7b Flashcards
Exothermic reactions
In an exothermic reaction, the energy released in bond formation is greater than the energy used in breaking old bonds. The leftover energy is released into the surroundings and the temperature rises.
Endothermic reactions
In an endothermic reaction, the energy required to break old bonds is greater than the energy released when new bonds are formed. The extra energy needed is absorbed from the surroundings and the temperature falls.
Endothermic reactions are much less common than exothermic reactions,
Eg: thermal decomposition reactions - The thermal decomposition of calcium carbonate is endothermic. Heat must be supplied to make calcium carbonate decompose into calcium oxide and carbon dioxide. The equation for this reaction is:
CaCO2 → CaO + CO2
Reaction profiles
A reaction profile is a graph that shows how the energy in a reaction change as the reaction progresses. The graph starts at the energy level of the reactants and finishes at the energy level of the products. These two points are usually joined by a smooth curve.
three useful pieces of information you can find from a reaction profile
- The overall energy changes.
- Whether the reaction is exothermic or endothermic
- The activation energy
The overall energy change
The overall energy change of a reaction is the difference between the energy of the reactants and the energy of the products. You can find the overall energy change of a reaction from a reaction profile by looking at the difference in height between the reactants and the products
Whether the reaction is exothermic or endothermic
Reaction profiles show the relative energies of the reactants and the products, so you can use them to work out whether a reaction is exothermic or endothermic.
In an exothermic reaction, the reactants have more energy than the products, because energy is released during the reaction. This means the reaction profile will start high and finish lower than where it started.
In an endothermic reaction, the products have more energy than the reactants, because energy is taken in during the reaction. This means the reaction profile will start low and finish higher than where it started
The activation energy
Reaction profiles don’t normally go straight from the reactants to the products -the graph will curve upwards before it starts to go down again. This is because some energy usually has to be put in to break the bonds in the reactants and get the reaction started.
The activation energy (E) is the minimum amount of energy the reactant particles need when they collide with each other in order to react. The greater the activation energy, the more energy that is needed to start the reaction. If the energy input is less than the activation energy there won’t be enough energy to start the reaction, so nothing will happen.
You can find the activation energy of a reaction from its reaction profile by looking at the difference between where the curve starts (the reactants) and the highest point on the curve
energy transferred vs temperature change
The amount of energy transferred during a reaction is proportional to the temperature change of that reaction. So you can use the temperature change as a measure of the heat energy transferred.
How to measure a temperature change
You can follow the change in temperature of a reaction mixture as a reaction takes place. Here’s how:
1. Put a polystyrene cup into a beaker of cotton wool (the cotton wool gives insulation to help limit energy transfer to or from the reaction mixture).
2. Add a known volume of your first reagent to the cup.
3. Measure the initial temperature of the solution.
4. Add a measured mass/volume of your second reagent and use the thermometer to stir the mixture.
5. Put a lid on the cup to reduce any energy lost by evaporation.
6. Record the maximum or minimum temperature (depending on whether it’s increasing or decreasing) that the mixture reaches during the reaction.
7. Calculate the temperature change.
Dissolving salts in water
You can measure the temperature change when dissolving salts in water by adding the salt to a polystyrene cup of water and measuring the change in temperature when the salt has dissolved.
Examples
* Dissolving ammonium chloride decreases the temperature of the reaction mixture it’s endothermic.
* Dissolving calcium chloride causes the temperature of the solution to rise it’s exothermic.
Neutralisation reactions
In a neutralisation reaction, an acid and a base react to form a salt and water.
Examples
* Most neutralisation reactions are exothermic: e.g., HCI + NaOH → NaCl + H₂O
* However, the neutralisation reaction between ethanoic acid and sodium carbonate is endothermic.
Displacement reactions
In a displacement reaction, a more reactive element displaces a less reactive element in a compound. These types of reactions are accompanied by a release of energy- they’re exothermic.
Example - Zinc powder and copper sulphate react in a displacement reaction forming zinc sulphate and copper.
Precipitation reactions
Precipitates are insoluble solids which can sometimes form when two solutions are mixed together. All precipitation reactions are exothermic.
Example - The reaction between lead(II) nitrate solution and potassium iodide forming a lead iodide precipitate would result in an increase in the temperature of the surroundings.
Units of energy
Energy is usually measured in joules (J). Large energy values are often given in kilojoules (kJ) - there are 1000 joules in a kilojoule.
When measuring energy transfer in reactions, the amount of energy released or absorbed will depend on how much reactant is used. As a result, energy transfer is usually measured in kilojoules per mole of reactant (kJ mol-¹), so that comparisons can be made between different reactions.
Bond energy calculations
Not all bonds are the same strength-it requires more energy to break some bonds than others. Every chemical bond has a particular bond energy associated with it.
e.g. A carbon-carbon (C-C) bond has a bond energy of 348 kJ mol. This means that it takes 348 kJ of energy to break one mole of C-C bonds. It also means that 348 kJ of energy is released when one mole of C-C bonds is formed.
formula to work out the overall energy change
Energy change = Energy of bond breaking - Energy of bond making
break - make
(think breakfast)
