Energetics (OCR GCSE Chemistry A (Gateway)): Flashcards
Exam code: J248
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Define exothermic reaction.
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Define exothermic reaction.
An exothermic reaction transfers energy to the surroundings, causing the temperature of the surroundings to increase.
True or False?
Combustion is an example of an exothermic reaction.
True.
Combustion releases energy to the surroundings, so the temperature increases — this is the definition of an exothermic reaction.
Define endothermic reaction.
An endothermic reaction takes in energy from the surroundings, causing the temperature of the surroundings to decrease.
Why does the temperature of the surroundings fall during an endothermic reaction?
The temperature of the surroundings falls because energy is transferred from the surroundings into the chemical system. The surroundings lose thermal energy to drive the reaction.
In an exothermic reaction, energy is .......... to the surroundings. In an endothermic reaction, energy is .......... from the surroundings.
In an exothermic reaction, energy is transferred to the surroundings. In an endothermic reaction, energy is transferred from the surroundings.
Define activation energy.
Activation energy is the minimum amount of energy required for a collision between particles to result in a chemical reaction.
On a reaction profile for an exothermic reaction, how does the energy of the products compare to the energy of the reactants?
On a reaction profile for an exothermic reaction, the products have lower energy than the reactants. The overall energy change is negative and the arrow points downwards.
True or False?
In a reaction profile for an endothermic reaction, the products are drawn at a higher energy level than the reactants.
True.
Energy is absorbed from the surroundings in an endothermic reaction, so the products end up at a higher energy level than the reactants.
The .......... is the minimum energy needed for particles to react when they collide, and it is shown on a reaction profile as the vertical distance from the energy level of the reactants to the peak of the energy profile.
The activation energy is the minimum energy needed for particles to react when they collide, and it is shown on a reaction profile as the vertical distance from the energy level of the reactants to the peak of the energy profile.
Give two examples of endothermic reactions.
Examples of endothermic reactions include:
Thermal decomposition (e.g. decomposing calcium carbonate by heating)
Citric acid reacting with sodium hydrogencarbonate
Define bond energy.
Bond energy is the mean amount of energy required to break a chemical bond, or the mean amount of energy released when that bond is formed.
Is breaking a chemical bond an exothermic or endothermic process? Explain why.
Breaking a chemical bond is an endothermic process because energy must be absorbed from the surroundings to separate the bonded atoms.
True or False?
Forming a new chemical bond releases energy to the surroundings.
True.
Bond formation is an exothermic process — energy is released to the surroundings as the new bond is made.
Energy change = energy .......... − energy ...........
Energy change = energy taken in − energy given out.
If the energy taken in to break bonds is greater than the energy released when bonds form, is the overall reaction exothermic or endothermic?
If the energy taken in is greater than the energy released, the overall reaction is endothermic. The energy change is positive.
What does a negative energy change value tell you about a reaction?
A negative energy change tells you the reaction is exothermic. More energy is released when bonds form in the products than is absorbed when bonds break in the reactants.
True or False?
Bond energy values are the same for every molecule containing that bond.
False.
Bond energy values are mean (average) values. The exact energy varies slightly depending on the molecular environment of the bond.
To calculate energy change using bond energies, add up the bond energies of all bonds in the .......... (energy in) and subtract the bond energies of all bonds in the .......... (energy out).
To calculate energy change using bond energies, add up the bond energies of all bonds in the reactants (energy in) and subtract the bond energies of all bonds in the products (energy out).
Why is it useful to draw the displayed formula of a reaction before doing a bond energy calculation?
Drawing the displayed formula lets you identify every bond in both the reactants and products, reducing the chance of missing a bond and getting the wrong energy change.
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