C6 + C7 - Energy changes and Chemical reactions Flashcards
Why and how do chemical reactions occur?
Chemical reactions occur so that elements can achieve a more stable energy state by gaining a full outer shell of electrons
* done by chemical bonding, where old bonds are broken and new bond are formed
* it involvs the transfer of thermal energy into and out of reaction mixtures
Exothermic reactions
Thermal energy is transferred to the surroundings so the temperature of the surroundings increases
* energy is transferred from the chemical energy store of the chemical system to the surroundings and so the energy of the system falls
Examples of exothermic reactions (name 2)
- Combustion
- Oxidation
- Neutralisation
Endothermic reactions
Thermal energy is taken in from the surroundings so the temperature of the surroundings decreases
* energy is transferred to the chemical energy store of the chemical system and so the energy of the system increases - this means the energy change is positive
Examples of endothermic reactions
- Electrolysis
- Thermal decomposition
- First stages of photosynthesis
Reaction pathway diagrams
Graphical representations of the relative energies of the reactants and products in chemical reactions
Energy level diagram properties (on both exo. and endo. reactions)
- Y-axis: energy of the reactants and products
- X-axis: reaction pathways
- Overall energy change: the difference in height between the energy of reactants and products
Exothermic reaction pathways diagram
- If more energy is released than is absorbed, then the reaction is exothermic
- More energy is released when new bonds are formed than energy required to break the bonds in the reactants
- The change in energy is negative since the products have less energy than the reactants
- Therefore an exothermic reaction has a negative ΔH value
Endothermic reaction pathway diagrams
- If more energy is absorbed to break bonds than is released to form new bonds, this reaction is endothermic overall
- The change in energy is positive since the products have more energy than the reactants
- Therefore an endothermic reaction has a positive ΔH value, which is shown on the energy level diagrams and in calculations
Activation enegry
The initial increase in energy represents the activation energy (Ea), which is the minimum energy that colliding particles must have in order to react
Bond breaking
Endothermic process
Bond forming
Exothermic process
Enthalpy change formula
Enthalpy change (ΔH) = Energy taken in - Energy given out
Physical change
Physical changes (such as melting or evaporating) do not produce any new chemical substances
* easy to reverse
Chemical change
During chemical changes, new chemical substances are formed that have very different properties to the reactants
* usually difficult to reverse
Rates of reaction factors (4)
- Concentration of the reactants in solution or the pressure of reacting gases
- Temperature at which the reaction is carried out
- Surface area of solid reactants
- The use of a catalyst
How does an increased concentration or pressure affect reaction rate?
With increased concentration of a solution or increased pressure of a gas, the rate of reaction will increase
How does surface area affect reaction rate?
With increased surface area of the solid, the rate of reaction will increase
How does temperature affect reaction rate?
With increased temperature, the rate of reaction will increase
How does a catalyst affect reaction rate?
With a catalyst, the rate of reaction will increase
Collision theory
Collision theory states that in order for a reaction to occur:
* The particles must collide with each other
* The collision must have sufficient energy to cause a reaction i.e. enough energy to break bonds
Successful collisions
Collisions which result in a reaction
* If they have sufficient energy (i.e. energy greater than the activation energy), they will react, and the collision will be successful
Unsuccessful collisions
When the colliding species do not have enough energy to break the necessary bonds (i.e. they collide with energy less than the activation energy)
Factors affecting number of successful collision (4)
- Number of particles per unit volume
- Frequency of collisions
- Kinetic energy of particles
- Activation energy - fewer collisions will have an energy that exceeds higher activation energy and fewer collisions will be successful
Process of investigating the effect of surface area on the rate of reaction (5 steps)
- Add dilute hydrochloric acid into a conical flask
- Use a delivery tube to connect this flask to a measuring cylinder upside down in a bucket of water (downwards displacement)
- Add calcium carbonate chips into the conical flask and quickly put the bung back into the flask
- Measure the volume of gas produced in a fixed time using the measuring cylinder
- Repeat with different sizes of calcium carbonate chips (lumps, crushed and powdered)
Results of investigating the effect of surface area on the rate of reaction
Smaller sizes of chips cause an increase in the surface area of the solid, so the rate of reaction will increase
* This is because more surface area of the particles will be exposed to the other reactant so there will be more frequent and successful collisions, increasing the rate of reaction
Process of investigating the effect of concentration of a solution on the rate of reaction (6 steps)
- Measure 50 cm3 of sodium thiosulfate solution into a flask
- Measure 5 cm3 of dilute hydrochloric acid into a measuring cylinder
- Draw a cross on a piece of paper and put it underneath the flask
- Add the acid into the flask and immediately start the stopwatch
- Look down at the cross from above and stop the stopwatch when the cross can no longer be seen
- Repeat using different concentrations of sodium thiosulfate solution (mix different volumes of sodium thiosulfate solution with water to dilute it)
Results of investigating the effect of concentration of a solution on the rate of reaction
- With an increase in the concentration of a solution, the rate of reaction will increase
- This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions, increasing the rate of reaction
Process of investigating the effect of temperature on the rate of reaction (5 steps)
- Dilute hydrochloric acid is heated to a set temperature using a water bath
- Add the dilute hydrochloric acid into a conical flask
- Add a strip of magnesium and start the stopwatch
- Stop the time when the magnesium fully reacts and disappears
- Repeat at different temperatures and compare results
Results of investigating the effect of temperature on the rate of reaction
With an increase in the temperature, the rate of reaction will increase
* This is because the particles will have more kinetic energy than the required activation energy, therefore more frequent and successful collisions will occur, increasing the rate of reaction
Process of investigating the effect of a catalyst on the rate of reaction (5 steps)
- Add hydrogen peroxide into a conical flask
- Use a delivery tube to connect this flask to a measuring cylinder upside down in a tub of water (downwards displacement)
- Add the catalyst manganese(IV) oxide into the conical flask and quickly place the bung into the flask
- Measure the volume of gas produced in a fixed time using the measuring cylinder
- Repeat experiment without the catalyst of manganese(IV) oxide and compare results
Results of investigating the effect of a catalyst on the rate of reaction (5 steps)
- Using a catalyst will increase the rate of reaction
- The catalyst will provide an alternative pathway requiring lower activation energy so more colliding particles will have the necessary activation energy to react
- This will allow more frequent and successful collisions, increasing the rate of reaction
