Energy changes & rates flashcards

Key points

• Exothermic: energy is transferred to the surroundings, so their temperature rises.
• Examples: combustion, neutralisation, respiration, and the oxidation inside hand warmers.
• Endothermic: energy is taken in from the surroundings, so their temperature falls.
• Examples: thermal decomposition, and the reactions in sports cold packs.

Key points

• Use a polystyrene cup with a lid, standing in a beaker: the polystyrene cuts the energy lost to the surroundings.
• Measure the starting temperature of the first solution, add the second, stir and record the highest temperature reached.
• Keep the volumes and concentrations the same each time, so only the variable you are testing changes.
• A rise means exothermic; a fall means endothermic.

Acid and alkali mixed

Key points

• It plots the energy of the reactants, the products and the barrier between them.
• Activation energy: the minimum energy colliding particles need to react. The arrow runs from the reactants up to the peak.
• The overall energy change is the gap between the reactants and the products.
• Products lower than reactants: exothermic. Products higher: endothermic.

Key points

• Breaking bonds takes energy in: it is endothermic.
• Making bonds gives energy out: it is exothermic.
• Exothermic overall: making the new bonds releases more energy than breaking the old ones takes in.
• Endothermic overall: breaking the bonds takes in more energy than making the new ones releases.

Key points

• Add up the energy taken in to break every bond in the reactants.
• Add up the energy given out making every bond in the products.
• A negative answer means exothermic; a positive one means endothermic.

Formula

H₂ + Cl₂ → 2HCl

Key points

• Rate is how quickly the reactants are used up or the products form.
• A gas given off: collect it in a gas syringe and record the volume at regular times.
• Or put the flask on a balance and record the mass lost as the gas escapes.
• A precipitate forming: time how long a cross under the flask takes to disappear. A shorter time means a faster rate.

Key points

• Units are amount over time, such as cm³/s or g/s.
• On a graph of product against time, the steeper the line, the faster the reaction.
• The line is steepest at the start and flattens as the reactants are used up. It is flat when one runs out.

Formulae

50 cm³ of gas in 20 s

Key points

• Draw a tangent: a straight line that just touches the curve at that time.
• The rate at that moment is the gradient of the tangent: change up ÷ change across.
• Read the two changes from points far apart on the tangent, to make the reading more accurate.

Formula

Tangent at 30 s

Key points

• Disappearing cross: mix sodium thiosulfate with hydrochloric acid over a cross. The sulfur made turns the mixture cloudy.
• Time how long the cross takes to disappear, for several concentrations of acid.
• Or react magnesium with acid of each concentration and collect the hydrogen in a gas syringe.
• Keep the volumes, the temperature and the mass of solid the same each time.

Key points

• Particles react only when they collide with at least the activation energy.
• The rate depends on how often these successful collisions happen.
• At a higher temperature the particles move faster, so they collide more frequently.
• More of those collisions also have at least the activation energy, so the rate rises for both reasons.

Key points

• Higher concentration: more particles in the same volume, so collisions are more frequent.
• Higher pressure of a gas: the particles are closer together, so collisions are more frequent.
• Larger surface area (a powder, not lumps): more particles are exposed, so collisions are more frequent.
• None of these makes the particles move faster: only temperature does.

Key points

• A catalyst speeds up a reaction without being used up, so a small amount works again and again.
• It provides a different pathway with a lower activation energy, so more collisions are successful.
• On a reaction profile, the peak is lower; the reactants and products stay at the same energy.
• Enzymes are biological catalysts. In industry, catalysts save energy and money, as reactions run fast at lower temperatures.

Key points

• The products can react to make the reactants again. The sign ⇌ shows this.
• The direction can be changed by changing the conditions, such as heating.
• If the forward reaction is exothermic, the backward reaction is endothermic, by exactly the same amount of energy.

Copper sulfate

Key points

• At equilibrium the forward and backward reactions happen at exactly the same rate.
• So the amounts of reactants and products stay constant, though both reactions keep going.
• It is only reached in a closed system, where nothing can get in or out.
• A catalyst speeds up both directions equally: equilibrium is reached sooner, with no more product.

Key points

• A system at equilibrium shifts to oppose any change made to it.
• Raise the temperature: it shifts in the endothermic direction, taking in the extra energy.
• Lower the temperature: it shifts in the exothermic direction.
• So if the forward reaction is exothermic, a higher temperature gives less product.

Key points

• Raise the pressure on gases: it shifts towards the side with fewer gas molecules.
• Lower the pressure: it shifts towards the side with more gas molecules.
• Add more of a reactant: more products form until a new equilibrium is reached.
• Remove a product: more reactants react to replace it.

Making ammonia