The Rate and Extent of Chemical Change Flashcards
Why is there a ‘trade-off’ in reaction rates in industry?
- In industry, faster reaction rates are often better as they yield more product in a given amount of time.
- However, it may be expensive and dangerous to generate the conditions required for very fast reactions.
- Therefore, reaction rates used in industry are the result of a trade-off between speed, cost, and safety.
How can the rates of reactions be calculated using graphs?
Rates of reactions at specific times can be calculated using graphs:
- Draw a tangent to the curve (straight line that touches the curve at just 1 point).
- Work out the gradient of the tangent:
Rate of reaction =
changein x /changein y
Why are fast reaction rates beneficial in industry?
Greater product yield in a given time
What is collision theory?
- Collision theory explains how reactions occur and why they occur at different rates.
- Particles must collide with sufficient energy for chemical reactions to happen. This energy is called the activation energy.
- We can increase the rate of a reaction by increasing the frequency (number) of collisions and/or the energy of reactant particles.
What factors can be increased to raise the rate of reactions?
- The concentration of dissolved reactants.
- The pressure of gas reactants.
- The temperature.
- The surface area of solid reactants.
What are catalysts?
Catalysts are substances that increase the rate of chemical reactions by lowering the activation energy. They do this by providing a different reaction pathway.
They often come as powders, pellets or fine gauzes because these types of substance have particularly high surface areas.
What are the advantages of using catalysts in industry?
Cost-effective: catalysts are not used up in chemical reactions, so can be reused. They can also reduce the temperature and pressure required for reactions to occur.
Environmental advantages: by reducing the necessary temperatures and/or pressures, fewer fossil fuels need to be burned, thus reducing negative environmental impact.
What are the disadvantages of using catalysts in industry?
Toxicity: many catalysts are toxic (e.g. transition metals), and these can escape into the environment and contaminate ecosystems.
What are the characteristics of reversible reactions?
The reactants can combine to give the products and the products can combine to give the reactants.
If the forward reaction is exothermic, the reverse reaction is endothermic.
Energy is conserved during the chemical reactions.
What is a closed system?
A closed system is a system where reactants and products can neither be added nor removed.
What are the characteristics of dynamic equilibrium?
If a reversible reaction happens in a closed system, a dynamic equilibrium will eventually be reached.
- At dynamic equilibrium, the rate of the forwards reaction equals the rate of the backward reaction.
- The equilibrium is dynamic because both the forward and backward reactions are still taking place.
- The concentrations of reactants and products do not change.
What is Le Chatelier’s principle?
- Le Chatelier’s principle says that if any of the conditions of a reversible reaction at equilibrium are changed, the system will change to oppose this change.
- This principle is used to predict the outcome of any changes imposed on a system at equilibrium.
What effect does changing temperature have on the equilibrium?
- Decrease temperature: the position of equilibrium will shift in the exothermic direction, increasing the products of the exothermic reaction and decreasing in the products of the endothermic reaction.
- Increase temperature: the position of equilibrium will shift in the endothermic direction, increasing the products of the endothermic reaction and decreasing in the products of the exothermic reaction.
What effect does changing pressure have on the equilibrium?
- Increase pressure: the position of equilibrium will shift to favour the reaction that produces the fewest gas molecules.
- Decrease pressure: the position of equilibrium will shift towards the side of the reaction that produces the most gas molecules.
What effect does changing concentration have on the equilibrium?
- Increase the concentration of a reactant: this will shift the position of equilibrium towards the products.
- Increase the concentration of a product: this will shift the position of the equilibrium towards the reactants.
