the rate and extent of chemical change Flashcards
Define rate of reaction
- How fast the reactants are changed into products
Name the equation used to calculate/measure the rate of reaction
- Rate of reaction = amount of reactant used / Time
- Rate of reaction = amount of product formed / Time
What can the quantity of reactant or product be measured by?
- Mass in grams or volume in cm3
What are the units of rate of reaction?
- g/s or cm3/s
What are the units for rate of reaction in terms of moles?
- You can also use quantity of reactants in terms of moles (instead of mass or volume) and therefore, units for rate of reaction in mol/s
How do you find the rate of reaction graphically?
- To measure the rate of reaction: draw tangents to curves and use the slope of the tangent
- Calculate the gradient of a tangent to the curve on these graphs as a measure of rate of reaction at a specific time
Describe the relationship between the gradient of the line and the rate of reaction
- The steeper the line on the graph, the faster the rate of reaction
- Over time the line becomes less steep as the reactants are used up
Name the factors which affect the rates of chemical reactions
- Concentration
- Pressure
- Surface area
- Temperature
- Catalysts
What does the rate of a chemical reaction depend on?
- Collision frequency of reacting particles
- Energy transferred during a collision
Define collision theory
- Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
Define activation energy
- The minimum amount of energy that particles must have to react
What happens when you increase the concentration of reactants in solution?
- If a solution is more concentrated, it means there are more particles in the same volume of water
- This means that the same number of particles occupy a smaller space
- Increases the frequency of collisions and so increases the rate of reaction.
What happens when you increase the pressure of reacting gases?
- If pressure increases, it means there are more particles in the same volume of gas
- This means that the same number of particles occupy a smaller space
- Increases the frequency of collisions and so increases the rate of reaction
What happens when you increase the surface area of solid reactants?
- When you break up a solid into smaller pieces it will increase its SA:V ratio
- This means for the same volume of solid, the particles have more area to work on
- Increases the frequency of collisions and so increases the rate of reaction.
What happens when you increase the temperature of the reactants?
- When the temp increases, the particles move faster (gain kinetic energy)
- Increases the frequency of collisions and makes the
collisions more energetic, and so increases the rate of reaction.
Define a catalyst
- Catalysts are substances that speed up chemical reactions without being changed or used up during the reaction
Define an enzyme
- Enzymes act as catalysts in biological systems
Are catalysts included in the equation for a reaction?
- No, catalysts are not included in the equation for a reaction
What is the purpose of a catalyst?
- Decrease the activation energy; this increases the proportion of particles with energy to react
- Provide a different pathway for a chemical reaction that has a lower activation energy.
What is a reversible reaction?
- Where in some chemical reactions, the products of the reaction can react to produce the original reactants
- The direction of the reaction can be changed by changing the conditions
(e.g. for forwards reaction use hot conditions and for reverse use cool)
Describe what happens in a reversible reaction
- As the reactants react, their concentrations fall, so the forward reaction will slow down
- But as more products are made, and their concentrations rise. the backwards reaction will speed up
- After a while, the forward reaction will be at the same rate as the backwards causing equilibrium
What happens if a reversible reaction is at equilibirum?
- There is no overall effect
- The concentrations of reactants and products have reached a balance and won’t change
What symbol is used in a reversible reaction?
- Use the symbol ⇌ instead of → to represent a reversible reaction
Give an example of a reversible reaction
- The Haber Process: hydrogen + nitrogen ⇌ ammonia
What happens if the equilibrium of a reversible reaction lies to the right?
- The conc of products is greater than reactants
What happens if the equilibrium of a reversible reaction lies to the left?
- The conc of the reactants is greater
Describe the energy changes in a reversible reaction
- If a reversible reaction is endothermic one way, it is exothermic in the opposite direction.
- The same amount of energy is transferred each way (just for one the energy will be lost and for the other the same amount will be gained)
- The energy transferred from the surrounding by the endothermic reaction is equal to the energy transferred to the surroundings during the exothermic reaction
Give an example of endothermic and exothermic in reversible reactions
- Thermal decomposition of hydrated copper sulfate
- hydrated copper sulfate <> anhydrous copper sulfate + water
- If you heated blue hydrated copper sulfate crystals, it drives the water off and leaves WHITE anhydrous copper sulfate powder (endothermic)
- If you add water to the white powder, you get blue crystals (exothermic)
When is equilibrium reached in a reaction?
- When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction.
Explain Le Chatelier’s principle
- The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction.
- If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change
What does the position of equilibrium depend on?
- Pressure
- Temp
- Concentration
What is the effect of changing concentration on equilibrium in a reversible reaction?
- If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again.
What happens if the concentration of reactants is increased in a reversible reaction?
- If concentration of reactants is increased: position of equilibrium shifts towards products so more product is produced until equilibrium is reached again
- (decrease reactants by making more product)
What happens if the concretion of products is increased in a reversible reaction?
- If concentration of products is increased: position of equilibrium shifts towards reactants so more reactant is produced until equilibrium is reached again
What happens if the concentration of products is decreased in a reversible reaction?
- System tries to increase it again by reducing the amount of reactants
What happens if the temperature is increased equilibrium in a reversible reaction?
- If temperature is increased: equilibrium moves in the direction of the endothermic reaction (e.g. if forwards reaction is endothermic and temperature is increased, equilibrium shifts right to produce more product)
- To decrease the heat, meaning more products for endothermic and fewer for exothermic
What happens if the temperature is decreased in a reversible reaction?
- If temperature is decreased: equilibrium moves in the direction of the exothermic reaction to produce more heat
- You will get more products for exothermic and fewer for endothermic
For an exothermic reaction, what happens when you increase/decrease the temperature?
- An increase in temperature - decreases yield of reaction
- A decrease in temperature - increases yield of reaction
For an endothermic reaction, what happens when you increase/decrease the temperature?
- An increase in temperature - increases yield of reaction
- A decrease in temperature - decreases yield of reaction
What is the effect of pressure changes on equilibrium in a reversible reaction?
- In gaseous reactions, an increase in pressure will favour the reaction that produces the least number of molecules as shown by the symbol equation for
that reaction.
What happens if pressure is increased in a reversible reaction?
- If pressure is increased: equilibrium shifts to side of equation with fewer moles of gas
(e.g. N2 + 3H2 ⇌ 2NH3, left side has 4 moles of gas (1+3) and right has 2 moles of gas. If you increase the pressure equilibrium moves right as there are fewer moles of gas on the right hand side, making more product)
What happens if pressure is decreased in a reversible reaction?
- If pressure is decreased: equilibrium will shift to side of equation with more moles of gas (e.g. for previous example equilibrium would move left
If a reaction produces a… An increase in pressure…
A decrease in pressure…
…larger volume of gas (more moles) Decreases yield of reaction
Increases yield of reaction
…smaller volume of gas (fewer moles)
Increases yield of reaction Decreases yield of reaction