C3.1 Introducing Chemical Reactions Flashcards
How do you write formulae of metal elements
Written as empirical formulae
As metals exist as giant metallic lattices
How do you write the formulae of non-metal elements
Individual atoms
Weak intermolecular forces
Group 7 is diatomic molecules, attracted with weak intermolecular forces - 2 atoms covalently bonded together
Other non metal molecules exist as giant covalent structures such as carbon and silicon.
How do you write the formulae for ionic compounds
Total number of positive charges = total number of negative charges
Examples:
NaCl: 1 Na+ ion and 1 Cl- ion
With compound ions if there are more than 1 of that compound, you put it in brackets:
NaOH
Mg(OH)2
Law of conservation of mass
Same atoms are present at the start and end of a reaction
Just joined in a different way
Total mass stays the same during a chemical reaction
Why the mass seems to change during some reactions
Substances can leave or enter the reaction mixture in a non-enclosed system
Usually happens in an open container when the reaction involves a substance in the gas state
What are balanced equations and word equations
Word equations:
Simple model for a chemical reaction
Shows the names of reactants and products involved
Balanced equation: more detailed model
Shows how the atoms are rearranged in a reaction
Relative amounts of each substance involved
Need to consider ion charges from each periodic table group and diatomic elements
How do you write balanced equations
Balanced when there are equal numbers of atoms of each element on both sides of the arrow
Step 1: Write the word equation and write formula for each substance under
Step 2: Add numbers on the left of 1 or more formulae, if necessary, making equal numbers of atoms on each side
Need to consider ion charges from each periodic table group and diatomic elements
Use CHOMO (hint: not a FNAF character)
What are state symbols
Show the physical state of each substance in a chemical reaction
Written in subscript letters in brackets next to products/reactants
State symbol-meaning
(s) - solid
(I) - liquid
(g) - gas
(aq) - aqueous solution
What are half equations
Model for the change that happens to 1 reactant in a chemical reaction
Balance charges
Balance numbers of atoms
Make sure of diatomic elements (H, N, F, O, I, Cl, Br)
Example: 2Na(s) + Cl2(g) -> 2NaCl(s)
Atom to ion example: Cl2 + 2e- —> 2Cl- (reduction)
Ion to atom example: Cl2 —> 2Cl- + 2e- (oxidisation)
What are ionic equations
Show the ions present in a reaction mixture
Also includes the formulae of any molecular substances present
Example:
Default: HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)
Ionic equation: H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) - Na+(aq) + Cl-(aq) + H2O(l)
H2O is not split up as it is covalently bonded not ionic bonding.
What are spectator ions
Ions that appear in the same form on both sides of the equation.
They are in the reaction mixture but do not take part in the reaction
Net ionic equation
Leaves out spectator ions
Form an ionic equation using half equations (links to C3.4 Electrolysis)
- Write down the anode half equation: 2Cl- —> Cl2 + 2e-
- Write down the cathode half equation: 2H+ + 2e- —> H2
- Combine both equations, keeping same species on either side of arrow: 2Cl- + 2H+ + 2e- —> Cl2 + 2e- + H2
- Cancel out electrons on either side: 2Cl- + 2H+ —> Cl2 + H2 (2e- goes from either side)
What is a precipitate
Product when 2 solutions are mixed
Always in solid state
What is a mole
Unit for amount of substance
1 mole of anything contains the same number of things
How many entities are there in 1 mole
Avogardo’s constant = 6.02 x 10²³
6.02 x 10²³ entities in 1 mole
An entity is a molecule, and some substances are bonded, with different numbers of atoms eg. Oxygen
For that multiply: number of atoms in the molecule x the number of moles x 6.02 x 10²³
How do you measure a mole of a substance
Number of moles = Mass / Ar or Mr
How do you calculate masses of reactants and products
Mass (g) = Ar x moles
Mole calculation
Example: Nitrogen reacts with hydrogen to produce ammonia:
N2(g) + 3H2(g) -> 2NH3(g)
1) Calculate molar masses using Ar values:
Molar mass of N2 = 2x14=28g/mol
Molar mass of NH3 = 14 + (3x1) = 17g/mol
2) Calculate the amount of N2 in 84 g by the equation: mass = molar mass x amount
Amount = mass/molar mass = 84g/28g per mol = 3 mol
3) Calculate the moles of NH3 made from this amount of N2:
From the balanced equation:
1mol of N2 makes 2mol of NH3
So, 3 mol of N2 makes 6mol of NH3
4 calculate the mass of NH3 in this amount:
Mass = molar mass * amount
Mass = 17g/mol x 6mol = 102g
What is a limiting reactant
1 reactant is usually in excess in a reaction mixture
More is present than is needed to react with the other reactant, so some is left at the end
Other reactant is in a limiting amount, that gets used up first.
Amount of product formed is determined by the amount of the limiting reactant
Reactant with less moles
Calculating stoichiometry
Example: An oxide of copper is heated with excess hydrogen H2 forming 6.4g of copper and 0.9g of water. Calculate the balanced equation. Ar of copper=63.5 , Ar of water = 18
1) Calculate the amount of each measured substance:
Moles of Cu = mass/Ar = 6.4g/63.5 = 0.1 mol
Moles of H2O = mass/Ar = 0.9g/18 = 0.05 mol
2) Simplify the ratio of the substances:
Cu : H2O = 0.1:0.05 = 2:1
The right of the equation = 2Cu + H2O
Hydrogen is H2 so the equation must be Cu2O + H2 -> 2Cu + H2O
What charge do hydrogen ions have
+1 charge
What charge do metals in groups 1+2+3 form
positive ions
charge same as non IUPAC group number
What charge do transition metals have
Transition metals preproduction positive ions that usually have a 2+ charge, except from silver and iron(lll)
What charge do non metals in groups 5+6+7 have
negative
number of charges = 8 - their non IUPAC group number
What are the 7 diatomic elements
hydrogen (H)
nitrogen (N)
oxygen (O)
fluorine (F)
chlorine (Cl)
bromine (Br)
and iodine (I)
They are called diatomic elements because the atoms appear in pairs.
Metal + acid
Metal + acid => hydrogen + salt
-OH
Hydroxide
NO₃-
Nitrate
CO₃²⁻
Carbonate
SO₄²⁻
Sulfate
Electronic structure of bromine
2.8.18.7
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