c8 - rates of reaction Flashcards | Knowt (2024)

• a measure of how quickly a reactant is sued up / a product is formed

• mean rate of reaction = quantity of reactant used or product formed / time taken

• unit: g/s or cm3/s

• quantity of reactant/product can be measured by mass in grams or by volume in cm3

• slope of line (gradient) = rate of reaction

• the steeper the slope the faster the rate of reaction

• larger rate of reaction means more product being formed/reactant being used

• over time, rate slows down as less reactant is available to form products

• flat line → rate of reaction = 0 (has stopped)

• you many need to draw a tangent to the curve to determine the rate of reaction (gradient)

• chemical reactions can only take place when the reacting particles collide with each other and to do this they must have sufficient energy

• the rate of chemical reaction is determined by the frequency (number of successful collisions per second) of successful collisions

• the rate is initially high because more molecules available means more potential collisions

• if a solution is more concentrated, there are more particles within the same volume of substance

• if pressure is increased, the same number of particles collide within a smaller area

• as a result the number of collisions per second will be greater

• rate is proportional to the concentration and pressure

• 50 cm3 of hydrochloric acid into a conical flask

• fit the bung & delivery tube on top of flask

• fill a measuring cylinder half way with water and invert it over a trough half filled with water

• position the delivery tube so that it’s underneath the mouth of the inverted cylinder

• add a strip of magnesium ribbon to flask, replace bung and start stopwatch

• record volume of gas given of in intervals (e.g. every 10s)

• repeat with different concentrations of HCL

* risk assessment: wear safety goggles, and take care with glassware

1. add 10cm3 sodium thiosulfate to conical flask with 40cm3 of water (dilutes it)

2. place conical flask on printed black paper cross

3. add 10cm3 of hydrochloric acid, begin gently swirling flask and start stopwatch

4. looking down, stop the clock when the cross is no longer visible

5. record time taken for cross to disappear in seconds

6. repeat with different concentrations (ratio of sodium thiosulfate to water)

7. should be proved that higher concentration= faster cross disappears

* risk assessment: take care to not breathe in any sulfur dioxide fumes from flask

• if a solid reactant is broken up into pieces, its SA: Vol ratio is larger meaning there is more area of particles to collide with in the reaction from the same volume

• reactions happen faster with increased temperatures

• particles gain kinetic energy so collide more frequently and energetically

• more successful reactions in a given time

• higher proportion of particles have energy greater than the activation energy

• increase rate of reaction without being used up themselves

• they provide an alternative pathway for the reaction that requires less activation energy

• activation energy is the minimum energy that particles need to collide (react)

• NOT included in chemical reaction equations

• when a reversible reaction occurs in a closed system equilibrium is reached when the forward and backward reaction are occurring at exactly the same rate

• initially forward reaction is fast, and backward slow but over time this reaches equilibrium

• if equilibrium lies to the right, concentration of products is greater than reactants

• if equilibrium lies to the left, concentration of reactants is greater

• position of equilibrium depends on temperature / pressure / concentration

• in reversible reactions, the products of the reaction can react to re-form the original reactants

• if a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction, and the same amount of energy is transferred in each case

• anhydrous = without water

• states that if a system is at equilibrium and a change occurs to any of the conditions, the system responds to counteract the change and restore equilibrium:

  • if concentration of reactants/products is changed, equilibrium is lost; so if concentration of products decreases, system restores equilibrium by reducing amount of reactants, or if amount of reactants increases, system opposes this by making more products

  • more reactant = more product and less product = more reactant used up (reduced)

  • if pressure on one side is increased, equilibrium shifts in direction of fewer molecules/if pressure is decreased, equilibrium shifts in direction of more molecules

  • decreasing temp = equilibrium shifts in exothermic direction to produce more heat so more exo. products than endo

  • increasing temp = equilibrium shifts in endothermic direction to reduce heat so more endo. products produced than exo

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