundings
3. You can reduce it a bit by putting the polystyrene cup into a beaker of cotton wool to give more insulation and putting a lid on the cup to reduce energy lost by evaporation
Example:
-Place 25cm cubed of dilute hydrochloric acid in a polystyrene cup, and record the temperature of the acid
-Place 25cm cubed of dilute sodium hydroxide solution in a measuring cylinder and record its temperature
-Add the alkali to the acid a stir
-Take the temperature of the mixture every 30 seconds, and the record the highest temperature it reaches
*Water and calcium oxide
*Citric acid and hydrogen carbonate
*Zinc and copper sulphate
*Sodium carbonate and ethanoic acid
How do you do calorimetry with combustion?
To measure the amount of heat energy produced when a fuel is burnt you can simply burn the fuel and use the flame to heat up some water. This method uses a metal container, usually made of copper because copper conducts heat so well
Method:
1. Use a screen to act as a draught excluder (and do not do it next to an open window), because it is important to make sure as much heat as possible goes into heating the water and reducing draughts is key here
2. Put 100cm cubed of water in the copper can and record its temperature
3. Half fill the spirit burner with ethanol and measure its mass (can set up same apparatus with propanol as the fuel)
4. Clamp the copper can so that the bottom of the wick is 5cm cubed above the wick of the spirit burner. Light the wick.
5. Heat the water, stirring constantly, until the temperature reaches about 50 degrees Celsius
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. You can then use the measurements you have taken to calculate the enthalpy change
What does equilibrium mean?
- The proportions of reactants and products are unchanging
- If a reversible reaction takes place in a closed system (none of the reactants or products can escape) then a state of equilibrium will always be reached
What does the term dynamic equilibrium mean?
- Both the forward and backward reactions are still occurring, but at the same rate
- This will happen only in a closed system
What is the effect of concentration on the equilibrium?
- If the concentration of reactant goes up, the equilibrium shifts to the RHS
- If the concentration of reactants goes down, the equilibrium shifts to the LHS
What is the effect of temperature on the equilibrium?
- The hotter the reaction the more likely it is to go to the endothermic reaction
1. Temperature increase, equilibrium shifts to the endothermic
2. Temperature decreases, equilibrium shifts to the exothermic direction
What is the effect of pressure on the equilibrium?
- The higher the pressure the more the reaction moves in the direction with less gas molecules (number)
1. Pressure increases: equilibrium shifts to the side with fewer gaseous molecules
2. Pressure decreases: equilibrium shifts to the side with more gaseous molecules
How do you answer equilibrium questions?
- Say SHIFT to the right/left
- WHY shift e.g. more gaseous molecules/lower temperature to counteract the increase in temperature)
(3. Result if asked for e.g. colour change)
What would be the effect of a catalyst on equilibrium?
- No change/effect
- This is because the catalyst increases the rate of the forward and reverse reactions by the same amount
Describe the reversible reaction of the dehydration of hydrated copper (II) sulphate. What is the equation?
- Heat some hydrated copper (II) sulphate crystals in a test tube until you have a white powder called anhydrous copper (II) sulphate
- Tip the powder onto a watch glass add water drop by drop (blue powder)
- Return the contents of the watch glass to the test tube and heat again
- CuSO4.5H2O (reversible reaction sign) CuSO4 + 5H2O
- Forward reaction is endothermic
- Backward reaction is exothermic
Describe the reversible reaction of the effect of heat on ammonium chloride. What are the equation?
- Heat a small spatula of ammonium chloride in a test tube
- Heat gently the bottom of the test tube only
- Use a loose mineral wool plug in the end of the tube to prevent ammonium chloride escaping
- Observe the top and bottom parts of the test tube whilst heating
- NH4CL (s) (white solid) (reversible reaction sign) –> NH3 (g) (colourless) + HCL (g) (not hydrochloric acid)
- Bottom of test tube: brown solid
- NH4Cl –> NH3 + HCL
- Top of test tube: White gas
- Cooler NH3 (g) + HCL (g) –> NH4Cl (g)
What is an exothermic reaction?
A chemical reaction in which heat energy is given out
What is an endothermic reaction?
A chemical reaction in which heat energy is taken in
What is Le Chatelier’s principle?
If a dynamic equilibrium is disturbed by changing the conditions, the reaction moves to counteract the change
