Energy changes Flashcards
2What is meant by the law of conservation of energy?
Energy is conserved in chemical reactions and cannot be created or destroyed. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place.
This explains why if a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.
What is meant by an exothermic reaction?
A reaction that transfers energy to the surroundings so the temperature of the surroundings increases.
EXOTHERMIC
- temp of surroundings increases
- temp of reaction increases
What is meant by an endothermic reaction?
A reaction that takes in energy from the surroundings so the temperature of the surroundings decreases.
ENDOTHERMIC
- temp of surrounding decreases
- temp of reaction decreases
Examples of exothermic reactions
- Combustion reactions
- Neutralisation reactions
- Oxidation reactions
Examples of endothermic reactions
- Reaction of citric acid and sodium -hydrogen carbonate
- Thermal decomposition
How are endothermic reactions used in every day life?
- Sports injury packs are based on endothermic reactions
How are exothermic reactions used in every day life?
- Hand warmers
- Self-heating cans for food and drink
Draw the reaction profile for an exothermic reaction.
Draw the reaction profile for an endothermic reaction and an exothermic reaction.
Why do the products have lessenergy than the reactants on the reactant profile of an exothermic reaction?
- Energy has been transferred from the reactants to the surroundings.
- So products will have less energy than reactants by that amount
The difference in energy between the reactants and the products is:
The amount of energy taken in/transferred to surroundings.
Why do the products have more energy than the reactants on the reactant profile of an endothermic reaction?
- Energy has been transferred from the surroundings to the products
- So reactants will have less energy than products by that amount.
Where is the activation energy on a reaction profile
The peak/max point
- Measure from reactant to peak
What is meant the activation energy?
The minimum energy particles must have to react.
Explain the concept of energy in reactions.
For particles to react energy is required so they can collide.
The minimum amount of energy particles must have to react = activation energy.
Making bonds?
- Exothermic
Releases energy, transferred to surroundings
Breaking bonds?
- Endothermic
Energy is required, taken in.
remember: BREAKing bonds ENDothermic
Break up = End
What does the energy value of a chemical bond tell us?
The energy required to break that bond.
True or false?
During a chemical reaction:
* energy must be supplied to break bonds in the reactants
* energy is released when bonds in the products are formed.
True
REMEMBER:
In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds (which is why -).
In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds (which is why +)
Describe a practical to investigate the variables that affect temperature changes in a chemical reaction.
- AIM; investigate temp change in a neutralisation reaction of HCL(hydrochloric acid) and sodium hydroxide.
In this experiment we add increasing volumes of sodium hydroxide to HCl, and each time we measure the max temp reached.
IDV - Sodium hydroxide
DV - maximum temperature reached
CV - volume of hydrochloric acid, concentrations of both the hydrochloric acid and sodium hydroxide solution.
- Use a measuring cylinder to measure 30cm^3 of dilute hydrochloric acid.
- Transfer this into a polystyrene cup and stand the cup inside a beaker (the beaker stops it from falling over).
- Use a thermometer to measure the temp of the acid and record in a table
- Use a measuring cylinder to measure 5cm^3 of sodium hydroxide solution
- Transfer this to to the polystyrene cup
- Immediately place a lid with a hole over the cup and put thermometer thru the hole in the lid
- Ensure the bulb of the thermometer is in the solution
- Use thermometer to gently stir solution
- Bc reaction is exothermic energy is released so temp of solution should increase
- Observe temp rise on thermometer, when reading stops increasing measure the highest temperature reached.
- Rinse out and dry the polystyrene cup and repeat experiment using 10cm^3 sodium hydroxide solution.
- Continue to repeat this experiment several more times and each time increase the volume of sodium hydroxide solution by 5 cm^3, until we reach a maximum of 40 cm^3
- Repeat all of this 1 more time, and calculate a mean for each volume of sodium hydroxide solution.
- At the end plot a graph of your results after recording results in a table.
OBSERVATIONS:
- As volume of sodium hydroxide solution is increased, max temp reached increases
- This is because there are more reactant particles of sodium hydroxide to react with hydrochloric acid
- Because this is an exothermic reaction more energy is released
- So max temp reached increases.
- At a point max temp begins to decrease
- So much NaCl added, not enough HCl
- Greater volume of solution
- Energy released in now more spread out within the greater vol of solution
- So less energy transferred to surroundings.
Why do we use a polystyrene cup with a lid during RP4?
- to reduce heat loss to the sides and the bottom bc polystyrene is a good thermal insulator
- lid reduces heat loss to the air
At a point, when we continue to add NaCl to HCl max temp transferred to surroundings begins to decrease, as shown on graph. suggest why.
- So much NaCl added, not enough HCl (1)
- Greater volume of solution (1)
- Energy released in now more spread out within the greater vol of solution (1)
- So less energy transferred to surroundings (1) - which is why max temp transferred to surroundings falls.
Give an example of an electrolyte
A solution of an ionic compound
An electrolyte is a liquid that can conduct electricity.
