C5 - Energy Changes Flashcards
Exothermic reactions mean what?
That heat is given out, because it releases thermal energy
“If the products of a reaction store more energy than the original reactants, then they must have taken in the difference in energy between the products and the reactants from the surroundings during the reaction.”
What is this referring to?
Endothermic reactions
Endothermic reactions mean what?
That energy is taken in from the surroundings, so it gets colder
Give some examples of exothermic reactions
Combustion (burning fuels), neutralisation reactions (acid + alkali), many oxidation reactions (e.g. sodium + water. The reaction releases energy and the sodium moves about on the surface of the water as it is oxidised)
Give some examples of endothermic reactions
Less common, but:
Citric acid + sodium hydrogencarbonate, thermal decomposition (like heating calcium carbonate causes it to decompose into calcium oxide (also called quicklime) and carbon dioxide)
Give some everyday uses for exothermic and endothermic reactions
Exothermic - some hand warmers use the oxidation of iron in air (with a salt solution catalyst) to release energy
Self- heating cans of hot chocolate and coffee also rely on exothermic reactions between chemicals and their bases
Endothermic - used in some sports injury packs - the chemical reaction allows the pack to become instantly cooler without a freezer
The overall amount of energy will never change, even if the reaction is exothermic or endothermic. True or false?
True. This is because energy is always conserved in reactions, it can’t be created or destroyed, only moved around. This means the amount of energy in the universe is the same
Rearrange these instructions for the required practical of measuring energy transfer:
1) You can reduce it a bit by putting the polystyrene cup into a beaker filled with cotton wool to give more insulation, and putting a lid on top of the cup to reduce energy lost by evaporation
2) You can measure the amount of energy released by a chemical reaction (in a solution) by taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction
3) The biggest problem with energy measurements is the energy lost to the surroundings
4) This method works for neutralisation reactions or reactions between metals and acids, or carbonates and acids
2) You can measure the amount of energy released by a chemical reaction (in a solution) by taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction
3) The biggest problem with energy measurements is the energy lost to the surroundings
1) You can reduce it a bit by putting the polystyrene cup into a beaker filled with cotton wool to give more insulation, and putting a lid on top of the cup to reduce energy lost by evaporation
4) This method works for neutralisation reactions or reactions between metals and acids, or carbonates and acids
5) You can also use this methods to investigate what effect different variables have on the amount of energy transferred - e.g. the mass or concentration of the reactions used
How can you test the effect of acid concentration on the energy released in a neutralisation reaction between hydrochloric acid and sodium hydroxide?
1) Put 25 cm3 of 0.25 mol/dm3 of hydrochloric aacid and sodium hydroxide in separate beakers
2) Place the beakers in a water bath set to 25C until they’re both the same temperature
3) Add the HCl followed by the NaOH to a polystyrene cup with a lid, with a thermometer in the lid
4) Take the temperature of the mixture every 30 seconds, and record the highest temperature
5) Repeat these steps with different concentrations of hydrochloric acids, and record the effects
What do reaction profiles look like roughly?
Energy (y axis) by progress of reaction (x axis) | | | bouncy line thing | |\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
| |. . . . . . . \_\_\_ | ----------/ . . . \ | . . . . . . . . . . . \ | . . . . . . . . . . . \ | . . . . . . . . . .. . .\\_\_\_\_\_ | |\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
Energy (y axis) by progress of reaction (x axis)
Is this exothermic or endothermic?
Exothermic, because the activation energy (between first 2 horizontal lines) is less than the energy released (between first and last horizontal lines)
It releases more energy, therefore it is exothermic
THE DIFFERENCE IN HEIGHT REPRESENTS THE OVERALL ENERGY CHANGE IN THE REACTION PER MOLE
Energy must always be supplied to do what?
To break bonds
During a chemical reaction, old bonds are broken and new ones are formed. Energy must be supplied to break existing bonds - so bond breaking is an endothermic process. Energy is released when new bonds are formed - so bond formation is an exothermic process
Energy must be supplied to break existing bonds - so bond breaking is an endothermic process.
Energy is released when new bonds are formed - so bond formation is an endothermic process.
Which is true?
The first one
Energy must be supplied to break existing bonds - so bond breaking is an endothermic process.
Energy is released when new bonds are formed - so bond formation is an EXOthermic process.
In exothermic reactions, the energy released by forming bonds is…
Greater than the energy used to break them
In endothermic reactions, the energy used to break bonds is…
Greater than the energy released by forming them
